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.

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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.

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.

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

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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.

The science team for NASA's Kepler planet-hunting mission nearly doubled their list of confirmed planets beyond our solar system in one fell swoop today, announcing the discovery of 26 planets spread among 11 star systems. Their sizes range from just a little bit bigger than Earth to super-Jupiter-size, but they're all closer to their parent stars than Venus is to our own sun.

The accelerating pace of discovery is matched by the diversity seen in the worlds discovered so far, one of the Kepler mission's co-investigators, Harvard astronomer Dimitar Sasselov, told me today.

"There is more diversity out there than our limited imaginations could come up with, which is good," he said.

The $600 million Kepler mission, launched in 2009, now has a list of 61 confirmed planets, and another 2,326 planetary prospects that have yet to be confirmed. At this rate, Kepler's worlds could soon account for the majority of the exoplanets detected beyond our solar system — a tally that now stands at more than 700.

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"Prior to the Kepler mission, we knew of perhaps 500 exoplanets across the whole sky," Doug Hudgins, Kepler program scientist at NASA Headquarters, said in a news release. "Now, in just two years staring at a patch of sky not much bigger than your fist, Kepler has discovered more than 60 planets and more than 2,300 planet candidates. This tells us that our galaxy is positively loaded with planets of all sizes and orbits."

The Kepler space telescope searches for other worlds by staring at more than 150,000 stars in that fist-sized patch of sky, straddling the constellations Cygnus and Lyra. Kepler's instruments can detect the faint dips in starlight that occur on a regular basis as a planet passes over the disk of its parent star, as seen from Earth. By analyzing the patterns of those passes, also known as transits, Kepler's scientists can figure out the orbit and the size of a potential planet — but not its mass.

An alternative method has to be used to confirm that Kepler is actually looking at a planet rather than something else, such as mutually eclipsing binary stars. The mission's early discoveries were confirmed by looking at the candidates' stars with ground-based telescopes and checking for the telltale gravitational wobbles that are caused by big, close-in planets.

Transit timing variations
Most of the planets added to the list today were confirmed using a different backup method. The Kepler mission's astronomers analyzed subtle changes in the intervals between the transits, caused when multiple orbiting planets exert gravitational pull on each other. The resulting data on acceleration and deceleration can be used to confirm the planets' existence and calculate their masses.

"By precisely timing when each planet transits its star, Kepler detected the gravitational tug of the planets on each other, clinching the case for 10 of the newly announced planetary systems," said Dan Fabrycky, an astronomer at the University of California at Santa Cruz and the lead author for a paper confirming four of the planetary systems, known as Kepler-29, 30, 31 and 32.

Other newly confirmed planetary systems include Kepler-25, 26, 27 and 28, described in a paper with Fermilab's Jason Steffen as lead author; and Kepler-23 and 24, which was the focus of research led by the University of Florida's Eric Ford. In today's release, Ford said the transit timing variation method "dramatically accelerated" the pace of planetary discovery.

Yet another study, led by Jack Lissauer, a planetary scientist at NASA's Ames Research Center, detected five planets around Kepler-33, a star that is older and more massive than the sun. Those five planets range in size from 1.5 to five times the width of Earth, and they're all closer to their parent star than Mercury is to our own sun.

"The approach that was used to verify the Kepler-33 planets shows that the overall reliability of Kepler's candidate multiple transiting systems is quite high," Lissauer said in today's release. "This is a validation by multiplicity."

Five of the newly confirmed planetary systems (Kepler-25, 27, 30, 31 and 33) contain a pair of worlds that are bound together in a 1:2 resonance. That means the inner planet makes two circuit for every one circuit made by the outer planet. Four other systems (Kepler-23, 24, 28 and 32) have two planets that are linked in a 2:3 resonance, like Pluto and Neptune in our own solar system.

"These configurations help to amplify the gravitational interactions between the planets, similar to how my sons kick their legs on a swing at the right time to go higher," Steffen said.

Fifteen of the 26 planets announced today are Neptune-size or smaller, and the orbital periods range from six to 143 days. The planets' distances from Earth range from 623 light-years (for Kepler-25) to 4,064 light-years (for Kepler-29).

