When a comet dips near the sun, it’s a spectacular and rare sight from here on Earth. Our solar system is brimming with these small chunks of ice, though, orbiting a great distance from the sun in the far-out Oort Cloud. Given comets’ ubiquity here, scientists figure that other planetary systems likely have them as well.
Astronomers from the Main Astronomical Observatory (MAO) of the National Academy of Sciences of Ukraine in Kyiv recently published a discovery of five new exocomets—comets orbiting a star other than the sun—in the journal Astronomy & Astrophysics, using data from the Transiting Exoplanet Survey Satellite (TESS). They also independently confirmed a handful of exocomets that were previously detected by other researchers.
Within our solar system, comets are studied as relics of the past, providing clues to the chemistry of how Earth and its neighbor planets formed. They’re a particularly important component of Earth’s story as well, because comets are thought to have brought water to Earth, setting the stage for our planet to burst with life.
“I think exocomets are exciting and important for the same reason that comets in our own solar system are important,” says Andrew Vanderburg, a physicist at the Massachusetts Institute of Technology who was not involved in the new finding. “Comets have clearly played an important role in the development of our solar system…. If the story of our own solar system depends so much on exocomets, how can we hope to understand other planetary systems without knowledge of their comets?”
These newly discovered exocomets are swirling around a star named Beta Pictoris, a well-studied favorite of astronomers that lies only around 65 light-years from Earth—fairly nearby in cosmic terms. Beta Pictoris (a.k.a. Beta Pic) is much younger than the sun, only 10 to 40 million years old compared to the solar system’s 4.5 billion years, making it a useful snapshot of what happens during a planetary system’s youth. This star is orbited by a gas giant planet 11 times larger than Jupiter (called Beta Pic b) and an enormous disk of dust almost 40 billion miles in diameter, known as a debris disk.
Debris disks represent the “elderly” era of planet-forming disks—the later stage in the complex dance of dust and gas that forms full-fledged planets like those around the sun. These disks are often chaotic and violent places, with chunks of rock and protoplanets flying about and colliding. That’s where exocomets come in. In younger planetary systems like Beta Pic, comets swoop near their stars much more often because everything in the disk is still shifting about before objects settle into their final configuration.
In fact, this discovery isn’t even the first time exocomets have been seen around Beta Pic—the first detection with TESS came in 2019, and earlier studies deduced that Beta Pic actually has two different groups of exocomets with different properties. The new exocomets add to a growing pile of exocomet discoveries around multiple stars from both TESS and its predecessor, the planet-hunting Kepler Space Telescope. Although these observatories aren’t the first to spot exocomets in some form, they’re the first to directly detect them through transits—small dips in the amount of light we see from a star as a comet passes in front. First spotted in Kepler data in 2017, comet transits are steeper and more lopsided than exoplanet transits, in part a result of the comet’s long tail.
Transits reveal how big the exocomet is, whereas other discovery methods can measure the comet’s speed and orbit. When combined, all this information provides a fuller picture of what’s going on with exocomets—how they’re born and how they change. Building up a large catalog of exocomet transits, around many different stars, will allow astronomers to uncover patterns in the data, potentially revealing trends caused by the process of planet formation.
Jonathan Marshall, a postdoctoral scholar at Taiwan’s Academia Sinica Institute of Astronomy and Astrophysics, hopes that this new wave of exocomet discoveries will shed light on stars that dim unexpectedly, known as “little dippers.” The light from these stars fluctuates, very similar to what astronomers see around Beta Pic, but the dips are less frequent and much larger. Little dippers are also further away, making it hard, if not altogether impossible, for astronomers to capture a photograph to further investigate the dimming’s cause. “Using the Beta Pic system as a template provides us with another example, on top of the solar system … that can be applied to little dippers to deduce the internal workings of those more extreme exocometary systems,” Marshall explains.
Exocomets may even offer an explanation to the inexplicable dimming of KIC 8462852, better known as “Boyajian’s star” after its discoverer Tabetha Boyajian, an astrophysicist at Louisiana State University. This star gained fame when a far-fetched hypothesis that this dimming could be the first signs of an alien megastructure took root. Boyajian herself thinks the explanation is more natural, but has yet to pin down one solution. “We are still working on a model that will explain all of the data for this star,” she says. “In my opinion, exocomets are still the best of the theories, but there are still a lot of open questions we have no good answer for.”
If there’s one takeaway from the discovery of more exocomets, it’s that they’re definitely common, at least around a star like Beta Pic. And scientists expect TESS to discover many more in the future. TESS’s power comes from its ability to observe “millions of stars at a time,” Vanderburg says—a much more efficient way to search the skies for exocomets than sifting through stars one by one. The long-awaited James Webb Space Telescope, which launched in late 2021, may also provide new insight into exocomets.
“Unfortunately, our group obtained only our first results in exocomet science. Then came the war,” says Yakiv Pavlenko, chief research fellow at Ukraine’s MAO, on behalf of the team of Ukrainian researchers behind the new result. “After the war, we will continue. We would like to thank all the astronomers of the world who support us in words and deeds.”