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Astronomy and space

Astronomy and space

Interstellar comet 2I/Borisov comes from a binary star 13 light-years away, say astronomers

11 Oct 2019
Borisov comet
Coming into focus: 2I/Borisov appears as a blur of light in this telescope image, but astronomers are learning more about interstellar object every day. (Courtesy: IAU)

Comet 2I/Borisov, recently confirmed as a visitor from interstellar space, could have its origin in a star system 13 light-years away, say astronomers. Extrapolating from the relatively scant orbital parameters determined so far, and accounting for the gravitational effects of hundreds of nearby stars, astronomers in Poland have projected the comet’s path back in time. They found that, about one million years ago, 2I/Borisov and the double star system Kruger 60 passed within a few light-years of each other at a very low relative velocity. Observations of the comet as it travels through the solar system will improve our understanding of its orbit and allow the astronomers to test their hypothesis more thoroughly.

Not counting cosmic dust grains found on Earth and captured in space, 2I/Borisov is only the second interstellar object that we know of. The first was the pencil-shaped body named ‘Oumuamua, which was spotted shooting through the solar system in September 2017. ‘Oumuamua caused great excitement when it was first discovered and some astronomers even speculated that it could be some sort of alien spacecraft. While that hypothesis has been discounted, much about this object remains a mystery. It was already heading away from the Sun when it was first spotted, so there was little time for detailed observations. Although some outgassing was inferred from unexpected changes in its orbit, it stubbornly refused to emit anything that could be measured directly.

This time things are different with 2I/Borisov. Discovered at the end of August 2019, 2I/Borisov is still on the inbound leg of its trajectory, and it will not reach perihelion (its closest approach to the Sun) until early December. This means that astronomers will have a year or so in which to make observations, and some of these will be measurements of the comet’s orbit with a view to determining its origin. In a preprint posted on the arXiv preprint server, Piotr Dybczyński, and colleagues Adam Mickiewicz University and the Space Research Center of the Polish Academy of Sciences report the first such study.

Complex problem

“Starting with the current position of 2I/Borisov, we traced its motion backwards, looking for a star that appeared to be close to it and with a small relative velocity,” says Dybczyński. This is more challenging than it sounds, however. The comet, the Sun, and every other star in the Milky Way pursue their own individual orbits around the galactic centre, with the path of each body influenced by the gravity of all of the others. Add in the fact that 2I/Borisov’s orbit is still relatively undetermined, and that the stars’ distances and motions are known only imprecisely, and the problem quickly becomes complex.

Dybczyński and colleagues modelled 2I/Borisov’s route through space under the gravitational influence of 648 star systems—including the Sun—that they identified as being close enough to affect it. A co-orbiting pair of red dwarf stars known collectively as Kruger 60 emerged as a potential home system for the comet. A series of 10,000 simulations, in which the astronomers varied the comet’s orbital parameters and Kruger 60’s location and velocity, suggested a closest approach of 5.7 light-years at a low relative velocity of 3.4 km/s.

Given this distance is somewhat greater than than the distance to the nearest star (Proxima Centauri just 4.22 light-years away), Kruger 60 might seem an unlikely source for the comet. According to Dybczyński, however, some residual uncertainties mean it is still a reasonable proposal. For one thing, says Dybczyński, the Solar System’s cometary cloud is believed to extend beyond 1.5 light-years and because Kruger 60 is a double system, its own cometary cloud (if it exists) might be larger. He also points out that our current knowledge of Kruger 60’s kinematics is poor.

The claim finds cautious support from Alan Fitzsimmons of Queen’s University Belfast, UK, who was not involved in the study. “The small velocity of the comet relative to Kruger 60 when it passed it is certainly suggestive of possible association,” says Fitzsimmons, “but the current miss distance, if true, would rule out an origin at that star. Yet it is still a contender, as our knowledge of the trajectory of 2I/Borisov is still evolving.” He also points out that any estimates of the comet’s past trajectory will have to be reconsidered if its current orbit is found to be affected by outgassing.

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