Great expectations
Sasselov, who has just come out with a book about the Kepler quest titled "The Life of Super-Earths," marveled that so many of the newfound worlds are in multiple-planet systems. He recalled that when the Kepler mission was proposed to NASA, more than a decade ago, "there was one little sentence that said maybe two or three of the systems will have multiple transiting planets."

None of the planets announced today would be conducive to life as we know it, because their orbits are so close to their parent stars. It's likely to be just a matter of time before Kepler achieves its main goal — confirming the existence of Earth-size planets in Earthlike orbits around sunlike stars. Unfortunately, it may be a matter of more time than initially expected.

Funding for the Kepler mission is due to run out in November, but the mission's scientists "don't have enough to statistically complete the core goal of the mission," Sasselov said. It turns out that the data collected by the telescope is "noisier" than expected. That means more observations will be required to confirm the mission's trickiest planetary finds.

The Kepler team has applied for a four-year extension, and is currently waiting for a decision from NASA executives.

More about the planet search:

• 160 billion planets in the Milky Way?!
• Three newfound worlds are smaller than Earth
• Flash interactive: How other worlds are found
• Two new 'Tatooine' planets with two suns
• SETI researchers check signals in exoplanet study
• Cosmic Log archive on planets

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The planetary confirmations are described in four research papers:

• "Transit Timing Observations from Kepler: II. Confirmation of Two Multiplanet Systems via a Non-parametric Correlation Analysis'' by E.B. Ford et al., The Astrophysical Journal.
• "Transit Timing Observations from Kepler: III. Confirmation of 4 Multiple Planet Systems by a Fourier-Domain Study of Anti-correlated Transit Timing Variations," by J.H. Steffen et al., Monthly Notices of the Royal Astronomical Society.
• "Transit Timing Observations from Kepler: IV. Confirmation of 4 Multiple Planet Systems by Simple Physical Models," by D.C. Fabrycky et al., The Astrophysical Journal.
• "Almost All of Kepler's Multiple Planet Candidates are Planets, and Kepler-33 5-planet System," by J. Lissauer et al.

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 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.

Even the astronomers on the science team for NASA's Kepler planet-hunting mission are marveling at the new worlds they're finding.

There's certainly a lot to marvel at: Just this week, Kepler astronomers announced the discovery of not just one, but two binary-star systems that have at least one planet each, reviving visions of the double sunset on Luke Skywalker's home world in the "Star Wars" saga. Another group of scientists drew on data from Kepler to detect the three smallest exoplanets yet discovered, including one just about the size of Mars.

The revelations at the American Astronomical Society's winter meeting in Austin, Texas, demonstrated that the number and diversity of the planets being found beyond our own solar system is growing by leaps and bounds. An august group of space commentators, including yours truly, celebrated the diversity during today's Weekly Space Hangout. And astronomers were celebrating in Austin as well.

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"Any kind of system you can think of, if it doesn't violate the laws of nature, it probably exists somewhere out there," Virginia Trimble, an astronomer at the University of California at Irvine, told reporters. "So as long as people think up new techniques, they will also find new types of planets. There will surely be lots of new, neat stuff in the coming years."

In an email, Berkeley astronomer Geoffrey Marcy, who has been in on the planet quest for 20 years and is a member of the Kepler science team, went positively gushy over the latest findings ... but also pointed out that the quest has really just begun:

"The NASA Kepler space telescope has discovered well over 2,000 strong candidate planets around other stars.  No exoplanet survey is even close to this coverage and statistical integrity.

"For each of those exoplanets Kepler finds, we have detailed knowledge of the planet's orbital period and the planet's orbital distance from its host star. More impressively, we know the planet's size (diameter) quite accurately. For some of the planets, we have also measured their mass and density, with some planets found to be definitively solid.

"With this wealth of information about over 2,000 planets, we continue to study the occurrence of planets around other stars. This work gives us a census of planets in the Milky Way galaxy. The 2,000 exoplanets is still too few to give an accurate answer. A useful census of humans on Earth requires that well over 2,000 people be surveyed. So it is with planets in the Milky Way galaxy. A useful census requires that thousands be sampled, and with accuracy. We desire integrity in our surveys of planets and people.

