Pressing the Search for Earth-Life Planets

Feb. 24, 2003 -- Are we alone? Could there be other life flourishing on some distant planet?

Although efforts to send people into space are stalled following the Columbia tragedy, the drive continues to try to understand realms that astronauts like the Columbia crew had just begun to probe.

One way scientists hope to understand what or, perhaps even who exists beyond our solar system, is by searching for planets like our own. And even NASA, amid its soul-searching, is intensifying that search with planned space missions designed to find faraway planets with newly refined techniques.

"In the next year or two we expect to have many more planets discovered," said Demitar Sasselov, head of an astronomy team at the Harvard-Smithsonian Center for Astrophysics that recently found a new planet.

Seeing Planets in Wobbles and Blinks

Sasselov's team is not alone. Since 1995, teams of astronomers across the country have detected about 100 large planets zipping around stars in far-distant solar systems. The discoveries are the result of new, indirect methods that sense planets by carefully observing their stars.

One method, developed in the late 1980s, detects extra-solar planets by observing the wobble of a star as it slightly jiggles due to the gravitational pull of its orbiting planet. While this technique has helped find a trove of new planets, it's only sensitive enough to find Jupiter-sized ones, since only large planets tug enough on their stars to be detected.

As far as the search for extraterrestrial life is concerned, finding Jupiter-sized planets is important, but less interesting than finding smaller ones.

"A planet of Jupiter's size has 300 times the mass of Earth and is a large ball of gas so it wouldn't be habitable — at least in ways we'd think of," said Greg Laughlin, an astronomer at the University of California in Santa Cruz.

A second method that researchers have just begun to refine could help in the search of distant planets more like our own.

So-called transit searches examine the slight dimming of a star's light caused as a planet moves between the star and telescopes on Earth. The dimming is so slight it has been compared to seeing the shadow cast by a mosquito buzzing in front of a search light nearly 200 miles away.

"The limitation is that you're not able to detect very slight dimming of a star from the ground because the atmosphere of the Earth makes it difficult to get highly precise measurements," said Sasselov, whose team recently used the transit method to find the planet OGLE-TR-56b orbiting its star 5,000 light years away.

In order to get a clearer view, NASA plans to perch the search in space.

Scanning From Space

In October 2006, NASA will launch a space system that will constantly scan stars for distinctive dimming signs of faraway planets. Focusing on 10 million stars at a time, the Kepler system, equipped with a telescope designed to measure brightness, will zero in on about 150,000 stars that fit criteria of suitable, life-hosting solar systems.

Since Kepler will orbit above the Earth's atmosphere, its telescope will have a clear view and should be able to detect very subtle changes in brightness caused by smaller planets passing in front of their suns.

Sasselov, who is a member of the Kepler mission, expects that the telescope will find about 1,000 Jupiter-sized planets in its first year and about a dozen Earth-sized planets after four years of searching.

"We'll have a big computer that will keep tags on each one of the stars," he said. "Kepler will snap digital images that will tell us when they blink."

Once Kepler concludes its four- to five-year search, NASA hopes to send even more powerful telescopes into space that could probe the atmosphere of key planets for signs of water, oxygen and chemicals that are produced by life.

Meanwhile, other astronomers have been calculating the odds that scientists will find life-friendly planets.

What We’ll Find

Kristen Menou, an astronomer at the University of Virginia in Charlottesville, recently created computer models of remote solar systems by studying the Jupiter-sized planets discovered so far.

"You can look, given the architecture of a planetary system, and ask whether an Earth-like planet can exist in this orbit," said Laughlin, who has created similar computer models.

As a fellow at Princeton University, Menou and colleagues there considered factors including a discovered planet's size and orbit to conclude if an Earth-sized planet might also fit within the system.They concluded about a quarter of distant solar systems discovered so far could host Earth-like abodes.

But confirming such predictions will require patience.

Several planets likely haven't been detected yet because they orbit their stars over longer time periods, says Laughlin. A longer orbit creates a longer lag between wobbles or dimmings in their stars — and years to confirm their existence.

Yet it's planets with longer orbits that likely belong to systems hosting planets like Earth.

Our Ideal Jupiter

For example, our Jupiter takes 11 years to orbit the sun and it exists in what seems like a perfect place to protect Earth. It's not too close to the sun so that it would jettison our planet from the solar system, and yet it remains close enough so its orbit protects our planet from incoming comets.

As time goes on, Laughlin and others are confident more discoveries will make the prospect of extraterrestrial life seem more likely.

Although the first steps to finding such planets will be limited to Earth-based astronomy and orbiting, unmanned telescopes, Sasselov believes astronauts will eventually join in the search.

"Doing what the astronauts do every time they fly around the Earth is an important step in going even further," he said. "It may seem like science fiction now, but someday they may go to the planets we find today. For better or for worse, we are a curious species.

"And so far, that has served us well."