Since they first began finding distant planets in the 1990’s, astronomers have found alien worlds orbiting everything from Sun-like stars to dim red dwarf stars to pulsars, the super-dense remnants of stars that exploded long ago. Lately, though, they’ve been finding planets around one other kind of celestial body: stars that never quite achieved star-hood. They’re known as brown dwarfs—too big to be called planets themselves, but too small to ignite the thermonuclear reactions that allow fully certified stars to shine.
Only a tiny handful of planets orbiting brown dwarfs have ever been found, but it’s not just rarity that makes the new discoveries so intriguing. Writing in the journal Astronomy and Physics, an international team reports the tentative discovery of a brown-dwarf planet just 6.6 light years away—not merely right next door in cosmic terms, but practically in the next room. “If confirmed,” says lead author Henri Boffin, of the European Southern Observatory (ESO), “it would be one of the closest planets to Earth, except for the planet around Alpha Centauri B [4.3 light-years away].” That, he says, “would allow for exquisite follow-up with next generation instruments.”
But that’s not all. The evidence for this possible planet came from a detection method that astronomers have been trying to master since the 1960’s, with uneven results. The technique is known as astrometry.
The gravity of an orbiting planet tugs on its parent star, making the star wobble back and forth in space. If the planet’s orbit is edge-on as seen from Earth, you can’t see the star’s wobble directly; you have to measure tiny changes in its color as it approaches us, then moves away, then approaches again. This kind of wobble measurement isn’t easy, but it’s how the first planets were found, and it’s gotten more and more accurate over time. (Edge-on orbits can also let astronomers watch as the planet moves right in front of its star, dimming the starlight—the technique used by the now-crippled Kepler space telescope.)
If the planet’s orbit is face-on, however, as though we’re looking down on it from above, the star’s wobble will be side-to-side—but measuring this motion has proven to be insanely difficult. A couple of planets were “discovered” this way in the 1960’s, but they turned out to be bogus.
The brown dwarfs written up in the new paper are in just this inconvenient face-on position, but in this case, the astronomers had three advantages. First, while the brown dwarfs are incredibly dim (there are two, orbiting each other with a period of about 20 years), they are really close by. Second, the dwarfs, known as Luhman A and B and themselves discovered early this year, are between 30 and 50 times as massive as Jupiter. That makes them light enough to be yanked around significantly by a planet that isn’t all that much lighter (it weighs in at about 10 Jupiter masses, so it’s not remotely Earthlike, and it orbits just one of the two brown dwarfs). And third, the scientists had access to the ESO’s powerful Very Large Telescope, located in the clear-skied high desert of northern Chile.
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This trifecta of good fortune allowed the changes in the brown dwarfs’ position on the sky to be measured with unprecedented accuracy. “It’s like a person in Paris being able to measure the position of someone in New York with a precision of 10 centimeters [about four inches],”says Boffin. He cautions that the detection can’t be considered rock-solid yet. “We only have hints that there is a third object in this system,” he says. The reason is that the scientists have been watching for just two months, and the planet may take up to a year for a full orbit. Ideally, they will keep monitoring the system for at least that long, just to be sure.
Assuming the planet is confirmed, astronomers will have to wait a bit for the next step. The coming generation of giant ground-based telescopes and powerful new space telescopes isn’t on line yet. But by the end of this decade, these impressive instruments could give observers a direct look at this surprising new addition to the planetary family.