NASA file

The Viking 1 lander sent back America's first pictures from the Martian surface in 1976. This picture shows off the lander's U.S. flag and Bicentennial logo as well as the planet's landscape.

Thirty-six years after an experiment conducted by NASA's Mars Viking lander sparked controversial claims about the presence of life on the Red Planet, NASA's next Mars mission could conceivably hint that those claims were correct after all.

At least that's the hope held by the experiment's principal investigator, Gil Levin, who is keeping the Mars Viking flame alive even in retirement. He still thinks Viking was "the most remarkable unmanned mission ever," but he worries that its legacy will be lost amid the scientific shuffle.

"Twenty or thirty years from now, when the economy permits NASA to rise again, there will be missions to Mars, and they will find life, and they will take credit for it and not mention Viking at all," he told me.

It might not take 20 or 30 years to bring Viking back into the spotlight, however. NASA's $2.5 billion Mars Science Laboratory mission is due to deliver the car-sized Curiosity rover to the Red Planet in August — and although the space agency insists that Curiosity doesn't have the capability to detect life, Levin believes it could show that his experiment was on the right track when it detected the chemical traces of organic activity.

GilLevin.com

Gil Levin was principal investigator for the Mars Viking probe's Labeled Release experiment.

Hopes of confirming the presence of life on Mars were riding high when the twin Viking landers touched down on Mars in 1976. The scientific payload included the Labeled Release apparatus, designed by Levin and his colleagues, as well as three other life-detection experiments. The Labeled Release experiment, or LR, was set up to take a bit of Martian soil and add a drop of water containing nutrients tagged with radioactive markers. The air above the mix was then monitored to see if it gave off a radioactive gas such as carbon dioxide or methane. That could be read as an indication that organisms in the soil were metabolizing the nutrients.

If the experiment came up with a positive response, a duplicate soil sample — the control — was heated to a temperature that should have been high enough to destroy microbes, but not to destroy any strong chemicals that might have produced a similar response sans life. 

The good news for Levin and the other life-hunters was that the LR experiment came out positive, and the control experiment came out negative. The bad news was that two of the other experiments came out negative, but they were based on different assumptions about potential Martian life. The really bad news was that the fourth experiment, conducted by Viking's Gas Chromatograph - Mass Spectrometer device, or GCMS, didn't detect any organic molecules in the soil.

The failure to find any organics led most scientists to assume that there was nothing living in the soil. Most scientists assumed that the LR findings were just a fluke. But not Levin.

"If these results are precisely the same as the results from biological entities on Earth, that's hard to get around," he told me. Dozens of explanations have been put forward for the LR results — for example, that the Martian environment is so chemically reactive, due to ultraviolet radiation, that the nutrients were broken down without life playing a part. Levin, however, says those explanations don't match up with the results produced during the LR experiments and the control experiments.

Hoping for new evidence
This might have ended up as one of those cold cases where nobody totally convinces everybody. But Levin says Curiosity's impressive array of scientific equipment could provide some hot new evidence. It has a suite of instruments known as Sample Analysis at Mars, or SAM, which is capable of detecting organic molecules in Martian soil or atmosphere. Another instrument suite, called ChemCam, can fire a laser blast at a soil or rock sample up to 23 feet (7 meters) away and use a spectroscopic imager to analyze the chemical composition of the vaporized material.

"I predict that one or more of these instruments, possibly all of them, will indeed find organic matter that the Viking GCMS missed," Levin said.

Finding organic molecules is not the same as finding life. After all, organic compounds have been detected within the interstellar stuff of distant galaxies, and it wouldn't be earth-shattering to detect them on Mars as well. But it would answer the main objection raised about the LR results.

Even more telling evidence could come from Curiosity's high-resolution cameras. Some of the pictures taken during the Viking mission showed colored patches on Martian rocks that were a fair spectrographic match for the color of lichen on earthly rocks. "The spectra were identical, but of course the images were not sharp enough to be able to make a conclusion, and everybody pooh-poohed it," Levin said.

Curiosity's color cameras will have much better resolution, and Levin said they "could detect sufficient detail to establish whether these might be lichenlike organisms." It might even be possible to take multiple looks at the same rocks, and track whether their appearance goes through the kinds of changes one would expect from lichen.

Levin said lichen, which is one of the hardiest types of organisms on Earth's surface, could conceivably have hitchhiked from Earth to Mars on meteorites. "Preserved, frozen, they could survive the entry to Mars and grow under Martian conditions," he told me.

The long search for life
The scientists who are in charge of Curiosity and the Mars Science Laboratory say that they're aiming for the same goal that Levin has in mind, but they argue that the search for life on Mars has to follow a step-by-step process.

"What the world needs to understand is that this is really the very beginning of a very systematic and deliberate form of exploration," Caltech's John Grotzinger, principal investigator for Mars Science Laboratory, told me. "The era of 'Star Trek' exploration is not over, but ... one must be more deliberate about it, because that's the way we do it on Earth, and we know that works."

Levin, however, thinks the evidence to come will show that Viking was working correctly 36 years ago. "To suggest that we should go back and start at a lower level ... means we throw away a billion dollars, in 1976 dollars. That's about $5 billion or $6 billion today that we don't have," he said.

He'd like to see a future Mars mission duplicate the LR experiment with a few added technological twists, including a check to see whether the active agent that Viking detected in the soil shows a preference for lefthanded or righthanded versions of the same molecule. Levin says that characteristic, known as chiral preference, would be strong confirmation of life, "since chemistry cannot distinguish chirality and reactions occur equally with both 'mirror images.'"

Levin also thinks the findings from Viking should be given another good, hard look.

"Let's convene a panel of astrobiologists," Levin said. "Let's have Levin present his data. Let's have the antagonists present their data. Let's examine this trove of data which we've never examined fairly."

Will that happen in Levin's lifetime? The researcher is now 88 years old, and nobody lives forever. But he's hoping that when the next episode in the saga of the search for life on Mars plays out ... maybe in the next few months ... the Viking missions will get their share of the spotlight.

"The stories increasingly omit any mention of Viking," Levin said. "I think Viking should be lauded rather than ignored."

More about Viking and the Mars saga:

• How the hunt for Mars life evolved
• Study suggests Viking found organics on Mars
• Were life's building blocks picked up on Mars?
• Did Viking probes find Mars life ... or kill it off?
• Did life on Earth actually come from early Mars?

Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding Cosmic Log's Google+ page to your circle. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for other worlds.

This image shows a type of bacteria called GFAJ-1 that was said to incorporate arsenic in its cellular machinery.

Researchers say they ran a more rigorous version of the experiment that sparked a yearlong debate over the prospects for arsenic-based bacteria — but found no trace of arsenic within the organisms' DNA.

The findings, submitted to the journal Science this week and distributed openly via the ArxiV.org website, serve as the most definitive refutation to date of the "weird life" claims that caused such a stir in December 2010. "They match with what basically all the scientists had concluded a year ago," University of British Columbia microbiologist Rosie Redfield, the paper's senior author, told me.

Redfield had criticized the original study from the start, suggesting that the arsenic detected in a strain of bacteria known as GFAJ-1 was not actually incorporated into the machinery of life but was merely the result of insufficient purification. "We were much more meticulous about purifying the DNA before we analyzed it," she said today.

She and her colleagues worked with the same bacteria used for the original research, which had astrobiologist Felisa Wolfe-Simon as lead author and was published in Science. The bacteria were bred to live in a high-arsenic environment, with virtually no phosphorus present. The aim was to see whether arsenic compounds known as arsenates, which are typically poisonous to life as we know it, could be substituted for chemically similar phosphorus compounds known as phosphates. If that turned out to be the case, that would suggest that alien life forms could operate using biochemical processes radically different from Earth's.

