Giant comet found in outer solar system

The discovery of the comet estimated to 100-200 kilometers across was made by Penn researchers following a comprehensive search of data from the Dark Energy Survey. Comet Bernardinelli-Bernstein is the most distant comet ever discovered and possibly the largest seen in modern times.

an image of the night sky with an object circled and annotated with Bernardinelli-Bernstein (C/2014-UN271)
Comet Bernardinelli-Bernstein (circled) is estimated to be about 1000 times more massive than a typical comet, making it arguably the largest comet discovered in modern times. It was discovered using images for the Dark Energy Survey (DES) by the 570-megapixel Dark Energy Camera (DECam) at Cerro Tololo Inter-American Observatory in Chile. These images show the comet in October 2017, when it was 25 au away, 83% of the distance to Neptune. (Image: Dark Energy Survey/DOE/FNAL/DECam/CTIO/NOIRLab/NSF/AURA/P. Bernardinelli & G. Bernstein (Penn)/DESI Legacy Imaging Surveys)

A giant comet has been discovered by Penn astronomers Pedro Bernardinelli and Gary Bernstein following a comprehensive search of data from the Dark Energy Survey (DES). The comet is an icy relic flung out of the solar system by migrating gas giants. Based on the amount of sunlight it reflects, the comet is estimated to 100-200 kilometers across, or about 10 times the diameter, and 1,000 times the mass of most comets and arguably the largest discovered in modern times.

Bernardinelli, a recent Ph.D. graduate, and Bernstein, the Reese W. Flower Professor of Astronomy and Astrophysics, found the comet, called Bernardinelli-Bernstein (with the designation C/2014 UN271), hidden among data collected by the 570-megapixel Dark Energy Camera on the National Science Foundation’s Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory (CTIO) in Chile. As one of the highest-performance, widest-field astronomical imagers in the world, the Dark Energy Camera was designed specifically for DES, which finished collecting data in 2019.

DES’ goal is to map millions of galaxies across one-eighth of the entire sky, but during its six-year mission it also observed many trans-Neptunian objects, or TNOs, icy bodies that reside in the solar system beyond the orbit of Neptune.

Bernardinelli and Bernstein used 15–20 million CPU hours at the National Center for Supercomputing Applications and Fermilab to identify more than 800 individual TNOs from among the more than 16 billion individual sources detected in 80,000 exposures taken as part of DES. Thirty-two of those detections belonged to C/2014 UN271.

Comets are icy bodies that evaporate as they approach the warmth of the sun, growing a “coma” and tail that are clouds of gas and dust. The DES images of the object collected from 2014–18 did not, however, show any coma. Within a day of the announcement of its discovery via the Minor Planet Center, astronomers at two other observatories took fresh images of the object and found that it has grown a coma in the past three years, officially making it a comet.

Its current visit to the inner solar system began at a distance of more than 40,000 astronomical units (au) from the sun, or 40,000 times farther from the sun than Earth is. This is 0.6 light-years or roughly 1/7 of the distance to the nearest star. For comparison, this is 1,000 times more distant than Pluto’s average distance of 39 au from the sun. This means that Comet Bernardinelli-Bernstein was ejected during the early history of planets into the Oort Cloud of icy small bodies that envelops the solar system.

This comet could be the largest member of the Oort Cloud ever detected. “This is a much-needed anchor on the unknown population of large objects in the Oort Cloud and their connection with early migration of the ice/gas giants soon after the solar system was formed,” says NOIRLab astronomer Tod Lauer.

an artists rendering of a large rock traveling across the sky
This illustration shows Comet Bernardinelli-Bernstein as it might look in the outer Solar System. It has an extremely elongated orbit, journeying inward from the distant Oort Cloud over millions of years. (Image: NOIRLab/NSF/AURA/J. da Silva)

Comet Bernardinelli-Bernstein is currently much closer to the sun; it was first seen by DES in 2014 at a distance of 29 au (4 billion kilometers, roughly the distance of Neptune), and as of June 2021 it was 20 au (3 billion kilometers, the distance of Uranus) from the sun. This comet was discovered at greater distance than any previous incoming comet. It will reach its closest point to the sun (known as perihelion) in 2031, when it will be around 11 au away, just outside of Saturn’s orbit.

“Comets’ behavior is very hard to predict. It’s even harder than predicting the 76ers’ playoff performance,” says Bernstein. “Despite the comet’s great size, it won’t get close enough to Earth to be visible to the naked eye.” The best guess is that skywatchers will require a good amateur telescope to see it, even at its brightest, the researchers say.

“We have the privilege of having discovered perhaps the largest comet ever seen, or at least bigger than any well-studied one, and caught it early enough for people to watch it evolve as it approaches and warms up,” says Bernstein. “It has not visited the planets in more than 3 million years.”

Comet Bernardinelli-Bernstein will be followed intensively by the astronomical community to understand the composition and origin of the massive relic from the birth of the solar system’s planets. Astronomers suspect that there may be many more undiscovered comets of this size waiting in the Oort Cloud far beyond Pluto and the Kuiper Belt.

“Astronomers have long hypothesized that the outer solar system is home to a vast number of comets that are left over from when our solar system formed. These Oort Cloud objects, which are thought to exist at distances up to thousands of times farther from the sun than Pluto, are extremely difficult to detect,” says Penn astronomer and associate professor Cullen Blake about the significance of this work. “Comet Bernardinelli-Bernstein is the rare Oort Cloud object with an orbit that brings it in much closer to the sun, where we can see it, presenting an invaluable opportunity to directly study an icy relic from the birth of our solar system."

Bernardinelli says that the Vera C. Rubin Observatory, an even more powerful survey telescope under construction in Chile, “will continuously measure Comet Bernardinelli-Bernstein all the way to its perihelion in 2031 and probably find many, many others like it.”

This research was reported to the Minor Planet Center.

Pedro Bernardinelli recently completed his Ph.D. in the Department of Physics and Astronomy in the School of Arts & Sciences at the University of Pennsylvania, co-advised by Bernstein and Masao Sako, with support from a School of Arts & Sciences’ Dissertation Completion Fellowship. Their earlier work on this project was supported by the National Science Foundation.

Gary Bernstein is the Reese W. Flower Professor of Astronomy and Astrophysics Professor in the Department of Physics and Astronomy in Penn’s School of Arts & Sciences.

The Dark Energy Survey is a collaboration of more than 400 scientists from 25 institutions in seven countries. Funding for the DES Projects has been provided by the United States Department of Energy Office of Science, the U.S. National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, ETH Zurich for Switzerland, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, the Center for Cosmology and AstroParticle Physics at The Ohio State University, the Mitchell Institute for Fundamental Physics and Astronomy at Texas A&M University, the Financiadora de Estudos e Projetos, the Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro, the Conselho Nacional de Desenvolvimento Científico e Tecnológico and Ministério da Ciência e Tecnologia, the Deutsche Forschungsgemeinschaft, and the collaborating institutions in the Dark Energy Survey, including the University of Pennsylvania.