Largest treasure trove of exocomets discovered to date in alien solar system

Thirty alien comets have been spotted transiting the young star Beta Pictoris, their long tails lighting up the skies of fledgling planets forming there.

The comet discovery was made using NASA’s Transiting Exoplanet Survey satellite (TESS), which monitors dips in starlight as planetary bodies pass in front of — or transit through — their star. Beta Pictoris, which lies 63.4 light-years away, is home to a dusty planet-forming disk that was discovered in 1983 by IRAS, the infrared astronomy satellite. The disc contains at least two planetstwo gas giants, and spectral observations collected as early as 1987 suggested evidence that comets (or “falling evaporating bodies” as they were called at the time) were releasing dust and gas into the disc.

In 2019, astronomers led by Sebastian Zieba from the University of Innsbruck in Austria used TESS to discover three “exocomets” in transit Beta Pictoris. Now another team, led by astronomer Alain Lecavelier des Etangs, of the Paris Institute of Astrophysics in France, has observed 30 exocomets in the Beta Pictoris system, including the three previously discovered.

“These additional exocomet detections are very useful because we are now seeing many different comets of different sizes, which means we can start to compare their size distribution – how many small ones we see versus larger ones,” said Matthew Kenworthy, astronomer in Leiden. Observatory in the Netherlands and a member of both research teams, told

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Although Beta Pictoris is not the first star around which exocomets have been discoveredit is the first star for which it has been possible to measure the size distribution of comets.

Depending on the size of their tails and the amount of dust they produce (using Comet Hale-Bopp as a model), Lecavelier des Etangs’ team was able to measure the diameter of each comet’s nucleus or core, finding them between 2 and 8.7 miles (3 to 14 kilometers) in diameter, with more small comets than larger comets. In fact, the size distribution of exocomets closely matches the size distribution of comets in our solar system.

This model suggests that the processes that formed the exocomets around Beta Pictoris are the same that formed the comets in our solar system; in turn, how comet formation plays into planet formation is also likely the same. Astronomers can learn more about how planets in the solar system, including Earth, formed by studying exoplanetary systems.

Beta Pictoris seen in infrared light (shown here in false color). A coronagraph erased the brightness of the star. The edge of the protoplanetary disk can be seen, as well as the light from one of its planets (the white dot near the center). (Image credit: ESO/A.-M. Lagrange et al.)

The dominant pattern of planet formation, at least for rocky planets, is that they are the result of collisions and mergers between smaller bodies – comets, asteroids and planetesimals. If the gravity of the bodies involved in a collision is strong enough, it can mold the resulting debris into a larger body. If the colliding objects are too small, then there won’t be enough gravity to merge them, and instead they will fragment and scatter, resulting in smaller bodies rather than larger ones. This is exactly what astronomers see in the size distribution of Beta Pictoris exocomets.

“The size distribution is remarkably similar to that predicted for a population of debris resulting from collision and fragmentation cascades,” Lecavelier des Etangs told

The young star Beta Pictoris, located 63.4 light years from the sun, is home to a protoplanetary disk, at least two planets and a host of comets. (Image credit: ESO/Digitized Sky Survey 2)

For scientists, it is reassuring to get more evidence that planets form in the same way around different stars, so that we can draw direct comparisons with our solar system.

“The formation of planets and comets is linked,” Kenworthy said. “So our first measurement of a comet’s distribution outside our solar system advances our understanding of these processes.”

Comets are also believed to be important for life, potentially providing water and biological building blocks to the surface of planets. Beta Pictoris is only about 25 million years old, and this process of comet impacts bringing the ingredients of life to hitherto unseen terrestrial planets may have already begun.

Comet Hale-Bopp, which visited our inner solar system in 1997, was used as a template for dust production on comets Beta Pictoris. (Image credit: ESO/E. Slawik)

For Lecavelier des Etangs, the observations provide a sense of personal justification. In 1999, a few months before the first planet was even discovered by the transit method, he predicted that one day exocomets would transit Beta Pictoris.

“It gives me deep satisfaction to see that the prediction I made over 20 years ago has come true,” he said. “But the most important thing is that not only was it good, but it was useful!”

A paper (opens in a new tab) describing the new discovery was published Thursday, April 28 in the journal Scientific Reports.

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