"Three weeks ago, the Kepler team announced the first two Earth-size planets. Only Kepler has sensitivity to Earth-size planets. [Now we have announced] the first Mars-size planet around another star. ... These discoveries by Kepler will mark an historic moment in the history of science, approaching the trans-oceanic voyages of the 15th century and the first steps on the moon.   Kepler is indeed finding new worlds."

The Kepler mission identifies potential new worlds by looking for the telltale dips of starlight that occur when a planet passes over the disk of its parent sun. Other methods are used to confirm the mass of alien planets, including a method that checks for a characteristic gravitational wobble in stars that have planets. And yet another method, called microlensing, was used in another study released this week that estimated there could be 160 billion planets in the Milky Way. There's a chance that estimate will turn out to be too high. There's a better chance it'll turn out to be too low. But in either case, astronomers now recognize there could be tens of billions of new worlds out there.

Some of those worlds no doubt will have the conditions conducive to life as we know it. Studying such planets could help us one of the deepest questions we have about the universe: Are we alone?

But in order to do that, the quest has to continue. Right now, funding for the $600 million Kepler mission is due to run out in November, and discussions about an extension are under way. Theoretically, Kepler could gather enough data by November to detect Earth-size planets in Earth-scale orbits around sunlike stars, but an extension would provide scientists with more confidence about their existing candidates — and also give them the chance to cast a wider net.

Chances are the mission will be extended. "It would seem to me just nuts to have it out there and turn it off," one astronomer, Greg Laughlin of the University of California at Santa Cruz, told Space.com. But the success of Kepler (and its European counterpart, COROT) should get people talking about what to do for an encore. So brace yourself for an alphabet soup of exoplanet-mission acronyms ranging from EChO to MPF to PLATO to WFIRST.

Odds and ends from the week in space:

• Hey, kids! Want to keep up with the lengthening list of exoplanets? Check out Hanno Rein's free Exoplanet app for iPhone, iPad and iPod Touch. There are planet catalogs for other mobile platforms as well, such as Exoplanet Catalog for Android and the Astronomy app for Windows Phone. Got more apps? Add your recommendations in a comment below.
• Speaking of apps, Powellware's newly released Mars Images app is getting some good reviews. The app for iPhone/iPad/iPod Touch/Android delivers the latest images from NASA's Opportunity rover on Mars.
• Remember the big radio-telescope array that Jodie Foster was plugged into when she heard the alien transmissions in the movie "Contact"? The real-life telescope complex in New Mexico where those scenes were filmed has been known as the Very Large Array, but during the AAS meeting, the National Radio Astronomy Observatory announced that it'll be renamed the Karl G. Jansky Very Large Array to honor Karl Jansky, the founder of radio astronomy. The name was selected from among 23,331 suggestions submitted by 17,023 people from more than 65 countries, the NRAO said. The new moniker will no doubt be shortened to the Jansky VLA, or the Jansky Array.
• If you've got an hour to spare, watch the Weekly Space Hangout video above, in which I and other Web-based worthies hold forth on a variety of out-of-this-world topics. And if you've got another hour to spare, perhaps while you're exercising at the gym, listen to last week's "Virtually Speaking Science" podcast, featuring my chat with Ig Nobel impresario Marc Abrahams. And stay tuned for the Feb. 1 installment of "Virtually Speaking Science," when we'll be talking about science policy and politics.

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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 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.

A statistical analysis based on a survey of millions of stars suggests that there's at least one planet for every star in the sky, and probably more. That would add up to 160 billion planets or so in the Milky Way.

"We conclude that stars are orbited by planets as a rule, rather than the exception," an international research team reports today in the journal Nature.

The estimate may sound amazing: Just a year ago, the world was wowed by the claim that at least half of the 100 billion or more stars in the Milky Way possessed planets, yielding a figure of 50 billion planets. The latest survey now suggests that there's an average of 1.6 planets per star system, which would work out to 160 billion. But perhaps the most amazing thing about the findings is ... astronomers don't find them amazing at all.

