The Ice Giant Arrives: A Giant Comet from the Oort Cloud

Comets are like lonely believers on a pilgrimage, moving slowly and steadily towards the direction of the holy land, even at the cost of their lives. A pilgrimage can last from a few years to tens of thousands of years, and the one we are going to introduce today has been traveling for millions of years!

Artist's impression of Comet C/2014 UN271 in the outer solar system

Image source: NOIRLab / NSF / AURA / J. da Silva

In August 2013, the 4-meter Blanco Telescope at the Pan American Observatory at Cerro Tololo in Chile launched an ambitious project, the Dark Energy Survey (DES). Recently, when processing survey data from 2013 to 2019, astronomers Pedro Bernardinelli and Gary Bernstein from the University of Pennsylvania accidentally discovered more than 800 icy objects beyond the orbit of Neptune, one of which was particularly large and eye-catching.

Soon after the discovery, astronomers immediately organized telescopes in South Africa and Namibia to conduct follow-up observations. In June this year, the coma structure of the object was observed in near space, confirming that it is an active comet. Because the first observation was in October 2014, it was numbered 2014 UN271 according to the asteroid naming rules. After confirmation, the prefix "C/" was added to indicate a comet, and the name of the discoverer was added according to the comet naming convention. The full name is C/2014 UN271 (Bernardinelli-Bernstein) Comet.

Imaging observations of Comet C/2014 UN271 from Las Cambres Observatory

There are tens of thousands of comets observed by humans in the vast starry sky. The reason why C/2014 UN271 has aroused great interest among scientists is that it breaks two records in human's current understanding of comets.

The biggest

The discoverer estimates that the size of the nucleus of C/2014 UN271, which is composed of ice and dust, is about 100-200 kilometers, which is almost the size of a dwarf planet. In comparison, the nucleus of Hale-Bopp, the Great Comet of the Century that passed through the night sky of the Northern Hemisphere in 1997, was only about 40 kilometers. Therefore, C/2014 UN271 is a veritable "giant". At the same time, it is estimated that the mass of C/2014 UN271 is more than 10 trillion tons, which is about 1,000 times the average mass of an ordinary comet. Therefore, C/2014 UN271 is not only the largest comet discovered so far, but also the largest celestial body from the Oort Cloud.

The longest detection distance

When it was first observed in 2014, C/2014 UN271 was about 29 AU (astronomical units, 1 AU is about 150 million kilometers) from the Sun, which is close to the distance between Neptune and the Sun, so it is also the farthest comet discovered so far. The reason why it can be observed from such a far distance is closely related to its huge size and possible activity that increases its brightness.

The comet is currently about 19 AU from the Sun. Over the next decade, C/2014 UN271 will fly toward the inner solar system, where the ice it carries will sublimate more intensely under the influence of increasingly strong solar radiation, causing its brightness to continue to increase. In 2031, this massive comet will reach its perihelion, close to the orbit of Saturn, at about 10 AU, and then embark on its journey back to the cold, dark edge of the solar system.

Coming from ancient times

Another special feature of this comet is that it comes from the deep Oort Cloud. The Oort Cloud is considered to be a thick cloud of primordial matter and planetesimals surrounding the solar system, extending from 1,000 AU to the edge of the solar system (about 100,000 AU). The aphelion of the orbit of C/2014 UN271 is located in the depths of the Oort Cloud, 40,000 AU away. Its distance from the sun is equivalent to 15% of the distance between the sun and Proxima Centauri, which is more than 260 times farther than the most distant man-made spacecraft "Voyager 1" (153 AU from the sun).

Solar system structure and the distance from the Oort Cloud to the Sun ▏ Image source: wiki

This primitive comet, which has traveled from the outer solar system for more than 1.5 million years, will provide us with important information about the early history of the solar system and the formation and evolution of planetesimals. Its activity indicates that it may contain highly active ices such as carbon monoxide and methane. In the next decade, as it gets closer to the sun, we will obtain more and more accurate observational data to study its composition, activity, and physical properties, and learn about the celestial bodies and materials distributed at the edge of the solar system.

Future Detection

C/2014 UN271 takes about 3-5 million years to complete a full solar system voyage. When it last flew to the planetary system of the solar system, humans were still in the process of evolution; and we have no idea what state humans will be in when it visits next time. Like many long-period comets from the Oort Cloud, this return of C/2014 UN271 may be the only chance for humans to catch a glimpse of it.

Therefore, scientists also expect that artificial spacecraft can conduct close-range high-precision detection of C/2014 UN271 before and after it reaches perihelion. Although the European Space Agency's Comet-Interceptor mission has confirmed that it will not use this comet as a detection target, probes heading to Jupiter, Saturn and the edge of the solar system may be able to capture relevant information about C/2014 UN271 at a closer distance. Scientists are full of expectations!

About the Author

Zhao Yuhui

Researcher at the Planetary Science and Deep Space Exploration Laboratory, Purple Mountain Observatory, Chinese Academy of Sciences. His research areas include dynamics and formation mechanism of small solar system bodies, physical properties, activity mechanism and long-term evolution of comets, etc.

Rotating Editor-in-Chief: Wang Ying

Editor: Wang Kechao