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Earth-Like Planets Fill the Galaxy


Kepler telescope's field of view

This is Kepler's field of view superimposed on the night sky. Credit: Carter Roberts

LONG BEACH, Calif.—Look up on a starry night. Almost every one of those tiny pricks of light is home to an unseen world. Our Milky Way galaxy is full of planets—100 billion or more—and many of those planets are Earth-like rocks (although our solar system still appears to be an oddball). Such are the major findings that astronomers are announcing here at the semi-annual meeting of the American Astronomical Society, where the halls are crackling with excitement as we all bear witness to a hidden, rocky universe beginning to coalesce out of the darkness.

The great explosion of planetary information is coming courtesy of the Kepler telescope, which has been peering at one small slice of the night sky to search for momentary dips in brightness that happen when a planet passes in front of its host star. Kepler scientists announced that they have found an additional 461 planet candidates, bringing the total number of such Kepler-found candidates to 2,740. (These objects all look like planets, but could potentially turn out to be something else like a double-star system upon further examination. "It's likely that 90 percent or more of these candidates are going to be bona fide planets," according to astrophysicist Natalie M. Batalha of NASA Ames Research Center.)

Most of Kepler's new planet candidates aren't the big Jupiter-like planets that early planet scans were sensitive to—they're Earth-like planets or so-called "super-Earths," planets about twice the diameter of Earth.

Of course, Kepler can only find planets that are aligned just so—the planet must pass directly between its host star and us. There's no reason to think that most planets are lined up this way. "For every transiting planet that we identify there are 10 to 100 more that aren't transiting," said Batalha. The question becomes: how many planets are out there that we don't see? The answer: lots.

"Almost all sun-like stars have a planetary system," said Francois Fressin, an astronomer at the Harvard-Smithsonian Center for Astrophysics who has been exploring statistical models of Kepler data. "If you travel to a sun-like star it will have a planet. We can't say if it will be welcoming, but it will have a planet." What would an unwelcoming planet be? Something very close to its star, and therefore very hot. Those close-up planets whip around their stars in a matter of days or weeks, which means that Kepler has seen them cross in front of their stars many times by now. Fressin's recent work has shown that about one in six stars is home to a rocky, Earth-like planet that orbits its star within 85 days or less. For longer-period planets, we just have to wait for more observations.

What about Earth-like planets with Earth-like orbits? Of the 461 new planet candidates, 51 of them are in the so-called "habitable zone," the Goldilocks region around the star that's at just the right temperature for liquid water to exist. And one of these new planet candidates has all three of the qualities we're looking for in a twin Earth: it's in the habitable zone, it's only 1.5 times the size of Earth, and it's orbiting a sun-like main sequence star.

This last attribute is important, because most stars are not, in fact, like our sun. Most stars in the galaxy are so-called red dwarfs­­--small, dim, cool stars that are our galaxy's "silent majority," according to John Johnson of the California Institute of Technology. Red dwarfs make up 70 percent of all stars in the galaxy, and these are absolutely full of planets, says Johnson--on average, about one per star. Summing up all the red dwarfs in the galaxy and all the planets that they host, we can estimate that the Milky Way is home to at least 100 billion planets. "Our solar system is rare among the galaxy's population of planetary systems," says Johnson, "because our star is not a red dwarf." But with 100 billion possibilities to choose from, who would bet that there's one not like us peering back through that darkness?


The views expressed are those of the author and are not necessarily those of Scientific American.

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