More 'Earths' Likely Out There

Irene Klotz, Discovery News Sept. 7, 2006 —
Planetary systems beyond the solar system that contain Jupiter-sized planets close to their suns have a good chance of spawning habitable earthly worlds, a new study concludes. Computer models suggest massive planets that orbit their mother stars closer than Mercury circles our sun churn up raw material as they migrate inward, setting the stage for rocky planets like Earth to form in their wake. The baby Earths could be bathed in oceans that have been delivered by small icy bodies traveling from the outer edges of dust discs from which solar systems are formed, researchers with the University of Colorado at Boulder, Pennsylvania State University and NASA's Goddard Space Flight Center in Greenbelt, Md., conclude in a study published in this week's issue of the journal Science "These gas giants cause quite a ruckus," said Sean Raymond with the Laboratory for Atmospheric and Space Physics at the University of Colorado. "We now think there is a new class of ocean-covered, and possibly habitable, planets in solar systems unlike our own."Previous computer models suggested material for planet formation would be tossed out of a solar system as giant Jupiter-class planets moved toward their parent stars. But after an extensive eight-month simulation consolidating 200 million years of theoretical planetary evolution, the researchers conclude that proto-planetary disks containing more than 1,000 rocky and icy bodies as big as Earth's moon commonly form habitable Earths. If the results are true, roughly one in every three of the planetary systems discovered so far outside our solar system could harbor planets like Earth orbiting at the proper distance from a sun to support life. The finding prompted the researchers to conclude that the fraction of known planetary systems that could be life-bearing may be "considerably higher than previous estimates," according to the Science article. "I think there are definitely habitable planets out there," Raymond said. "But any life on these planets could be very different from ours. There are a lot of evolutionary steps in between the formation of such planets in other systems and the presence of life forms looking back at us." To date, astronomers have discovered about 200 giant planets in orbit around stars similar to our sun. The census sample is expected to grow considerably when a fleet of new observatories — such as NASA's Kepler and Terrestrial Planet Finder and the European Space Agency's COROT and Darwin — are built and put into service over the next decade.

Beta Canum Venaticorum, Turnbull's top prospect. It's a sunlike star about 26 light-years away in the northern constellation Canes Venatici. Astronomers have been looking for planets around the star but have found none to date.
HD 10307, another sunlike star about 42 light-years away. It has nearly the same mass, temperature and metal content as our sun — plus a companion star. HD 211415, which has about half the metal content of the sun and is a bit cooler.
18 Scorpii, a popular target for proposed planet searches. The star is almost an identical twin of the sun, Turnbull says.
51 Pegasus, which was the first normal star beyond our solar system known to have a planet. The Jupiterlike planet was detected in 1995, and Turnbull believes 51 Pegasus could harbor Earthlike planets as well. Epsilon Indi A, about 11.8 light-years from Earth, leads Turnbull's list. It's a star somewhat cooler and smaller than our sun, and was recently found to have a brown-dwarf companion. "Star Trek" fans consider it the home of the Andorian race. In the original "Star Trek" series, it was the base of operations for an evil entity called "Gorkon." Epsilon Eridani, 10.5 light-years away, is a star somewhat smaller and cooler than our sun, and is already known to have at least one planet. By some science-fiction accounts, Epsilon Eridani is the parent star for Vulcan, Mr. Spock's home planet on "Star Trek." However, Trekkers have come to favor another star in the same constellation... Omicron 2 Eridani, also known as 40 Eridani, is now cited in most "Star Trek" literature as Mr. Spock's home turf. It's a yellow-orange star about 16 light-years away, and is roughly the same age as our sun.
Alpha Centauri B is part of the triple-star system closest to our own sun, just 4.35 light-years away. It's long been considered one of the places in the Milky Way that might offer terrestrial conditions — and it's often cited in science-fiction tales, including Isaac Asimov's Foundation series. Tau Ceti is in the same brightness category as our sun. It's metal-poor, compared to the sun, but long-lived enough for life forms to evolve. It has also served as a locale for science-fiction works ranging from Ursula Le Guin's "The Dispossessed" to the TV show "Earth: Final Conflict."