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Planet probe spots hot prospects

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:

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.

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