Astronomers have observed an object that looks like the interstellar object ‘Oumuamua – but unlike the first known interstellar visitor, this one has likely been an inhabitant of the solar system for billions of years.
When the interstellar object 1I / ‘Oumuamua was first sighted in the solar system in 2017, we had never seen anything like it. Among many inexplicable curiosities was its shape: with a 6:6:1 ratio, it’s basically an out-of-this-world pancake – unlike anything we’ve seen in the solar system.
But, after all, it may not be a class alone. At the Division of Planetary Sciences meeting in October, Ari Heinze (now at the University of Washington) reported preliminary results for an asteroid, the 2016 AK 193 , which behaves remarkably like the ‘Oumuamua: it topples like the interstellar object did, and it looks like be as elongated as.
But unlike ‘Oumuamua, this asteroid is not a newcomer; it has probably revolved around the Sun for billions of years. Time will tell whether its origins clarify the mysteries of ‘Oumuamua.
Astronomers observed ‘Oumuamua exiting the solar system at high speed (it had already rotated around the Sun when they spotted it), and its highly variable brightness immediately indicated two strange things: First, it was staggering instead of spinning on an axis. . Second, its shape has been surprisingly stretched.
The shape was a puzzle because up to that point, the longest asteroid astronomers have observed was 216 Kleopatra, which is shaped like a dog bone (it’s probably a contact binary) with a 3:1 ratio.
However, when Heinze added measurements from the 2016 AK 193 , taken using the Gemini Observatory in 2020, to a few days of observations made in 2016, he realized that what he had found was a staggering object approximately the same size as ‘Oumuamua, and possibly so elongated too.
“The ‘Oumuamua was a big mystery because there has literally never been anything like it before,” says Darryl Seligman (University of Chicago), who was not involved in the observations. “So if there’s anything close to that in the solar system, just from that point of view, it’s worth following up.”
Tracking will take some patience, however – the 2016 AK 193 is a close-to-Earth object, but that doesn’t mean it’s been close to Earth all the time. It won’t get close enough to look again until 2029, says Heinze. When that happens, it will approach 0.05 Earth astronomical unit, appearing with a magnitude of 18.
“Hopefully there will be some detailed studies then (eg radar imaging and a lot more photometry),” says Heinze, “but there’s very little prospect of useful additional measurements in the intervening seven years.”
Still, the mere existence of the asteroid is intriguing because it’s far from being interstellar – it’s probably been in the solar system for billions of years. The ‘Oumuamua, on the other hand, is probably younger, less than 100 million years old based on his arrival trajectory.
“Oumuamua’s age is subtly the strangest thing,” says Seligman. Objects in the galaxy orbit its center, and the older an object, the more likely it is that it gravitationally bounced off another object outside the galactic plane. The ‘Oumuamua, however, arose almost exactly along the galactic plane, which means that any theories about its nature and origin must also explain its youth.
In part, this is why many of the ‘Oumuamua’s explanations focused on exotic ices: Seligman himself speculated that it was made from hydrogen ice, recently released from an interstellar cloud. Others have suggested that it was made of nitrogen ice, cut from a Pluto-like body. The strong sunlight would eliminate such ice cream as a bar of soap in the shower, says Seligman, reshaping the body into a mere sliver of its former self.
It is also possible that the object contains more mundane carbon monoxide ice, such as distant solar system objects and such as 2I / Borisov, the second interstellar object. The observations did not find a coma near ‘Oumuamua, but as Seligman points out, “Comets turn on and off all the time.” But carbon monoxide ice doesn’t ablate as quickly, so that scenario doesn’t explain the shape either, he adds.
Future observations from the AK 193 2016 will help put these ideas to the test. Its current near-Earth orbit is unstable, Heinze notes, and will only last about 100 million years. It’s possible that this object belonged to the main belt before a gravitational encounter redirected it, and that push may even have precipitated its remodeling. More and better observations will help us understand your past.
While 2029 might seem like a long time to wait for the follow-up, it’s better than what we’ll get from ‘Oumuamua. It is already leaving the solar system and will pass through Neptune’s orbit next year. So far, no theory explains all our observations of this object. A solar system analogue might be as close as we can get.