160 billion planets in the Milky Way?!

http://cosmiclog.msnbc.msn.com/_news/2012/01/11/10115679-160-billion-planets-in-the-milky-way

By Alan Boyle
Jan. 11, 2012

A statistical analysis based on a survey of millions of stars suggests that there's at least one planet for every star in the sky, and probably more. That would add up to 160 billion planets or so in the Milky Way.

"We conclude that stars are orbited by planets as a rule, rather than the exception," an international research team reports today in the journal Nature.

The estimate may sound amazing: Just a year ago, the world was wowed by the claim that at least half of the 100 billion or more stars in the Milky Way possessed planets, yielding a figure of 50 billion planets. The latest survey now suggests that there's an average of 1.6 planets per star system, which would work out to 160 billion. But perhaps the most amazing thing about the findings is ... astronomers don't find them amazing at all.

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"Results from the three main techniques of planet detection are rapidly converging to a common result: Not only are planets common in the galaxy, but there are more small planets than large ones," Caltech astronomer Stephen Kane, a member of the team behind the findings reported in Nature, said in a news release from the Space Telescope Science Institute. "This is encouraging news for investigations into habitable planets."

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All these figures are surrounded by wide bands of uncertainty. For example, the researchers say their estimate of 1.6 planets per star system could actually be anywhere between 0.7 and 2.5. But the lead author of the Nature study told me that his team's estimate is the best guess yet.

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Whether the actual number of planets in the Milky Way is 70 billion or 250 billion, it's a big, big number — 10 to 30 planets for every human on Earth. And the number doesn't even count worlds that are less than five times as big as Earth (such as Mercury, Venus, Mars and our own planetary home), inside the orbit of Venus or beyond the orbit of Saturn (such as Uranus, Neptune and the icy dwarfs on the solar system's edge).

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