During the summer of 2019, amateur astronomer Gennadiy Borisov spotted a rogue comet that was born in a star system far beyond the Sun. The comet, named 2I/Borisov, was only the second interstellar interloper ever identified in our solar system, following the sighting of the bizarre object ‘Oumuamua in 2017.
While ‘Oumuamua sparked raging controversies over its strange nature and possible origins, 2I/Borisov has fascinated scientists for the opposite reason: it looked pretty much like local comets that orbit the Sun, suggesting that the visitor may hail from a very similar system to our own.
Now, independent findings from two research teams have strengthened this connection between our solar system and the mysterious birthplace of 2I/Borisov, while also revealing a surprising new twist about homegrown and interstellar comets.
Piotr Guzi and Michał Drahus, two scientists at Jagiellonian University in Poland, announced the “unexpected finding” of nickel vapor in the atmosphere of 2I/Borisov, according to a study published on Wednesday in Nature. Though it is well-known that heavy metals such as iron and nickel exist as solids in the interiors of comets, these elements had only ever been only seen in a gaseous form when comets make close approaches to stars, causing extreme heating that sparks sublimation of the metals into vapor.
That’s why Guzi and Drahus were caught off-guard by clear evidence of gaseous nickel in 2I/Borisov’s coma, or atmosphere, even though the interstellar comet was about twice as far from the Sun as Earth at the time of the team’s observations in January 2020. At that distance, the comet’s temperature was estimated at about -93°C, well short of the 425°C conditions that normally produce nickel gas.
Still, the team’s observations—obtained by the sophisticated X-shooter spectrograph at the European Southern Observatory’s Very Large Telescope—showed evidence of gaseous nickel across several nights, “effectively eliminat[ing] the possibility of confusion with a background source or instrumental artefacts,” according to the study.
“Atomic nickel was a truly unexpected finding, because observation of the gaseous form of any metal was thought to be limited only to hot environments, such as sungrazing comets passing a few solar radii from the Sun surface,” Guzi said in an email.
“The two brightest nickel lines were clearly visible in our combined spectrum, but since we did not know about any typical emissions at these wavelengths, we first made sure that they are present in the spectra taken each individual night, not present in the sky background and that they are not artefacts,” he continued. “Still, it took us a lot of time to find out their true nature.”
To potentially explain the discovery, Guzi and Drahus propose that heavy metal vapor might be released at cold temperatures by a process known as photodissociation, in which photons (light particles) stimulate a nickel-containing molecule on the surface to sublimate into gas.
The team’s results would be intriguing enough on their own, but they also happen to coincide with the publication of a decades-long compositional survey of comets in our solar system, led by University of Liège astronomer Jean Manfroid. With the help of observations collected since 2002, Manfroid’s team discovered that nickel and iron “are ubiquitous in cometary atmospheres, even far from the Sun,” according to a study also published on Wednesday in Nature.
“We learned about the other paper when ours was already under review,” Guzi said. “The fact that the presence of nickel was detected both in the coma of the first known interstellar comet and a set of comets native to our planetary system is simply astonishing.”
“It opens up a new perspective for understanding how the formation of planetary systems proceeds in the galaxy since comets are thought to be the leftovers from this process,” he added.
The new research also bolsters the case that 2I/Borisov may have originated in a star system much like our own, given that these studies identify yet another property that the interstellar visitor shares with our own local comets. If scientists discover more interstellar comets that look like 2I/Borisov, it could hint at the existence of many solar system analogues across the Milky Way.
“We might expect that the conditions at the place of their birth, as well as the chemical composition of the interstellar cloud their home planetary system originated from, were similar to the interstellar cloud from which our Solar System was born,” Guzi said.
“It would be interesting to investigate the composition of other interstellar comets in the future to compare them with 2I/Borisov,” he continued. “Unfortunately, we do not know when such an object will appear. The other fascinating question is what is the source of metal atoms in the gases surrounding the cometary nucleus.”
Fortunately, these tantalizing questions might be resolved in the coming years as next-generation observatories increase the rate at which interstellar visitors are spotted. In particular, the Vera C. Rubin Observatory in Chile, which is due to begin operations within the next few years, is expected to accelerate detections of these mysterious interlopers.
A larger census of interstellar objects will provide vital information about their distant home systems, shedding light on just how unique our own cosmic neighborhood is in the scope of the Milky Way.
“It is a significant time in the history of planetary science because we have an opportunity to investigate objects that were born in distant planetary systems and were ejected from their homes finding their way to our cosmic neighborhood,” Guzi concluded.