Scientists Find Bizarre Distant Worlds

S A N   D I E G O, Jan. 10, 2001 -- Two “clearly bizarre” planetary systems foundin the orbits of distant stars are puzzling astronomers and raisingnew questions about how planets form.

Planet hunters at the University of California, Berkeley, reported Tuesday that a star 123 light years away is being circledby two objects, one of which may be the biggest planet ever foundoutside the solar system. Around another star, the astronomersfound two planets moving in lockstep, gravitational harmony.

The astronomers announced their discoveries at the nationalmeeting of the American Astronomical Society.

Profound Theoretical Implications

Geoffrey Marcy, leader of the planet-searching team, said a starcalled HD168443 is being circled by a planet about 17 times moremassive than Jupiter. It is by far the largest planetary-likeobject yet found beyond the solar system.

“This is more massive than a planet and it defies theconventional definition for a planet,” said Marcy.

The object is big enough to be called a brown dwarf, which issometimes called a “failed star.” Brown dwarfs are usuallydefined as stellar objects that did not collect enough mass toignite the nuclear fires that causes a star to shine. Generally, anobject 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, aCarnegie Institution of Washington astronomer who is a member ofthe planet-hunting team. “To call it a brown dwarf sweeps themystery under the rug. It is a mystery system.”

“This defies explanation,” said Marcy. “We don’t know if itis a brown dwarf or some type of hybrid.”

The team has discovered more than 30 planets outside the solarsystem. Other teams have found about 20 additional ones. Most suchplanets are two to three times more massive than Jupiter. NoEarth-sized planets have been found.

“This is one of the most exciting discoveries yet,” saidDouglas N.C. Lin, a planetary formation expert from the Universityof California, Santa Cruz, who is not a member of the Marcy team.“This discovery has profound theoretical implications.”

Huge Gas Balls

Although the planetary object is 17 times more massive thanJupiter, Lin said, “it is possible that it formed in the same waythat Jupiter formed in our solar system.”

Planets are thought to form by gravitationally attracting gasand dust in a cloud surrounding a developing star. But planets thatbecome too large can destabilize a planetary system. Butler saidthe HD168443 system is “extremely stable.”

Marcy said a second planet in the HD168443 system has seventimes the mass of Jupiter and orbits closer to the central star. Hesaid both planets are probably huge gas balls, much like Jupiterand Saturn.

“If you could fly a space ship by it, you would see an objectvery much like our Jupiter,” he said.

The second discovery announced Tuesday is of two smaller planetsin closely linked orbits around a star called Gliese 876, a smallstar 15 light years from Earth.

Debra Fischer, a member of the Marcy team, said one planet isabout half the mass of Jupiter and the other is 1.9 times moremassive.

Fischer said the planets are gravitationally locked in what iscalled a resonate orbit. One circles the star every 30 days, andthe other takes 60, forming a near-perfect 2-to-1 ratio.

“We don’t know how they could have gotten into thatconfiguration,” said Marcy.

“These two resonate planets seem to be humming in harmony,”said Fischer. “They are like two harmonic notes on a stringedinstrument.”

Lin said that once such planets are in resonance, they “move asa pair,” responding to the gravitational tug of the companion.