In their paper, Wolfe-Simon and her colleagues said they saw evidence that the bacteria could be bred to live in the arsenic-rich environment, and that arsenates were detected in "macromolecules that normally contain phosphate, most notably nucleic acids and proteins."

Redfield and her colleagues were able to grow the bacteria amid high arsenic levels, under special conditions, but they found that the arsenic wasn't necessary for the bacteria's survival — and that the highly purified DNA from the bacteria did not contain detectable levels of arsenate.

Redfield noted that some arsenate stuck to the DNA even after what she thought would be sufficient purification, but was removed during a second round of washing. "That shows that arsenate does persist through steps in the DNA purification, but in a form that will wash away," she told me.

The researchers acknowledged that arsenate might occasionally get into the bacteria's biological machinery.

"Given the chemical similarity of arsenate to phosphate, it is likely that GFAJ-1 may sometimes assimilate arsenate into some small molecules in place of phosphate, such as sugar phosphates or nucleotides. Our results do not rule out the possibility that such assimilation could be beneficial," they wrote. "When it comes to DNA synthesis, however, GFAJ-1 does not appear to productively assimilate any arsenate."

Open review for results
The scientists behind the original study have said they would refrain from commenting on follow-up research until the peer-review and publication process is completed. Wolfe-Simon did not immediately respond to an emailed request for comment, but Science News' Rachel Ehrenberg quoted her as saying she and her colleagues never actually claimed that arsenate was being incorporated in GFAJ-1’s DNA.

"As far as we know, all the data in our paper still stand,” Science News quoted Wolfe-Simon as saying in an email. “Yet, it may take some time to accurately establish where the [arsenic] ends up."

That response left Redfield figuratively scratching her head — and literally wondering "WTF??" in a Twitter update. She pointed to several references in the original Science paper referring to DNA, including a sentence saying that the measurements "specifically demonstrated that purified DNA extracted from +As/-P [high-arsenic, low-phosphorus] cells contained As [arsenic]."

The paper written by Redfield and her colleagues is open for review and comment even in advance of its consideration for journal publication. That's consistent with Redfield's advocacy of an "open science" approach to research, as reflected in the regular updates posted to her RRResearch blog. Thanks to the blog, avid followers of the #ArsenicLife issue have known for weeks that the original results couldn't be replicated.

Redfield said she has received assurances that freely distributing the draft paper won't hurt the prospects for publication in Science — which goes against the traditional grain for peer-reviewed publication.

"What's happening, and I'm really pleased by this, is that interested people are reading the manuscript, and they're putting comments on it," she observed.

Redfield said she and her colleagues appreciated the feedback being posted to her blog by experts — as well as by non-experts. "Their comments are going to let us polish the manuscript to make it more accessible to non-experts," she told me.

More about the arsenic-life debate:

• A year later, the debate still percolates
• Nature: Study challenges existence of arsenic-based life
• Chemical & Engineering News: The arsenic aftermath

In addition to Redfield, the authors of "Absence of Arsenate in DNA From Arsenate-Grown GFAJ-1 Cells" include M.L. Reaves, S. Sinha, J.D. Rabinowitz and L. Kruglyak.

Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding Cosmic Log's Google+ page to your circle. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for other worlds.

This artist's impression shows Earth alongside the super-Earth known as 55 Cancri C, which is thought to be a little more than twice as wide as our planet and 7.8 times as massive.

Two years ago, Harvard astronomer Dimitar Sasselov stunned the world when he claimed there might well be 100 million Earth-size planets in the Milky Way. To some, the number sounded shockingly high. But the torrents of data that have come in from planet-hunters since then suggest that, if anything, the estimate was almost laughably low.

Just this month, researchers reported that there are probably more planets than stars in our galaxy, which would bring the total count well past the 100 billion mark. What's more, astronomers say the planets toward the lower end of the scale — "super-Earths" that are up to 10 times as massive as our own planet — are likely to be more common than Jupiter-scale planets.

"Small planets are really much more abundant than big planets," Sasselov told me last week.

Planet-hunters have already identified more than two dozen super-Earths beyond our solar system, including a batch of 16 announced on a single day last September. A couple of weeks ago, scientists spread the news about three planets smaller than Earth, and last week the science team for NASA's Kepler space telescope mission added still more super-Earths to the list.

That kind of planetary plenitude has even had an impact on the funny pages: "I don't know why this isn't the only thing people are talking about!" one character told another last week in the Arlo & Janis comic strip.

Basic Books

"The Life of Super-Earths" focuses on how the hunt for alien worlds and artificial cells will revolutionlize life on our planet.

It's the main thing that Sasselov is talking about, for more than one reason. He's a co-investigator for the $600 million Kepler mission, the director of the Harvard Origins of Life Initiative, and the author of a new book titled "The Life of Super-Earths." In the book, he makes the case that super-Earths could be as hospitable to life as our own planet, and perhaps even more so. Super-Earths that lie in the habitable zones around their parent stars — that is, the zones where water can exist in liquid form — would be prime candidates in the search for signs of extraterrestrial life.

"Life is not rare, it seems," the Bulgarian-born astronomer says.

Sasselov talked about the Kepler mission, the plenitude of planets and its implications for the search for alien life during our wide-ranging interview. Here's an edited transcript of last week's Q&A:

Cosmic Log: Do you look back at your estimate from two years ago and just shake your head at the idea that you were guessing so low? Were people making a fuss over something that now seems obvious?

Dimitar Sasselov:I feel that I was on the right track. Basically, yes, we have on one hand an even larger number of planetary candidates than I anticipated two years ago. The numbers went up. However, there is also a result which cancels those large numbers. There is a fly in the ointment. The caveat is that as it happens, most of our planetary candidates and confirmed planets are in relatively short orbits.

That means two things. First of all, they don’t directly tell you what the exact prediction about planets in the habitable zone should be.

Second, a lot of our small-planet candidates are in compact, multi-planet systems. Planets are closely packed next to each other, and these planets usually are within the orbit of Mercury around a star which is not that different from the sun. So there must be something extraordinary about the way they formed. It's quite possible that the formation and evolution required to create such architectures in planetary orbits is different in some fundamental way from planetary formation and early evolution in our solar system.

Jon Chase / Harvard

Dimitar Sasselov is a professor of astronomy at Harvard University.

So it is still a question mark as to what these planets are telling us, and what they are made of.

For the Kepler-11 system, we have the mean density of the planets. Those little planets are very low-density planets. They’re nothing like a bigger version of Earth. They have envelopes of hydrogen, or probably hydrogen and helium. They're like mini-versions of Neptune and Uranus. There are no planets like that in our solar system, so we don't know much about them.

It’s a cautionary tale there. Yes, there may be plenty of planets that are just two to three times more massive than our own Earth. But their mean density may be very low, because they formed farther out and migrated inward, and ended up in the moderate temperature regions of their planetary systems.

What would happen if we have a very large number, maybe billions, of super-Earth-size planets in the habitable zones — but half of them, or even nine out of 10 of them, are these mini-Neptunes? Would I consider them Earthlike? Definitely not, because they don't have the same geochemistry.

So while on one hand, the numbers have gone beyond my expectations, the diversity has gone beyond my expectations, too. And that means we might have a lot of planets with something different from an Earthlike geochemistry. Looking at the physics and the geochemistry is the only way we can go to the next step — and that is the search for signatures of life.

Q: What is the next step? How do you go from Kepler and planet detection to getting at the more fundamental questions?

