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Thursday, November 14, 2013

A fifth of Sun-like stars may have Earth-like planets

How many Earth-like planets could there be in our galaxy? According to Erik Petigura and Geoffrey Marcy from the University of California Berkeley and Andrew Howard from the University of Hawaii, lots. It looks like up to 20% of all Sun-like stars have at least one Earth-like planet.

There are two criteria for determining that a planet is ‘Earth-like’. First, the planet must have a rocky surface and be more or less the same size as the Earth. Gas giants like Jupiter need not apply. Second, the planet has to orbit its star within the ‘habitable zone’. That is, the planet must orbit under conditions that allow for liquid water on the surface. If a planet is too far away from its star (too cold) all liquids are permanently frozen solid. If a planet is too near (too hot), any water will evaporate out into space.

For four years, the Kepler Space Telescope scoured the skies, using the ‘transit method’ to find exoplanets. Although the telescope can no longer continue that mission, it has provided an enormous amount of information and cosmologists are still pouring over the data. So far, 603 planets have been confirmed from Kepler, ten of which are Earth-like. That sounds like a ratio of 1 in 60. How did the astronomers get to 1 in 5 stars having an Earth-like planet?

As good as it was, Kepler was never going to find all the planets that are out there. After all, the transit method works by detecting the periodic dimming of light that occurs when a planet passes in front of its star. If, from our vantage point, there is no transit, then there is no planet detection. Even if a planet does eclipse its star, there’s still a chance that the Kepler failed to spot it. 

The scientists created computer models to correct for these missed planets. According to their algorithm, they calculate that about 22% of Sun-like stars have a rocky planet between one and two times the size of the Earth that receives between one and four times as much light as we do. 

At first glance, this seems like an unreasonably high percentage. However, keep in mind that many if not most stars have more than one planet. Among the planets in a star’s orbit, there very well could be at least one smallish, rocky one. That’s certainly the case in our solar system. 

If the predictions are accurate, the nearest Earth-like planet might be as close as twelve light years from Earth, a distance that a highly advanced civilization might one day traverse. But probably not us, at least not anytime soon. The fastest rockets we have today would take over ten thousand years to get there.

Petigura EA, Howard AW, & Marcy GW (2013). Prevalence of Earth-size planets orbiting Sun-like stars. Proceedings of the National Academy of Sciences of the United States of America PMID: 24191033.