There's No Place Like Home? 

Not so fast, Dorothy! It looks as if Earth may not be so special after all. Second in a two-part series.

Page 3 of 6

Others followed quickly after that, and the media couldn't get enough. Marcy did interviews on every television network and with nearly every talking head. "Every day there was another camera crew or two coming through, and they would often sit here all day, because some times we would discover planets while they were filming," he recalls.

In 1999, the team had another victory. By then Marcy was teaching at Cal, and he asked Fischer, then a postdoc, to model what was going on around a star called Upsilon Andromedae. Marcy and Butler had already discovered a Jupiterlike planet orbiting it roughly every five days, but they suspected there might be a second one. Try as she might, though, Fischer couldn't plot the orbital period for the second planet — there was some background noise in the data, and the numbers just weren't working out. On a hunch, she separated out the background noise, and discovered that it represented the movement of yet a third planet. And there it was: the first multiplanet solar system other than our own.

Today 23 stars are known to have multiplanet systems, Marcy says. There's reason to believe there are many more — that in fact, many of the stars that seem to have a single gas giant orbiting them actually contain multiple smaller planets that haven't been detected yet.

One reason, Fischer says, is that computer models show that as many planets as gravity will allow are packed into our own solar system. Each planet is surrounded by its own gravitational "personal space," and these are pressed up against each other. Try to squeeze another in, and the whole system falls apart, with planets booting each other out of orbit. So knowing that our own solar system is nearly "gravitationally saturated," she says, "What do we think when we see a star that has a Jupiter out here that we can detect, but nothing else? What I would think is that there is something out there, and the something else is stuff that's really interesting — it's the stuff that we can't detect yet." In other words, small rocky planets like our own.

Of course, it takes more than rocks of a certain size to make a planet that can support life. Two years ago, the Berkeley team had discovered a Neptune-size planet circling the star Gliese 436. This May a Belgian astronomer at the University of Liège announced that he had observed it crossing in front of its host star, creating a tiny eclipse. It had dimmed the star's light a fraction of a percent, enough to provide a crucial new measurement: the planet's diameter, and therefore its density (scientists already knew its mass). Although the planet, dubbed Gliese 436b, is 22 times Earth's size, Marcy says, "We know the density of this planet to be about 40 percent of the density of Earth. The Earth is pure rock, so this planet is rock with something a little less dense than rock. Almost certainly what that substance is is water." In fact, astronomers estimate that Gliese 436b is about half water.

The first rocky planet with proof of water is a huge deal since, as far as we know, biochemistry can't happen without it. In fact, NASA's dictum in searching for life is "Follow the water."

Might this planet be habitable? It's hard to say. Its water is likely to exist in an unusual form — mostly vapor at the surface, and, toward the interior of the planet, so crushed by its own pressure that it forms a crystalline solid, a sort of "hot ice." Marcy speculates that Gliese 436b is probably so covered in water that there are no continents, therefore no place for land-dwelling life.

The bigger stumbling block is temperature. The ne plus ultra of planet hunting would be to find a water-bearing planet in the "habitable zone," an orbital sweet spot the right distance from the host star to permit liquid water. (It's also sometimes called the "Goldilocks zone," because it's not too hot, and not too cold.) Gliese 436b is too close to its host star, which is why its surface water is likely superheated steam.

Yet given how common rocky planets are believed to be, Marcy says, it's only a matter of time before researchers start finding them in the sweet spot. "Within our Milky Way galaxy alone I estimate that there are fifty billion rocky planets," he says. "A tenth of them are lukewarm in the Goldilocks habitable zone. That makes something like five billion water-laden rocky planets — five billion just within our Milky Way galaxy alone!"

Here's the kicker, he says: "My guess is that biochemistry springs up on all of them. The amino acids combine into proteins, the proteins eventually coordinate themselves into replicating molecules ... they are able to use up energy and resources, and voilà, you have life. My guess is there is life on billions of planets just within our Milky Way galaxy alone. Primitive life, maybe single-celled life and no more."

Complex life, Marcy says, probably arises less frequently, but still has fantastic odds of being out there. "Our universe as a whole has hundreds of billions of galaxies, most of which are more or less like our Milky Way galaxy with its five billion Earthlike planets," he says, "so there's an uncountable number of habitable worlds with liquid water, with continents, lakes, oceans, ponds, waterfalls, and no doubt fishlike species that spawn upstream, albeit around a star that's in the Andromeda galaxy or some such. It sounds science fiction-y, but how can it not be with all of the trillions, billions of billions of Earths out there? Billions of billions. Some of those Earths are going to make salmon, elk, cheetahs, and symphony-writing critters."

But then he brings up a worrisome problem. If the universe is teeming with life, and some of it is intelligent enough to write symphonies, why hasn't it ever called to say hello?

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