Starting in 2008, a Hawaiian telescope called Pan-STARRS 1 has been taking a survey of the night sky. This telescope, like many others around the world, was built primarily for the purpose of identifying and cataloging Near Earth Objects (NEOs). These objects have attracted a lot of scientific and popular interest in the past few decades because of two reasons. Firstly, these objects have a significant potential to strike the Earth, causing a natural disaster comparable to a major volcano or earthquake. Secondly, unlike the multitude of other disasters that are always threatening humanity, NEOs are the one disaster that could conceivably be prevented with today’s technology; gravity tractors, nuclear weapons, or kinetic impacts are all possible prevention methods.
In October 2017, Pan-STARR made a typical observation: a small smear in the camera, indicating an object moving quickly across the sky (at least relatively quickly compared to most objects). The observation was announced with the normal amount of fanfare—none at all—and other NEO-tracking telescopes turned to look for the object. Other observations were reported on subsequent nights, so scientists could begin to estimate its orbital path. As more and more measurements came in, the projected orbit of the object—now called A/2017 U1—became increasingly elongated until, on October 22, observations from the Canada France Hawaii Telescope pushed the expected “orbit” into interstellar space. Suddenly, A/2017 U1 became a very interesting object.
The asteroid, which has since been renamed as ‘Oumaumau (Hawaiian for “scout”), is now known to have originated outside our solar system. Instead, the 250 m long object is has been floating through space for hundreds of thousand or even hundreds of millions of years. It happened to fly close to the sun on September 9, then happened to come close enough to Earth for Pan-STARR 1 to spot it. For the next few weeks, scientists used every tool they could to study ‘Oumaumau, since it will never return to the sun again.
This is not a totally unexpected discovery. Many scientists had predicted that a large number of small planetesimals, especially comets, should be ejected from star systems during their planet formation stage. However, the particular nature of ‘Oumaumau is exciting because it defies many expectations that these interstellar objects (ISOs) were expected to have.
For instance, while the asteroid was always too far away to be resolved in a telescope with a size larger than one pixel, it displayed regular variations in brightness that researchers have interpreted as it being cigar-shaped, ten times longer than it is wide. This is much more elongated than any other asteroid that has ever been observed. It is spinning so fast and is so elongated, in fact, that the object must have some tensile strength, since gravity alone is not enough to prevent it from shedding material. Alternatively, ‘Oumaumau may have a difference in albino (reflectivity) that causes the brightness difference over the orbit.
‘Oumaumau is also unexpected in that it is a rocky asteroid, not an icy comet. Surrounding our solar system is a cloud of bodies called the Oort cloud. These are objects that were almost-but-not-quite ejected from the solar system, and they are almost all comets. Therefore, it was assumed that most interstellar objects would also be comets. The fact that the first interstellar body that is observed was an asteroid suggests that our solar system may be an exception, with other systems having asteroids in their outer reaches.
In light of these recent findings, scientists are now eager to spot and observe more ISOs. The prospect of another interloper coming along soon is less illusory than might be expected. A letter published in Nature by a number of scientists, lead by astro-biologist Karen Meech, suggests that there may have been previous detentions of ISOs, but that they were not properly identified. They estimate that, statistically, there should always be one ISO that is about 250 m in diameter closer to the sun than Earth.
The letter goes on to describe how previous ISO sightings could have been discounted; after a first observation, the orbit of an object is not known. Subsequent observation attempts must guess at the likely orbit of the object and search in the expected regions. Since ISOs are on hyperbolic, as opposed to elliptical, trajectories and move much faster than orbiting bodies, they would typically move beyond the limits of this search region. After failing to find the body a second time, the hopeful telescope operators would discount the first observation and move on to more promising NEO candidates.
‘Oumaumau is now speeding away from Earth at over 30 km/s, never to return. The observations that have been made have been enough to challenge current theories on stellar evolution and the normalcy of the solar system, but not enough to disprove them or offer concrete new ideas. New ISOs will be found and observed in much greater detail eventually. The bigger news, however, is that they are there. Just as the discovery of exoplanets is becoming normal and routine, their reveal less ground-breaking and statistical, a whole new type of astronomical body has been found. Be it decades or centuries in the future, the detection of these bodies, too, will become routine. Just in time, perhaps, for a new type of astronomical body to be discovered.