Comet Madness

Beta Pictoris has Comets

The southern hemisphere star Beta Pictoris has been one of the great “go to” places for exoplanet stories in the years since it was discovered to have a ring of debris around it by the IRAS satellite in 1983. To be specific, IRAS found excess infrared emission around the star, which indicated the presence of the disk. The disk has been imaged from ground-based observatories, and studied by Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer (FUSE), among others. The Hubble studies showed that Beta Pictoris has no one, but two disks of material, and it’s very likely that the materials in this disk are in the early stages of planetary formation.

In a spectacular study just released by the European Southern Observatory, astronomers using the HARPS “planet hunter” instrument at the La Silla Observatory in Chile have made a census of comets orbit Beta Pictoris. They studied about 500 of these “exocomets” and grouped them into two “families”. One is a group of old comets that have been around the star many times and have been heated and lost some of their mass. The other is a collection of young comets that are the result of the breakup of larger icy objects orbiting the star. This is the most extensive survey of comets around another star ever made and may well help us understand the role of comets in the early stages of solar system formation.

Beta Pictoris is a very young star system — only about 20 million years, which makes it an infant, as stars go. By contrast, the Sun is 4.5 billion years old, and the entire solar sysem may have looked very much like Beta Pic back when it was a baby.

The disk surrounding Beta Pictoris is filled with gas and dust, plus larger particles and planetesimals. As astronomers watched this star over the years, they noticed that the star itself would ever-so-slightly dim as something passed in front of it. The star does have a planet, but many of these dimmings did not correspond with that planet’s orbit.

What were they? It turns out that they were comets. Yes, comets can dim a star’s light as they evaporate. Their gases and dust are perfect “light traps” to capture starlight as it passes through, thus causing the star to seem just a little bit less bright for a short time. The science team studied the light to determine the types of comets the instrument detected.

So, the age of exoplanet discovery has expanded into the age of exocomet studies. And, as I recall from another news story, it joins the age of asteroidal dicovery around other stars. All of these ages not only help us understand the formation of planets and objects around other stars, but give us a fascinating look back at how our own system must have looked nearly 5 billion years ago.