The Sciences

Binary planetary systems caught in the act of forming!

Bad AstronomyBy Phil PlaitJul 2, 2009 11:00 AM


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Astronomers have discovered a young binary system where both stars are surrounded by thick disks of material that are in the process of forming planets! And it's a near thing, too -- this system almost didn't exist at all. First, the cool image:

On the right is a Hubble Space Telescope image of the two stars (collectively called, weirdly, 253-1536). In the optical, the disk enveloping the star on the left (called 253-53 a, so I'll just call it Star A) is obvious. It's dark because it blocks most of the light from the star, which is deeply embedded in the disk and can barely be seen. The star on the right (Star B) has a disk as well, but it's far smaller than the other star's disk, and swamped by the light of the star. So the components of this binary are like Jekyll and Hyde: one star is blocked by the dark disk, and in the other the disk is outglared by the bright star. The image on the left was made using the Submillimeter Array, or SMA. At this wavelength (almost out in the radio part of the spectrum) the warm dust in the disks is bright, and the stars are almost completely dark. The disk on the right becomes obvious. Using some relatively simple math, the mass of the disks can be calculated (basically by measuring the size and brightness of the disks): Star A's disk on the left has a mass of about 70 times that of Jupiter, and Star B's disk is about 20 times Jupiter's mass. Our entire solar system of planets (that is, everything except the Sun) has roughly twice the mass of Jupiter. So what we're seeing here is easily enough material to make a fully-fledged system of planets! In fact, this is the very first time a binary star, where both stars are detected in visible light, has been seen where each has a disk capable of making planets.

Very cool. And, actually, rather lucky for these stars. They are located inside the vast Orion Nebula, a star-making factory about 1300 light years from Earth. In the heart of the nebula is a cluster of stars containing extremely massive, hot, and bright stars. The starlight from those beacons is so fierce that it actually disrupts disks around nearby young stars; the ultraviolet light boils away the dust in a process called photoevaporation. As you can see in this image (which I took from the scientific journal paper about these observations) 253-1536 is located about a parsec away (more than 3 light years) from the center of the nebula, sparing it from the harshest effects of those bright stars. Had it been much closer, the disks around the two stars would have boiled away by now. In a few million years, both these stars may have actual planets orbiting them. Star B is a red dwarf, cool and dim, and it's not clear what type of star A is. Probably not terribly massive, and I'm guessing somewhat less massive than the Sun. Imagine what the sky would look like from such a planet! From Star A's planets, for example, Star B would be an intense red glare in the sky, far far brighter than Venus appears from Earth. The position of the other star in the sky would change slowly as the two stars complete their 4500 year long orbit. And if you look away from the other star, you'd be looking deep into the heart of the nebula, where a dozen or more stars would shine almost as brightly as the Moon does from Earth! And, of course, you'd see the nebula itself stretched across half your sky, glowing red, green, and white. I would sorely love to see such a thing. Wow. Whatever life that eventually evolves there would be very lucky to get such a view... and they'd have another advantage over us. The two stars of 253-1563 are separated by only about 400 times the Earth-Sun distance, about ten times the distance of Pluto from the Earth. If they really had the will, life there could visit the other system! It would be a technical achievement and difficult to be sure, but we're almost there ourselves. Hmph. I do believe I'm jealous of a hypothetical life form that won't even exist for billions of years, if it ever does! Come to think of it, though, by the time any life there has the tech savvy to build rockets, all those bright stars in the nebula will have long since exploded as supernovae... and worse, at a distance of only a few light years, those titanic explosions will do serious damage to any planets, and in fact could blow away those disks long before planets could form. So maybe planets never will get a chance to exist there. Wow, again: I went from jealous to sad awfully quickly. But such is life in the Universe. I suppose I should just be glad that we here on Earth are clever enough to create telescopes to give us a view of such a remarkable system, and that allows us to appreciate what we see... and what we've got already.

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