The number of extrasolar planets has jumped to 28, with the announcement Monday of six newly discovered planets orbiting sun-like stars.
The lightest of the six planets is about 75 percent as massive as Jupiter, while the largest is about six Jupiter masses. These planets orbit six different stars that are between 65 and 192 light-years from Earth. The stars are similar in brightness, size and age to the sun.
Unlike most planets found previously, scientists believe that five of the six new discoveries could theoretically sustain liquid water because they are just the right distance from their parent stars. Most of the other extrasolar planets lie outside the so-called "habitable zones" -- either too close or too far from their parent stars for the temperature to support liquid water.
The discoveries were made by a team led by Steven Vogt of the University of California, Santa Cruz; Geoffrey Marcy of the University of California, Berkeley and Paul Butler of the Department of Terrestrial Magnetism at the Carnegie Institution of Washington. The astronomers used the Keck 1 telescope in Hawaii to conduct their observations, which are part of a three-year, $200,000 project to search for planets around 500 nearby stars. The survey is funded jointly by NASA and the National Science Foundation.
Although astronomers have never actually seen any of the planets around other stars, they detect such bodies by measuring minute wobbles in the star caused by the gravitation of orbiting planets.
Although astronomers have long suspected that other stars must have planetary systems around them, it wasn't until five years ago that the first extrasolar planet was actually confirmed. The handful of planets that have been discovered around other stars since then has given scientists new models to understand how planetary systems evolve, Aizenman said.
One of the surprises that accompanied the planetary discoveries is the emerging picture that most planets seem to travel in oblong, not round, orbits.
One of the six new planets that Vogt, Marcy and Butler have found is a Jupiter-size orb that swings within about 30 million miles of its parent star at closest approach -- about one-third the distance from Earth to the sun -- then lopes wide out to more than two times the distance from the sun to Earth at the farthest point in its orbit. This orbit is the most eccentric of any planet yet found, said Morris Aizenman, executive officer of the National Science Foundation's Astronomical Sciences Division.
"We don't expect planets with eccentricity like this," Aizenman said.
Observing our own solar system, where all the planets have more-or-less circular orbits, supports the conclusion that most planets around stars move in circular orbits, he said. Observations of extrasolar planets, though, show that perhaps it is our own solar system that is unique.
"As we discover [planets around other stars] we're learning about things we didn't even dream about," Aizenman said.
Using our own solar system as a model for how other ones form "we would have thought initially, 'well, we have planets with solid cores nearby the sun and we would have the more massive gaseous planets farther out.' [In fact], we're seeing that this isn't necessarily the case," Aizenman explained.
It is still unclear where the Earth-size planets might be in these types of solar systems, he said. The next step is to increase the sensitivity of instruments to detect much smaller planets, and even satellites around the massive planets that we already know exist.
Vogt's, Macy's and Butler's findings will be published in the Astrophysical Journal.
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