It could give researchers more insight into the comet’s composition.

Earlier this year, researchers discovered the largest comet they have ever seen. The comet was given the name C/2014 UN271 (or: Bernardinelli-Bernstein) and is huge with an estimated diameter of about 100 kilometers (!). Most comets are less than 10 kilometers in size.


But now the comet turns out to be special not only because of its monstrous size. An new research shows that it also became active much earlier than expected (see box).

Comets are also known as dirty ice balls. They consist of a mixture of ice, dust and debris. At a great distance from the sun, such a mixture can withstand well. But that changes as the comet approaches the sun and begins to warm up. Ice that is on or just below the surface turns into gas due to the heat. And those gases rush away from the comet, carrying small dust particles in their haste as well. This creates the dust cloud that surrounds the comet’s nucleus, which is so characteristic of many nearby comets. When a comet is so affected by the sun’s heat and surrounded by a cloud of gas and dust, it is also referred to as ‘active’.

Previous observations indicated that the giant comet Bernardinelli-Bernstein is also active. The comet is currently rushing towards the sun and is therefore getting warmer. And observations revealed that at about 20 AU from the sun (that is, a distance 20 times greater than the distance between the sun and Earth) it already resulted in a fairly highly visible cloud of gas and dust. “Most objects from the outer reaches of the solar system – such as Kuiper belt objects – do not look different from stars through Earth-based telescopes. They’re just dots of light,” researcher Michael Kelley said “But we saw that C/2014 UN271 is a bit bigger and fuzzier and has a very short tail,”

24 AU

But new research now reveals that the monster comet has already become active at significantly greater distances from the sun. Even at nearly 24 AU, a cloud of gas and dust was already visible, researchers at the University of Maryland write in the magazine. The Planetary Science Journal. It’s exceptional. In fact; to date, only one active comet has been observed even further from the Sun: C/2010 U3 Boattini. The comet was observed in 2005 and was found to be active about 25.8 AU from the sun.


The researchers base their conclusions about Bernardinelli-Bernstein on images collected by the Transient Exoplanet Survey Satellite (short: TESS). The telescope is actually designed to detect exoplanets and to do so, for 28 days in a row, the telescope will stare at the same part of the universe, hoping to witness as yet unknown planets pass in front of their parent star and thus ensure that the brightness of that star temporarily decreases. The researchers combined 1000 images taken by TESS between 2018 and 2020 and searched for Bernardinelli-Bernstein. The researchers then ‘stacked’ the images showing the comet: they piled them up. Because the comet moves in the night sky, the images could not be stacked straight, but the individual images also had to be moved slightly each time to ensure that the comet in photo 1 was directly above the same comet in photo 30. This approach not only made the comet more visible with each image added to the stack, but also allowed the researchers to filter out noise in individual photos. And along the way, not only did the comet become more visible, but so did the hazy cloud of gas and dust around it.

23.8 AU

The images reveal that in 2018 the comet was already active 23.8 AU from the Sun. But not only that; the comet was still there two years later at 21.2 AU. And because the gas and dust cloud also looked the same at both times, the researchers assume that it is continuously active and that the gas and dust cloud spotted in 2018 and 2020 is not the result of a sporadic eruption, where in a short time a lot of gas and dust is released. This is further supported by more recent observations revealing that the comet will still be active in 2021.

Been active for a long time

Based on their observations, the researchers certainly do not suspect that the comet only became active in 2018. While there are no previous images of TESS — the planetary fighter launched in April 2018 — the researchers believe, based on the composition of the gas and dust cloud, the comet started warming several years and possibly even a decade before, releasing gas and gas. started to lose dust.

More insight into Bernardinelli-Bernstein’s activity is very welcome, because it can tell us more about the composition of this monster comet. The moment at which a comet becomes active is strongly dependent on the type of ice it houses. This can be the well-known water ice, but also carbon dioxide, carbon monoxide ice or other frozen substances, for example. Because one type of ice evaporates at lower temperatures than another, the moment a comet becomes active can reveal what type of ice it has on board. For example, the fact that Bernardinelli-Bernstein becomes active already at such a great distance from the sun and therefore at relatively low temperatures, cautiously indicates that the comet harbors carbon monoxide ice. Carbon monoxide ice can be found in many comets and can already sublimate at low temperatures – and therefore quite far from the sun.