NASA / JPL

An artist's conception shows an alien Earthlike planet.

It's just one data point among the 1,235 potential worlds identified by NASA's Kepler planet-hunting probe, but you can't help noticing it on a graph. The planetary candidate known as KOI 326.01 sticks out as the one object that's estimated to be the size of Earth or smaller, with an average temperature that's lower than water's boiling point.

If scientists confirm that what they're seeing actually exists, KOI 326.01 could go down as the closest analog to our own planet in the current crop of Kepler data. But that's a big if.

"It's a small object, a small candidate," William Borucki, a planetary scientist from NASA's Ames Research Center who heads Kepler's science team, said today during a news briefing at the American Association for the Advancement of Science's annual meeting in Washington. Astronomers don't even know the size of its parent star with sufficient precision, Borucki said.

These factors make the planet's existence and its characteristics "extremely difficult to confirm." He said further observations over the next few months might produce the data for that confirmation. Or maybe not.

The case of KOI 326.01 illustrates how tricky the planet-hunting business can get. MIT's Sara Seager, a member of the Kepler team, said the $600 million mission represents just one step toward figuring out the answers to the three big questions about worlds beyond our solar system: Do Earth-size planets exist out there? How common are they? Do they show signs of life? "The reality is that one telescope cannot answer all three questions," she said.

50 billion planets in our galaxy?
Kepler detects extrasolar planets by staring at 150,000 stars in a single patch of sky, centered on the constellation Cygnus, and detecting the faint dips in light as planets pass over the stars' disks.

Based on a statistical analysis of the data available so far, 44 percent of the 150,000 stars in the Kepler sample should have planets going around them, Borucki said. You could take that statistic and do some mathematical gymnastics to extend it to the entire Milky Way galaxy, which by conservative estimates has 100 billion stars. That would give you 44 billion stars in our galaxy with planetary systems — or the nice round number of 50 billion planets that was cited today by The Associated Press. That number has a high uncertainty factor, to be sure. But the bottom line is that there are almost certainly tens of billions of planets out there, including hundreds of millions of planets in habitable zones of outer space.

Borucki provided a more detailed breakdown:

• 10.5 percent of the stars in the sample are predicted to have Earth-size planets (that is, 50 percent to 125 percent as wide as Earth).
• 7.3 percent should have super-Earths (125 to 200 percent as wide as Earth).
• 20.8 percent should have Neptune-sized planets (two to six times as wide as Earth).
• 5.2 percent should have Jupiter-scale planets (more than six times as wide as Earth). All these numbers will get some additional tweaking, because they don't reflect the breakdown for multiple-planet systems.

The preliminary estimates suggest that roughly one out of every 200 stars should have a planet in the habitable zone, where life could theoretically exist. If you extend that statistic to 100 billion stars in the Milky Way, you come up with a figure of at least 500 million planets in habitable zones.

There's a lot of uncertainty about how many of those planets you could actually live on, because some of those worlds might be too big or otherwise unsuitable. For instance, on Kepler's current list of 1,235 candidates, 54 potential planets are in habitable zones, but only five of them are around Earth's size. KOI 326.01 appears to be the smallest of the five candidates. ("KOI," by the way, stands for Kepler Object of Interest. SolStation.com has the full rundown on Kepler's potentially habitable planet candidates.)

Earth-size planets and super-Earths would be considered the best prospects for alien life, but Borucki pointed out that even Jupiter-scale planets could have moons where life as we know it would do pretty well (as seen in the sci-fi movie "Avatar").

"There's a very rich ocean of planets out there to explore," he said.

A cautionary note
Seager cautioned the journalists gathered at the AAAS meeting not to expect too much from Kepler. The spacecraft was designed to provide data for a statistical survey of planet distribution, but not to point out specific targets for astrobiologists, SETI astronomers and starry-eyed space settlers. "Kepler never promised to say, 'That star has the Earthlike planet in an Earthlike orbit," she said.

