Letter From Discover

Monday, February 20, 2006

I Think I Saw A Planet

Seeing is believing. That may be both the problem and the answer to why no one seems to care how many planets are out there. By all rights, the feeling of shock should still be fresh. A mere decade ago, two Swiss astronomers disclosed the existence of the first planet found revolving around another star like our sun. Before that, the only planets we knew of anywhere in the entire universe were our own—nine planets in all. Ridiculous, said astronomer Carl Sagan: There must be millions of worlds just in the Milky Way, let alone in other galaxies. But, of course, no one had ever set eyes on any of them.

Today we know a couple of things about our universe that we didn't know before December 1995. First, there are planets circling many, perhaps most, of the stars in the Milky Way. Second, there are at least 11 planets here in our own solar system—including Sedna (discovered in 2004), which is slightly smaller than Pluto, and Xena (discovered in 2005), slightly larger than Pluto.

So why isn't anyone excited? Why aren't ordinary people buzzing about these discoveries at cocktail parties?

Perhaps the answer is that no one has actually seen another planet around another star. Instead, other planets have been detected by observing the wobbles of stars and inferring that the motions are caused by a planet orbiting the star. Moreover, star wobbles are most easily discovered when they're big, so big planets are what we've found. Many of them are gassy monsters larger than Jupiter.

Things are about to change, as you will read in Thomas Levenson's article, beginning on the next page. The amazing Giant Magellan Telescope under development in Tucson, Arizona, will be able to find lots of planets, many of them smaller than Jupiter. With an array of seven primary mirrors, this new observatory will be so powerful that it will no longer have to rely on the indirect tricks that astronomers have used to locate other planets. At long last, we will truly be able to see them.

But that's only half the story. In reporting the article, Levenson came across a surprise: the diffident suggestion by the mirror specialist behind the telescope, Roger Angel, that the Giant Magellan might actually find another Earth, depending on where it is sited. Many of his colleagues demur, believing that our pale blue dot—as Sagan used to call it—is so modest that a similar planet circling a distant star might lie beyond even the reach of the Giant Magellan Telescope.

So those of us eager to find our next home may have to wait for NASA's Terrestrial Planet Finder, a grand machine that will use multiple mirrors spread out in space. It has one goal—to search nearby stars for a planet that might support humans. The targets have already been selected. Imagine the headline in the daily newspaper: "Another Earth Found!" That would really get conversation humming.

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