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Friday, January 18, 2013

Other stars have asteroid and comet belts

You’re familiar with the eight planets in our solar system (sorry, Pluto), but that list is far from a complete inventory of the objects orbiting our sun. We also have two regions, or belts, full of asteroids and comets. It turns out that at least two nearby stars have the same swath of objects in proportionately the same places. This may mean that they also have planets in about the same places as the sun does.

To put things in perspective, let’s take a tour of our solar system. By definition, the Earth orbits the sun at a distance of one astronomical unit (1 AU). Mars is a little further out at 1.5 AU. Next comes the asteroid belt, which contains millions of objects, mostly tiny, swirling around the sun at 2.3 to 3.3 AU. Then comes Jupiter at 5.2 AU, followed by Saturn, Uranus and finally Neptune at 30 AU. The Kuiper belt begins just at the edge of Neptune’s orbit and spreads from 30-50 AU. Like in the asteroid belt, most of the Kuiper objects are small, though this time they tend to be frozen comets rather than rocky asteroids.

University of Arizona astronomer Kate Su and her colleagues have discovered that the stars Vega and Formalhaut have asteroid belts in the same places. These are extremely bright stars only about 25 light years from the Earth. Like our sun, they each have an inner ‘warm’ (negative 190 oF) asteroid belt and an outer ‘cool’ (negative 370 oF) comet belt. You can see from the diagram, that if our solar system were the same size as that of Vega’s these belts would be in the same positions (the actual size of the solar system at the scale of Vega’s system is shown in the middle).

Image: NASA/JPL-Caltech.

What makes this particularly interesting is that it strongly implies that there are planets between the two bands, just as there are in our solar system. Otherwise, what would have swept away the debris dividing those two regions? Because of the larger scale of those systems (Vega and Formalhaut are each about twice the size of our sun), there’s about a hundred AU between the inner and outer belts. According to the researchers, multiple planets are probably responsible for tidying up that section of space. The next task is to find them.