The reason is that Kepler has its limits: Mission scientists want to see signs of at least three planetary transits before they add a star system to their list of candidates. That implies that it would take three years of observations for an Earth-size planet in an Earthlike orbit around a sunlike star to become a candidate. The reason KOI 326.01 is already on the list is because it has an orbit that's much closer than Earth's, around a red dwarf star that's much dimmer than our sun.

Once a candidate is on the list, more sophisticated analysis has to be done to confirm that it's a planet rather than, say, a binary star. That may require different types of observations by ground-based telescopes, to pick up the signatures of gravitational interactions. Or it may require looking for subtle variations in the timing of the transits, which astronomers can use to deduce the masses of the planets.

"It generally takes a year [after the] data has come down before we have any results to tell anyone about," Borucki said.

Matthew Holman, a Kepler team member from the Harvard-Smithsonian Center for Astrophysics, said the mission's first habitable planets would likely be confirmed using the transit-timing method. That's how the Kepler team nailed down the find they announced this month, a planetary system that has six worlds packed into tightly spaced orbits.

Worthy of note
Holman listed a couple of other candidate systems worthy of note:

• KOI 314, a potential multi-planet system that may include a world three times as wide as Earth in the "near-habitable zone."
• KOI 730, which appears to be a four-planet system with two of those worlds sharing an orbit. The pattern suggests a complex 6-to-4-to-4-to-3 resonance. (Neptune and Pluto co-exist in crossed orbits largely because they're in a 3-to-2 resonance that keeps them far away from each other.)

Multiple-planet systems are where the action is when it comes to the planet search. Again, Kepler has its limits. Borucki and Seager noted that if an alien Kepler were to look at our own solar system from hundreds of light-years away, it would probably detect only one planet. That's because the planets are too spread out vertically — in Seager's words, we're not "co-planar enough."

Kepler's primary mission is due to run until November 2012, but if the money keeps flowing, the spacecraft could keep going until 2017 or later. And astronomers are already talking about missions that would follow in Kepler's footsteps, such as TESS, Plato and ExoplanetSat. Seager said there's a chance that the low-cost ExoplanetSat mission could be launched in 2012.

In the meantime, you'll be hearing plenty more about Kepler. Here are a couple of links on the lighter side of the planet-hunting mission:

• Check out this video visualization, which shows the 1,235 Kepler candidates as if they were orbiting a single star.
• Give a listen to my podcast about Kepler, originally presented in Second Life under the sponsorship of the Meta Institute for Computational Astrophysics.

Update for 1:25 p.m. ET March 29: Well, it turns out that KOI 326.01 isn't as Earthlike as Kepler's scientists originally hoped. Check out this item for the slightly disappointing reassessment.

Tip o' the Log to Lee Billings, who wrote about KOI 326.01 and other goodies from Kepler for BoingBoing two weeks ago.

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 Alan Boyle's book on Pluto and the search for planets, check out the website for "The Case for Pluto." 

Nature / NASA / Ames / JPL-Caltech / T. Pyle

This illustration, appearing on the cover of the journal Nature, shows the six planets of the Kepler-11 system as they might have looked up close during a triple transit observed on Aug. 26, 2010. The Kepler probe couldn't produce direct imagery of the planets, but it could detect the dip in starlight caused by the transit.

Astronomers behind NASA's $600 million Kepler mission say they’ve detected a star system that packs six planets inside a space that would fit within the orbit of Venus in our own solar system. It’s the marquee event for this week’s “big reveal” from the most sensitive planet-hunting probe ever launched.

It's also a huge puzzle for planetary scientists.

The worlds around a star now known as Kepler-11 rank as the "most densely packed planetary system known," said Jack Lissauer, a researcher at NASA's Ames Research Center. Lissauer is one of the principal authors of a study about the Kepler-11 system, published in this week's issue of the journal Nature.

The six planets range in size from two to four times as wide as Earth, with orbital periods that go from 10 to 118 Earth days. The lightest of the planets is only about twice as massive than Earth. But unlike the close-in "super-Earth" that was reported by the Kepler team last month, none of these worlds is anywhere near as dense as our own planet. If the Kepler team's figures are right, ice or gas must make up a significant proportion of their mass.

