spaceSpace and PhysicsspaceAstronomy

Comets From Outside Our Solar System Might Visit Us Often, Study Suggests


Stephen Luntz

Stephen has a science degree with a major in physics, an arts degree with majors in English Literature and History and Philosophy of Science and a Graduate Diploma in Science Communication.

Freelance Writer


Given our limited capacity to find small objects unless they come close, detection of Comet Borisov hints at a very large population of such objects passing beyond our current range. Image Credit: NASA, ESA and D. Jewitt (UCLA)

In the last four years, astronomers have spotted two visitors to the Solar System from elsewhere in the galaxy. Both provided an abundance of data that will be studied for many years – and left scientists wanting more. A new paper argues one of these guests, Comet Borisov, represents a class of objects we will see many more of in the near future, although it acknowledges error margins are huge.

Interstellar visitors could provide great insight into other star systems, but we have to find them first. Harvard University’s Amir Siraj and Professor Avi Loeb conclude in Monthly Notices of the Royal Astronomical Society we should have plenty of opportunities.


“Before the detection of the first interstellar comet, we had no idea how many interstellar objects there were in our solar system, but theory on the formation of planetary systems suggests that there should be fewer visitors than permanent residents,” Siraj said in a statement. “Now we’re finding that there could be substantially more visitors.”

We have seen only one interstellar comet, and thousands of locals, making the idea that visitors outnumber natives deeply counter-intuitive. However, Siraj points out the Oort cloud stretches to a distance of 160 trillion kilometers (100 trillion miles) from the Sun in every direction. Locals cluster closer in, but the vast majority of visitors will pass through its outer reaches with no chance of detection.

The possibility that lumps of rock and ice frequently pass by beyond the orbit of Neptune is intriguing, but of limited relevance. However, the paper goes further – suggesting even within the orbit of Saturn, there may be similar numbers of visitors to locals. We now detect many asteroids too distant to pose a threat, and never determine their orbits. Some may be long gone by the time anyone tries to follow up.

Siraj and Loeb’s calculations rely on estimates of the chances of detecting a smallish object that never gets close to the Sun. If we found one, there must be many more, and they have tried to quantify that.


Borisov was 97 percent oxygen and carbon by mass. If the authors’ estimates are right, around 1 percent of these elements in the galaxy could be contained in objects floating in interstellar space, unbound to any star.

Theoretically, the possibility of finding an interstellar comet dates back to the 18th century, when we started plotting cometary orbits. However, with only a few not terribly advanced telescopes watching the sky at the time, only the brightest comets were seen – all of which had paths indicative of coming from within the Solar System.

There have been many upgrades since then, but it was only in 2017 that Oumuamua made an appearance, with Borisov following two years later. The question was obvious: did these two discoveries reflect pure luck, or have there been many objects like this we lacked the instruments to find?

The question is likely to be answered soon, making this paper either look highly prescient or very wrong. Loeb has attracted plenty of notoriety for his insistence Oumuamua is an alien spaceship and aggressive behavior towards colleagues. However, his work on topics unrelated to alien civilizations remains more respected.


The Vera C. Rubin Observatory is planned to start operating in 2022, and should transform our capacity to detect faint moving objects of any origin. Within a few years, we should have a statistically useful sample size of visitors. Even before that, the Transneptunian Automated Occultation Survey may provide some useful data.



spaceSpace and PhysicsspaceAstronomy
  • tag
  • solar system,

  • asteroids,

  • Comets,

  • oort cloud,

  • Astronomy,

  • oumuamua,

  • borisov