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2013-01-08 06:04:04
By By ALICIA CHANG | Associated Press
LOS ANGELES (AP) Our Milky Way is home to at least 17 billion planets that
are similar in size to Earth, a new estimate suggests. That's more than two
Earth-size planets for every person on the globe.
Just how many are located in the sweet spot where water could exist is "simply
too early to call," said Francois Fressin of the Harvard-Smithsonian Center for
Astrophysics, who presented his work at an astronomy meeting Monday.
It's the first reliable tally of the number of worlds outside the solar system
that are the size of Earth, but the hunt for our twin is far from over.
Despite the explosion of exoplanet discoveries in recent years, one find
remains elusive: A planet that's not only the right size but also in the
so-called Goldilocks zone where it's not too hot or too cold for water to be in
liquid form on the surface.
The sheer number of Earth-size planets gives astronomers a starting point to
narrow down which ones are in the habitable zone.
Fressin and his team came up with their figure by conducting a fresh analysis
of data collected by NASA's Kepler spacecraft, which was launched in 2009 to
track down other Earths. They estimated at least one in six stars in the galaxy
hosts a planet the size of ours, translating to at least 17 billion Earth-size
worlds.
Using a different method, a team from the University of California, Berkeley
and University of Hawaii separately came up with a similar estimate. They
calculated 17 percent of distant stars have planets that are the same size as
Earth or slightly larger.
The findings were presented at the American Astronomical Society in Long Beach,
Calif.
Meanwhile, the Kepler spacecraft continues to spot planets as they pass between
Earth and the star they orbit. It found 461 new candidate planets, bringing the
total to 2,740 potential planets, said mission scientist Christopher Burke at
the SETI Institute.
Most of the new Kepler finds were driven by discoveries of Earth-size planets
and super-Earths. Four of those are thought to reside in the Goldilocks zone,
but more observations are needed.
Fressin said it's clear that rocky planets abound outside the solar system.
"If you look up on a starry night, each star you're looking at almost each
one of them has a planetary system," Fressin said.
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Follow Alicia Chang at http://twitter.com/SciWriAlicia
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Online:
Kepler mission: http://www.nasa.gov/kepler