Our first view of a rock from beyond our solar system was fleeting. Astronomers spotted the object, later called 鈥極umuamua, in October 2017, and it was quickly too distant for Earth-based telescopes to spot.
In late November 2017,聽researchers pointed the Spitzer Space Telescope towards where 鈥極umuamua should have been and stared for more than 30 hours. It saw nothing.
Immediately, the researchers wondered whether that was because of 鈥極umuamua鈥檚 strange acceleration 鈥� astronomers using the Hubble Space Telescope had found that it wasn鈥檛 just passively passing through the solar system, but聽something was pushing it.聽Many researchers agree that this unexpected boost was likely due to ice on 鈥極umuamua鈥檚 surface turning into gas as it passed close to the sun.
Advertisement
So when Spitzer looked at 鈥極umuamua, it wasn鈥檛 quite where the researchers had expected it to be when they planned the observations 鈥� the Hubble discovery came too late for them to change the plan. The telescope鈥檚 field of view聽should have聽been big enough to capture the object鈥檚 altered orbit anyway, but the interstellar visitor turned out to be too dim to spot.
Bad news and good news
鈥淭he fact that we didn鈥檛 see anything was disappointing, but Spitzer did an amazing job,鈥� says team member Carey Lisse at Johns Hopkins University in Maryland. 鈥淭his is the most significant non-detection paper I鈥檝e ever been a part of.鈥�
That鈥檚 because Spitzer鈥檚 observations were so sensitive that the researchers could set hard limits on the properties of 鈥極umuamua based on the fact that we couldn鈥檛 see it. 鈥淚t鈥檚 either聽smaller than we thought or more highly reflective than we thought,鈥� says David Trilling at Northern Arizona University, who led the observational team. They found that it likely has an average diameter less than 140 metres and is brighter than most asteroids and comets.
The observations also placed limits on the gas coming off of the object 鈥� it was releasing almost no carbon monoxide or carbon dioxide, which is unexpected based on what we know about comets in the solar system.
Added to what we already know about 鈥極umuamua 鈥� it likely has an elongated shape and is tumbling end-over-end 鈥� these findings make it a very strange object indeed.
鈥淏eing peculiar on so many counts says to me that we really don鈥檛 understand something fundamental about it,鈥� says Avi Loeb at Harvard University, who has previously suggested 鈥極umuamua could be a discarded part of an alien spaceship, a view generally not shared by other astronomers. Its very strangeness is what makes our limited data on it so frustrating, he says.
鈥淲e鈥檙e still struggling as a community to create a common story that isn鈥檛 completely speculative but explains all the weird things about this body,鈥� says Trilling. 鈥淭he path to unravelling the mystery is much less clear to me here than it is in any other part of astronomy and astrophysics.鈥�
Long-term visitors
One possible help, Loeb says, is the idea that if many interstellar objects pass through our solar system, as statistical studies suggest that they must, some probably get trapped here for millions of years.
He and Amir Siraj, also at Harvard have identified four asteroids on strange tilted orbits that may also be from outside our solar system. The pair traced these suspect space rocks by simulating the potential orbits of trapped interstellar objects and then comparing them to actual asteroid orbits.
If these are truly interstellar, they might be a treasure trove of information. 鈥淭he problem with Oumuamua is that it moves faster than any chemical rocket that we can launch, so we cannot catch up with it,鈥� he says. 鈥淔or a bound object, you have all the time in the world.鈥�
Even if those asteroids are not from afar, new telescopes and lessons from observing 鈥極umuamua mean that we will hopefully be able to get more information when another interstellar rock comes passing through. 鈥淲hen another one comes, whether it鈥檚 in a year or in ten years, we鈥檒l be ready,鈥� says Trilling. Nevertheless, 鈥極umuamua will always be the one that got away.
References: ,