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"I am not surprised by the numbers," Didier Queloz, a planet-hunter at the Geneva Observatory who was not involved in the survey, told me in an email. Back in 2008, Queloz was part of a different research team that concluded one-third of the stars like our sun harbored super-Earth-size planets — the kinds of planets that could support life.

Over the past couple of years, findings from a variety of planet-hunting missions — including NASA's Kepler space telescope, the European Space Agency's COROT telescope and ground-based telescope surveys — have reinforced the view that planets are plentiful.

"Results from the three main techniques of planet detection are rapidly converging to a common result: Not only are planets common in the galaxy, but there are more small planets than large ones," Caltech astronomer Stephen Kane, a member of the team behind the findings reported in Nature, said in a news release from the Space Telescope Science Institute. "This is encouraging news for investigations into habitable planets."

Six years' worth of data
The new findings draw upon six years' worth of data from two wide-field surveys known as PLANET and OGLE. These surveys use a network of telescopes around the world, scanning the night sky for very rare events in which the light from one star system is amplified by the gravitational-lensing effect of another star (and  perhaps a planet) passing in front of it. This particular planet-hunting method is known as microlensing, as opposed to the transit method (which looks for telltale dips in starlight as a planet crosses its parent star's disk) or the radial-velocity method (which looks for the slight gravitational wobble in a star that has a planet in orbit).

During their six years of searching, the microlensing researchers identified only three actual planets. But they combined those detections with seven earlier detections, plus all the data about non-detections, to arrive at an estimate of how probable it is that planets of different types would be found around a star.

They estimated that about 17 percent of the stars in the Milky Way should host planets in the Jupiter range (0.3 to 10 times as massive as Jupiter), 52 percent should have planets in the Neptune range (10 to 30 times Earth's mass), and 62 percent should have super-Earth-size planets (five to 10 times Earth's mass).

All these figures are surrounded by wide bands of uncertainty. For example, the researchers say their estimate of 1.6 planets per star system could actually be anywhere between 0.7 and 2.5. But the lead author of the Nature study told me that his team's estimate is the best guess yet.

"The average number we find is higher than estimates derived by other methods," said Arnaud Cassan, an astronomer at the Institut d'Astrophysique de Paris. That's because the microlensing method can detect planets as small as five times Earth's mass up to 10 times Jupiter's mass, in orbits ranging from 0.5 to 10 times as wide as Earth's. Other methods aren't that sensitive, Cassan said.

"If they could detect planets with a range farther out, our guess is that they would find more planets," Cassan said.

What are the chances?
The big issue would have to do with how precise the statistical analysis can be with such a small sample of actual detections. Microlensing events are so rare that coming upon even one is like winning the lottery, and that makes the numbers difficult to crunch. But after reviewing the Nature paper, Queloz told me that Cassan and his colleagues conducted "a very good statistical analysis of the microlensing surveys."

Whether the actual number of planets in the Milky Way is 70 billion or 250 billion, it's a big, big number — 10 to 30 planets for every human on Earth. And the number doesn't even count worlds that are less than five times as big as Earth (such as Mercury, Venus, Mars and our own planetary home), inside the orbit of Venus or beyond the orbit of Saturn (such as Uranus, Neptune and the icy dwarfs on the solar system's edge).

Still more revelations about planets beyond our own solar system are coming up this week, but the bottom line for all this is that there's a big cosmos out there — with plenty of opportunities for planets and even life to develop. And that'll always be amazing.

"We used to think that the Earth might be unique in our galaxy," Daniel Kubas, a colleague of Cassan's at the Institut d'Astrophysique de Paris and a co-author of the Nature paper, said in a news release from the European Southern Observatory. "But now it seems that there are literally billions of planets with masses similar to Earth orbiting stars in the Milky Way."