A: To me, the next big step is to go from discovery and detection of planets like our Earth, to understanding their geochemistry. We have to do that to be effective in searching for biosignatures. The way we would do the first step — that is, understanding geochemistry — is by finding enough planets that are close to us. Kepler's candidates are a little bit too far for a good follow-up on characterization. So in terms of a practical approach, we should be gearing up for surveys of the nearby population of stars, and discovering those nearby planets.

There, the news from Kepler is good, because the statistics are high. If the statistics were low, then it would take more of an effort. Once we make that survey, and we can practically accomplish that in the next 10 years, we can jump onto those planetary candidates, and do atmospheric analysis, and try to understand the diversity of their atmospheres. This is a necessary step to talk about the signatures of life. Otherwise, we'd be looking blindly.

Q: Some people might say, well, let's just look for oxygen or methane, or something we associate with life on Earth. 

A: That wouldn't be prudent at all. If we just look at biosignatures as we understand them on our own Earth today, they correspond to a particular moment in time in which the microbial communities on this planet have managed to change the atmosphere in a particular way. For about half of the history of life on Earth, the atmosphere wasn't anything like what it is today. It would be foolish to just assume that all life shares the same biochemistry and the same history.

Theoretically speaking, we should not assume that all planets that otherwise resemble Earth have the same geochemical cycle. There are alternatives.

Q: What sort of mission would work for this next step?

A: There are two approaches that need to be taken. The first one, when it comes to discovery, is a combination of space- and ground-based surveys. The space surveys would use smaller arrays of telescopes in orbit, and would scan the entire sky by observing the brightest stars, nearest to us, in a selective manner. But as opposed to concentrating in one direction, which was necessary due to the design of Kepler, we can select the nearest stars over the entire sky.

This can also be done from the ground for a particular subset of stars, which are the M stars. These stars are so much smaller than a sunlike star that the transits for Earth-size planets are much more prominent. You can see them using ground-based telescopes. You don't need to go to space. The trick is to do the whole sky and catch all those M dwarfs, and catch the transits.

Q: One of themes in your book is that we shouldn't limit the planet search to Earth-size planets, because the planets that are bigger than Earth — the super-Earths — might be more conducive to life than even our own planet. How can that be?

A: What we're finding out about super-Earths places them front and center as the most suitable places for life to emerge. These are planets that are only slightly bigger than Earth. In terms of size, we're talking about an average of 50 percent larger. In terms of mass, we're talking about two, three, five times as massive — maybe 10 in some cases, but overall, made of the same stuff.

Then you just compare the whole range of planets, from Mars to Earth to the largest super-Earths. In all different levels of comparison, the super-Earths end up being equal or slightly better when compared with Earth.

For example, one of the problems a planet could encounter is the ability to keep water liquid on the surface, and to have the good chemical exchange between the interior and the surface. That’s very difficult to do if you don’t have an atmosphere. An atmosphere in the habitable zone is difficult to keep, because it evaporates over the course of billions of years. If you have a small planet, made of rock but still low mass, like Mars is, eventually you lose more of your atmosphere than if you have a bigger planet. There is no negative factor, it is just more of a good thing.

Here's another example. A lot of people would say we have it good here on Earth because the moon keeps the axis of Earth's rotation more stable than it otherwise would be. It's the kind of momentum effect you get when you're on a bicycle — you can let the handlebars go and you still go straight. In a similar way, the existence of the moon out there cancels out the additional push and pull from the other planets, which could from time to time turn the axis of Earth dramatically and change the climate. This is what we think happened a few times on Mars. The more massive a planet is, the less vulnerable it would be to these effects.

Q: Is it always "the bigger, the better," until you get into a Neptune-class ice giant?

A: It's always the bigger the better. There's either no difference, or it's better. I didn’t find anything which was actually detrimental about having a big planet. Larger g-force, having more gravity on the surface, has a small effect when it comes to building biological structures, such as the membranes of cells. The list goes on and on. Everything gets better when you're slightly bigger.

Q: How long do you expect this book to stand up? I suppose that's an occupational hazard when you're writing about planet-hunting.

A: I would say it should stand up until we discover life out there on another planet, or in the lab when we manage to put it together as an artificial minimal cell. Then, of course, we'll open a whole new chapter in the history of science — and it will be so exciting that I wouldn't care. If a new book needs to be written, I will be happy to do so.

More about the planet search:

• NASA mission piles on the planets
• 160 billion planets in the Milky Way?!
• Three newfound worlds are smaller than Earth
• Flash interactive: How other worlds are found
• SETI researchers check signals in exoplanet study
• Millions of Earths? Talk causes a stir
• Cosmic Log archive on planets

Dimitar Sasselov will talk about the planet search during a book tour that takes him to Boston on Thursday and on Feb. 17, to New York on Feb. 6, San Francisco on Feb. 8 and Seattle on Feb. 10.

Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding Cosmic Log's Google+ page to your circle. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for other worlds.

Planetary Habitability Lab / UPR

This "periodic table" of exoplanets, including confirmed planets as well as candidates from NASA's Kepler mission, places exoplanets into 18 categories based on mass and temperature. The numbers keep track of how many worlds are in which categories. Click on the image to see a larger, more readable version.

Researchers have set up an online "periodic table" for extrasolar planets ranging from Hot Mercurians to Cold Jovians, with Earthlike worlds right in the middle. 

The Habitable Exoplanets Catalog, drawn up by the University of Puerto Rico's Planetary Habitability Laboratory, is aimed at pigeonholing the hundreds of worlds that are being identified by NASA's Kepler space telescope and other planet-hunting projects. Eventually, the tally of exoplanets is expected to mount into the thousands, and that's where researchers hope the proposed catalog will come in handy.

"One important outcome of these rankings is the ability to compare exoplanets from best to worst candidates for life," Abel Mendez, the laboratory's director and principal investigator for the project, said today in a news release.

Also today, Kepler's scientists said they've confirmed the existence of their first exoplanet solidly within the habitable zone of its solar system, where water could exist in liquid form at a pleasant 72 degrees Fahrenheit (22 degrees Celsius). That certainly sounds livable, but Mendez told me that the planet, known as Kepler-22b, doesn't quite fit into the sweet spot for habitability because it's closer in size to Neptune than to Earth.

"I confirmed its radius, and Kepler-22b is a low-end Warm Neptunian, very close to a Superterran," Mendez said in a Twitter back-and-forth from NASA's Ames Research Center in California, where he was presenting his research at the Kepler Science Conference.

Neptunians are likely to have a gaseous rather than a rocky composition, which might make it tough for life as we know it on Kepler-22b. However, the situation might be more hospitable on a moon orbiting the planet, just as it is in the movie "Avatar" for the inhabitants of Pandora, a fictional moon orbiting the gas giant Prometheus.

How the catalog was created
The Habitable Exoplanets Catalog sets up a matrix of 18 pigeonholes based on temperature and mass: Planets in the Hot Zone would be too close to their parent suns for water to exist in liquid form. Water would exist only as ice in the Cold Zone, but could take liquid form in the Warm Zone. The catalog sets up six categories of planetary mass: Mercurians (think Mercury), Subterrans (Mars-size), Terrans (Earth-size), Superterrans (up to 10 times as massive as Earth), Neptunians (Neptune-size) and Jovians (Jupiter-size).

To figure out which planets fit which categories, the catalog draws upon a variety of resources, including the Kepler database of candidates, the Extrasolar Planets Encyclopaedia, the Exoplanet Data Explorer, the Earth Similarity Index, the Habitable Zones Distance metric and the Global Primary Habitability index.

