On August 30, 2019, our solar system was visited by an alien comet.
Not only did comet 21/Borisov travel from points beyond our Sun and its surrounding planets, but new research suggests that it is one of the most pristine comets ever observed, holding clues to an ancient past of distant worlds.
The recent findings are detailed in a study published Tuesday in the journal Nature Communications and could have implications for future missions set to explore other interstellar visitors.
WHAT'S NEW — Ludmilla Kolokolova, a researcher at the University of Maryland’s astronomy department and co-author of the new study, explains how a comet is altered by its interactions with a star.
“In general, regular comets that we usually study have an orbit where they approach the Sun from time to time,” Kolokolova tells Inverse. “And when they approach the Sun, their surface changes.”
Comets are icy bodies of frozen gas, rock, and dust, material that likely dates back billions of years ago before the formation of planets and other celestial bodies.
As comets travel closer to the Sun, the star's powerful gravity can weaken them and break them apart as they draw near. Solar radiation, wind, and particles emitted by the Sun tend to evaporate carbon and other material found in comets. Most of the comets that scientists regularly observe in the Solar System have already been altered by their trips around our host star.
However, the team behind the new study found that, unlike those comets, 21/Borisov had a chemical composition consistent with being intact from its formation. This means it likely was flung out of its home system early in its history.
Here’s the background — Comet 21/Borisov was discovered by amateur astronomer Gennady Borisov in the sky above Crimea, Ukraine. Its trajectory indicated that it wasn’t in orbit around the Sun — and likely was an interloper from elsewhere. Tracing its exact origins is difficult.
The discovery marked the second interstellar comet observed by astronomers, following the elongated comet ‘Oumuamua (1I/2017 U1), which was discovered on September 9, 2017.
‘Oumuamua and 21/Borisov were the first interstellar objects captured as they paid brief visits to our solar system, allowing astronomers insight into how other planetary systems come together. However, there may be more waiting to be discovered as they pass through.
HOW THEY DID IT — Using the European Southern Observatory’s Very Large Telescope in Atacama Desert of Chile, the team behind the recent study observed comet 21/Borisov between December 25, 2019 and March 20, 2020.
They then compared 21/Borisov’s observations with comets from our own solar system. The team found that the dust particles of 21/Borisov were homogeneous in size and unusually small compared to other comets, with the exception of Hale-Bopp.
Hale-Bopp was discovered on July 23, 1995, and broke the record for the farthest comet from the Sun ever observed. Its composition indicated that it had made relatively few visits to the inner solar system. The 1995 visit may have been only its second.
The team discovered that 21/Borisov was remarkably similar in its pristine condition to Hale-Bopp, indicating that it may still have primordial materials from its formation. However, Kolokolova says it’s not direct evidence.
“We compared it with other comets and found that it reminded us of comet Hale-Bopp because they’re very similar,” Kolokolova says. “So if Hale-Bopp was pristine, then 21/Borisov is also pristine, or even more pristine because it shows some kind of undisturbed material around the nucleus.”
WHY IT MATTERS — Scientists believe that comets are made up of material that has been leftover from the formation of a star system, scraps from the building blocks of planets.
Observing comets that formed from within our own solar system informs us of the early history of planets like Earth and Jupiter. However, interstellar comets provide us with clues on neighboring star systems.
Neil Dullo Russo, a planetary astronomer at Johns Hopkins University who was not involved in the study, says that comet 21/Borisov gives scientists a different picture than what they’re used to.
“We have only measured comets from our solar system and we're trying to get clues as to what the chemical makeup was in the comet-forming regions of our solar system,” Russo tells Inverse.
“That's why measurements of these interstellar objects are so valuable — because we don't see them very often and it gives us a window into what chemical processes were happening in other solar systems than ours,” he adds.
WHAT'S NEXT — Unfortunately, comet 21/Borisov will not pass through our inner solar system again.
“It completely left the solar system and is now somewhere far in space,” Kolokolova says.
The interstellar comet is currently too far for further observations.
However, scientists are optimistic that they can discover more of these interstellar comets and be able to probe at them for more clues about alien star systems.
“Detection techniques of all sorts of objects in our solar system have increased,” Russo says. “So, the ability to detect these things when they're far enough away where we can plan campaigns to study them is increasing.”
The European Space Agency is working on a mission for a potential 2029 launch to rapidly launch toward interstellar comets.
The Comet Interceptor consists of a large spacecraft and two small, robotic probes. After the spacecraft launches into space, it will be set to a "parking" orbit, essentially waiting patiently in the depths of space until scientists identify a suitable comet for it to target.
After a target is identified, the spacecraft will be sent out on an intercept course, following the comet at a distance. The mothership will then deploy the two probes, which will travel closer to the comet and gather data and observations from up close.
“We really hope this mission brings us new data on interstellar objects,” Kolokolova says.
Abstract: Comet 2I/Borisov is only the second object observed within our Solar System but not gravitationally bound to it. However, while the first such object, 1I/‘Oumuamua, discovered in 2017, had an overall asteroidal appearance, 2I/Borisov shows clear evidence of cometary activity. Our observations show that the polarisation of 2I/Borisov is higher than what is typically measured for comets, and with a steeper phase angle dependence than observed for the small bodies of the Solar System. These polarimetric properties distinguish 2I/Borisov from dynamically evolved objects such as Jupiter-family and all short- and long-period comets in our Solar System. The only object with similar polarimetric properties as 2I/Borisov is comet C/1995 O1 (Hale-Bopp), an object that is believed to have approached the Sun only once before its apparition in 1997. Unlike Hale-Bopp and many other comets, though, comet 2I/Borisov showed a polarimetrically homogeneous coma, suggesting that it is an even more pristine object than comet Hale-Bopp.