Monday, September 14, 2009

Looking for Planets Around White Dwarfs

An asteroid passing too close to a white dwarf gets shredded by gravity
Image Credit: NASA / JPL-Caltech
Our Solar System is such a nice, ordered place. Eight (or nine or thirteen) planets have been happily circling our sun for 4.6 billion years, and the whole system is stable enough to life to have formed on at least one planet. And the solar system looks like it will continue to be a nice, stable place to live, at least for another six billion years. At that time, the sun will run out of hydrogen fuel, swell up into a red giant star (swallowing Mercury, Venus, and maybe Earth in the process), expel about half of its mass as a planetary nebula, and then shrink into a white dwarf. When the sun loses mass, its gravitational pull will weaken a little. This will cause the remaining planets to move outwards in their orbits a little bit, but they should remain bound to the white dwarf sun, forever circling the ashes of our star.

Now that we astronomers are finding planets around all kinds of stars, we can estimate that at least ten percent of stars have planets of some sort. Many of these planets are very close to their parent star, much closer than Mercury is to our sun. But many are further away, like Jupiter. And, like Jupiter, these planets should survive the death throes of their parent star, and be circling white dwarfs.

Here at Texas, my colleagues have been looking for planets around white dwarf stars, with only one possible success so far. It's a long, hard, slow process that involves looking for the gentle tug of gravity that the planet exerts on its parent white dwarf.

Another of my colleagues, Dr. Mukremin Kilic of Harvard University, has been looking for white dwarf planets another way. He's trying to detect the infrared light given off by giant planets, a heat left over from the very creation of those planets. A newborn giant planet can be as hot as a few thousand degrees on its surface, and even Jupiter still glows at a more feeble one hundred degrees Kelvin (a modest -300 degrees Fahrenheit). With an infrared light telescope, it may be possible to detect these planets around white dwarf stars.

Today, Dr. Kilic released a study that he and his collaborators completed using the Spitzer Space Telescope to look for extra infrared light coming from 14 individual white dwarf stars. They specifically targeted white dwarfs that came from stars three to five times more massive than the sun. This is because astronomers have noticed that bigger stars tend to have bigger planets, and that bigger stars don't live very long. That means that any planets around these stars should be bigger and brighter (because planets cool over time). They found no evidence for any planets.

Does this mean that white dwarfs just don't have planets? Not necessarily. Even though Kilic looked at what should be fairly bright and young planets, he and his team were still limited to finding planets about five times the mass of Jupiter. Planet searches around living stars have found that these behemoth planets are rare; it's more common to find smaller planets than bigger ones. Also, more massive stars become larger red giant stars, and so they can swallow planets out to much larger distances. While the Earth may or may not survive a red giant sun, Jupiter might have trouble surviving around a star five times the mass of the sun.

And perhaps Kilic and his collaborators may just have gotten unlucky. Roughly 10% of stars have planets, and they looked at 14 stars, so they might have only expected 1 star to have planets. And, in the funny way that statistics work, expecting one and finding none doesn't prove very much. But, this isn't the first work on planets around white dwarfs that Kilic has published; he and his collaborators have now searched about 40 white dwarf stars for planets, so they might have expected to find four. If you expect to find four planets and find none, then you may be on to something.

There's another possibility. Many planetary systems we find tend to be chock full of planets. Our own Solar System has four giant planets. Remember how I said that when the sun loses mass, the planets will move a little further away? In that process of moving away, it is possible for the planetary orbits to become unstable, and one or more planets could be flung out of the system. So, maybe big planets won't be found around white dwarfs because of this. We don't know.

Astronomers today are finding planets everywhere, and we do expect to find them around white dwarfs, too. But studies like Kilic's are showing that these remnant solar systems, fossils of planetary systems once similar to our own, are not easy to find. Perhaps they are just harder to see than we thought. Perhaps they didn't survive the death of their parent star. Time will tell. If nothing else, we just need to wait six billion years and watch what happens to Jupiter, Saturn, Uranus and Neptune. One way or another, we'll learn what happens to planets when their parent star dies.

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