Last year a comet heading towards the inner Solar System was recognized as probably the largest ever seen, closer in size to a minor planet than an ordinary comet. Early size estimates were vague but we now have something much more precise indicating its size to be 137 kilometers (85 miles) across – similar to the distance between New York and Philadelphia. 

“We have confirmed the estimate,” Dr Emmanuel Lellouch of the Paris Observatory told New Scientist. “It’s the biggest comet from the Oort Cloud ever found.”

The first images taken of Comet 2014 UN271/(Bernardinelli-Bernstein) were from 2014 when it was as distant as Neptune, although its significance was not recognized for another seven years. At the time it was thought to be anywhere from 100 to 370 kilometers (60-230 miles) wide, with even the lowest figure roughly matching Comet Sarabat, which was retrospectively estimated at 100 kilometers (60 miles) across after being seen in 1729. Later estimates of 150 kilometers (90 miles) still came with wide error bars. Although Bernardinelli-Bernstein is getting closer it will never cross the orbit of Saturn, making looking up safe but size measurements tricky.

Lellouch and co-authors used microwave radiation to seek precision unavailable in visible light. Their paper has been accepted for publication in Astronomy and Astrophysics Letters, with a preprint on ArXiv.org. Rather than attempting to measure how much of the sky 2014 UN271 takes up, Lellouch observed the amount of heat it is giving off and used that to calculate the comet's surface area.

Measuring the size of an object 100-200 kilometers (60-120 miles) wide when it is more than 3 billion kilometers (1.9 billion miles) away is intrinsically hard. It gets harder still when it is releasing gas and dust as its ice warms up, creating a cometary shroud around the solid object itself.

Using the Atacama Large Millimeter Array, Lellouch and co-authors studied C/2014 UN271 at four wavelengths between 1 and 2 millimeters skipping those in which dust is brightest. This allows them to conclude the contribution they were seeing from dust given off by the comet was almost certainly negligible – almost everything was from the nucleus itself. The comparative brightness in the different wavelength bands indicates a composition fairly typical of comets that reflect 5 percent of the light that falls on them. This allowed Lellouch and co-authors to estimate Bernardinelli-Bernstein would need to be 137±17 kilometers (85±10 miles) across to produce this much radiation. The uncertainty is a consequence both of not knowing how far from spherical the comet is, and residual uncertainty about its reflectivity.

For comparison, Hale-Bopp, the largest comet we have previously been able to measure reasonably precisely is 74±6 kilometers (46±4 miles). Although the estimate is towards the lower end of the original range, it still makes C/2014 UN271 larger than all but one of the active Centaurs – objects that orbit between Jupiter and Neptune and show occasional comet-like outbursts.

The authors note this is by far the most distant measurement of a comet's albedo (reflectivity), providing astronomers with an unprecedented opportunity to observe how it changes as Bernardinelli-Bernstein nearly halves its current distance from the Sun. This will help settle the question of whether the release of volatile material changes a comet's albedo.