One of the more pressing questions now is why the first object to have made this fascinating journey was an asteroid, not a comet.
A diagram showing the path of A/2017 U1. Caption and credit: NASA
Aswin Sekhar is an Indian astrophysicist based at CEED, the University of Oslo, Norway.
Oslo: Media outlets around the world have been abuzz with news about a newly discovered body in our Solar System that appears to have come from the neighbourhood of a different star. Since its discovery, astrophysicists have been conducting follow-up observations and immersing themselves in mathematical calculations that could tell us more about its origins and properties.
Theoretical astrophysicists predicted the existence of such interstellar visitors many years ago. “Many times over the years, astronomers have predicted comets that could come from way beyond our planetary system. Accurate observation and computation always showed these objects to be gravitationally bound to our Sun, even if only loosely, and orbiting [it] in the distant ‘Oort cloud of comets’,” said David Asher, an astronomer at Armagh Observatory and Planetarium in Northern Ireland, told The Wire. “This new body, [known only as] A/2017 U1, is totally different – the first asteroid or comet unambiguously demonstrated to be from further away.” He thinks it could have been ‘kicked out’ of a star system other than the one hosted by our Sun and left wandering the interstellar wasteland “for millions of years”.
The spotting of A/2017 U1 marks the first time in history when a distinct interstellar body was observed and confirmed. “They really do show that interesting discoveries can still be made,” Robert Weryk, from the Institute for Astronomy, University of Hawaii, and who first observed and reported the object, said – adding that astrophysicists now know that “these objects exist, confirming various theoretical work [that suggested] their existence.”
The path of the interstellar asteroid A/2017 U1, which passed by the Sun on September 9, 2017. It was moving too fast (at 25 km/s) to be trapped in any kind of orbit within the Solar System, and is likely to never return after this visit. Credit: NASA/JPL
“I think everybody was expecting to see an interstellar object doing a flyby with the Solar System, sooner or later,” Alessandro Morbidelli, at the Lagrange Laboratory, Observatory of Nice, said. “But everybody was expecting a comet. … As a general rule, the [reservoirs of comets] are more massive and, being further out, they are easier to eject [into] interstellar space.”
“Now that we know what to look for, we will certainly try to find more,” Weryk added.
This is an important discovery in the domain of solar system astronomy and exoplanetary astronomy. Alan Fitzsimmons, a noted expert on comets at Queen’s University, Belfast, explained: “It has been clear from studies of our own Solar System that it must have lost trillions of comets and asteroids over its lifetime, both when the planets formed and settled down into their current orbits, and later from the Oort cloud of comets that surrounds our Sun. If other planetary systems undergo similar evolution, then interstellar space should be populated with these orphan objects, and astronomers have waited for decades to see one entering our Solar System.”
So with A/2017 U1 having been confirmed, the wait has ended.
An artist’s impression of the location of the Oort cloud, a cloud of objects like asteroids and comets and the region of space from which comets flying by our Sun are thought to come. The Oort cloud exists only in theory; if it exists for real, we haven’t directly observed it yet. Credit: NASA
Apart from validating some ideas that has existed only on paper thus far, A/2017 U1 also stands to provide more clues about how bodies like itself are formed and how they might be ‘leaking’ out of some stars systems and entering others.
According to Fitzsimmons, “Spectroscopic studies of this object will allow its composition to be constrained, while the very act of discovery will give a first estimate of the number that exist between the stars.”
The question of ‘why an asteroid and not a comet’ is particularly pressing. Morbidelli, making an effort to provide a structural argument, says, “I think the key is the size distribution. The smaller [some] objects are, the more numerous they are. So the first object discovered is necessarily very small. Indeed this object is estimated to be about 150 metres [wide]. It is possible that comets this small do not exist.”
There are other, more intricate questions as well that will have to be answered before we can make full sense of our first (known) interstellar visitor – answers that could keep astrophysicists occupied for the next decade.