Searching for the Ends of Worlds

One of my last astronomy projects was a study of a star surrounded by the debris of two planets that suffered a catastrophic collision.

Two planets colliding around the binary suns of BD +20 307.


When asteroids or planets collide, the debris that results ends up orbiting the star. Our own solar system contains a small amount of this sort of dust, which is produced by collisions between small asteroids and by comets evaporating as they approach the Sun. This dust is known as the Zodiacal light. It shines by reflecting sunlight. producing a glow that can sometimes be seen in the morning sky.

Very rarely, collisions between larger asteroids, or even planets, occur around other stars. This creates immense amounts of dust that can absorb light from its star, and re-radiate it at longer wavelengths. Therefore, astronomers identify stars around which collisions have occurred by looking for stars that are unexpectedly bright at infrared wavelengths.

We don’t really know how often big collisions occur between asteroids and planets around the Sun. Until recently, we only had indirect information from our own Solar System to work with. For instance, we think that when the Earth was less than 50 million years old, a Mars-sized planet collided with it, breaking off a material from our planet to form the Moon. The dust kicked up by this collision would definitely have been visible to extraterrestrial astronomers.

I am not sure whether later impacts would have produced much dust. Between an age of about 400 and 700 million years, there is some geological evidence that the Moon was bombarded by a large number of asteroids, during a period referred to as the late heavy bombardment. Presumably, other inner planets would have been bombarded as well, although erosion and volcanism would have erased the signs. This could have produced dust that was visible to astronomers near distant stars.

At its current age, asteroids are believed to hit Earth every few hundred million years. These isolated events might (or might not) wreak havoc on Earth, but they are minor on cosmic scales. In the past 20 years, we have also seen Jupiter hit by comets not once, but twice. However, Jupiter is so massive that it attracts those sorts of collisions, and in any case, the asteroid simply gets absorbed into the planet, without polluting the surroundings with dust.

So, to get a better handle on how often collisions occur around stars of different ages, astronomers get a large catalog of stars (such as the Henry Draper catalog and its extensions, which contain the 359,083 brightest stars), and see whether any are unexpectedly bright in the infrared. We find that most stars around which collisions have occurred are young (10 to 100 million years old), because the orbits of their planets and asteroids haven’t settled yet. Old stars surrounded by the debris of annihilated planets are reassuringly rare.

Nonetheless, we have some good evidence that big collisions do sometimes happen, even around older stars. Over the summer I noticed a couple papers. Lisse et al., recently described the composition of dust around a 12 million year old star HD 172555. They infer that a large asteroid recently collided with a rocky planet around that star. Moor et al. report the discovery of four stars surrounded by dust. Three of them are less than 200 million years old (phew), and one of which is probably about 2 billion years old (HD 169666, uh-oh). Moor et al. do not elaborate much on the origin of the dust in their paper (frankly, in the preprint version, they didn’t do a particularly good job of presenting their conclusions).

My collaborators and I, however, do not suffer from similar restraint. I joined a project led by Ben Zuckerman, to study the dustiest Solar-type star known, BD +20 307. The amount of dust around the star led us to believe that two planets had collided recently, in an event of a magnitude similar to that which formed the Moon. We expected to discover the star was young, but we were in for a surprise. . .

I have written a piece about the result, and placed it here. You can also see our press release, if you want the quick version.