In a discovery sure to satisfy space artists and sci-fi buffs alike, astronomers have found an alien planet illuminated by three suns. There's just one problem: It shouldn't exist.
Over the past decade, dozens of so-called 'hot Jupiters' have been found: massive, gaseous planets in very tight orbits around their mother stars. How they get there is a puzzle. The inner parts of the planet-spawning disks of gas and dust surrounding new-born stars are not believed to contain enough mass to form giant planets. Gas giants are probably born further out, beyond some 400 million kilometers, where ice crystals can develop and accumulate into planetary cores that are massive enough to attract large amounts of gas from the disk.
So how do these far-out giants end up at the star's doorstep? The answer, according to theorists, is planetary migration: dramatic orbital changes due to the gravitational interaction of the young giant planet with the remains of the disk, or mutual interactions between planets.
Now enter the planet of a Sun-like star known as HD 188753. Located 149 light years away in the constellation Cygnus the Swan, it was discovered by planetary scientist Maciej Konacki of the California Institute of Technology in Pasadena with the 10-meter Keck telescope at Mauna Kea, Hawaii. The planet is a classic hot Jupiter, but its mother star is part of a close triple system, which should never have been able to give rise to giant planets in the first place. The reason? The combined gravity of the second and third stars would have kept the gas and dust disk of the primary star at a maximum radius of 200 million kilometers--too close for the formation of giant planets. And if these planets don't form, they obviously can't migrate.
"This discovery will make us look very hard at exoplanet formation scenarios," says planetary scientist David Trilling of the University of Arizona. But, he says, "I don't think it's the death sentence for migration." Trilling says the triple-star planet could belong to a class that formed in a different way, maybe even close to the star. There's also a remote chance it's not a planet at all but a brown dwarf star, he says. Future observations of other binary and triple-star systems may yield further clues. Until now, exoplanet hunters have focused on single, Sun-like stars. Says Trilling: "They have so far ignored at least half of all the stars."