"It is clear that such planets need not resemble the earth in any way, which adds to our incredible planetary diversity," astronomer Jonathan Fortney, a member of the research team from the University of California at Santa Cruz, told journalists. "If connections to our solar system can be drawn at all, the low-mass planets in the Kepler-11 system appear to be more like small Neptunes than massive Earths."

Such low-mass, low-density planets are "truly astounding," Alan Boss, a planetary scientist at the Carnegie Institution for Science, told me in an e-mail.

"No theorist would have been nutty (or brave) enough to have claimed that a planetary system like this one might exist," he said. "Yet it does."

Lissauer said he rated the findingas the "biggest thing in exoplanets" since the 1995 discovery of 51 Pegasi, the first extrasolar planet detected around a sunlike star. Yale astronomer Debra Fischer, a planet-hunting pioneer who was not involved in the Kepler-11 study, agreed. "With five low-mass planets in the system, this discovery is as momentous as 51 Peg was in 1995," she said today during a NASA news briefing.

"Kepler is actually reaching the milestone discoveries faster than certainly I anticipated," Fischer said. "Kepler has blown the lid off of everything that we know about extrasolar planets, and this week to me feels very different than last week did."

15 confirmed planets, 1,200 candidates
Today's announcement boosts the Kepler mission's count of confirmed planets to 15, less than two years after the van-sized spacecraft was launched from Cape Canaveral. From its Earth-trailing orbit, Kepler has been focusing on an area of the sky in the constellations Cygnus and Lyra, staring at more than 150,000 stars simultaneously. The probe's telescope documents the tiny dips in starlight that could signal the passage of a planet across a star's disk.

NASA / Nature

This graphic compares the sizes of Jupiter and Earth with the nine Kepler planets that were previously confirmed (above the line) as well as the six planets reported today (below the line). The term "RE" indicates the radius of the planet in terms of the radius of Earth.

Eventually Kepler should find thousands of new planets beyond our solar system, adding to a list that now amounts to more than 500. Just this week, the science team listed a fresh batch of 1,200 planetary candidates, including more than 50 planets that appear to be in the "habitable zones" of their respective star systems.

Not all of those candidates will be confirmed as planets. Some will turn out to be eclipsing binary stars or other phenomena that can mimic the dimming caused by a planet. But in time, astronomers expect to find hundreds of Earthlike planets in Earthlike orbits around sunlike stars. Kepler's census should shed new light on an age-old question: How prevalent are the conditions that could give rise to life in the universe?

If you were looking for life as we know it, you wouldn't look in the Kepler-11 system, which is about 2,000 light-years from Earth. Five of the planets orbit closer to their parent star than Mercury does around our own sun — much closer than the "habitable zone" where life as we know it could exist.

Finding so many worlds in one faraway planetary system is notable, but not unprecedented. Last year, astronomers using a different detection method found a star system that has at least five and perhaps as many as seven planets. Lissauer said the key distinction here is that the planets are so close to each other, circling in a disk that is proportionally flatter than a vinyl record.

"It's just totally unexpected to have this much material, to be able to get a planetary system where planets can be this close to another, that there can be so many of them, that they can be so flat," he said. "It's really a sense of extremes there."

The other curious thing about Kepler-11 is that the planets are close to the mass of Earth, but must be structured like ice giants or gas giants to account for their low density. "How in the world they formed is going to be a headache for theorists for some time to come," Boss told me.

NASA / Nature

This chart graphically shows the estimates of the new planets' radius and mass, as well as how they would fit in among the solar system's planets and other worlds that have been discovered (Kepler-10b and CoRoT-7b). The newly discovered planets (shown in blue) are more similar in composition to Uranus and Neptune than to Earth and Venus.

UC-Santa Cruz's Fortney said the findings suggest that the composition of planets up to 10 times as massive as our own can be "extremely variable."

Lissauer said it's "more likely than not" that astronomers will be detecting more of these mini-Neptunes than super-Earths. But that doesn't mean the search for alien Earths will be fruitless, as demonstrated by the Kepler mission's latest batch of planet candidates.

