Artist’s impression of exocomets orbiting the star Beta Pictoris. ( ESO/L. Calcada)
Artist’s impression of exocomets orbiting the star Beta Pictoris. ( ESO/L. Calcada)

Two Families of Exocomets Circle Nearby Star

Sometimes, it seems as though exoplanets have all the fun. But they’re not the only exo-things out there, and this week, exocomets are taking center stage. Almost 500 of them, in fact.

Those exocomets are all circling a young, nearby star called Beta Pictoris. Rather than flying randomly through their stellar system, though, the comets are organized into two distinct populations, astronomers report in Nature.

One of those faraway families appears to be older, and orbits nearer the star. The other looks fresher and orbits farther away.

“This can be explained if the first family is composed of aged comets influenced by the gravity of a massive planet,” says astronomer Flavien Kiefer of the Paris Institute of Astrophysics and Tel Aviv University, “while the other family is composed of fresh comets, which are most probably remnants of the fragmentation of one or a few bigger bodies.”

Beta Pictoris

Beta Pictoris is a young star, only about 23 million years old, and it lives roughly 63 light-years away. It’s got at least one giant planet, Beta Pictoris b, which is bigger than Jupiter and orbits about nine times as far from the star as Earth is from the sun.

For decades, astronomers have known that Beta Pictoris is surrounded by a large debris disk. Conveniently, we can see it edge-on, kind of like looking at the brim of a sombrero on someone’s head. That orientation makes it much easier to see the signatures of bodies orbiting the star because there’s a better chance they’ll pass between the star and Earth, rather than traversing the blackness of space.

Because of the disk and the star’s age, scientists surmised that watching Beta Pictoris would be a bit like watching our 4.5 billion-year-old solar system during its very early years, when the planets were still growing and shifting into their current configuration.

In the 1980s, astronomers studying Beta Pictoris noticed something strange: Every so often, the star’s light appeared to be filtered by something mobile. Eventually, they hypothesized that those brief blips were coming from the streaming tails of debris disk comets circling the star. As those comets moved across the face of the star, their trailing dust and gas absorbed a bit of the star’s light traveling toward Earth.

Since then, exocoments have been found orbiting about a dozen young stars, including Eta Corvi, 49 Ceti, and HD 172555, and some astronomers suspect that exocomets could be as common as exoplanets.

“Comets are essentially the left-over building blocks of planets, and thus they represent what materials were like in the early stages of planetary formation,” says UC-Berkeley astronomer Barry Welsh, who studies exocomets. “It is like going to a building site and picking up the leftover bricks that were destined to form the building.”

A Tale of Two Families

Recently, Kiefer and his colleagues studied eight years of data collected by the High Accuracy Radial velocity Planet Searcher, located in Chile. In those data were 1,000 observations containing the spectral signatures of nearly 500 evaporating, star-crossing comet tails around Beta Pictoris.

The movements and characteristics of those tails suggested that there were two distinct comet populations circling the young star.

One of these populations, calculated to orbit nearer the star, didn’t absorb as much light as the other, suggesting the comets were depleted and the tails were sparser. And they weren’t all following the same path, though they seemed to be trapped in the same place. That could happen if a large planet like Beta Pictoris b were perturbing those small bodies and sending them falling toward the star, Kiefer says.

The other family, orbiting farther from the star and all on similar paths, looked fresher. They were evaporating more, producing thicker tails that absorbed more starlight. A possible explanation for this population is the disintegration of a larger planetary body, Kiefer says.

“What is the chance that some comets evaporate a lot more gas and dust and are sharing almost the same orbit?” Kiefer asked. “Very small, unless there is a common origin!”

The Young Sun

The evidence for these two comet families is compelling, says Welsh, who wasn’t involved in the work. He notes that it’s not surprising to see this kind of organization in an exoplanetary system – and perhaps particularly not around Beta Pictoris. That system is not only the best studied of these exocomet-hosting stars, but its disk is oriented in a way that’s favorable for observations.

Or, Beta Pictoris could be an exceptionally active oddball.

But that seems a bit unlikely, given that we see similar behaviors among comets in our own solar system, which tend to form families with distinct orbits and compositions. Today, we don’t see many of these icy travelers in Earth’s sky. But once upon a time, when the sun was young and surrounded by a more chaotic disk, there would have been many more comets zipping around, being tugged on by the sun and pushed around by planets.

“This reinforces the feeling that when looking to Beta Pictoris and its environment, we are observing a somewhat younger version of our Sun, when it was 10-20 million years old and it just formed its planets,” Kiefer says.