First giant alien comet discovered orbiting another star

Astronomers have discovered, for the first time, an alien comet in orbit around a nearby star – and it is a whopper.

An artist imagines the scene as planets and comets form around the young star Beta Pictoris: image credit: NASA/FUSE/Lynette Cook

The extended object, termed an exocomet because it belongs in another star system, was detected over a five-week time interval as it passed in front of the star Beta Pictoris.

Beta lies about 63 light-years away, which is relatively close by cosmic standards. You can see it fairly easily with the unaided eye because it shines at magnitude 3.9, but unfortunately for northern readers, it lies in the far southern sky, so never rises above the horizon for most of the USA and Europe.

Astronomers led by Sebastian Zieba and Professor Konstanze Zwintz, of the University of Innsbruck, Austria, detected the exocomet. Their discovery is announced here.

They used data from the Transiting Exoplanet Survey Satellite (TESS), which was launched in 2018 to search 200,000 stars for new planets. The observations were made from from 19 October 2018 to 1 February 2019.

The exocomet, technically known to planetary scientists as a falling evaporating body, was identified by fades in the star’s light, involving three distinct dips over a 105-day period. The fades, lasting up to two days for the greatest, were uneven in nature.

These results appear to show that the body passing in transit across the face of the star was not an exoplanet, but rather a comet with a lengthy tail. We reported in 2013 how exocomets were expected to be common around other stars.

Beta Pictoris is a young star, just 12 million years or so old, and is known to be surrounded by a disk of dust and gas. This shows that its planetary system is still forming, and one giant world may already have formed.

Asteroid-like building blocks of planets, and comets will be far more abundant than in our own Solar System, which is more than 4.6 billion years old.

The team’s discovery confirms a prediction made 20 years ago by Alain Lecavelier Des Etangs, of the Institut d’Astrophysique de Paris in France, that transits of falling evaporated bodies could be detected.

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