More about the planet quest:

• Three newfound worlds are smaller than Earth
• Flash interactive: How other worlds are found
• Two new 'Tatooine' planets with two suns
• SETI researchers check signals in exoplanet study
• Microlensing turns up rocky, low-mass world
• Cosmic Log archive on planets

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In addition to Cassan, Kubas and Kane, authors of the Nature paper, "One or More Bound Planets Per Milky Way Star from Microlensing Observations," include J.-P. Beaulieu, M. Dominik, K. Horne, J. Greenhill, J. Wambsganss, J. Menzies, A. Williams, U. G. Jørgensen, A. Udalski, D.P. Bennett, M.D. Albrow, V. Batista, S. Brillant, J.A.R. Caldwell, A. Cole, Ch. Coutures, K.H. Cook, S. Dieters, D. Dominis Prester, J. Donatowicz, P. Fouqué, K. Hill, N. Kains, J.-B. Marquette, R. Martin, K.R. Pollard, K.C. Sahu, C. Vinter, D. Warren, B. Watson, M. Zub, T. Sumi, M.K. Szymanski, M. Kubiak, R. Poleski, I. Soszynski, K. Ulaczyk, G. Pietrzynski and L. Wyrzykowski.

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 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.

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.

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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

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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.

Computer simulations suggest that a giant planet was kicked out of our solar system billions of years ago, saving Earth in the process. But how solid are those simulations?

The concept appears in a paper written by David Nesvorny, a researcher at the Southwest Research Institute, and published in The Astrophysical Journal Letters. His findings aren't based on the discovery of an actual Planet X, but instead are the result of thousands of simulations re-enacting the dynamical development of our planetary system.

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Past simulations have shown that the solar system's current configuration is the result of a complex dance of the planets: About 600 million years into the solar system's existence, gravitational interactions caused a series of orbital shifts and scatterings. Astronomers believe that Jupiter moved inward and scattered many of the solar system's smaller bodies outward. Meanwhile, Saturn, Uranus and Neptune moved outward. Some models even suggest Uranus and Neptune switched places as they moved out.

This scenario explains phenomena ranging from the distribution of small bodies on the solar system's edge to the cratering rate on the moon. However, Jupiter's behavior in the model had to be tweaked: If Jupiter's orbit moved inward gradually, the giant planet would have stirred up the inner solar system too much. Earth's orbit could have been disrupted so much that it would have crashed into Mars or Venus.

"Colleagues suggested a clever way around this problem," Nesvorny said in a news release issued this week by the Southwest Research Institute. "They proposed that Jupiter's orbit quickly changed when Jupiter scattered off of Uranus or Neptune during the dynamical instability in the outer solar system."

This maneuver, known as the jumping-Jupiter hypothesis, would be less disruptive for the inner solar system. Nesvorny decided to test the idea by running billions of years' worth of simulations. He found that Jupiter did indeed do a quick orbital change as the result of gravitational encounters with Uranus or Neptune. But the encounters also scattered Uranus or Neptune all the way out of the solar system. "Something was clearly wrong," Nesvorny said.

Nesvorny ran the simulations again, this time with an additional giant planet that had a mass similar to that of Uranus and Neptune. Sure enough, Jupiter scattered the extra planet out of the solar system, and then quickly settled into an orbit that left the inner planets undisturbed. Nesvorny wrote that "it is roughly 10 times more likely to obtain a good solar system analog" if the extra planet is included.

How likely is it that a giant planet could be tossed out of the solar system? "This possibility appears to be conceivable in view of the recent discovery of a large number of free-floating planets in interstellar space, which indicates that planet ejection should be common," Nesvorny wrote.

No Planet X in sight
It's intriguing to imagine that there's a Planet X out there, cruising dejectedly through the interstellar wilderness. But so far, there's no empirical evidence to back up the theoretical claim.

"This paper is an example of what theorists do best: theorize," Alan Boss, a planetary scientist at the Carnegie Institution for Science, told me in an email. "The results are conceivable, but it is hard to imagine that they will be conclusive. There is a huge volume of possible initial conditions space for theorists to explore, and this is one particular example of an interesting possible path leading to a system similar to our own."

Boss said he was most concerned about the reference to the free-floating planets. He pointed out that the planets reported in the earlier research were around the mass of Jupiter, not Uranus or Neptune.

