A mysterious interstellar comet is revealing secrets about alien worlds that should not exist
A mysterious interstellar comet is revealing secrets about alien worlds that should not exist
When amateur astronomer Gennady Borisov aimed his telescope at the constellation Cygnus on August 30, 2019, he caught a smear of light moving against the fixed stars. Within days, orbital calculations confirmed the object was on a hyperbolic trajectory, unbound to the Sun. Something from another planetary system had entered our neighborhood. Within weeks, it had a name: 2I/Borisov, the second interstellar object ever detected and the first confirmed comet from another star.
Unlike its predecessor, the enigmatic OUmuamua, 2I/Borisov came close enough and moved slowly enough to study in detail. What astronomers found defied expectations built over decades of solar system observation.
In December 2019, NASA and the ALMA Observatory in Chile turned their instruments toward the approaching comet. Their findings, published in Nature Astronomy in April 2020, documented a carbon monoxide concentration 9 to 26 times higher than any comet measured in the inner Solar System. The ratio of carbon monoxide to water came in at 173 percent, more than triple the solar system average.
We have never seen anything like this in our own Solar System, said NASA astrophysicist Stefanie Milam. The carbon monoxide ratio is really dramatically high compared to water.
The ALMA team, led by Martin Cordiner and Stefanie Milam, observed the comet's coma across two nights in mid-December 2019. Their spectroscopic data detected water, hydroxyl, hydrogen cyanide, and carbon monoxide, with CO dominating the emission spectrum. The concentration gradient suggested the comet was releasing carbon monoxide from a subsurface reservoir, likely in the form of frozen ice.
The comet must have formed from material that was very rich in carbon monoxide ice, and this must have been preserved at very low temperatures, Cordiner said.
In a separate analysis of Hubble spectra, a team led by Dennis Bodewits at Lowell Observatory measured the comet's gaseous emissions between December 2019 and January 2020. Their findings aligned with ALMA's results, and they drew a striking conclusion: 2I/Borisov likely originated around a red dwarf star, where temperatures remain cold enough for carbon monoxide to condense into solid ice without being destroyed by radiation.
We think that 2I/Borisov formed around a red dwarf star, Bodewits told NASA. The cold conditions needed for high CO ice formation are most stable around such stars, which are smaller and cooler than the Sun.
But the chemical surprises did not end there. Lowell Observatory spectroscopy also found that 2I/Borisov exhibited a depleted C2 abundance, similar to a small and unusual subclass of solar system comets, combined with an elevated NH/OH ratio suggesting nitrogen chemistry that differs from what we see closer to home. As the comet approached the Sun, its carbon chemistry behaved unexpectedly: unlike solar system comets, which maintain stable surface compositions, 2I/Borisov's C2 abundance appeared to change in ways scientists attributed to a space-weathering crust formed during its long interstellar crossing burning off near the Sun. This was unprecedented, no existing cometary model predicted such behavior.
The discovery carries implications that extend well beyond a single object.
This is the first glimpse into the chemistry that shaped another planetary system, Milam said at a NASA press briefing. We want to know what kind of chemistry is happening in other planetary systems, and this is the first time we are actually getting that information.
The Science: What the Data Show
2I/Borisov was confirmed as interstellar in late September 2019. Its orbital eccentricity of 3.36 definitively established its hyperbolic path. It traveled through our Solar System at 33 kilometers per second, fast enough to have been ejected from its host system by a migrating Jupiter-sized planet. The Hubble Space Telescope measured its nucleus at approximately 0.61 miles (0.98 km) in diameter, roughly half the size of comet Hale-Bopp's nucleus but within the range of well-studied solar system comets.
The most significant ALMA observations took place December 15 to 16, 2019, when the comet was 2.4 astronomical units from the Sun. The team detected strong emission from carbon monoxide, hydrogen cyanide, and water, with the CO signal exceptionally strong relative to water.
Competing Explanations
Scientists have proposed two main theories for the CO excess.
The first, consistent with the red dwarf hypothesis, holds that the comet formed in the outer reaches of its host protoplanetary disk at temperatures cold enough for CO ice to condense and persist. In our own Solar System, comets formed in the Kuiper Belt and Oort Cloud contain significant CO, but not at the levels detected in 2I/Borisov. If the comet originated in a system with a more extended cold zone, it could have accumulated far more carbon monoxide than typical solar system comets.
The second theory suggests an insulating crust of water ice formed on the comet's surface during its long interstellar journey, trapping CO-rich material beneath. As water ice began to sublimate near the Sun, trapped CO would have been released, artificially inflating the measured ratio.
Both theories may contribute. We cannot yet determine which mechanism is responsible, the ALMA team noted in their press release. More observations of future interstellar comets will be needed to establish whether 2I/Borisov is typical or exceptional.
Open Questions
Three unresolved questions define the current scientific frontier around 2I/Borisov.
First, is the comet typical or exceptional? Bodewits and Noonan addressed this directly in their Nature Astronomy paper: It remains unclear whether the unusual composition of 2I/Borisov is typical of interstellar comets or represents a special case. The answer requires more interstellar comet detections and more opportunities for detailed spectroscopy.
Second, how was it ejected? The high transit speed of 33 km/s suggests the comet was thrown out by a migrating gas giant, similar to how Jupiter's gravity is thought to have ejected comets into the Oort Cloud in our own Solar System. But no one has confirmed the mechanism in another system.
Third, why did its chemistry change near the Sun? The anomalous C2 behavior and elevated NH/OH ratio suggest 2I/Borisov experienced something no solar system comet has, a surface transformation during its interstellar crossing that altered its outgassing chemistry as it approached the Sun. Whether this reflects a common feature of long-period interstellar objects or something unique to 2I/Borisov remains unknown.
The red dwarf connection remains the most widely cited origin theory. JHU APL released an analysis in April 2020 noting that the cold temperatures required for CO ice formation are most stable around red dwarf stars, making them the most likely birth environment for a CO-rich interstellar comet. The alternative proposed by the ALMA team, that the comet might represent a fragment of a dwarf planet with a CO-rich surface, does not require a specific host star type but also lacks a confirmed mechanism.
Looking Ahead
The case of 2I/Borisov demonstrates what happens when a known scientific framework confronts an object from outside it. For decades, astronomers used solar system comets as reference points for cometary chemistry everywhere. 2I/Borisov suggests those reference points may be incomplete. The comet arrived without warning, was studied intensively for months, and departed before every question could be answered.
The JWST followed up with observations of 3I/ATLAS, the third confirmed interstellar object, in 2024, using its NIRSpec instrument to analyze a more recent visitor's coma composition. Scientists are already preparing observation campaigns for the next confirmed interstellar comet. The next one to arrive may not wait decades.