It’s a long shot, but scientists are about to listen very closely for radio signals from our solar system’s first known interstellar visitor
Ever since its discovery in mid-October as it passed by Earth already outbound from our solar system, the mysterious object dubbed ‘Oumuamua (Hawaiian for “first messenger”) has left scientists utterly perplexed. Zooming down almost perpendicularly inside Mercury’s orbit at tens of thousands of kilometers per hour—too fast for our star’s gravity to catch—‘Oumuamua appeared to have been dropped in on our solar system from some great interstellar height, picking up even more speed on a slingshot-like loop around the sun before soaring away for parts unknown. It is now already halfway to Jupiter, too far for a rendezvous mission and rapidly fading from the view of Earth’s most powerful telescopes.
Astronomers scrambling to glimpse the fading object have revealed additional oddities. ‘Oumuamua was never seen to sprout a comet-like tail after getting close to the sun, hinting it is not a relatively fresh bit of icy flotsam from the outskirts of a nearby star system. This plus its deep red coloration—which mirrors that of some cosmic-ray-bombarded objects in our solar system—suggested that ‘Oumuamua could be an asteroid from another star. Yet those same observations also indicate ‘Oumuamua might be shaped rather like a needle, up to 800 meters long and only 80 wide, spinning every seven hours and 20 minutes. That would mean it is like no asteroid ever seen before, instead resembling the collision-minimizing form favored in many designs for notional interstellar probes. What’s more, it is twirling at a rate that could tear a loosely-bound rubble pile apart. Whatever ‘Oumuamua is, it appears to be quite solid—likely composed of rock, or even metal—seemingly tailor-made to weather long journeys between stars. So far there are few if any wholly satisfactory explanations as to how such an extremely elongated solid object could naturally form, let alone endure the forces of a natural high-speed ejection from a star system—a process thought to involve a wrenching encounter with a giant planet.
These bizarre characteristics have raised eyebrows among professional practitioners of SETI, the search for extraterrestrial intelligence, who use large radio telescopes to listen for interstellar radio transmissions from other cosmic civilizations. If ‘Oumuamua is in fact artificial, the reasoning goes, it might be transmitting or at least leaking radio waves.
So far limited observations of ‘Oumuamua, using facilities such as the SETI Institute’s Allen Telescope Array, have turned up nothing. But this Wednesday at 3 p.m. Eastern time, the Breakthrough Listen project will aim the West Virgina-based 100-meter Green Bank Telescope at ‘Oumuamua for 10 hours of observations in a wide range of radio frequencies, scanning the object across its entire rotation in search of any signals. Breakthrough Listen is part of billionaire Yuri Milner’s Breakthrough Initiatives program, a collection of lavishly-funded efforts aiming to uncover evidence of life elsewhere in the universe. Other projects include Breakthrough Starshot, which intends to develop and launch interstellar probes, as well as Breakthrough Watch, which would use large telescopes to study exoplanets for signs of life.
“With our equipment at Green Bank, we can detect a signal the strength of a mobile phone coming out of this object,” Milner says. “We don’t want to be sensational in any way, and we are very realistic about the chances this is artificial, but because this is a unique situation we think mankind can afford 10 hours of observing time using the best equipment on the planet to check a low-probability hypothesis.” Besides being simply a search for signs of aliens, Breakthrough Listen’s efforts could also narrow down the possibilities for ‘Oumuamua’s composition by looking for signs of water vapor sublimating from any sun-warmed ice lurking beneath the object’s red, desiccated surface.
Avi Loeb, an astrophysicist and Breakthrough advisor at Harvard University who helped persuade Milner to pursue the observations, is similarly pessimistic about prospects for uncovering aliens. There are, he says, arguments against its artificial origins. For one thing, its estimated spin rate seems too low to create useful amounts of “artificial gravity” for anything onboard. Furthermore, ‘Oumuamua shows no sign of moving due to rocketry or other technology, instead following an orbit shaped by the gravitational force of the sun. Its speed relative to the solar system (about 20 kilometers per second) also seems rather slow for any interstellar probe, which presumably would cruise at higher speeds for faster trips between stars. But that pace aligns perfectly with those of typical nearby stars—suggesting ‘Oumuamua might be merely a piece of galactic “driftwood” washed up by celestial currents.
Then again, Loeb says, “perhaps the aliens have a mothership that travels fast and releases baby spacecraft that freely fall into planetary system on a reconnaissance mission. In such a case, we might be able to intercept a communication signal between the different spacecraft.”
Several years ago Loeb and two colleagues performed a speculative calculation estimating the interstellar abundance of ‘Oumuamua-sized space rocks based on the density of stars in the Milky Way and the vagaries of planet formation. That calculation, Loeb says, suggests the number of such space rocks is at least a hundred thousand times too low to account for ‘Oumuamua’s detection. Simply put, objects like ‘Oumuamua should be far too rare for our current telescopes to have any reasonable chance of spotting one. Newer studies gauging the odds find that for ‘Oumuamua’s detection to not be an astronomically unlikely fluke, there must be a sizeable population of such objects continuously passing through our solar system. This in turn suggests that more-capable future observatories, such as the Large Synoptic Survey Telescope, will find many more when they begin operations in the 2020s.
“Typically in astronomy we don’t see things that are rare—if we see one, that means there’s a lot more out there,” says Breakthrough Listen’s lead scientist Andrew Siemion, who is also director of the Berkeley SETI Research Center. “So, while this is most likely a natural object, if we don’t eventually see any more, that would indeed be very strange and would increase interest from a SETI perspective.”
Either way, Siemion says, “‘Oumuamua’s presence within our solar system affords Breakthrough Listen an opportunity to reach unprecedented sensitivities to possible artificial transmitters and demonstrate our ability to track nearby, fast-moving objects. Whether this object turns out to be artificial or natural, it’s a great target.”
And if, against all odds, the Green Bank Telescope detects signals from this mysterious interstellar interloper—what happens then? Breakthrough Listen’s leaders assure us they would keep no secrets. First, the team at Green Bank would immediately re-observe ‘Oumuamua to confirm the signal. Next, they would reach out to astronomers around the world who could target the object with other radio telescopes. “We quite literally have a little Rolodex just for that,” Siemion says. “And at that moment this would become public. There’s no way to keep something like this a secret, because it requires us calling everyone we can. We tend not to ‘cry wolf’ about these things.”