Multiple Suns?
Published on Mar 31, 2007 at 9:25 pm.
2 Comments.
Filed under extrasolar planets.
It has long been a favorite in science fiction stories for the stories to be set in binary star systems. After all, that makes it all the more alien, right? You look up in the sky, and there are two suns, instead of just one. Even one of the early seens in the first Star Wars (OK, it’s episode four, but to me, it is still the first one!) has Luke Skywalker watching the two suns of Tatooine setting. And after all, why not have planets around binary stars? What’s more, at least half of all stars are binaries (there is some evidence that the smallest stars might be more likely to be solo, but certainly stars anything at all like the Sun are more likely binary as not).
But, some years back, the thinking changed. Orbits are hard to compute when there are multiple gravitational sources. In fact, the three body problem is still an area of active research in physics. But, with computers, we can start to get a grip on how these complex orbits evolve.  Twenty years ago, computers were showing us that planetary orbits don’t seem to be stable in binary star systems. In fact, while I was in graduate school, I did a project where I moved mass from the Sun to Jupiter (on the computer of course!) just to see what would happen. Long before Jupiter even got to be a brown dwarf, the Earth’s orbit began to go haywire. The technical term is “chaos.” Well, this just seemed to solidfy the idea that binary stars can’t have planets.
Then, astronomers started finding planets around other stars. But, the biggest shock came when some binary stars began to be found to have planets. Just a year ago, Deepak Raghavan (Georgia State University) found that 29 of the then known extra-solar planets were in binary star systems! That number has now grown to about 50 extrasolar planets being in binary star systems. But, these were all in wide binaries (with the two stars much farther apart than our whole Solar System is across. Quickly theorists got to work and showed that if the binary stars stay far enough apart, then the gravitational disruption of the second star would not be too big to prohibit planets forming. Evidence continues to mount that planets form much more quickly than had first been thought, and that makes it even less likely that the secondary star would prohibit planets forming. All of the observed extrasolar planets in binary star systems orbit stars that are separated at least 50 times farther than the farthest planet is from our Sun. But, theorists also showed that if the stars are very close together (a fraction of the distance that Mercury is from the Sun), then planet orbits could be pretty stable farther out (say well past where Jupiter is located in our Solar System). But, still, the case that I did for my project seems out: that is, a binary star with the companions only about 5 AU apart.
Now, David Trilling (University of Arizona) has even more evidence that planets can form around binary stars. He and his team have found evidence of dust disks in about 40% of the binary stars that they studied using the Spitzer Space Telescope. This is in infrared telescope that shows the glow of the dust disks. These disks are believed to be either involved in forming planetary systems or the leftovers from such formation. Seen from a great distance, planets around our own Sun would be hard to detect, the the interplanetary medium, dust, would be easier to detect. That’s sort of ironic, don’t you think? The largest things in the Solar System are hard to see, but the smallest things make their presense known! But, what makes their finding even more interesting is that many of these dust disks were around tight binary systems. So far, we haven’t found any planets around such systems, but that is probably because planets of such stars would have to be quite far out, and the farther the planets are out, the harder they are to find using the techniques that we have available today. But, the fact that the dust disks are there suggests that planets are possible.
So, perhaps the science fiction writers were right after all. There are planets out there with double sunsets!
-Astroprof
(Image courtesy of NASA)
A Ler…-- Rastos de Luz on April 2, 2007 at 12:11 pm: 1
[…] “Multiple Suns“, no Astroprof’s Page. Ainda sobre o mesmo tema, “Double sunsets and sunlit nights“, no The Planetary Society Blog. […]
Thomas Houck on January 11, 2008 at 6:28 am: 2
Of course there are binary star systems with planets; system in which the two stars are close or far apart. Simply because there are billions of stars in our own galaxy multiplied by the billions of galaxies gives odds that almost anything we can imagine exists and probably things we cannot or have not image exist. Open your minds to the possibilities, not another can of beans.