The initial classification of more than 1,600 confirmed planets and yet-to-be-confirmed candidates puts only 16 potential worlds in the habitable categories — that is, Warm Subterrans, Warm Terrans and Warm Superterrans. But that list will grow: The Kepler team announced today that its tally of candidates has risen to 2,326, based on the first 16 months of the space telescope's mission. Forty-eight of those candidates are said to lie in their stars' habitable zones.

"The tremendous growth in the number of Earth-size candidates tells us that we're honing in on the planets Kepler was designed to detect: those that are not only Earth-size, but also are potentially habitable," Natalie Batalha, Kepler's deputy science team lead at San Jose State University, said in a NASA news release. "The more data we collect, the keener our eye for finding the smallest planets out at longer orbital periods."

Mendez and his colleagues are working on software to keep the Habitable Exoplanets Catalog updated. "The computers are doing the job," he told me. "I am trying to automate everything, but it takes time."

Right now, the world in the database that's judged most similar to Earth is a candidate known as KOI 736.01, which is 1,750 light-years away and is estimated to have a surface temperature of 55 degrees F (286 Kelvin). But the top prospect for surface habitability is KOI 255.01, a Warm Superterran that's 1,169 light-years away with a surface temperature of 86 degrees F (303 K). Some researchers believe super-Earths can be even more conducive to life than Earth.

Gliese 581d, a world that orbits a red dwarf just 20 light-years from Earth, shows up among the Sweet 16 on both lists.

The search revs up
So what's next? "I hope this database will help increase interest in building a big space-based telescope to observe exoplanets directly and look for possible signatures of life," Jim Kasting, a planetary scientist from Penn State, said in the Planetary Habitability Laboratory's news release.

A habitability index could help scientists set the priorities for future observations, but they don't necessarily need to wait until a new super-space telescope is launched. During the Kepler conference, the California-based SETI Institute announced that it was once again searching planetary systems for radio signals that could serve as evidence of extraterrestrial intelligence. Some of Kepler's planetary candidates are among its first targets.

"For the first time, we can point our telescopes at stars and know that those stars actually host planetary systems — including at least one that begins to approximate an Earth analog in the habitable zone around its host star," Jill Tarter, director of the institute's Center for SETI Research, said in a news release. "That's the type of world that might be home to a civilization capable of building radio transmitters."

Tarter and her colleagues makes use of the Allen Telescope Array, a network of radio antennas in northern California that had to be put into hibernation due to money troubles. The SETI Institute was able to restart work at the array thanks to contributions made by the public through the SETIStars.org website, as well as funding from the U.S. Air Force to assess the array's utility for space situational awareness (that is, monitoring the skies for hazardous asteroids and space debris).

Tarter said the highest priority would be given to Kepler planets that are located within their stars' habitable zones. But the search for extraterrestrial intelligence won't stop there.

"In SETI, as with all research, preconceived notions such as habitable zones could be barriers to discovery," she said. "So, with sufficient future funding from our donores, it's our intention to examile all of the planetary systems found by Kepler."

More about the planet quest:

• Which alien worlds are most livable?
• City lights could point to E.T.'s home
• Super-Earth on the 'edge of habitability'
• Interactive: How scientists search for planets
• Astronomers find 18 alien planets, and they're huge

Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

ESO

One of the several planets within the Gliese 581 star system, called Gliese 581d, ranks among the most potentially habitable alien worlds on a new scale.

Astronomers have come up with a livability index for alien planets and moons, and the winners are ... Titan in our own solar system, and the Gliese 581 planets in the extrasolar league.

Rating systems for Earthlike and habitable planets may not make much difference now, but the developers of the Earth Similarity Index and the Planet Habitability Index say they could be crucial in the years ahead.

"With a new generation of telescopes and missions on the way, the discovery of many more exoplanets can be expected," they write in a paper to be published in the December issue of the journal Astrobiology. "That, in turn, will drive the need for a classification scheme for assigning astrobiological potential for exoplanets based on estimates derived from quantitative data of their probability for supporting life."

If such a scheme could truly reflect whether or not a given planetary environment is habitable, that could drive the priorities for exploration in our own solar system, as well as high-resolution observations of extrasolar systems.

Habitability indexes have been in the works for at least the past couple of years. Traditionally, astrobiologists have focused on three conditions that appear essential for life on Earth: organic compounds, the presence of liquid water, and an energy source such as the sun or undersea volcanoes. But in the search for alien Earths, those conditions aren't easily determined, and they may even be irrelevant.

The newly proposed indexes take a two-track approach to the classification challenge.

"The first question is whether Earthlike conditions can be found on other worlds, since we know empirically that those conditions could harbor life," Dirk Schulze-Makuch, an astrobiologist at Washington State University who is one of the study authors, said in a news release. "The second question is whether conditions exist on exoplanets that suggest the possibility of other forms of life, whether known to us or not."

The Earth Similarity Index looks at the size, density and orbital distance of a planet or moon, as well as the size and temperature of its parent star, and compares those parameters with Earth's. Earth has the maximum global ESI of 1. Mars has a 0.70 rating, and Mercury is the next on the list with 0.60. For what it's worth, the dwarf planet Pluto and Neptune's moon Triton register a measly 0.075 and 0.074, respectively. And Enceladus, the icy Saturnian moon that is thought to harbor a subsurface ocean and perhaps life, is right down there with them at 0.094.

Looking beyond the solar system, the researchers worked up ESI values for a variety of extrasolar planets. The top finishers were Gliese 581g (whose existence is in dispute) with 0.89, and Gliese 581d with 0.74.

But that's just the first part of the job: The researchers' Planet Habitability Index looks at a different set of factors: Does the planet have a rocky or frozen surface? Is there an atmosphere, and how thick is it? How about a magnetic field? How much energy is available, either through tidal flexing or from the parent star? Could there be organics present, and is a liquid solvent available for chemical interactions?

By those measures, Earth has a relative PHI of 0.96, which is nearly as close as you can get to the maximum of 1. Based on what's known about the rest of the solar system, the runner-up is not Mars, as you might expect, but the Saturnian moon Titan (0.64 vs. 0.59 for Mars). The Jovian moon Europa is next on the list (0.47), but Enceladus (0.35) ranks lower than Venus, Jupiter and Saturn (0.37).

The authors stress that expectations based on earthly life may not apply to extraterrestrial environments.

"Habitability in a wider sense is not necessarily restricted to water as a solvent or to a planet circling a star,” they write. "For example, the hydrocarbon lakes on Titan could host a different form of life. Analog studies in hydrocarbon environments on Earth, in fact, clearly indicate that these environments are habitable in principle. Orphan planets wandering free of any central star could likewise conceivably feature conditions suitable for some form of life."

So how does the Gliese 581 system's PHI look? Gliese 581g's value was estimated at 0.45, 581d registered 0.43, and 581c came in at 0.41. By that scale, the chances of finding life in a red-dwarf system 20.5 light-years away (or sustaining life if we ever get there) are about as good as they are for Europa. OK, but not great.

It's important to keep a couple of things in mind about this research: First of all, there's a fair amount of speculation about the various factors and their relative value for habitability. Further observations may shift the values for those factors, as well as the mathematical formula into which they're fed.

Perhaps more importantly, the numbers game can't take the place of actual observation and exploration. The ESI and PHI may well turn out to be thought experiments like the Drake Equation, which takes your assumptions about a variety of cosmic factors (How many planets like Earth come into existence every year? How likely is it that intelligent civilizations arise on alien Earths? How long do they last?) and turns them into a number. At least that's the message from David Morrison, director of the Carl Sagan Center for the Study of Life in the Universe, headquartered at the SETI Institute in Mountain View, Calif.

