A New Earth?

The ultimate in astronomical matters would be the discovery of a new planet similar to the Earth. This would be a whole new environment, whole new species as well as many other intriguing philosophical matters. Astronomers have found a new, potentially habitable Earth-sized planet. It is one of two new planets discovered around the star Gliese 581, some 20 light years away from our own. The planet, Gliese 581g, is located in a "habitable zone"—a distance from the star where the planet receives just the right amount of stellar energy to maintain liquid water at or near the planet's surface.

The 11- year study, published in the Astrophysical Journal and posted online at arXiv.org, suggests that the fraction of stars in the Milky Way harboring potentially habitable planets could be greater than previously thought—as much as a few tens of percent.

Gliese 581 is a red dwarf star, located 20.3 light years away from Earth in the constellation Libra. It is currently the 87th closest known star system to the Sun. Observations suggest that the star has at least six planets: Gliese 581 e, b, c, g, d and f.

The star system first gained attention after Gliese 581 c, the first low mass extrasolar planet found to be near its star's habitable zone, was discovered in April 2007. It has since been shown that under known terrestrial planet climate models, Gliese 581 c is likely to have a runaway greenhouse effect, somewhat like Venus, and hence is probably not habitable. A subsequently discovered planet Gliese 581 d, somewhat like Mars, may be just inside or just outside the outer boundary of the habitable zone (depending in part on the greenhouse properties of its atmosphere).

The new study brings the total number of planets around Gliese 581 to six and, like our own solar system, they orbit their star in nearly circular orbits. The scientists, members of the Lick-Carnegie Exoplanet Survey, collected 11 years of radial velocity data on the star. The radial velocity method looks at a star's tiny movements in response to the gravitational tug from orbiting bodies. The team tracked the motion of the planets to a precision of about 1.6 meters per second.

"Our calculations indicate that the planet is between 3.1 and 4.3 Earth masses, has a circular 36.6-day orbit, and a radius estimated between 1.2 and 1.5 Earth radii," remarked co-author Paul Butler of Carnegie's Department of Terrestrial Magnetism.

The surface gravity is similar to Earth's and is estimated at 1.1 to 1.7 times stronger than the Earth's.

Habitability depends on many factors, not just the temperature. The gravity has to be strong enough to hold an atmosphere, for instance, and the temperature must be lower than about 26° F somewhere on the planet. The researchers estimate that the surface temperature of the newly discovered planet is between -24° F and 10° F. The surface would be blazing hot on the side facing the star and freezing cold on the dark side. The planet might be tidally locked to the star—with one side always facing the star, and the other side always dark and cold. This serves to stabilize the planet's surface climates, according to Steven Vogt, co-author and professor of astronomy and astrophysics at UC Santa Cruz. The most habitable zone on the planet's surface would be along the line between shadow and light, with surface temperatures decreasing toward the dark side and increasing toward the light side.

Temperatures on Earth vary tremendously, and life can thrive in very extreme environments, ranging from Antarctica, where the temperature can get to -94 ° F, to extremely hot hydrothermal vents, which roil at 235 ° F.

There is only so much that can be calculated or estimated at a distance of 20 light years. Still it is exciting to know there are other worlds that may harbor life like Earth. It would be a different world with a reddish sun.

For further information: http://www.eurekalert.org/pub_releases/2010-09/ci-php092810.php