The structure of Comet Borisov
If it was possible to slice through the interloper, what would be found within?
“It has the features of a comet – a coma and a tail of dust – which require an ice-rich composition” Paul Kalas
Solar System comets as they approach the Sun. Future observations may discover jets of gas shooting out in specific directions from the nucleus, but a particularly exciting possibility is that the nucleus will break apart into many fragments.”
The chances of the latter happening are as low as ten per cent, but that is still high enough. In the meantime, and ahead of such a possible fate, data from Hubble is being combined with that collated on the ground, and the hope is that scientists will get a better estimate of the shape and size of the nucleus. Measurements across different wavelengths will also tell researchers what the comet is made of. Meanwhile, Kalas has a student at Berkeley working on the potential trajectory of the comet, but it'll be a while before we have conclusive answers.
“The comet isn't interacting much with our Solar System,” affirms Mutchler. “The Sun's gravity is barely modifying its course and it doesn't get near any planets. The orbital path is interesting in that it gives a hint of where the object may have come from, but that is a difficult task which relies on a few assumptions.”
Nevertheless, some studies have already worked on giving a set of clues. Polish researchers believe Comet 2I/Borisov's path leads back to a binary red dwarf star system called Kruger 60, which is 13.14 light years from the Sun. The study also suggested that the comet passed 5.7 light years from the centre of Kruger 60 a million years ago, travelling at 12,348 kilometres (7,670 miles) per hour.
Such a small relative velocity of the two objects during this approach was, they concluded, not indicative of the comet merely passing by. Rather it most likely originated from that star system, they said, although the findings are not confirmed.
One of the difficulties in studying the comet is the trickiness of getting decent data. It's not easy to pinpoint the core of an object that, when imaging through a telescope, is blurred. More than that, determining the origin of an interstellar comet entails figuring out the position of the Milky Way's stars in the required timeframe when we don't yet have a full star map of the galaxy. What we do know is that it has an extremely hyperbolic orbit, which points to its interstellar status as opposed to an elliptical orbit typical of ‘domestic’ comets.
We do have great photographs, though. The first colour image was snapped on the night of
9-10 September using the Gemini Multi-Object Spectrograph on the Gemini North Telescope that's on Mauna Kea in Hawaii. It clearly showed the aforementioned pronounced comet tail. Yet getting that photo was no easy task. “We really had to scramble for this one, since we got the final details at 03:00 and were observing it by 04:45,” says Andrew Stephens, who works at the observatory.
He also explained the image was possible because of Gemini’s ability to rapidly adjust observations and observe objects such as Comet 2I/Borisov with its very short window of visibility. Meanwhile, Hubble began observing the comet on 12 October when it was 418 million kilometres (260 million miles) from Earth. It will continue doing so as it makes its way through the Solar System and presumably leaves in the direction of the constellation of Telescopium. But what other mysteries will it help to uncover as it does so?
One of the key reasons why this comet is so intriguing is because of its potential to explain how
comets and other objects from outside of the Solar System. This in turn can shed better light on the origins of objects closer to home.
“We don't know exactly how planets form in our own Solar System or elsewhere,” says Mutchler. “So it is interesting that this comet seems similar to comets in our own Solar System, since they are one of the building blocks for planets. It hints that perhaps the planet-building process is similar in other star systems, but again we'd like to see many more examples before concluding anything.”
The very fact that we now have two different interstellar objects to study is certainly pleasing scientists, but it has also thrown up a mystery of its own. “It's a puzzle why these two are so different," says David C. Jewitt of the University of California, Los Angeles, leader of the Hubble team who observed the comet.
“They likely have different origins and history, so they would each tell a different story,” enthuses Mutchler. “But they are just two objects representing trillions of objects that have likely been ejected by their parent stars into interstellar space, so we can't even assume that these two reflect the majority very well. We'll need to discover and study thousands more before we start to get a balanced idea of what is out there.”
The expectation is that we will see more interstellar objects very soon. After all, we have now been able to gather observations and evidence of two in two years. “Advances in technology are allowing astronomers to monitor the entire sky for transient events such as interstellar comets that zip through our
Solar System very quickly and unpredictably,” explains Kalas. “Amateur astronomers like
Borisov are also able to use their own equipment, which is becoming ever more sophisticated.”
Borisov used a 0.65-metre telescope that he designed and built himself, which makes the discovery even more remarkable.
It cements the fact that with effort, talent and the right gear, any amateur astronomer could survey the night sky for other mysterious interstellar objects, although a good number of them are beyond the capabilities of modern-day telescopes. Still, it won't stop the search, especially when two diverse objects have been seen so far.
“Nobody knows how many there are out there in interstellar space, but planet-formation models imply that most of the small objects would get ejected as planets form and migrate,” says Mutchler. “There should be many more 'rogue' comets and asteroids than the ones still orbiting their parent star, but they are spread out in the enormous volume of space between stars, so are understandably rare. We have more sky surveys underway and planned though, which should discover many more asteroids and comets in our own Solar
System. More interstellar visitors are expected.”