"That is not the situation being investigated [in Nesvorny's paper]. That is not to say that the scenario could not occur, but only that they cannot use the free-floating Jupiter-mass objects to support their scenario," he said. "If astronomers find lots of free-floating Neptune-mass objects, that would be another story."

More strange planetary tales:

• Three weird planets found around sunlike star
• Why odd alien planets travel in backward orbits
• Nearby stars could pose threat to solar system
• Don't fret over Planet X

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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.

It's been almost a year since astronomers suggested that Eris, the icy world whose discovery prompted Pluto's controversial reclassification in 2006, wasn't as big as they originally thought. Now the official word has leaked out unofficially: Pluto just might be the largest dwarf planet after all — although Eris is still seen as more massive.

The latest measurements were reported last week in Nantes, France, at a joint meeting of the American Astronomical Society's Division of Planetary Sciences and the European Planetary Science Congress. But as the Planetary Society's Emily Lakdawalla explains, it took a while for the report to become public, due to worries about the journal Nature's rules on embargoes and confidentiality.

Here are the statistics: Based on measurements made last November during the dwarf planet's occultation of a faraway star, Eris' diameter is estimated at 2,326 kilometers (1,445 miles). A similar set of measurements, published in 2009. estimated that Pluto was at least 2,338 kilometers (1,453 miles). When you include the margin of error, Pluto is essentially Eris' equal in size.

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"It could be smaller, it could be larger; basically, it is a twin," Lakdawalla quoted Paris Observatory astronomer Bruno Sicardy, the lead researcher for the Eris measurements, as saying at the conference.

Lakdawalla held back from reporting what Sicardy said because she was asked to. The research paper about the measurements is under consideration for publication in Nature, and Sicardy said the journal's editors told him he could discuss the results only if he instructed his audience not to report them publicly. The implication was that Sicardy's paper would be tossed out if his team's findings appeared in the press.

The audience was all abuzz about the findings, of course, but Lakdawalla said she wouldn't "break anything until somebody else breaks it."

She did, however, refer to the zipped-lip situation in a Twitter message to Embargo Watch's Ivan Oransky. Long story short, Oransky checked with Nature and was told that "researchers with papers in submission at a Nature journal can certainly present at a scientific meeting but shouldn't court the press." Oransky blogs about the back-and-forth today on Embargo Watch, but the bottom line is that Sicardy needn't have feared having his paper rejected, as long as he confined his public remarks to the presentation.

If Nature sticks to the reported publication plan, the paper will be published on Oct. 26. Today, a lot of the details came out not only on Lakdawalla's blog, but also on Scientific American's Observations blog — which is interesting, because Scientific American is part of the Nature Publishing Group. (SciAm's John Matson helpfully included a link to Sicardy's conference report.)

So what else do Sicardy and his colleagues say? Although Pluto and Eris are roughly the same size, Eris is more massive, which implies it's "mainly composed of rocky material, with a relatively thin ice mantle," the astronomers say. They suggest that Eris once had a thicker layer of ice, most of which was "blasted away" as the result of a catastrophic cosmic collision.

Sicardy and his colleagues also note that when you factor in Eris' distance, its observed brightness and its relatively small size, the dwarf planet stands out as one of the brightest bodies in the solar system, after the Saturnian moons Tethys and Enceladus. They suggest that the dwarf planet is so bright because it has a surface layer of nitrogen or methane frost, due to the freezing-out of its atmosphere.

A similar freeze-out might well happen on Pluto as it heads out to the farthest point of its orbit around the sun. Eris, meanwhile, is coming closer to the sun — and at some point the nitrogen or methane might thaw back into the atmosphere.

The two worlds seem destined to stand in the planetary pantheon as separated twins — in possession of moons, seasons, their own distinctive geologies and potentially some kind of cryovolcanic activity. Should they really be regarded as non-planets, or is it better to see them as a different class of planets? I argue for the latter in my book, "The Case for Pluto," but I'd love to hear what you think. Please feel free to add your comments below.

