Giant comet was active way farther from the sun than expected, scientists confirm

One of the largest comets ever discovered was active long before scientists had expected, according to NASA's industrious planet-hunting telescope. 

Comets are made up of dust and ice left over from the early days of the solar system. When a comet passes near the sun, its ice starts to vaporize and form an envelope known as a coma, making it an active comet. However, the distance from the sun at which a comet becomes active largely depends on what kind of ice it contains — water, carbon dioxide or carbon monoxide, for example. 

A new study led by astronomers at the University of Maryland shows that Comet Bernardinelli-Bernstein (BB) became active much farther from the sun than expected. So far out, temperatures are too cold for water ice to turn to vapor. Therefore, the findings can help determine what exactly the comet is made of and provide new insight on the conditions of the early solar system, according to a statement from the university. 

Related: The 'megacomet' Bernardinelli-Bernstein is the find of a decade. Here's the discovery explained.

Early estimates suggest Comet BB could be up to 62 miles (100 kilometers) in diameter, among the largest comets discovered to date; scientists first spotted the object when it was beyond the planet Uranus. 

By comparison, most comets are under a mile (1 km) wide and are discovered much closer to the sun. In fact, scientists have only spotted one other active comet so far from the sun, and it was much smaller than Comet BB, according to the statement. 

"These observations are pushing the distances for active comets dramatically farther than we have previously known," Tony Farnham, lead author of the study and an astronomer at the University of Maryland, said in the statement.

Using data from the Dark Energy Survey — an international effort to study the sky over the Southern Hemisphere — astronomers first discovered the bright nucleus of Comet BB in June 2021. However, at that time, the observations did not have a high enough resolution to show the comet's coma. 

Instead, the recent study used images taken by NASA's Transient Exoplanet Survey Satellite (TESS), which launched in 2018 and spends most of its time looking for planets orbiting nearby stars. But to conduct that work, the telescope captures longer exposures and, in turn, a more detailed view of the sky, according to the statement, and that can be used for a variety of objects. 

The researchers combined thousands of TESS images taken between 2018 and 2020 to get a clearer view of the comet and the hazy glow of dust that surrounds it. By layering the images so that the comet was aligned in each frame, the researchers revealed the comet's coma, proving it was active at the time, according to the statement. 

Scientists had previously spotted activity on the comet in images taken when it was about 20 times Earth's distance from the sun. (Scientists call Earth's average distance from the sun an astronomical unit or AU; the measurement denotes about 93 million miles or 150 million km.)

TESS' observations show that Comet BB was active as much as 23 AU from the sun, although the researchers suspect that similar observations taken a couple years earlier could have detected a coma — if only TESS had been at work at the time.

Their findings were published Nov. 29 in The Planetary Science Journal.

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