Here's what Morrison told me in an email:

"Very interesting. Discussing such conceptual indexes is a good way to organize our thinking about worlds that may be suitable for life. But it doesn’t actually add value, in my opinion. For the Earth Similarity Index, we already have thought that liquid water, and a solid surface, and enough gravity to hold on to a substantial atmosphere, are important indications of habitability. Hence the interest in Earth-size planets within the habitable zone (meaning surface liquid water is possible). To go further, as by considering the composition of the atmosphere, we are quickly into the effort to identify life by its chemical signatures, not just habitability. The broader habitability index in also interesting, but we just don’t know how to define habitability. And if Titan is an example, we may never have the data on exoplanets that could distinguish the hydrocarbon liquid lakes that we see on Titan.

"Bottom line: This (like the Drake Equation) is a good teaching tool. It helps is to organize our thoughts. But I doubt it will be very useful as a research tool, because we know so little about what properties truly define habitability. Without a much better idea of what alien life is like, we don't know how to define habitability. And probably nature is much more creative than we can imagine."

What do you think? Where would you target the search for extraterrestrial life, and what criteria would you use to prioritize the targets? Feel free to weigh in with your comments below.

More about the search for alien life:

• A new equation for life
• Case builds for habitable alien planet
• Super-Earth on the edge of habitability
• More about astrobiology from Cosmic Log

In addition to Schulze-Makuch, the authors of "A Two-Tiered Approach to Assessing the Habitability of Exoplanets" include Abel Mendez, Alberto G. Fairen, Philip von Paris, Carol Turse, Grayson Boyer, Alfonso F. Davila, Marina Resendes de Sousa Antonio, David Catling and Louis N. Irwin.

Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or following the Cosmic Log Google+ page. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

David A. Aguilar / CfA

If an alien civilization builds brightly lit cities like those shown in this artist's conception, future generations of telescopes might allow us to detect them.

Astronomers suggest that artificial illumination creates a signature that could point to the existence of civilizations on other worlds — and they say we should get started on a survey of the edges of our own solar system, just in case.

The suggestion comes from Harvard's Abraham Loeb and Princeton's Edwin Turner, in a research paper submitted to the journal Astrobiology. A version of the paper appears on the arXiv.org preprint server and sparked a write-up today on Technology Review's Physics arXiv Blog.

Loeb, who chairs Harvard's astronomy department and is affiliated with the Harvard-Smithsonian Center for Astrophysics, acknowledged that detecting aliens by looking for the glow of their cities would be a long shot. But he pointed out that the cost of the exercise would be low.

"We say that we can piggyback on existing surveys that people are doing anyway. There's no need to use extra resources. ... My philosophy is simple: If we can do it, why not do it and check? Why put blinders on ourselves?" Loeb told me today.

Here's how the idea could work: An object's brightness varies with distance, but the relationship between those two factors will depend on whether the brightness is due to reflected sunlight or due to illumination from the object itself. For a self-illuminated object, the brightness varies by a factor of 1 over the distance squared, but "if you have an object that reflects light from another source ... the flux dies out like 1 over the distance to the fourth power," Loeb said.

Monitoring the changes in the brightness of an object on the edge of our solar system, in a broad disk of icy material known as the Kuiper Belt, could provide a "very simple test" to determine whether extraterrestrials have turned on the lights, Loeb said.

"We conclude that existing telescopes and surveys could detect the artificial light from a reasonably brightly illuminated region, roughly the size of a terrestrial city," on a Kuiper Belt object, Loeb and Turner write.

NASA

The lights of Cairo, Alexandria and the Nile shine through the night on Oct. 28, 2010, as seen from the International Space Station. Astronomers say such illumination could serve as a tip-off in the search for civilizations on other worlds.

How likely is it that E.T. would be found on the edges of our own solar system? Not that likely, but Loeb and Turner speculate that it could happen. "Artificially lit KBOs [Kuiper Belt objects] might have originated from civilizations near other stars," they write. "In particular, some small bodies may have traveled to the Kuiper Belt through interstellar space after being ejected dynamically from other planetary systems."

In addition to the E.T. search, Loeb said the Kuiper Belt survey would also be useful for studying how Kuiper Belt objects reflect light at different points in their orbits. "Even if the answer is, 'No, there is nothing peculiar,' we can still learn something from doing that," he told me. "And if there's something out there worth finding, that could change our perception of our place in the universe."

The technique could conceivably be extended to other stars once next-generation telescopes such as the James Webb Space Telescope and the Giant Magellan Telescope come online, over the next decade or so. There's been a lot of debate over whether the traditional search for radio signals from alien civilizations might be fruitless if E.T. moved beyond analog radio transmissions — and the search for artificial illumination could be worth checking out as a new frontier.

Someone could even try looking for the spectral signature of artificial light. (Do aliens use incandescent bulbs, compact fluorescent or LEDs?) But that particular kind of search would not be easy.

"For this signature to be detectable, the night side needs to have an artificial brightness comparable to the natural illumination of the day side," Loeb and Turner write. And when you consider that Earth's day side is about 600,000 times brighter than the night side, that means E.T. would have to cope with one heck of an electric bill.

What do you think about the search for E.T.'s city lights? Feel free to add your comment below.

More about the search for extraterrestrial intelligence:

• Donations revive the SETI quest
• Gallery: Four decades of SETI
• Alien-hunters add super-Earths to their list
• A new idea in the search for E.T.'s footprints
• More from Cosmic Log about aliens ... and about SETI

Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding me to your Google+ circle. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for other worlds. 

A study that reviews a host of sci-fi scenarios for contact with extraterrestrials stirred up such a ruckus today that NASA had to step in and distance itself from the research. The controversy focuses on the idea that E.T. could well decide that we're a threat to interstellar order, and therefore we have to be stopped before we spread.

The report itself, published in the journal Acta Astronautica, covers ground that's familiar to dedicated fans of E.T. lore. For example, the premise of the 1951 sci-fi classic "The Day the Earth Stood Still" is that universalist-minded aliens see our civilization as so rooted in violence that it's better to snuff us out than let us ruin the neighborhood. (The 2008 remake, starring Keanu Reeves, recycled that idea with an environmental theme.)

Then there's the "Hitchhiker's Guide to the Galaxy" scenario, in which Earth is destroyed merely to make way for a new stretch of intergalactic infrastructure.

"At the heart of these scenarios is the possibility that intrinsic value may be more efficiently produced in our absence," the researchers write.

The most familiar sci-fi scenario is the one in which the aliens are as selfish and territorial as we are, and want to wipe us out or enslave us and take our stuff. Think "War of the Worlds" or "Independence Day." In such cases, the researchers note that there's the potential for big payoffs ... if we prevail.

"Humanity benefits not only from the major moral victory of having defeated a daunting rival but also from the opportunity to reverse-engineer ETI [extraterrestrial intelligence] technology," they write. Indeed, New York Times columnist Paul Krugman joked last weekend that a fake alien invasion might be just the thing to spark an economic turnaround.

The researchers touch on more benign scenarios as well — for example, the "Star Trek" scenario, in which helpful aliens welcome us into the United Federation of Planets because we're all basically good guys (as opposed to those evil Klingons, until they become good guys, too). And then there's something like the "E.T." scenario, in which the aliens mostly just want to stay out of our way.

The 33-page study reflects at length on the potential risks.

"The possibility of harmful contact with ETI suggests that we may use some caution for METI [sending messages to extaterrestrial intelligence]," the researchers write. "Given that we have already altered our environment in ways that may be viewed as unethical by universalist ETI, it may be prudent to avoid sending any message that shows evidence of our negative environmental impact. The chemical composition of Earth's atmosphere over recent time may be a poor choice for a message because it would show a rapid accumulation of carbon dioxide from human activity. Likewise, any message that indicates widespread loss of biodiversity or rapid rates of expansion may be dangerous if received by such universalist ETI."