"There could very well be as many true Earth analogs as people have previously suspected," Lissauer told me. "Over the next two, three, four years, Kepler will be weighing in on that. It just takes a long time for us to collect enough data to address that question directly."

To untangle the complicated orbital dynamics of the Kepler-11 system, scientists tracked the apparent planets' movements for months. Usually, astronomers can confirm the existence of distant planets by checking for slight gravitational wobbles in the star that they orbit. But in Kepler-11's case, the planets were too small and the star was too far away to use that method. Instead, the Kepler team worked out the timing of the planets' movements so precisely that they could figure out their masses using those calculations alone.

"The timing of the transits is not perfectly periodic, and that is the signature of the planets gravitationally interacting," Daniel Fabrycky, a postdoctoral researcher at UC-Santa Cruz, explained in a news release. "By developing a model of the orbital dynamics, we worked out the masses of the planets and verified that the system can be stable on long time scales of millions of years."

The same method should come in handy as Kepler gathers more data on potential alien Earths. "We expect to be doing a lot of those analyses," Fabrycky said. 

In addition to Lissauer, Fabrycky and Fortney, the authors of the Nature paper, "A Closely Packed System of Low-Mass, Low-Density Planets Transiting Kepler-11," include Eric B. Ford, William J. Borucki, Francois Fressin, Geoffrey W. Marcy, Jerome A. Orosz, Jason F. Rowe, Guillermo Torres, William F. Welsh, Natalie M. Batalha, Stephen T. Bryson, Lars A. Buchhave, Douglas A. Caldwell, Joshua A. Carter, David Charbonneau, Jessie L. Christiansen, William D. Cochran, Jean-Michel Desert, Edward W. Dunham, Michael N. Fanelli, Thomas N. Gautier III, John C. Geary, Ronald L. Gilliland, Michael R. Haas, Jennifer R. Hall, Matthew J. Holman, David G. Koch, David W. Latham, Eric Lopez, Sean McCauliff, Neil Miller, Robert C. Morehead, Elisa V. Quintana, Darin Ragozzine, Dimitar Sasselov, Donald R. Short and Jason H. Steffen.

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 Alan Boyle's book on Pluto and the search for planets, check out the website for "The Case for Pluto."

An artist's conception shows the rocky planet Kepler-10b

NASA's Kepler spacecraft has detected a rocky planet that's one of the closest analogs to Earth — except for the fact that it's way too close to its sun.

Rocky worlds have been detected around alien stars before, but Kepler-10b is the first of what's expected to be hundreds of Earth-scale planets found by the Kepler mission. It's too hot to sustain life as we know it, but it buoys hopes for finding other Earths and "super-Earths" that may be more habitable.

Natalie Batalha, an astronomer from San Jose State University who is part of the discovery team, said Kepler-10b is a "scorched world." The temperatures on the planet's sun-facing side would be 2,500 degrees Fahrenheit (1,370 degrees Celsius), almost hot enough to melt iron. Temperatures on the dark side would be too chilly for life, and Batalha said there was no chance that the planet could hold onto an atmosphere.

Kepler-10b's diameter is 1.4 times that of Earth, and its mass is 4.6 times Earth's, Batalha said. That makes it one of the smallest worlds ever found in a distant planetary system like our own. But if Kepler-10b were in our own solar system, it would orbit more than 20 times closer to the sun than Mercury — so close that it makes a complete orbit in just a little more than 20 hours.

Geoffrey Marcy, an astronomer at the University of California at Berkeley who is one of the pioneers in the effort to detect planets beyond our solar system, said the discovery "will be marked as among the most profound scientific discoveries in human history." Marcy explained that Kepler-10b served as a "planetary missing link" between the giant planets that dominate the list of more than 500 distant worlds found to date, and the Earth-size worlds that scientists hope to find in the future.

The find was reported today at the American Astronomical Socyty's winter meeting in Seattle, and a research paper on the discovery has been accepted for publication in the Astrophysical Journal.

How the world was found
The $600 million Kepler mission, launched in March 2009, looks for distant planets by staring at a patch of sky between the constellations Cygnus and Lyra. What it's looking for is the faint dimming of starlight that's produced regularly when a planet passes over the bright disk of the star it orbits. Kepler is monitoring 150,000 stars for those telltale signals, and in principle, it should be able to find Earthlike planets in Earthlike orbits around sunlike stars.

