lunes, 28 de mayo de 2007

28 New Exoplanets Discovered

An artist's concept of the Neptune-sized planet GJ436b (right) orbiting an M dwarf star, Gliese 436, at a distance of only 3 million miles. With a density similar to that of Neptune, the exoplanet is an ice giant and probably has a rocky core and lots of water that forms ice in the interior under high pressure and temperature. Credit: Copyright Lynnette Cook

Astronomers have discovered 28 new planets outside of our solar system, increasing to 236 the number of known exoplanets, one of which is a hybrid Earth-Neptune object with a rocky core surrounded by a shell of ice.

"We added 12 percent to the total in the last year, and we're very proud of that," said Jason Wright of the University of California at Berkeley. "This provides new planetary systems so that we can study their properties as an ensemble."

The planets are among 37 new objects spotted within the past year. Seven of the objects are failed stars called brown dwarfs, with masses that dwarf the largest, Jupiter-sized planets but too small to sustain the nuclear reactions necessary for stellar ignition.

John Johnson of the University of California at Berkeley and his colleagues presented the findings here today at a meeting of the American Astronomical Society (AAS).

Astronomers don't directly spot extrasolar planets, but rather look for stellar wobbles caused by orbiting planets. The planet's size and distance from the parent star affect how strong or weak of a wobble, and more sophisticated techniques for measuring the stellar wobbles has led to an ever-lengthening list of such outer planets. Now they can detect wobbles of a meter per second compared with the 10-meter limit just 15 years ago.

Planet profiles

One of the exoplanets, a red M dwarf just 30 light-years from Earth, was discovered two years ago, but recent observations have allowed astronomers to pin down its mass, radius and density. The ice-giant planet circles the star Gliese 436 (GJ 436) and has a radius and density that are surprisingly similar to that of Neptune. Weighing in at 22.4 Earth-masses, the exoplanet is the first Neptune-sized planet observed to transit a star.

"It must be 50 percent rock and about 50 percent water, with perhaps small amounts of hydrogen and helium," said head of the planet-search team Geoffrey Marcy, also of UC Berkeley. "So this planet has the interior structure of a hybrid super-Earth/Neptune, with a rocky core surrounded by a significant amount of water compressed into solid form at high pressures and temperatures."

Its 2.6-day orbit around GJ 436 means the hybrid planet circles very close to its star, just 3 percent of the sun-Earth distance and making it a hot Neptune. Unlike most giant planets found with such close ties to their stars, this planet has an eccentric orbit. The elongated orbit suggests the parent star could have another planetary companion with a more distant orbit.

"I'm sure people will immediately follow up and try to measure the atmospheric composition of this planet," Wright said.

Bigger is better

At least four of the newly spotted planets belong to multiple-planet systems, supporting the idea that at least 30 percent of all planet-parent stars have more than one planetary companion. Since smaller planets and those outside our solar system are trickier to detect, Wright predicts this percentage will continue to rise as detection methods improve.

Three of the just discovered planets circle stars that boast masses between 1.6 and 1.9 that of our sun. Called A and F stars, massive stars weighing 1.5 to 2.5 solar masses are typically difficult to detect because they rotate fast and have pulsating atmospheres.

Due to their extreme rotational velocities and high temperatures, A and F stars only jitter slightly from orbiting planets and so surveys tend to pick up those affected by super-massive planets and brown dwarfs in short-period orbits.

Johnson discovered that the cooler "retired" A stars, which have nearly burned all of their hydrogen and remain stable for a short stint, are quiet enough to allow astronomers to measure their planet-caused wobbles.

The findings indicate massive stars are more likely to harbor Jupiter-sized planets than are lower-mass stars, Johnson said.