The Sciences

A tiny wobble reveals a massive planet

Bad AstronomyBy Phil PlaitMay 29, 2009 4:00 PM


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For the first time ever, an extrasolar planet has been detected due to its physically slinging around its parent star.

Imagine two children, one big and one small, facing each other and holding hands. They swing around each other, each making a circle. The bigger kid makes a smaller circle, and the smaller kid makes a larger circle. If you stand off to the side, you'll see each child alternately approaching and receding from you as they make their circles on the ground. In effect, this is how we've been finding most of the planets orbiting other stars. As a planet orbits its parent star it tugs on that star gravitationally, and we see a very slight Doppler shift in the light of that star as it approaches and recedes from us. If only we could magnify the system hugely, then we would actually see the star make a little circle as the planet orbited it. Well, this new discovery did just that! The star is called VB10, and lies about 20 light years away. That's close! Only a handful of stars are closer. It's a very small, faint star: classified as an M8 dwarf, it's barely massive enough to maintain the pressure and temperature in its core needed to fuse hydrogen into helium. If it were any less massive it would be a brown dwarf, a starlike object that is not actively fusing elements in its core. The planet is massive, about six times the mass of Jupiter. All of these things together -- the close distance, the low mass of the star, and the high mass of the planet -- combined happily for this discovery. The massive planet tugs on its star, which is low mass so it makes a relatively big circle, and the nearby location makes it easier to see that motion. Astronomers watched the system very carefully for 12 years. As the star was swung around by the planet, the physical motion was seen as a wobble in the location of the star. The position of the star had to be measured with exacting precision, which ain't easy. Even a slight warp in the detector itself can throw off the measurements, as can a hundred other possible errors. I've done work like this myself -- not exactly like this, but trying to get very high-precision positions on a detector -- and it's frustratingly difficult. This discovery is an amazing achievement.

Artiost's impression of the planet orbiting VB10. Image credit: NASA/JPL-Caltech

And the system itself is pretty interesting. For one thing, although the star is far more massive than the planet, physically they are about the same size! When a planet gets to be about as massive as Jupiter, its size remains the same as mass is added to it; instead of getting bigger it gets denser. So if you took two Jupiters and smashed them together, you'd get one planet with twice the mass and twice the density, but about the same size as the original planets. The star is very dense, so even though it's much more massive than its planet, they are about the same size. That would have to look pretty weird if you were there. VB10b, the planet, orbits the star pretty close in, about 50 million km (30 million miles) out... closer than Mercury orbits the Sun! The star is so dim, though, that the planet would only be about room temperature at its cloud tops. It's possible there are other, less massive planets closer in yet, though I doubt it. The gravity of the super-Jupiter would probably disrupt their orbits. As it happens, VB 10 is a flare star; it sometimes erupts in ginormous X-ray flares so violent we can detect them here on Earth, 200 trillion kilometers away! So any planet orbiting that star would be cooked to a crisp. It's ironic that such dim bulbs can produce such enormous blasts of energy, but the eruptions are magnetic in nature, much like the Sun's own solar flares. These kinds of stars can have powerful magnetic fields, capable of much violence. Perhaps the most remarkable thing about this system is that VB10 is the lowest mass star known to have a planet. It may keep that record for some time; there aren't too many stars with a lower mass, or else they wouldn't be stars at all! So no mater how you look at this, it's a pretty cool discovery. And it adds another arrow in our quiver of methods to detect other planets. It's a difficult one to be sure, and it takes a lot of patience, but now we know it works.

Comparison chart of the sizesof the star and planet. Click to embiggen.

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