In short, let's keep our environmental bad habits on the down low, so as not to get the sad-Keanu E.T.'s on our case.

The basis of the brouhaha
By themselves, these ideas are not all that, um, alien. For years, sci-fi author David Brin has advised keeping quiet about our existence, and celebrity physicist Stephen Hawking agrees. U.N. officials and scientific experts also say the messages we direct toward any aliens we come across would have to be carefully managed.

So what's the big deal? Well, one of the authors of the paper, Shawn Domagal-Goldman, happens to be a postdoctoral student working at NASA Headquarters — and that highly tenuous connection to the world's most influential space agency sparked a huge wave of scare headlines. It started with The Guardian's story, and rolled onto The Drudge Report's webpage with a headline reading "NASA REPORT: Aliens may destroy humanity to protect other civlizations..." Another variant was this one: "NASA: Aliens May Destroy Humanity Over Greenhouse Gases."

Eventually, NASA had to send out a Twitter update saying "Yes, @drudge and @guardiannews are mistaken about an 'alien' report. It's not NASA research. Ask the report's author...." The space agency followed up later with two more tweets, emphasizing that it was not involved in the study and saying that Fox News and CNN "have it wrong."

In each case, NASA linked to a lengthy clarification and apology from Domagal-Goldman, who made clear that the study was not a "NASA report," that no NASA funding was expended on it, and that he spent none of his working hours on writing the paper. He said his two co-authors, Seth Baum and Jacob Haqq-Misra of Pennsylvania State University, "put in the vast majority of work on it."

"It was just a fun paper written by a few friends, one of whom happens to have a NASA affiliation," Domagal-Goldman wrote.

He admitted that including the NASA affiliation turned out to be a "horrible mistake":

"I did so because that is my current academic affiliation. But when I did so I did not realize the full implications that has. I'm deeply sorry for that, but it was a mistake born out of carelessness and inexperience and nothing more. I will do what I can to rectify this, including distributing this post to the Guardian, Drudge and NASA Watch. Please help me spread this post to the other places you may see the article inaccurately attributed to NASA.

"One last thing: I stand by the analysis in the paper. Is such a scenario likely? I don't think so. But it's one of a myriad of possible (albeit unlikely) scenarios, and the point of the paper was to review them. But remember — and this is key — it's me standing for the paper ... not the full weight of the National Aeronautics and Space Administration. For anything I have done to mis-convey that to those covering the story, to the public, or to the fine employees of NASA, I apologize."

This isn't the first case where the NASA connection has become entangled in scientific speculation. In March, the space agency took great pains to distance itself from NASA researcher Richard Hoover's claims to have found evidence of outer-space organisms in meteorites.

In Domagal-Gordon's case, the substance was far less controversial. As I've tried to point out above, the views expressed in the paper aren't that far off from the typical science-blog fare. I'm willing to bet a goodly sum of quatloos that Domagal-Gordon will go on to have a fine career in science ... and also that this won't be NASA's last P.R. kerfuffle over E.T.

More about aliens:

• Hollywood remakes an alien
• What would you ask the aliens?
• Why we love to fear E.T.
• The alien files on Cosmic Log

Connect with the Cosmic Log community by "liking" the log's Facebook page or following @b0yle on Twitter. You can also add me to your Google+ circle, and check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

Henry Bortman / 2010

Astrobiology researcher Felisa Wolfe-Simon works with samples at California's Mono Lake.

Last updated 8:30 p.m. ET

After five months of battles in the blogosphere, the debate over whether life can be based on an alternate biochemistry is playing out on the highest levels of peer-reviewed research.

Back in December, the journal Science sparked a ruckus by publishing an online report from researchers who claimed that they had coaxed bacteria from California's Mono Lake to live on arsenic rather than phosphorus. That's a big deal, because phosphorus is thought to be one of the six elements essential for life as we know it (along with carbon, hydrogen, nitrogen, oxygen and sulfur). Arsenic, on the other hand, is typically seen as a potent poison.

The researchers, led by astrobiologist Felisa Wolfe-Simon at the U.S. Geological Survey, suggested that such an alien biology could exist in environments beyond Earth that are traditionally thought to be inimical to life — for example, the hydrocarbon seas of Titan, Saturn's largest moon.

That sounds like a wonderful vision, but the claims from Wolfe-Simon and her team instantly came under attack from other chemists and microbiologists. In a flurry of blog postings and Twitter tweets, the critics took aim at what they saw were fatal flaws in the team's methodology. Wolfe-Simon and her colleagues defended their work in a Q&A issued via Science, but said they preferred to pursue the debate through the traditional peer-review process.

Now that process has taken a great leap forward. Today Science posted eight peer-reviewed technical comments from the critics to its Web site, along with a response from the original research group. The journal said all these papers would be printed, along with the original study (which has so far been available only online), in next week's edition.

"There's a lot of stuff that's happened," Wolfe-Simon told me today. "It's been a real challenge for me and my co-authors. ... We think this is evidence that, really, science is moving forward faster."

She held to the original claim that molecules of arsenic were incorporated into the machinery of life, replacing at least some of phosphorus. "We would argue that our conclusion is still viable," she said. "We never claimed 100 percent substitution, and in a way that point was misconstrued."

Wolfe-Simon said more evidence has been amassed to back up the arsenic-life claims over the past five months. However, the fresh evidence had to be held back for future publication. Wolfe-Simon said she was constrained from reporting new data in today's online response to the critics, which was a source of frustration for her. Science insisted on that to keep the cycle of response and counter-response from spinning out of control.

It's not unheard of to publish technical comments and responses in the wake of a controversial paper. Science did exactly that this week, with regard to a study claiming that microbes consumed all the methane that leaked from last year's Deepwater Horizon spill in the Gulf of Mexico. But it's very unusual to publish a research paper, eight critiques of that paper and a follow-up response to those critiques in the same issue of a scientific journal.

Science's editors said they did not expect their data dump "to be the final word on the subject."

"The fact that we received so much feedback to the Wolfe-Simon paper suggests to us that science is proceeding as it should," the editors said in a statement. "The study involved multiple techniques and lines of evidence, and the authors felt their conclusion was the most plausible explanation for these results when considered as a whole. We hope that the study and the subsequent exchange being published today will stimulate further experments — whether they support or overturn this conclusion."

The criticisms — and the responses from Wolfe-Simon and her colleagues — thus set the ground rules for the debate, which will likely continue for months and years to come. Here's a quick rundown of some of the issues involved:

Criticism: The "arsenic-eating" bacteria, known as GFAJ-1, were grown under conditions that still had trace amounts of phosphorus, and it's more likely that the microbes used that trace phosphorus rather than the arsenic. The arsenic-life researchers claimed there was so little phosphorus left that the bacteria couldn't possibly have survived on it — but under extreme conditions, some individual microbes have been found to survive on that little.  

Response from the team: They point out that they checked bacteria under three conditions: high phosphorus and low arsenic; high arsenic and low phosphorus; low arsenic and low phosphorus. If the "arsenic-eating" bacteria were actually living off the low levels of phosphorus, they should have done as well in the low-arsenic / low-phosphorus environment. But they didn't. The team also says the survival rate on an ultra-low-phosphorus level should be compared based on wider populations, and not based on individual extremophile microbes.

Criticism: In the course of breeding bacteria to make them live in a high-arsenic environment, the team might have actually created bacteria that adapted to the low-phosphorus concentrations by processing the chemicals differently. That would explain why the high-arsenic / low-phosphorus bacteria did so much better than the low-arsenic / low-phosphorus bacteria.