More about dwarfs and other planets:

• Pluto debate is about more than one little world
• Interactive: The new solar system
• Eris looks a lot like Pluto
• Eight decades of Pluto
• The Pluto files on msnbc.com

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New techniques for analyzing decade-old images from the Hubble Space Telescope are helping astronomers track planets that went undiscovered at the time. So far, the techniques have confirmed the existence of planets that were found in the meantime using other methods — but astronomers will be checking hundreds of stars in hopes of making brand-new discoveries.

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Remi Soummer, an astronomer at the Space Telescope Science Institute in Baltimore who led the new study, compared the technique to a "time machine" for seeking out planets beyond our solar system.

The key to the time machine is a huge database of observations made in the '90s by the Hubble Space Telescope's Near Infrared Camera and Multi-Oblect Spectrometer, or NICMOS. The instrument was used back then to look for dusty planetary disks and brown dwarfs. NICMOS focused on the regions around hundreds of stars, using a coronagraphic disc to block out the glare of the stars themselves.

The images were then processed to remove any remaining glare and bring out dim details. But back then, astronomers "did not have the cleanup techniques that we have now," Soummer told me today. Now Soummer and other astronomers are taking a second look at the NICMOS targets with improved image-processing software, and they're finding objects that were missed the first time around.

The star HR 8799, which is 130 light-years away in the constellation Pegasus, serves a classic example. NICMOS took a look at the star in 1998, but the imaging software available at the time didn't pick up any planets. In 2008 and 2009, a team led by Christian Marois of Canada's National Research Council analyzed ground-based imagery of the star and spotted three planets. The same team detected a fourth planet in 2010.

Spurred by the planet discoveries, the University of Montreal's David Lafrenière and his colleagues used upgraded software to find one of those four planets in the old NICMOS picture. Soummer, Marois and others followed up by locating two more of the planets. The fourth, innermost planet can't be seen in the NICMOS image because it's on the edge of the coronagraphic disc.

"From the Hubble images, we can determine the shape of their orbits, which brings insight into the system stability, planet masses and eccentricities, and also the inclination of the system," Soummer said in a Hubblesite news release. The results from his team are to be published in the Astrophysical Journal.

The three outermost planets make one orbit around HR 8799 roughly every 100, 200 and 400 years — so being able to see where the planets were a decade ago will give astronomers an extra data point for calculating the orbits more precisely. That's why the technique works like a time machine: It's as if you could go back to 1998 and see where the planets were back then. "It's 10 years of science for free," Soummer said.

But that's just the beginning. "What's really exciting now is that we're going to apply the same method to a bunch of other stars, and hopefully we'll make some discoveries of our own," said Brendan Hagan, a member of the research team who recently graduated from Goucher College in Baltimore.

Soummer said his team plans to analyze about 400 other stars in the NICMOS archive with upgraded image-processing software, which should improve image quality by a factor of 10.

"Once the code is ready, it's going to be a very intensive computing process," he told me. "It's going to take a few weeks to go through everything." Soummer plans to make several passes through the data, then compare the NICMOS results with other imagery to confirm the existence of new extrasolar planets.

The Extrasolar Planets Encyclopaedia currently lists 690 worlds that orbit other stars, and Soummer can hardly wait to add to the tally. "We have this huge wealth of data," he said, "and it's ready to be analyzed."

More about exoplanets:

• Real-life 'Star Wars' planet seen
• Fifty new alien worlds revealed
• 'Super-Earth' just might support life
• Interactive: How scientists search for other worlds
• Looking for alien Earths? Here they come

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In addition to Soummer, Marois and Hagan, the authors of "Orbital Motion of HR 8799 b, c, d Using Hubble Space Telescope Data From 1998: Constraints on Inclination, Eccentricity and Stability" include Laurent Pueyo, Adrien Thormann and Abhijith Rajan.

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.

Scientists laid out the results of an all-sky asteroid survey conducted by NASA's Wide-field Infrared Survey Explorer, or WISE. Sorry, no Planet X has been discovered yet ... which is good news, come to think of it. "Planet X is not coming to get us," said Amy Mainzer, principal investigator for the probe's NEOWISE mission.

