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.

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