Response from the team: They saw no evidence that the bacteria's biochemistry processed phosphorus in the way that was suggested, but acknowledged that the chemical pathways used by GFAJ-1 "are important avenues for future investigation."

Criticism: The molecular bonds involving arsenic would simply not be strong enough to hold up in alternate forms of DNA and other biochemical building blocks. What's more, phosphorus is far more abundant than arsenic in the solar system, and most of the arsenic available on rocky planets would be available in a form that is structurally quite different from phosphorus. These considerations point to the unlikelihood of life arising on Earth or elsewhere with an arsenic-based biochemistry.

Response from the team: It's conceivable that the arsenic bonds in large biomolecules are more resistant to a breakdown than the bonds in smaller molecules. "GFAJ-1 may have evolved specific strategies to cope with this issue, such as stabilizing structures," the team wrote.

Criticism: The team didn't devote enough attention to guarding against contamination of their samples and purifying the DNA that they analyzed. What's more, the uncertainties surrounding the measurements may not allow the team to make definite conclusions.

Response from the team: Wolfe-Simon and her colleagues recap the procedures they used and say they "were sufficient to remove any impurities." They also cite multiple techniques that cross-checked their results, through radiolabeling as well as high-resolution mass spectroscopy. They agree that further analysis of the DNA "would be a useful future experiment" because it could shed further light on the chemistry involved. They reworked their calculations on some of the analysis to respond to some of the criticisms about averaging, and said the data still supported their conclusions.

The bottom line is that the debate will continue, with more researchers getting into the act. Wolfe-Simon's team says samples of GFAJ-1 are being made available to other labs upon request, through the Oremland Laboratory at the U.S. Geological Survey.

"We look forward to working with our peers to replicate our observations and to test our hypotheses along the lines suggested by [one of the critics, Stefan] Oehler and others," the team writes.

One of the most vocal critics, University of British Columbia microbiologist Rosie Redfield, said today that she was still unconvinced:

"The authors don't report any new experiments. Most of their responses take the form of 'our interpretation could be correct on this point if...' In many cases there is indeed a small possibility that it could, but there are so many of these points of interpretation, each with only a very small probability of being correct, that I don't think anyone will find the arguments convincing."

Redfield said the team's responses to her comments about contamination were "in some ways the most scientifically valid, as they provide information about their media and DNA purification." She promised to have more about that on her blog later today.

So the blogosphere beat goes on. What do you think? Are you intrigued by this latest chapter in the grand scientific debate, or has the whole subject of arsenic life lost its appeal? Either way, please feel free to add your comments below.

Update for 8:30 p.m. ET: I've added some comments from Wolfe-Simon above after chatting with her this afternoon. She said she and her colleagues took Redfield's concerns about potential contamination very seriously. "Her criticisms are definitely valid," she said. "One of the first things we went back and did was look at all the ways we can get [phosphorus levels] down to zero."

But she said the key observations would come when scientists look at GFAJ-1's molecular machinery, to confirm that arsenic really is being incorporated into DNA, lipids and other molecules where phosphorus is usually found. "The question that people are really asking is, 'Show me the money. Let's see those biomolecules,'" she said.

Wolfe-Simon said she and her co-authors have been getting offers of help from other researchers in fields ranging from molecular biology to astronomy. She's also been getting supportive messages from lots of folks, including a 7-year-old girl who told her she wants to do research at Mono Lake when she grows up. "If I can put my peg on the wall, if we can ask the right questions ... she's going to answer the questions," Wolfe-Simon said.

Meanwhile, Redfield has posted an additional blog item that details her concerns about contamination. She's not satisfied with the response that Wolfe-Simon and her colleagues provided. "Overall, the most striking aspect of the authors' formal response is that they never admit to having made any mistakes or having done anything badly," she writes. "This is a bit disconcerting, given how many concerns were raised."

Update for 9:30 p.m. ET: An additional post from Rosie Redfield addresses how to test arsenic-life claims. Meanwhile, science writer Carl Zimmer looks at the big role that online discussion played in the arsenic-life debate, and Nature's Erika Check Hayden rounds up reactions from other researchers. One theme: Is it worth spending time and effort to try replicating the findings?

The story so far:

• December 2010: Life as we don't know it ... on Earth?
• 'Weird life' claims spark a backlash
• 'Weird life' reveals science at work
• 'Weird life' researchers answer critics
• February 2011: Arsenic debate just won't die
• April 2011: Alien life revisited

Science is making all 10 papers accessible with free online registration. You can see the whole list on Science Express, the journal's rapid-publication website, and here's an item-by-item menu:

• Comment by James Cotner and Edward Hall
• Comment by Rosemary Redfield
• Comment by B. Schoepp-Cothenet, W. Nitschke, L.M. Barge, A. Ponce, M.J. Russell and A.I. Tsapin
• Comment by Istvan Csabai and Eors Szathmary
• Comment by Stefan Oehler
• Comment by David Borhani
• Comment by Steven Benner
• Comment by Patricia Foster
• Response by Felisa Wolfe-Simon, Jodi Switzer Blum, Thomas Kulp, Gwyneth Gordon, Shelley Hoeft, Jennifer Pett-Ridge, John Stolz, Samuel Webb, Peter Weber, Paul Davies, Ariel Anbar and Ronald Oremland
• "A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus" by Felisa Wolfe-Simon et al.

You can connect with the Cosmic Log community by "liking" the log's Facebook page or following @b0yle on Twitter. Also, give a look to  "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

Univ. of St. Andrews photoillustration

On a world that spins around two dim suns, the vegetation may well look black to human eyes.

Researchers suggest that vegetation on an alien planet like Tatooine in "Star Wars" might well look black or gray to human eyes. But they probably wouldn’t seem devoid of color to the eyes of the aliens — assuming they have eyes, that is.

The conjecture comes from a paper presented by the University of St. Andrews' Jack O'Malley-James at the Royal Astronomical Society's National Astronomy Meeting in Wales. O'Malley-James is working on a Ph.D. project to assess the potential for photosynthetic life in multiple-star systems with different combinations of sunlike stars and red dwarfs.

On Earth, the leaves of plants generally look green because two types of chlorophyll absorb the reddish and bluish wavelengths in the visible-light spectrum. Those red and blue wavelengths drive the photosynthetic process by which plants convert the sun's energy into chemical energy. In contrast, the green wavelengths are reflected into the RGB optical sensors known as our eyes.

Scientists surmise that the birds and bugs may see plants quite differently, with greater sensitivity to different shades of green and the ability to sense ultraviolet wavelengths as well.

O'Malley-James suggests that in different corners of our galaxy, plants could evolve to take advantage of different combinations of wavelengths, depending on the light coming from their parent sun ... or suns. The possibilities become particularly intriguing for a planet in a multiple-star system — like Tatooine, Luke Skywalker's fictional home planet in the "Star Wars" movie saga.

J. O'Malley-James / Univ. of St. Andrews

On planets orbiting red-dwarf stars, the vegetation may have more photosynthetic pigments in order to make use of a fuller range of wavelengths, giving them a "black" appearance. Here are some earthly examples of dark plants and flowers.

"If a planet were found in a system with two or more stars, there would potentially be multiple sources of energy available to drive photosynthesis. The temperature of a star determines its color and, hence, the color of light used for photosynthesis. Depending on the colors of their starlight, plants would evolve very differently,"  he said in a news release.

Statistics show that more than 25 percent of sunlike stars and 50 percent of the red dwarfs in our galaxy are found in multiple-star systems. Armed with such statistics, O'Malley-James and his colleagues ran computer simulations to determine the optimal strategy for photosynthesis over a wide spectrum (heh, heh) of planetary alignments.