The even better news is that there appear to be significantly fewer threatening near-Earth objects than previously thought. "We believe that the hazard to the earth may be somewhat less," Mainzer. an astronomer at NASA's Jet Propulsion Laboratory, reported today during a news briefing at NASA Headquarters in Washington.

Mainzer and her colleagues shared all this good news after taking the most accurate census to date of the asteroids in Earth's orbital vicinity, within 120 million miles (195 kilometers) of the sun. Results from NEOWISE (which stands for Near-Earth Object WISE) are being published in The Astrophysical Journal, with Mainzer as lead author.

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WISE was launched in 2009 to scan the full sky twice in infrared wavelengths, which provide a clearer picture of dark asteroids than visible-light observations — just as infrared-sensing goggles improve a soldier's night vision on Earth.

The space telescope observed more than 100,000 asteroids in the main belt between Mars and Jupiter, plus at least 585 near Earth. That sampling was sufficient to produce an estimate of how many asteroids of various sizes exist in the target area — just as a population census provides a good demographic picture even though the census-takers didn't knock on every door.

Mainzer said the WISE results confirmed that astronomers worldwide "have now found more than 90 percent" of the near-Earth asteroids that are wider than a kilometer (0.6 miles). That's the sort of space rock that scientists believe could create a mass-extinction event of the type that killed off the dinosaurs 65 million years ago. The NEOWISE team's computer model suggests that there are 981 of such asteroids, compared with previous estimates of around 1,000. Astronomers currently have identified 911 of these "planet-buster" asteroids, none of which pose a threat to Earth over the next few centuries.

The 90 percent figure is important because Congress specified that figure in 1998 as the initial goal for NASA's Spaceguard program.

"The risk of a really large asteroid impacting the Earth before we could find and warn of it has been substantially reduced," Tim Spahr, the director of the Minor Planet Center at the Harvard Smithsonian Center for Astrophysics, said in NASA's news release.

However ... there's another category of bad-news asteroids, ranging in size from 100 meters (330 feet) to a kilometer in width. These midsize rocks could wipe out a metropolitan area or create a "cosmic Katrina" if they were to hit in just the wrong place. The NEOWISE team estimated that there are 19,500 of such rocks in the near-Earth zone.

The good news is, that's a significantly smaller figure than previous estimates of 35,000 midsize near-Earth objects. The bad news is, astronomers are currently tracking 5,500 asteroids in that size range, which suggests that 14,000 or so remain to be detected. And there are an estimated million asteroids smaller than 100 meters capable of causing lesser amounts of damage if they were to blast through Earth's atmosphere.

In 2005, Congress revised the Spaceguard Survey's initial goal to call on NASA to find 90 percent of the near-Earth objects that are at least 140 meters (460 feet) wide, and Mainzer said tracking down all those rocks is "going to keep us busy for a long time."

"NEOWISE was just the latest asset NASA has used to find Earth's nearest neighbors," Lindley Johnson, program executive for the Near Earth Object Observation Program at NASA Headquarters, said in today's release. "The results complement ground-based observer efforts over the past 12 years. These observers continue to track these objects and find even more."

During today's briefing, Johnson said the results of asteroid surveys would feed into NASA's plans to send astronauts to a near-Earth asteroid sometime in the mid-2020s. In fact, the asteroids that come closest to Earth and pose more of a potential threat would be of "particular interest for exploration destinations," he said.

As for Planet X, or Nibiru, or Nemesis ... Mainzer said there's not been any sign of a large object that could bring about a doomsday in 2012 or anytime soon. "We don't think that there's anything hazardous in the outer solar system," she said.

But that doesn't mean there's nothing interesting out there. Mainzer pointed out that the $320 million WISE mission has already detected 100 brown dwarfs in Earth's vicinity, including some of the coldest brown dwarfs ever seen. Although the WISE spacecraft was shut down earlier this year, the data analysis continues, and there's still a chance that other strange cosmic objects could come to light.

"We're still working on that now," Mainzer said.

More about asteroids:

• Hunt for new worlds goes into overdrive
• Interactive: The new solar system
• Asteroid early-warning system proposed
• Interactive: Close encounters of the asteroid kind

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Last updated 5:42 p.m. ET.

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.