A detailed study of comets orbiting the young nearby star Beta Pictoris is published today in the journal Nature, and it reveals striking similarities to the comets found in our solar system.
Over the past 30 years, 11 stars have been identified that appear to have comets orbiting around them, known as exocomets (akin to exoplanets).
In this new work, a team of French astronomers has discovered that most of Beta Pictoris’ exocomets can be separated into two distinct families – a group of old, decrepit comets and another that is made up of fresh, active, new exocomets.
It is the first time that cometary families around another star have ever been characterised, and the results draw striking parallels with the families of comets that move in our own solar system.
Young star, bright disk
Beta Pictoris (or Beta Pic) is a young star – a mere toddler, in astronomical terms – less than 20 million years old and 63 light-years from our sun. Somewhat more massive and more luminous than our sun, it is easily visible with the unaided eye, shining close to the second brightest star in the night sky, Canopus.
In 1983, the Infrared Astronomical Satellite (IRAS) detected that the star was brighter in the infrared than it should be. This revealed it to be surrounded by a thick disk of debris, like a far denser version of the asteroid belt.
Indeed, the edge-on disk is so substantial that it has since been imaged by professional and amateur astronomers!
Since that discovery, astronomers have paid particularly close attention to the Beta Pic system, as it appears to be very similar to how we think our own solar system would have looked, back in its youth.
How to find an exocomet
On many occasions in the past 30 years, astronomers have observed that the light coming from Beta Pic varies subtly and unpredictably. Those small variations are thought to be the result of active comets, shedding dust and gas to create magnificent comae and tails (similar to what’s been observed during comet Siding Spring’s recent flyby of Mars).
As the exocomets pass in front of Beta Pic, the light from the star dims slightly, as it is absorbed and scattered by the comet’s gas and dust. By looking at the star’s spectrum, it reveals the tell-tale imprint of the cometary material. This allows astronomers to study the motion of the comets around the star, as well as the nature of the material they shed.
As technology has improved, our ability to study these subtle variations in the light from Beta Pic has increased dramatically. Over the past decade, the research team has gathered more than 1,000 observations of the star using the HARPS instrument on the European Southern Observatory’s 3.6m telescope, at La Silla Observatory, in Chile.
The researchers have been able to isolate a sample of 493 exocomets orbiting the star. The observations are of such exquisite detail that the motion of the comets around the star can be observed.
Surprisingly, among all these comets it has been found that the majority can be broken into just two separate families – and each one has a direct analogue in our own solar system.
Dim and depleted comets at the end of their lives
The first family of exocomets move on short period orbits. Their motion is controlled by the massive planet Beta Pictoris b. This planet is a true behemoth, weighing in at between four and 11 times the mass of Jupiter.
As the exocomets whip around the star, they lose material with every pass, rapidly becoming depleted of their ices.
As a result, the comets fade with time, and rather than shedding vast comae and tails, they fizzle rather than bloom. Such comets are relatively plentiful in our own solar system where we know them as the “Jupiter-family comets”.
To date, we know of almost 300 such comets in our own solar system. They all whip around the sun with periods of just a few years, but very few ever become bright enough to be visible with the unaided eye.
They are mostly burnt-out husks that likely originated in the trans-Neptunian region. From their origin in that reservoir, they bounced back and forth through the outer solar system (just like celestial pinball), until they were finally pushed inwards and captured by Jupiter.
Such comets are a testament to the awesome mass and gravitational influence of Jupiter, our solar system’s biggest, baddest bully.
To see a similar population of objects orbiting Beta Pic is an amazing result, and gives the system a strong resemblance to our own. In the youth of our solar system, when significantly more debris remained after the formation of the planets, there would have been far more Jupiter-family comets sputtering their way through their dotage, just as is the case around Beta Pic.
The discovery of Beta Pic b-family comets may also suggest the presence of further, as yet undiscovered, massive planets in the system, orbiting at a greater distance than Beta Pic b itself.
In our solar system, while Jupiter controls the Jupiter-family comets, they are handed to it by the gravitational influence of the other giant planets. Without those planets, the Jupiter-family would be far more sparsely populated.
Bright shards of a shattered behemoth
The second family of comets discovered around Beta Pic are far different. Bright, overflowing with volatile material, and following near-identical paths around their parent, they are likely to be pieces of a giant comet that fragmented in the recent past.
Once again, these objects closely mirror a famous family of cometary fragments found in our own backyard. The Kreutz sungrazing comets are thought to be remnants of a giant cometary nucleus that was at least 100km in diameter, but fragmented more than 2,000 years ago.
Unlike the faint Jupiter-family comets, Kreutz sungrazers are often astonishingly spectacular. The brightest comets ever observed were all members of that family – moving around the sun on orbits that last several hundred years. At perihelion, their closest point to the sun, they skim our star’s surface as they swing by causing all but the biggest to disintegrate.
Fragmentation is actually a common outcome for comets that swing so close to the sun. The Kreutz sungrazers are merely the most famous of a number of cometary families known in our solar system.
Another is the Taurid stream, which is the source of most of the meteoroidal material that enters Earth’s atmosphere. That stream’s influence is actually currently visible in the form of the Taurids meteor shower.
A mirror to the solar system’s youth?
The two populations of exocomets discovered orbiting Beta Pic highlight a system that looks remarkably like our own – albeit perhaps one with all the dials turned up to 11, and still very active and youthful.
A giant planet, bigger and brasher than our own Jupiter, dominating the behaviour of a population of short period comets. At least one disk of debris, analogous to our asteroid belt, shepherded by the planet(s) in the system, and even the shattered remains of a giant comet, moving in concert.
The juvenile planetary system around Beta Pictoris speaks toward one of the most exciting questions in modern astronomy – how common are planetary systems like our own? The more closely we look, the more that system fits the bill.
For such a system to be so close – astronomically speaking its sitting right on our doorstep – then it seems highly likely that we will find many more in the years to come.