ANCHORAGE, ALASKA—Brown dwarfs are a sorry lot. If it wasn't bad enough that these Jupiter-sized "failed stars" are born too lightweight to become suns, now it turns out they also run the risk of being swallowed whole by their parent stars. The finding, reported here yesterday at the 220th meeting of the American Astronomical Society, may shed light on the future fate of Earth and our planetary neighbors.
Dozens of brown dwarfs have been discovered orbiting normal stars. But astronomers have always wondered about the paucity of close-in brown dwarfs: While many giant planets have been found in small orbits, whirling around their sunlike stars in just a few days, the more massive brown dwarfs appear to shun these intimate relationships.
Planetary scientist Tristan Guillot of the Côte d'Azur Observatory in Nice, France, has an explanation for this "brown dwarf desert." At the meeting, he argued that brown dwarfs in tight orbits get devoured by their sunlike parent stars. "Some fall into their stars very quickly," he says.
The reason: If a brown dwarf orbits close to its star, both objects raise tides, just as the moon raises tides on Earth. In the Earth-moon system, tides cause the moon to slowly drift away from Earth - a side effect of the fact that Earth's rotation is much quicker than the moon's orbital motion. However, in a star-brown dwarf binary, the star's rotation is much slower than the brown dwarf's orbital motion, so the companion is dragged inward. The more slowly the star rotates, the stronger this effect is, says Guillot. "Stars that are hotter than our sun spin much faster," he notes, "so they drag their companions less efficiently," enabling the brown dwarf to survive.
Indeed, the few close-in brown dwarfs that have been found orbit stars that are hotter and more massive than our sun, and spin faster. At the same meeting, astronomer Thomas Beatty of Ohio State University, Columbus, announced the discovery of just such a system with the small KELT telescope in Arizona: a brown dwarf 27 times as massive as Jupiter, orbiting its hot parent star every 30 hours.
But even this brown dwarf will one day get swallowed. "Right now, the star and the brown dwarf are locked in a honeymoon phase, where they both turn the same face to each other all the time," says Beatty, "but in the future, the star will swell up and engulf its companion completely."
Incidentally, that's the same fate that awaits Mercury and Venus, the two innermost planets in our own solar system, when the sun grows into a bloated red giant star some 5 billion years from now. "It's unclear whether or not the Earth will survive this red giant phase," says Guillot, "but our theoretical models may help to shed more light on that."