SAN DIEGO Two "clearly bizarre" planetary systems found in the orbits of distant stars are puzzling astronomers and raising new questions about how planets form.
Planet hunters at the University of California, Berkeley reported Tuesday that a star 123 light-years away is being circled by two objects, one of which may the biggest planet ever found outside the solar system. Around another star, the astronomers found two planets moving in lockstep, gravitational harmony.
The astronomers announced their discoveries at the national meeting of the American Astronomical Society.
Geoffrey Marcy, leader of the planet-searching team, said a star called HD168443 is being circled by a planet about 17 times more massive than Jupiter. It is by far the largest planetary-like object yet found beyond the solar system.
"This is more massive than a planet and it defies the conventional definition for a planet," said Marcy.
The object is big enough to be called a brown dwarf, which is sometimes called a "failed star." Brown dwarfs are usually defined as stellar objects that did not collect enough mass to ignite the nuclear fires that causes a star to shine. Generally, an object must acquire a mass greater than 13 times that of Jupiter, the largest planet in the solar system, to start the fires burning. Yet, the object orbiting HD168443 is larger.
"We have never seen anything like this," said Paul Butler, a Carnegie Institution of Washington astronomer who is a member of the planet-hunting team. "To call it a brown dwarf sweeps the mystery under the rug. It is a mystery system."
"This defies explanation," said Marcy. "We don't know if it is a brown dwarf or some type of hybrid."
The team has discovered more than 30 planets outside the solar system. Other teams have found about 20 additional ones. Most such planets are two to three times more massive than Jupiter. No Earth-sized planets have been found.
"This is one of the most exciting discoveries yet," said Douglas N. C. Lin, a planetary formation expert from the University of California, Santa Cruz, who is not a member of the Marcy team. "This discovery has profound theoretical implications."
Although the planetary object is 17 times more massive than Jupiter, Lin said, "it is possible that it formed in the same way that Jupiter formed in our solar system."
Planets are thought to form by gravitationally attracting gas and dust in a cloud surrounding a developing star. But planets that become too large can destabilize a planetary system. Butler said the HD168443 system is "extremely stable."
Marcy said a second planet in the HD168443 system has seven times the mass of Jupiter and orbits closer to the central star. He said both planets are probably huge gas balls, much like Jupiter and Saturn.
"If you could fly a space ship by it, you would see an object very much like our Jupiter," he said.
The second discovery announced Tuesday is of two smaller planets in closely linked orbits around a star called Gliese 876, a small star 15 light-years from Earth.
Debra Fischer, a member of the Marcy team, said one planet is about half the mass of Jupiter and the other is 1.9 times more massive.
Fischer said the planets are gravitationally locked in a what is called a resonate orbit. One circles the star every 30 days, and the other takes 60, forming a near perfect 2-to-1 ratio.
"We don't know how they could have gotten into that configuration," said Marcy.
"These two resonate planets seem to be humming in harmony," said Fischer. "They are like two harmonic notes on a stringed instrument."Lin said that once such planets are in resonance, they "move as a pair," responding to the gravitational tug of the companion.
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