For Alien Life-Seekers, New Reason to Hope

 

Serge Bloch

By NATALIE ANGIER

Published: June 24, 2008

Twinkle, twinkle, all you stars

With your Earths, Neptunes and Mars.

I sing hello across the void

To your Pluto, O.K., plutoid.

Whatever the name, the laws

demand it

You’re a star. Show me your planet.

For those of us who still mourn the demise of the “Star Trek” franchise and its vision of the cosmos as a thrillingly multicultural if occasionally lethal nightclub, the announcement last week that many Sun-size stars in our galaxy are girdled with Earth-size planets was, frankly, transporting.

The newly detected worlds are far too close to their stellar parents to have much chance of harboring even microbial life, let alone anybody capable of looking boss in spandex. Nevertheless, the discovery gave astronomers and alien life-seekers heart. For one thing, the planets are encouragingly compact. In the past decade, astronomers have found some 250 extrasolar planets, but most have been forbiddingly Jovian: celestial gas bags presumed to have no solid surface and hundreds of times the mass of Earth. In the new report, Michel Mayor of the Geneva Observatory and his colleagues said they had found 45 planets that were only a few times as massive as our beloved blue base, which means that they, like Earth, are probably built of rock.

The tally is proportionally impressive as well: roughly one in three stars surveyed showed signs of harboring stony planets, and other researchers performing similar studies said the figure might be more like one in two. And though the 45 planets on the Geneva list are all “star-huggers,” as one astronomer put it, with orbital periods of 2 to 50 days — even Mercury needs nearly three months to circumnavigate the Sun — researchers are confident that other rocky planets remain to be found at Earthier distances from their suns.

Sara Seager, a planetary theorist at the Massachusetts Institute of Technology, said astronomers hunt for planets by detecting telltale wobbles they induce in their host stars, a method that selectively nets the too big or too near. Nevertheless, she said, “the fact is, as soon as astronomers started looking for low-mass planets, they found a whole bunch, and that’s a real breakthrough.” Just imagine the orgy of moderation that a more inclusive scan would reveal.

To some theorists, the new results virtually guarantee the existence of other Earthlike worlds.

“Suppose you have a tribe, and the most noticeable members are the warriors, because they’re adventuresome, they roam around, they’re the first to be spotted,” said Douglas N. C. Lin, a professor of astronomy and astrophysics at the University of California, Santa Cruz. “But you know that for every warrior, there’s a family behind the warrior.”

Dr. Lin continued, “Just as you can extrapolate from the warriors you see what the size of the larger population deep in the woods may be, so the presence of these short-period, super Earths implies that there are clusters of other planets farther out.” Potentially pleasant planets at that. “I would imagine that a significant fraction of ordinary Sunlike stars, maybe more than 10 percent, have habitable planets around them,” Dr. Lin said.

Whether habitable or abominable, planets are inescapable. “You make a star, you’re probably going to get planets,” said Seth Shostak, a senior scientist at the SETI Institute in Mountain View, Calif. “They’re like those knives that get thrown in for free when you order a Veg-o-matic.”

When a cloud of dust and gas collapses to make a new star, spinning faster and faster as it shrinks, competing forces of gravity, pressure and rotation cause some of its midriff to flatten into a disk, rather as the skirt of a skater flies into a circle as she pulls in her arms for a twirl. The planets in turn condense from the dust, gas and ice of that central disk, in sequences that researchers have just begun to model. In Dr. Lin’s view, planetary evolution is a kind of Darwinian affair, as embryonic planets compete to enlarge themselves with heavy metal “food” from the disk, while struggling not to be consumed by a sibling or pulled into the mother star.

If planets abound, scientists suspect that life abounds, too, at least of the microbial kind. After all, they said, life arose here relatively quickly, maybe 800 million years after Earth’s condensational birth — and then stayed unicellular for the next three billion-plus years.

Eager to identify other candidate Gaias, astronomers have high hopes for the Kepler spacecraft to be launched in February. Kepler will take a different approach in its planetary scan, Dr. Seager said, searching not for stellar wobbles but for “tiny drops in brightness,” possible signs of a planet transiting across the distant Sun’s face. Kepler will track 100,000 stars for four years, enough to detect the occasional crossing of any planets with leisurely orbits like ours. “It will be akin to the great age of exploration, the explorers of the 16th century,” Dr. Shostak said. “We will nail down what fraction of stars have planets,” and more important, “what fraction of those planets are small, terrestrial planets.” .

With that comprehensive planetary atlas in hand, we can pick out the places most worthy of follow-up probes: planets that are relatively close, and closest in kind to the one we know best. We can look for rocky planets that follow stable paths, and are laced with clouds of water vapor that hint at liquid oceans below, and, can it be, atmospheric oxygen, the voice of a biosphere. “Oxygen is so reactive that it shouldn’t be in the atmosphere unless it’s being produced by something like photosynthesis,” Dr. Seager said. “It’s a huge indicator of life.”

We may never visit these worlds bodily, but who knows what we may manage by proxy. “We could send something the size of a golf ball,” Dr. Shostak said. “We could send something with robotic eyeballs, noses, ears, fingertips, all the senses that make things interesting, obviate the need to get into a rocket but have the adventure just the same.” May we live long and prosper, with our secondhand heads in the stars, but our mortal feet safe on the ground.