UCSC Astronomy

Stepchild. The orbit of HD 80606b may have been skewed by an early encounter with its star's binary partner (inset).

One Planet, Two Parents?

Astronomers have found that an extrasolar planet's orbit is so elongated and tilted that its path was probably gravitationally shaped by two stars instead of the usual, single central star. The discovery represents the first example of a new category of alien worlds.

Located about 190 light-years from Earth, the planet HD 80606b is a gas giant four times more massive than Jupiter. The planet's unusual orbit grazes its parent star and also swings it out to nearly the distance between Earth and our sun. Even stranger, that orbit is tilted high above the star's equator and axis of rotation. After the planet was discovered in 2001, astronomers had speculated that these bizarre characteristics were caused by the gravitational interference of the star's binary partner, located less than 200 billion kilometers away. But they lacked conclusive evidence.

So astronomers at eight observatories from Massachusetts to Hawaii tracked the planet over 11 hours as it passed in front of its star. The star appeared shortly after sundown at each location and remained visible only for a couple of hours, so each telescope could contribute only a portion of the data.

Based on those observations, reported in the current issue of The Astrophysical Journal, the authors confirm what astronomers had been thinking about HD 80606b. Previous predictions are "exactly what was observed" by the research team, says astronomer Greg Laughlin of the University of California, Santa Cruz, who was not affiliated with the study. If that's the case, HD 80606b opens the ledger of a new type of exoplanet, one that is gravitationally linked to two stars.

The multiple-telescope approach "will become more common and indeed necessary to study other long-transiting planets with large orbital periods," says astronomer and co-author Eric Ford of the University of Florida, Gainesville. It's particularly true in the search for Earth-like planets in the habitable zone around stars, he says, which will have similar transit times.

Planetary scientist Jack Lissauer of NASA's Ames Research Center at Moffett Field, California, says that, based on his own solar system modeling research, the team's conclusion is the most plausible explanation for HD 80606b's strange orbit. It's possible that HD 80606b could have been gravitationally perturbed by a neighboring massive planet and kicked into its current orbit, he says, but this is unlikely given the presence of the companion sun.

Posted in Space, Physics