Kepler-10b, circling a star 560 light-years from Earth, was one of the mission's first good candidates for an Earth-scale planet. Its signature showed up in data collected while the spacecraft was being commissioned for science operations, just a couple of months after launch. Scientists collected eight months' worth of readings pointing to Kepler-10b's existence, but they needed to confirm that the planet was really there and get a better estimate of its mass and size.

For the planet's mass, they turned to the W.M. Keck Observatory's 10-meter telescope in Hawaii. The Keck telescope detected the right pattern of tiny wobbles in the movement of the parent star — which is similar to the mass and size of the sun but is more than 8 billion years old (as opposed to the 4.6 billion-year age of the sun).

To confirm the planet's size, astronomers had to figure out the width of the parent star. They resorted to analyzing high-frequency oscillations in the star's brightness that are caused by "starquakes." The oscillations can serve as an indication of how big the star is, just as the pitch of a pipe organ's musical note could be used to estimate the size of the pipe making that tone.

Batalha said the size of the star could be determined to an accuracy of 4 to 6 percent. The researchers combined that measurement with the others to confirm the planet's mass and size as well as its density.

"All of our very best capabilities have converged on this one result," she said.

Where Kepler-10b fits
The Kepler team reported that the exoplanet's density is 8.8 grams per cubic centimeter, which is far denser than Earth's 5.5 grams per cubic centimeter. Batalha said the best explanation for that density is that Kepler-10b is a rocky planet like Earth, only bigger. However, Kepler-10b's proximity to its star means that it would look nothing like Earth. The heat might well be blasting away rock, sending flurries of debris into space. She said mountains wouldn't have much chance to rise up, but canyons could be carved into the planet's surface by flowing lava.

Batalha recalled that a century ago, astronomers were looking for a planet that might orbit our own sun within Mercury's orbit, known as Vulcan. "The thing that came to me is, wow, this is our planet Vulcan," she said. 

A couple of years ago, a European planet-hunting probe called CoRoT detected a similar "lava planet," which has been designated CoRoT-7b. That planet is thought to be a little larger than Kepler-10b (1.8 times as wide as Earth, vs. 1.4 for Kepler-10b), and Batalha said there were still some uncertainties surrounding CoRoT-7b's size estimate. Uncertainties also surround the reported discovery of an Earth-scale planet known as Gliese 581g.

Berkeley's Marcy said the Kepler find would likely "go into textbooks" around the world, due in part to the innovations that were used to nail down the planet's vital statistics. But there's more to come: Batalha said that there might be yet another planet in the Kepler-10 system with an orbital period of 45 days. Those observations still had to be confirmed, she emphasized.

She also noted that the Kepler mission has turned up more than 300 other yet-to-be-confirmed planet candidate, with most of them thought to be smaller than Neptune. More revelations are likely to come to light in February, when the next big batch of Kepler data is due to be released. Although Kepler-10b may be a milestone, it's by no means the end of the road for planet-hunters.

"The discovery of Kepler 10-b is a significant milestone in the search for planets similar to our own," Douglas Hudgins, Kepler program scientist at NASA Headquarters in Washington, said today in a news release. "Although this planet is not in the habitable zone, the exciting find showcases the kinds of discoveries made possible by the mission and the promise of many more to come."

More about the planet quest:

• Interactive: The search for other worlds
• Looking for alien Earths? Here they come
• Rare exoplanet has twin 'Star Wars' sunset

Correction: Ugh, did I really say two hours per orbit? I meant 20 hours, or 0.84 Earth days. These conversions always get me in trouble. Sorry about that.

Update for 3:35 p.m. ET Jan. 11: Kepler researcher Natalie Batalha told me that the research paper on this discovery lists the Kepler-10 star's age as 11.9 billion years, plus or minus 4 billion years. Some reports have gone with the 11.9 billion-year figure, but Batalha prefers to say that the star is "more than 8 billion years old," and so that's what I'm going with.