The first visitor from another solar system has just been spotted

AEONS ago, perhaps long before Earth itself existed, a hunk of rock circling a star somewhere in the Milky Way was thrown out of its orbit so violently that it was ejected from its natal system. Thus began a journey that would, in time, take it within an astronomical hair’s breadth of humanity’s home planet. On October 19th this visitor was spotted by Rob Weryk of the University of Hawaii in pictures produced by Pan-STARRS 1, a telescope on Haleakala. It thus became the first interstellar interloper into Earth’s solar system to be spied by astronomers.

Its origin is clear from its speed. When spotted, it was travelling at 25.5km per second. That is too fast for it to have a closed, elliptical orbit around the Sun. Nor could its velocity have been the result of an encounter with a planet giving it an extra gravitational kick, for it arrived from well above the ecliptic plane, close to which all the Sun’s planets orbit. Dr Weryk’s object, now named A/2017 U1 (the “A” stands for asteroid), thus almost certainly arrived from interstellar space.

Observations from other telescopes have confirmed A/2017 U1’s extrasolar origins. After swinging around the Sun, as the diagram shows, it passed about 25m km below Earth on October 14th, before speeding back above the ecliptic plane. It is now heading out of the solar system towards the constellation of Pegasus, at a speed of 44km per second.

Sci-fi buffs may find this tale familiar. One of the great works of 20th-century science fiction, “Rendezvous with Rama”, by Arthur C. Clarke, starts similarly. Rama, as the object in the novel is dubbed, turns out to be an uncrewed alien spacecraft, 54km long. It, too, arrives from the void, loops around the Sun, and vanishes into the distance again. Sadly, A/2017 U1 is no spacecraft. It is a rock about 400 metres across. But it still has an interesting story to tell.

Hello and goodbye

Models of planet formation suggest that interstellar objects such as A/2017 U1 are likely to be icy rocks known as comets, formed on the periphery of distant solar systems, rather than dry rocks, known as asteroids, dislodged from such systems’ interiors, which are places where any comet-like volatiles will have been driven off by the heat of their parent stars. Indeed, A/2017 U1 was first classified as a comet. But the absence of a tail of gas and dust, produced when comets fly close to the Sun, and analysis by Alan Fitzsimmons of Queen’s University in Belfast of sunlight reflected from its surface, suggest that surface is mostly rock.

One explanation is that over many millennia cosmic rays have transformed the icy, volatile chemicals that would be expected to stream off a comet into more stable compounds. Another is that the Sun is not the first star A/2017 U1 has chanced upon, and that the volatile materials have been boiled off by previous stellar encounters. Or it could be that the object actually was dry to begin with—perhaps once orbiting its parent star in an equivalent of the solar system’s asteroid belt and then having been ejected by an encounter with a Jupiter-like planet.

Another puzzle is why nothing like A/2017 U1 has been seen before. Theories of planet formation suggest such objects should be a reasonably common sight. Perhaps the theories are wrong. Or it could be that these interstellar visitors have been overlooked in the past, and that there will be a spate of such sightings in future.

The proof that interstellar wanderers like A/2017 U1 really do exist also touches on the question of how life got going on Earth in the first place. Though most researchers think it evolved in situ from non-living chemicals, a few favour the idea that this evolution happened elsewhere and that living things, in the form of bacteria, were carried to Earth fully formed, inside objects of this sort.

Whether life could survive such a journey is moot. Outer space has a temperature close to absolute zero, is full of harmful radiation and is of course a vacuum. But some forms of life are remarkably resilient, even to these sorts of extremity. Experiments that may shed some light on the matter are being planned as part of efforts to send unmanned, miniature space probes to stars close to the solar system (see article).

As to the rock itself, it surely deserves a more memorable name than the one it sports at the moment. And a quick look at the list of existing asteroid names instantly suggests one. Perhaps in expectation of a discovery like this, the International Astronomical Union, which approves such names officially, has not yet called an asteroid “Rama”. How about it, chaps?