Startled Astronomers, Fishing for Planets, Hook a Whopper
A California-based team announced Tuesday that is has found the most bizarre solar systems yet, raising new questions about the exact nature of what constitutes a planet, how planets interact and how solar systems form in the first place.
One of the systems contains a monstrous body with at least 17 times the mass of Jupiter--so large that it stretches the notion of what a planet may be.
The other system contains two planets “humming in harmony” as they spin in orbits so synchronized that scientists compare them to nested Russian dolls.
“Both planetary systems are quite unique, and a bit frightening,” said Geoffrey W. Marcy, an astronomer at UC Berkeley who leads the prolific planet-hunting team that has detected about two-thirds of the 55 extrasolar planets found since 1995. “This shakes my confidence that I knew what the full range of planets were.”
Astronomers cannot see the planets directly because the objects do not give off their own bright light, as stars do. Instead, the scientists use telescopes to detect the wobble in a star’s orbit that is caused by the gravitational tug of a planet.
What astronomers actually see are changes in the spectra of light given off by stars as they move back and forth. This “Doppler signature” is the same phenomenon that causes a firetruck siren to wail at different frequencies as the vehicle moves past.
Current technology--even the 10-meter Keck telescope, the world’s largest, allows astronomers to detect only larger planets, because the larger pull they exert on stars is sufficient to make a visible wobble.
One planet with the mass of Saturn was found in 1999, and most of the others have had two or three times the mass of Jupiter. Still, these are far smaller than the newly detected behemoth, which has at least 17.2 and possibly 40 times Jupiter’s mass.
(Astronomers can only approximate the measurement of mass. And, in a counterintuitive twist, a planet that has as much as 80 times the mass of Jupiter might actually be smaller than Jupiter because of the stronger effect of gravity in pulling material toward the planet’s center.)
“This is the whopper,” R. Paul Butler, an astronomer at the Carnegie Institute of Washington and a longtime collaborator of Marcy, said at a briefing about the new findings that was held at the annual American Astronomical Society meeting in San Diego. “This is literally off our scale.”
The system orbits the sun-like star HD 168443, which is 123 light-years from Earth in the constellation Serpens.
The finding feeds directly into a contentious debate over whether large extrasolar planets are indeed planets, or whether they are brown dwarfs--big balls of gas that glow briefly but are not large enough or hot enough to sustain nuclear fusion and become stars.
It is not clear if the “whopper” is a very large planet, a very small brown dwarf or something entirely new. “I call it a mystery object at this point,” Butler said.
Planetary theorists have generally said planets must have less than 12 or 13 times the mass of Jupiter, which would be too small to start nuclear fusion. Brown dwarfs, in contrast, can range from that mass to about 84 times the mass of Jupiter, at which point they would be classified as stars.
But such neat categories miss the point, said Butler, who argues that, since this new object orbits a star and is near another planet, it might be more like a planet. “To call it a brown dwarf,” he said, “is simply sweeping all the mystery under the rug.”
Hal Levison, an expert on planetary formation at the Southwest Research Institute in Boulder, Colo., agreed that using mass to differentiate between planets and brown dwarfs may prove invalid, especially because there is no way yet to glean clues about the formation of these mystery objects.
Levison, who heard about the new object for the first time Tuesday, said the finding would lead him to make changes in his computer models of solar system formation. “I was surprised,” he said. “The whopper really is a whopper.”
The other system of two smaller planets was detected around the dim red dwarf star Gliese 876, just 15 light-years from Earth, in the constellation Aquarius.
In this system, the planets are moving in near-perfect synchronism. The inner planet orbits the star every 30 days, while the outer planet orbits every 60.1 days. The two planets gravitationally “shepherd” each other, pulling each other back in line should one step out of sync, said Berkeley team member Debra Fischer.
This gravitational dance could help astronomers with the “very much unsolved” questions of how planets migrate and interact with each other after they are formed, said team member Jack Lissauer, an astronomer at Silicon Valley’s NASA Ames Research Center.
The slight lack of synchronism between the planets could be a powerful clue. Did they form in step but start slipping apart? Or are they getting closer and closer to synchronism as time moves on?
Such synchronism has been detected between various smaller objects, such as the moons of Jupiter and satellites near Saturn’s rings, and has been seen between Neptune and tiny Pluto. But it has never been found between two relatively large planets.
The movement suggests that solar systems--even our own stately group of nine planets--might not be as stable over billions of years as previously believed.
If the “whopper” is a planet, it raises many difficult questions for scientists about how planets may form. “This is going to bug astronomers,” said team member Steve Vogt, a UC Santa Cruz astronomer.
Douglas N.C. Lin, an expert on planetary dynamics at UC Santa Cruz agreed, added: “I’ll lose sleep over this.”