“Our simulations suggest that planets in multi-star systems may host exotic forms of the more familiar plants we see on Earth," O'Malley-James reported. "Plants with dim red dwarf suns for example, may appear black to our eyes, absorbing across the entire visible wavelength range in order to use as much of the available light as possible. They may also be able to use infrared or ultraviolet radiation to drive photosynthesis. For planets orbiting two stars like our own, harmful radiation from intense stellar flares could lead to plants that develop their own UV-blocking sunscreens, or photosynthesizing microorganisms that can move in response to a sudden flare."

But even if the plants reflected none of the visible-light wavelengths, extraterrestrial gardeners might well have their own special appreciation for an ultraviolet bloom, or leaves that are variegated in the thermal infrared.

I know it sounds like a flight of fancy, but this is just the kind of flight I enjoy the most. The subject reminds me of the scene from "Battlestar Galactica" where Brother Cavil complains about the "ridiculous gelatinous orbs" in his head. "I want to see gamma rays!" he shouts. "I want to hear X-rays!" Which new senses do you think the aliens might have ... and which do you wish you could have? Feel free to weigh in with your own conjectures in the comment section below.

More about alien perspectives:

• Want to call E.T.? Keep it simple, stupid Earthling
• Rare exoplanet has 'Star Wars' twin sunset
• How to find aliens: Follow the photosynthesis
• Alien plants get new twist in world of 'Avatar'
• Plants on other planets might not be green

O'Malley-James' supervisors on the Ph.D. project include Jane Greaves of the University of St. Andrews, John Raven of the University of Dundee and Charles Cockell of The Open University.

Join the Cosmic Log community by clicking the "like" button on our Facebook page or by following msnbc.com science editor Alan Boyle as b0yle on Twitter. To learn more about my book on Pluto and the search for planets, check out the website for "The Case for Pluto." 

Science / AAAS

A photomicrograph shows a strain of bacteria called GFAJ-1 that was said to incorporate arsenic into its cellular machinery.

Last updated 2:15 p.m. April 2:

Is there life beyond Earth? Over the past few months, scientists have repeatedly suggested that there could be — but the science behind those suggestions remains frustratingly murky and controversial.

Astrobiology's X-Files were the subject of a talk I gave on Saturday in the Second Life virtual world, at the invitation of the Meta Institute for Computational Astrophysics. Here's the vidcast of the talk — which gives you a taste of how Second Life works as well as how the search for extraterrestrial life works.

Arsenic life
This talk came exactly four months after researchers shook up the scientific world with claims that they were able to get the cellular machinery of microbes from California's Mono Lake working with arsenic instead of phosphorus. That's an amazing result, because arsenic is supposed to be poisonous to living things. If organisms on Earth could be tweaked in such a dramatic way, perhaps life could arise in other environments that don't seem conducive to life as we know it ... the Saturnian moon Titan, for example.

The implication of the research, published in the journal Science, would be that we might be missing strains of "weird life" that just might exist under our noses. (Perhaps literally under our noses, as a "second Genesis" that has gone undetected.)

The study ran into a lot of skepticism from the start. Some microbiologists and chemists have faulted the research team's laboratory techniques, or the conclusions that the team drew from their data. In response, the research team insisted their science was sound — but also encouraged their detractors to run their own experiments and report the results. Science pledged to publish a follow-up.

That follow-up is still in the works, but commentaries on the "arsenic life" are showing up in peer-reviewed journals such as BioEssays and FEMS Microbiology Letters. These papers have sparked a secondary controversy: Does scientific criticism really count if it's just on the Internet?

The BioEssays paper sees no "fatal flaw" in the original paper, and the paper's authors contend that Internet-only discussions "are not components of the peer-reviewed literature and thus are not placed on record as part of the official scientific discourse." The Microbiology Letters commentary complains about "the magic and nonsense that floods cyberspace."

As you can imagine, that's sparked a lot of counter-criticism from the folks who have been using the blogosphere and Twittersphere as a sounding board for their own review of the research. To get that side of the story, check out the postings from Rosie Redfield at the University of British Columbia, Zen Faulkes from the University of Texas-Pan American and Michael Eisen from the University of California at Berkeley (who attended an informal seminar given by Felisa Wolfe-Simon, the lead author of the arsenic-life study).

R. Hoover / Journal of Cosmology

A field-emission scanning electron micrograph shows one of the filaments that was found in the Ivuna CI1 carbonaceous meteorite. The filament looks similar to those seen in earthly cyanobacteria.

Meteorite life
Less than a month ago, NASA astrobiologist Richard Hoover published a paper in the online-only Journal of Cosmology, suggesting that a number of meteorites contained microbes that could have come from outer space. Once again, the study created a splash, in large part because of the NASA connection. There was quite a furor over whether or not Hoover was misinterpreting what he was seeing, and some critics pointed out that the research had been submitted to (and rejected by) other, better-known journals before it wound up in the Journal of Cosmology.

The story went big on a Saturday, but by the following Monday, executives at NASA disavowed the research, and the debate quickly died down. The Journal of Cosmology's editors said they were selling off the publication. Hoover, who has had a long and distinguished career as a researcher at NASA's Marshall Space Flight Center, faced sharp questions about his academic credentials.

Today, Hoover came in for an added dose of indignity: The James Randi Educational Foundation named him one of the year's "five worst promoters of nonsense," alongside anti-vaxxer Andrew Wakefield, televangelist Peter Popoff, TV doctor Mehmet Oz and the CVS pharmacy chain (for offering homeopathic remedies). The last thing Hoover needs right now is a "Scientist Pigasus Award" from the Amazing Randi.

NASA / LPI

Some scientists have suggested that tiny wormlike structures seen within the Mars meteorite known as ALH84001 may be "nanofossils" of biological origin.

Life on Mars
You could argue that the sharp debate over the prospects of detecting microbial life from beyond Earth began 15 years ago, with Science's 1996 publication of research about "nanofossils" found in a meteorite from Mars. Some might go two decades further back, to the much-debated life-detection experiments that went to Mars aboard the Viking landers.

Even after 15 years, the microfossil debate is still percolating. The researchers behind the original study have been setting out other lines of evidence to argue that they're seeing the fossilized traces of ancient organisms rather than modern-day contamination from Earth, or geological shapes that just happen to look like critters.

Other studies, conducted as part of NASA's Phoenix Mars Lander mission, have shown the presence of perchlorate, a chemical that could be associated with particular kinds of exotic life on Earth. Those findings have revived discussions over what Viking found (or failed to find).

Although the debate over past life-on-Mars experiments is continuing, most astrobiologists say it's going to take additional  studies on the Red Planet to resolve the controversy. That's the goal of an experiment being proposed by MIT and Harvard researchers, known as the Search for Extra-Terrestrial Genome, or SETG. Right now the researchers are facing one big challenge: They don't yet have a spot on a future Mars probe.

Even if SETG's genome sequencer went to Mars and detected a snippet of DNA or RNA, would that serve as sufficient evidence that life arose on other planets? Or would such a claim end up in the same limbo that surrounds earlier claims for alien life. I suspect that the latter would be the case — but what do you think? Feel free to weigh in with your comments below, and check out Saturday's hourlong presentation.

More controversies in astrobiology:

• Strange find on Titan sparks chatter about life
• Mars methane mystery: What's making the gas?
• Search for alien life may take giant leap forward
• What exactly is life, anyway?

Join the Cosmic Log community by clicking the "like" button on our Facebook page or by following msnbc.com science editor Alan Boyle as b0yle on Twitter. To learn more about my book on Pluto and the search for planets, check out the website for "The Case for Pluto."