The Universe Just Got Weirder
12:00 p.m. Jan. 9, 2001 PST
SAN DIEGO -- Scientific planet hunters have found two new planetary systems, neither of them much like ours and one of them downright bizarre, researchers said on Tuesday.
One star has two planets locked in harmonic orbits -- one twice as fast as the other -- while the second star's satellites are huge, one of them so big that it challenges the very definition of a planet, the scientists said at the annual meeting of the American Astronomical Society.
While astronomers have found dozens of planets outside our solar system over the last five years, these two new systems are only the second and third with more than one planet orbiting a star.
These discoveries are part of a long-term project to search for planets among 1,100 stars within 300 light years of Earth, which in cosmic terms is fairly close. A light year is about 6 trillion miles, the distance light travels in a year.
These new systems initially fooled astronomers, who previously believed each star had only one possible planet in its thrall. The planets are not literally seen, but are detected by a characteristic wobble of the stars they orbit. The putative planets' gravitational pull causes the wobble.
That the sun-like star, HD 168443, (123 light years away in the constellation Serpens) has two monster planets orbiting it has been confirmed, according to astronomer Geoffrey Marcy of UC Berkeley and Paul Butler of the Carnegie Institution of Washington.
Both of these planets are big, but one is so huge that it makes the whole system "truly bizarre," Marcy said in an e-mail interview.
"The outer companion (of this star) is so massive, between 17 and 40 times the mass of Jupiter, that it seems too large for a conventional planet," Marcy said.
"We frankly don't know what name to give it. Is it a planet or brown dwarf (a dim failed star) or something that formed in the protoplanetary disk, but gobbled an unusual quantity of gas in that disk?" Marcy said. "We simply don't know."
The other star with two planets circling it is Gliese 876 in the constellation Aquarius, a mere 15 light years from Earth.
Gliese's satellites are more seemly in size, with one planet at least half the mass of Jupiter and the other with nearly twice Jupiter's mass.
What makes them remarkable is their resonant orbits. The smaller planet orbits the star twice for every one 60-day orbit of the bigger planet, the astronomers said. This synchrony may have led astronomers to believe there was just one planet around the star, according to astronomer Jack Lissauer of the NASA Ames Research Center.
But their closeness to each other may help scientists learn more about planets in our own solar system, he said.
"They're tugging on each other much closer than any other pair that we've seen," Lissauer said, referring to the planets' gravitational pull. He said this proximity may let astronomers determine the actual masses of these two bodies, rather than just the minimum masses they have for most extrasolar planets.
"The 2-to-1 ratio (in the planets' orbits) probably implies that the planets formed separately with different periods," Marcy said. "But one planet moved from its original location. As it moved, it gravitationally shepherded the other planet in the current 2-to-1 ratio."
He said this was similar to the planets Neptune and Pluto in our solar system, which have an orbital period of 3-to-2. Neptune moved and is shepherding Pluto, Marcy said. More information on these discoveries can be found online at http://www.exoplanets.org
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