Of all the rocks and lumps of ice in the universe, we have only seen two that have visited our own solar system from the outside

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And now scientists have taken a closer look at one of them – the interstellar comet 2I / Borisov

In addition to the planets, there are millions of other rocks and many thousands of comets buzzing around the sun. Most are located in the asteroid belt between Mars and Jupiter, but there are many floating around in space between the planets.

Scientists know of 25,000 asteroids orbiting near Earth, so-called Near-Earth Asteroids, according to NASA.

The vast majority of these comets and asteroids are probably remnants of the early solar system, more than 4.5 billion years ago. These are rocks and ice that did not end up as part of anything bigger.

But there is another, very little used category: Interstellar objects. These are things that have come from another solar system, and travel fast enough not to be bound in orbit around the sun.

Most likely they are also remnants from other solar systems, which have been ejected from the orbit around the home star.

The sun’s gravity causes them to swing past the solar system, before they are at full speed out again.

Between the stars

They are difficult to detect, and so far researchers know of only two examples. The first was discovered in 2017, and was named ‘Oumuamua. This discovery attracted a great deal of attention around the world, because ‘Oumuamua did not behave like anything else in the solar system.

This is how an artist imagines that ‘Oumuamua looks, if it is a natural asteroid. It is about 400 meters long, with a dark red color.
(Image: ESO / M. Grain fairs)

It can be oblong and thin, and behaves a bit like a comet – without looking like a comet. Researchers have come up with many explanations for what it could be. Israeli astronomer Avi Loeb believes one must also consider the possibility because it originates from an extraterrestrial civilization.

But another visitor from another solar system has been discovered. In 2019, amatørastronomen Gennadiy Borisov en komet who turned out to be the second visitor. It was less mysterious than ‘Oumuamua’, and is more clearly a comet.

But not an ordinary comet

It was named 2I / Borisov, and now several research groups have made further investigations of this comet. It turns out that it is not similar to the comets found here in the neighborhood.

Two studies in the journal Nature Astronomy goes through several observations of the comet, among other things made with the ALMA telescope and the VLT telescope in Chile. Among other things, the researchers have examined the dust and ice that comes from 2I / Borisov, to see what it is made of, and how it differs from our own comets.

One study examined light that was filtered through the comet tail. This comet tail consists of dust and ice that disappears from the comet due to the influence of the sun. The scientists report a special signature from this light, which indicates that the comet is very unaffected.

This may mean that the comet has never been exposed to so much solar radiation before, and that this is the first time it is near a star. The researchers also say that the signature is reminiscent of the comet Hale-Bopp, which visited our part of the solar system in 1997.

But 2I / Borisov seems even more untouched, the researchers believe.

Hale-Bopp may have been in the sun sometime before 1997, the researchers believe. It has an extremely long orbit around the sun, and will not be in the inner solar system until around the year 4385.

Comet Hale-Bopp seen from Earth in 1997. It was very visible for a while

Comet Hale-Bopp seen from Earth in 1997. It was very visible for a while
(Image: Philipp Salzgeber / CC BY-SA 2.0)

The second study argues that the dust comes from 2I / Borisov as small lumps, which is also different from the comets from the solar system. Here the dust tends to be finer. This says something about the conditions in the solar system where the comets were created, the researchers believe.

References:

Bagnulo mfl: Unusual polarimetric properties for interstellar comet 2I/Borisov. Nature Communications, 2021. DOI: 10.1038/s41467-021-22000-x. Summary

Yang mfl: Compact pebbles and the evolution of volatiles in the interstellar comet 2I/Boriso. Nature astronomy, 2021. DOI: 10.1038/s41550-021-01336-w. Summary

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