Blue moons? Kepler-22b offers NASA habitable world hopes

— -- Star Wars toured "Tatooine," Star Trek visited "Vulcan" and Avatar plundered "Pandora."

And NASA? NASA's Kepler space telescope team this month unveiled "Kepler-22b." A planet some 600 light-years away, Kepler-22b circles its star squarely in a "habitable zone" — the orbital distance where a world's surface temperature would neither boil nor freeze water, perhaps allowing oceans to survive as on Earth. Water is widely seen as one of life's vital ingredients by planetary scientists.

Catchy names, clearly, aren't a priority in astronomy. Other proposed habitable zone worlds reported by astronomers (among the more than 700 planets detected in the last two decades orbiting nearby stars) sport monikers such as "55 Cancri f" and "HD 85512 b.

But at least some solace comes from the Kepler space telescope team's estimate that just in our Milky Way galaxy alone, some 500 million planets likely orbit inside their star's habitable zone.

"We have many candidates in that region," said Kepler principal scientist William Borucki of NASA's Ames Research Center in Moffett Field, Calif., at a briefing unveiling Kepler-22b to his colleagues earlier this month. At his briefing, Borucki showed a chart depicting more than 50 possible habitable zone planets, as well as Kepler-22b, among the 2,326 planetary candidates detected by Kepler since its 2009 launch.

How has Kepler piled up so many planet candidates? The $591 million space telescope gazes unblinkingly at roughly 170,000 stars within 3,000 light years (one light year is about 5.9 trillion miles) along the " Orion Spur" of stars in our Milky Way. Kepler detects planets by spotting dips in starlight, eclipses called "transits," that they cause when they circle in front of their stars.

Science fiction fans hoping that Kepler-22b is another Earth may need the solace that many more habitable planets may be out there. That's because the early indications are that Kepler-22b's habitable zone isn't all that habitable for that particular world.

For one thing, Kepler-22b isn't really Earth-like. At 2.4 times the width of Earth, Kepler-22b seems more like a smaller version of the gas-shrouded world Neptune in our own solar system, according to planet hunter Geoff Marcy of the University of California, Berkeley. Where Neptune is about 17 times heavier than Earth, Kepler 22b is likely about 14 times heavier than Earth, estimates astronomer Francesco Pepe of Switzerland's Geneva Observatory, who attended the Kepler briefing. At that weight, Kepler-22b likely has an atmosphere nothing like Earth, likely making it uninhabitable.

The Kepler team for example estimated that Kepler-22b, if it had an atmosphere like Earth's, would enjoy balmy average temperatures of 72 degrees Fahrenheit, "a little warmer than a nice day here in California," Borucki said. But even the draft paper written by Borucki and colleagues describing the discovery admits this is "not very likely."

"No, too big," says planetary scientist Lena Noack of Germany's Institut für Planetenforschung (Institute for Planetary Research), for at least one other reason. In research presented at the American Geophysical Union meeting in San Francisco the same week as the Kepler briefing, Noack found that planets need to weigh less than five times as much as Earth to possess plate tectonics, the continental movements that characterize our world.

Plate tectonics causes earthquakes, but also releases volatile gasses, nutrients and minerals important for life. Pointing in our own solar system to Venus, which is almost Earth's size but seems to lack plate tectonics, Noack says: "Earth might be unique."

Looks pretty bleak, science fiction fans. But hold on, there is some hope still for Kepler-22b. Earth, after all, has a moon, one about one-fourth as wide as our planet. And so might Kepler-22b, or the other habitable-zone candidate planets.

"Kepler-22b is certainly a great target for moon hunters," says astronomer David Kipping of The Hunt for Exomoons with Kepler (HEK) project, which already rates the newly-discovered world a "high-priority" target . "Any moon of mass greater than about a third of the Earth's mass should be massive enough to hold onto its own atmosphere and thus be a habitable exomoon," Kipping says, by e-mail.

Basically, if a planet and its moon are spaced far apart, Kepler might see two transits, at least in cases where the space telescope is sensitive enough to detect Mars-sized planets, ones about half-as-wide as Earth, Kipping says. But if the moon and planet are close, astronomers might see a "triple eclipse" in their transit data. "This is when during the transit of the planet across the star, the moon passes in front or behind of the planet too," Kipping says.

The result would be a camel-backed double bump in the amount of starlight blocked from a star during a transit by a planet, resulting from the time when a moon is eclipsed. "So wide or close, the moon tends to always reveal her presence," Kipping says.

Kepler team scientist Jack Lissauer, also of NASA's Ames Research Center, is a little more cautious, saying, "if the planet (and) moon were very close, it would be difficult to tell the difference between the pair and a single larger planet."

We'll see. Kipping hopes to present some preliminary reports from the HEK project early next year at the American Astronomical Society meeting in Austin. Let's hope he has a HEK of a chance.

So hang in there, space fans, "where there's life, there's hope," as folks sometimes note in science fiction films. "I hope so," Noack said, even after pouring cold water on Kepler-22b. "I still hope to find life somehow out there."