THE QUEST FOR INTERSTELLAR COMETS

To date we have witnessed three confirmed interstellar objects pass through our Solar System in the space of 8 years, Oumuamua in 2017, 2I/Borisov in 2019 and of course 3I/ATLAS in 2025. But the question we need to ask is: are these truly the first interstellar objects we have seen? Neil Norman thinks they might not be.

Neil Norman runs the Facebook group CometWatch.

My fascination with this subject began in 1988 when I watched Sir Patrick Moore talking about comets and he mentioned a comet named C/1956 R1 (Arend–Roland). This famous ‘spiked’ comet put on a decent naked-eye performance in spring of 1957. It was spiked because it displayed a prominent anti-tail, a stream of cometary material that appears to point towards the Sun rather than away from it. In this respect it was rather similar to 3I/ ATLAS, which also displayed an anti-tail.

In his commentary, Moore stated that this comet was on an orbit that would see it leave the Solar System and forever roam the emptiness of space until one day it encounters another star system. This got my young mind wondering: if comets are ejected from our system then surely we should see other comets falling in?

The quest begins….

There are a couple of criteria to help find potential interstellar objects. The first is orbital eccentricity and the second is chemical composition.

Orbital Eccentricity: This simply defines how elliptical the comet’s orbit is and this ranges from a value of zero, which is a perfect circle, to a value of just less than one, which is highly elliptical. All of these orbits remain gravitational bound to the Sun even if the orbital periods are huge, hundreds or even millions of years. For example, comet 1P/Halley has an eccentricity of 0.967 and an orbital period of 76 years. Comet C/1975 V1 West had an eccentricity of 0.9999 and a period of 558,000 years. Any comet with an eccentricity greater than one is on a hyperbolic orbit, which means that it is not bound to the Sun, because it has enough velocity to escape the Sun’s gravitational influence. The three confirmed interstellar objects have all been hyperbolic in nature. Their eccentricities are given in Table 1.

Object

eccentricity

1I/‘Oumuamua

1.20

2I/Borisov

3.35

3I/ATLAS

6.14

Object

speed at 30 AU from Sun

1I/‘Oumuamua

27 km/s

2I/Borisov

33 km/s

3I/ATLAS

58 km/s

(1P/Halley

3.1 km/s)

To give a little perspective on these values: at 30 times the Earth’s distance from the Sun (a measurement known as 30 AU), which is before the gravitational influence of the planets can add any meaningful momentum, these objects are already travelling at outstanding velocities. (See Table 2, in which comet 1P/Halley is included for comparison.)

Chemical Composition: ‘Oumuamua caught us somewhat off-guard in 2017 and, because it did not display conventional cometary behaviour, the composition was never truly determined. 2I/ Borsiov did display typical cometary behaviour but some compositional surprises were found when compared to an average comet from our system. Carbon monoxide (CO) was a major constituent compared to water. Borisov had a CO:H 0 ratio of between 7:20 and 21:20 during its apparition, Solar System comets are typically 4 percent at 2.5 AU from the Sun but those heading in from the Oort cloud do typically have higher values of 10–24 percent. This indicates Borisov formed in a CO- rich region of another planetary system.

2 There were other chemical differences too. Carbon molecules are prevalent in our comets but were depleted in Borisov. Conversely, it had more ammonia (NH ) than a typical Solar System comet. Neutral nickel to iron (Ni:Fe) was consistent with our comets, which would

3 indicate a similar formation around its parent star but the dust composition for Borisov was more like compact pebbles rather than the fine dust seen in our comets. 3I/ATLAS had a carbon dioxide (CO ) to water ratio of 8:1, which is up to 16 times higher than a typical Solar System comet. A high production of atomic nickel vapour was detected without iron emission lines, something never seen before. Iron lines were only detected when the comet was close to the Sun. There were high amounts of methanol (CH OH) detected, which is only found in trace amounts in our comets.

23So, with all this in mind, are there other comets we have seen that could have come from interstellar space?

FEATURE

Potential Candidates

C/1956 R1 (Arend–Roland): As already mentioned, the comet put on a bold show with a dusty tail stretching for some 15 degrees or more. The comet was extensively documented by astronomers for 520 days. Renowned comet specialist, Zdenek Sekanina, Charles University in Prague, concluded that Arend–Roland either had strong non- gravitational forces altering its orbit, which could have come from its evaporating ices as it neared the Sun, or it was of interstellar origin.

C/1853 R1 (Bruhns): This comet was discovered near the constellation of Lynx on 12 September 1853 by C.Bruhns (Berlin). It made a close approach to the Sun of 0.17 AU on 17 October. When the orbit is tracked back to around 1600, the eccentricity consistently averages a value of 1.0008. As of 1 January 2026, the comet is some 184 AU from the Sun on its outward journey with an eccentricity of 1.0006. The fact that its eccentricity has remained hyperbolic all through its journey strongly suggests that this comet was an interstellar interloper.

96P/Machholz: This comet was discovered by Donald Machholz on 12 May 1986 and has been an enigma ever since. Its orbit suggests it was captured by Jupiter around 4,000 years ago and may be a fragment from a larger body.

Aside from the orbit, it raises red flags due to its chemical composition. 96P/Machholz is carbon depleted by as much as 72 times that of an average comet from the Solar System. This can be explained by three hypotheses. One, 96P was captured from interstellar space. Two, it formed in a frigid part of the Solar System or three, repeated close passes by the Sun have depleted the majority of its cyanogen ice.

Option three is plausible but hardly definitive. Comet 2P/Encke is not as depleted in cyanogen yet orbits the Sun closer than Machholz. Option two is difficult to reconcile because it would then be hard to get Machholz into its current orbit.

Therefore option one is the strongest: Machholz originally formed in a planetary system that was intrinsically carbon depleted.

C/2018 V1 (Machholz-Fujikawa-Iwamoto): Discovered on 7 November 2018 by no less than three amateur astronomers within hours of each other, when the comet was in Virgo at a distance of 0.80 AU and 1.23 AU from the Sun and Earth respectively, the comet featured a slightly hyperbolic orbit. According to a 2019 study by Carlos and Raul de la Fuente Marcos, Complutense University of Madrid, Spain, there is a 72.6 percent chance of this comet originating from interstellar space about a million years ago. They conclude “Our results suggest that comets coming from interstellar space ... may not be rare.”

Rubin’s haul

From now on we may be able to find one or two more interstellar comets every year.

The Vera Rubin Observatory is about to embark on a mammoth 10-year survey of our skies known as the Legacy Survey of Space and Time (LSST). If estimates are to be believed, it will discover one or two interstellar comets every year.

It will also detect more regular Solar System comets than ever before. This is good news for astronomers eager to study the systematic differences between regular comets and interstellar ones. It should also help determine whether some of our own comets were stolen from the Sun’s siblings when it formed in a tightly-knit stellar community 4.6 billion years ago.

While the true origin of the comets mentioned in this article may remain uncertain forever, one things is certain: with modern technology evolving at a rapid pace, interstellar objects could very well become a regular observing target.

Neil Norman runs the Facebook group CometWatch.