Astronomers have carried out new observations of the giant Comet C/2014 UN271 (Bernardinelli-Bernstein) using the Hubble Space Telescope and have now confirmed what they suspected from previous data: this object is truly something enormous when it comes to frozen space rocks.

As reported in The Astrophysical Journal Letters, Hubble observations place Comet Bernardinelli-Bernstein as the largest comet ever discovered. Comets have huge tails that can stretch for millions of miles, but their nucleus is a solid "dirty snowball" of ice and dust. Consistent with previous observations from the Atacama Large Millimeter/submillimeter Array, this comet’s nucleus is around 130 kilometers (80 miles) across, a third wider than the previous record holder, comet C/2002 VQ94, and around the size of the state of Rhode Island.

"This is an amazing object, given how active it is when it's still so far from the Sun," the paper's lead author Man-To Hui of the Macau University of Science and Technology, said in a statement. "We guessed the comet might be pretty big, but we needed the best data to confirm this."

Now, we know the comet's nucleus is around 50 times larger than most known comets' nuclei, with a mass of 500 trillion tons. As NASA points out, that's a hundred thousand times greater than the mass of a typical comet found much closer to the Sun.

The comet is traveling from the Oort cloud, the vast region at the edge of the Solar System where many cometary bodies reside. Frozen space rocks start to heat up and spew out dust and gas the closer they get to the Sun. Comet Berardinelli-Bernstein is getting closer to the Sun, but will never get closer than 10.9 AU (astronomical units, or the distance between Earth and the Sun), or 1 billion miles. That’s roughly just outside the orbit of Saturn, and it will reach that point in 2031.

The challenge in measuring it was how to discriminate between the dusty coma released by the comet and its nucleus. Even though it is over 3 billion kilometers from the Sun, with a temperature hundreds of degrees below zero, the faint light of the Sun is still enough to sublimate carbon monoxide on its surface. The comet is still too far away for the nucleus to be measured directly so the observations had to be compared with a model that allowed for removing the coma and estimating just the nucleus. 

The comet also has a very dark nucleus – “blacker than coal” according to co-author David Jewitt – something also seen in Comet 67P, which was visited by Rosetta. Future observations will refine these size measurements.   

"This comet is literally the tip of the iceberg for many thousands of comets that are too faint to see in the more distant parts of the solar system," added Jewitt, a professor of planetary science and astronomy at the University of California, Los Angeles. "We've always suspected this comet had to be big because it is so bright at such a large distance. Now we confirm it is."

The comet trajectory is almost 90 degrees to the plane of the Solar System where the eight planets' orbits are located. Its elliptical orbit takes it about half a light-year away from the Sun and it has been falling inward for over 1 million years.