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Many "Earths" Are Out There, Study Says

Brian Handwerk
for National Geographic News
April 6, 2005
 
A new study of known planetary systems outside our solar system gives a theoretical boost to the search for extraterrestrial life. Researchers in England say that half of the systems could harbor habitable, Earthlike planets.

Barrie Jones, an astronomer at the Open University in Milton Keynes, England, co-authored the new study. He said, "We were particularly interested in the possible survival of 'Earths' in the habitable zone."

"This is often called the Goldilocks zone—where the temperature of an 'Earth' is just right for water to be liquid at its surface. If liquid water can exist, so could life as we know it."

The location of a system's habitable zone depends on how bright and hot the that system's star is. The zone can shift over the eons as the star ages and becomes brighter and hotter.

Jones collaborated with Open University colleagues Nick Sleep and David Underwood. The team used computer models to map the habitable zone in some 130 known exoplanetary systems—star-planet formations found outside our solar system.

The researchers examined the interplanetary gravitational pull found there. Gravity among planets plays a key role in how they align themselves in relation to their star.

Gravitational buffering from larger planets, for example, could pull an Earthlike planet from an orbit that would otherwise fall in the sweet spot, or Goldilocks zone, that is conducive to life. (Not too hot, not too cold.)

Many of the systems being discovered have giant [planets] parked dangerously close to the habitable zone," Jones said. Imagine if Jupiter were much closer to our own planet, say just beyond the orbit of Mars.

Using computers to model distant star and planet systems, the team mapped the gravitational "disaster zones" that accompany each giant planet.

Smaller, Earthlike planets that orbit in these disaster zones, would eventually collide with the giant planet or be hurled outward into distant, cold regions of the exoplanetary system.

With these criteria in mind, the team evaluated each exoplanetary system.

The researchers looked for planets that orbited in habitable zones from their stars—both in the present and in the distant past. That historical view was important, as it would allow any potential life-form—at least life as we know it—time to evolve, in theory.

New Discoveries

Over the past decade about 130 extrasolar planets have been discovered, and the number is steadily rising.

None of the distant planets are visible by modern telescopes, however. Scientists rely on indirect methods to detect the planets. For example, some are identified by the "wobble" their gravity induces in the stars they orbit.

Many of these exoplanets are large planets that resemble the gas giants of our solar system: Jupiter, Saturn, Uranus, and Neptune.

"We just don't have the technology at the moment to detect Earth-size planets," Jones said. He added that it may be ten years until astronomers detect such and analyze their atmospheres.

Only then can scientists determine if they "are potentially habitable or even inhabited," Jones said.

Geoscientist James F. Kasting hopes to be among the first researchers to directly observe such planets. The Pennsylvania State University professor is part of a NASA Jet Propulsion Lab project dubbed Terrestrial Planet Finder C (TPF-C).

It will likely be a decade before TPF-C can spy Earthlike planets. When it does, Kasting suspects they might be located around stars that currently reveal no planets at all.

"I personally think that the systems we've seen so far are not the best candidates for having Earthlike planets," Kasting said. "There are a lot of stars out there which could also have Earthlike planets and may be even more likely candidates."

"With the observations we've used so far, we wouldn't detect planets looking at our own solar system. We just don't have the observations yet."

Perhaps 150 subgiant or red giant stars lie within a hundred light-years of Earth. (Subgiants and red giants are stars in the later stages of their evolution.)

Some say those 150 stars may lie close enough to Earth that we may one day launch planet-finding space missions to their systems.

But long before any such efforts take place, new imaging technology will probably be available. Most likely it could add to the tally of known exoplanets—without a space mission. The number already seems to grow on a monthly basis.

"It's still philosophical at this point, but I'm an optimist," Kasting said. "I think we'll end up finding a lot of terrestrial planets. Over the next five years or so, I think people using Doppler techniques [observing shifts in electromagnetic waves] will find systems that look much more like ours in terms of giant planets."

For now, the possibility that life inhabits any such distant worlds remains purely theoretical.

Jones, the Open University astronomer, said, "We do believe that if you form Earth-mass planets in the Goldilocks zone, there is no reason we know [of] why those planets couldn't be habitable."

"We've offered a tantalizing possibility: We've shown that 'Earths' could indeed exist in the Goldilocks zone of many of the systems we already know of," he said. "The next job is to see if they are really there."

Jones and his colleagues described their study in the April 1 issue of The Astrophysical Journal.

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