Ukrainian Astronomers Discover “Exocomets” Around Another Star News and Research
When a comet falls close to the sun, it is a spectacular and rare sight from here on earth. However, our solar system is filled with these small pieces of ice, which orbit a large distance from the sun in the distant Oort cloud. Given the comets’ ubiquitous presence here, scientists believe that other planetary systems are likely to have them as well.
Astronomers from the Main Astronomical Observatory (MAO) at the National Academy of Sciences of Ukraine in Kyiv recently published a discovery of five new exocometers – comets orbiting a star other than the sun – in the journal Astronomy and astrophysics, using data from the Transiting Exoplanet Survey Satellite (TESS). They also independently confirmed a handful of exocometers previously discovered by other researchers.
In our solar system, comets are studied as relics from the past, which provide clues to the chemistry of how the earth and its neighboring planets were formed. They are also a particularly important part of the Earth’s story, as comets are believed to have brought water to Earth, setting the stage for our planet to be blown to life.
“I think exocometers are exciting and important for the same reason that comets in our own solar system are important,” said Andrew Vanderburg, a physicist at the Massachusetts Institute of Technology who was not involved in the new discovery. “Comets have clearly played an important role in the development of our solar system … If the history of our own solar system depends so much on exocometers, how can we hope to understand other planetary systems without knowledge of their comets?”
These newly discovered exocometers swirl around a star called Beta Pictoris, a well-studied favorite of astronomers located just about 65 light-years from Earth – quite close in cosmic terms. Beta Pictoris (aka 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 orbits a gas giant planet eleven times larger than Jupiter (called Beta Pic b) and a huge dust disk that is almost 40 billion miles in diameter, known as a garbage disk.
Junk discs represent the “older” era of planet-forming discs – the latter stage in the complex dance of dust and gas that form full-fledged planets like those around the sun. These disks are often chaotic and violent places, with rock fragments and protoplanets flying around and colliding. This is where the exocometer comes in. In younger planetary systems like Beta Pic, comets sweep near their stars much more often because everything on the disk is still moving before objects settle into their final configuration.
In fact, this discovery is not even the first time an exocometer has been seen around Beta Pic – the first discovery with TESS came in 2019, and previous studies concluded that Beta Pic actually has two different groups of exocometers with different properties. The new exocometers add a growing pile of exocomet discoveries around several stars from both TESS and its predecessor, the planet-hunting space telescope Kepler. Although these observatories are not the first to detect exocometers in any form, they are the first to directly detect them through transits – small declines in the amount of light we see from a star as a comet passes in front. Comet passengers were first discovered in Kepler data in 2017 and are steeper and more skewed than exoplanet passages, partly a result of the comet’s long tail.
Transits reveal the size of the exo comet, while other detection methods can measure the comet’s velocity and orbit. When combined, all this information provides a more complete picture of what happens to exocometers – how they are born and how they change. Building a large catalog of exokomet passages, around many different stars, will allow astronomers to detect patterns in data, which could potentially reveal trends caused by the planet formation process.
Jonathan Marshall, a postdoctoral fellow at Taiwan’s Academia Sinica Institute of Astronomy and Astrophysics, hopes that this new wave of exocometer discoveries will shed light on stars that are unexpectedly attenuated, known as “small dippers”. The light from these stars fluctuates, much like what astronomers see around Beta Pic, but the cases are less frequent and much larger. Small immersers are also further away, making it difficult, if not completely impossible, for astronomers to take a photograph to further investigate the cause of the attenuation. “Using the Beta Pic system as a template gives us another example, on top of the solar system … that can be applied to small dippers to derive the internal functions of the more extreme exochometric systems,” Marshall explains.
Exocometers may even offer an explanation for the inexplicable downgrade of KIC 8462852, better known as the “Star of Boyajian” after its discoverer Tabetha Boyajian, an astrophysicist at Louisiana State University. This star became famous when a long-held hypothesis that this dimming could be the first signs of an extraterrestrial megastructure took root. Boyajian himself thinks the explanation is more natural, but has not yet found a solution. “We are still working on a model that will explain all the data for this star,” she says. “In my opinion, exocometers are still the best of theories, but there are still many open questions to which we have no good answer.”
If there is one thing to take away from the discovery of more exocometers, it is that they are definitely common, at least around a star like Beta Pic. And researchers expect TESS to discover many more in the future. TESS’s power comes from its ability to observe “millions of stars at a time,” says Vanderburg – a much more efficient way to search the sky for exocometers than to sift through stars one by one. The long-awaited space telescope James Webb, which was launched at the end of 2021, can also provide new insight into exocometers.
“Unfortunately, our group only got our first results in exochrome science. Then came the war,” said Yakiv Pavlenko, chief researcher at Ukraine’s MAO, on behalf of the team of Ukrainian researchers behind the new result. “After the war, we will continue. We want to thank all the astronomers in the world who support us in word and deed. ”