In December, astronomers checked every inch of an interstellar object that had entered our solar system for evidence of artificial technology. They spent weeks pouring over the data, looking for radio signals that would suggest the object, known as ‘Oumuamua, may be something other than a just a strange space rock.
They didn’t find anything.
The Breakthrough Listen Initiative, a $100 million effort in the search for intelligent extraterrestrial life, did not detect radio emissions from the object, according to a new paper published on arXiv, a repository for papers approved for future publication in journals, on Tuesday. ‘Oumuamua caught everyone by surprise in October as the first known interstellar object to be spotted in our solar system.
“From the start, we knew it would be a long shot, like any other SETI experiment,” said Emilio Enriquez, an astronomy Ph.D. student on the Breakthrough Listen team and the lead author of the paper.
The decision to check ‘Oumuamua for artificial technology came from Yuri Milner, the Russian billionaire and tech investor who established and is funding Breakthrough Listen. Astronomers released their first results from the observations in mid-December, based on an analysis of just one chunk of the radio data. “Indeed, nothing has popped up, but we’re busy churning through the data we’ve collected so far,” Andrew Siemion, the director of the Berkeley SETI Research Center who leads its Breakthrough Listen Initiative, said at the time. The latest paper includes the analysis of the full dataset. Still nothing.
The new paper also reports the team found no evidence of water on ‘Oumuamua, like other groups of astronomers studying the object. (Some suspect ‘Oumuamua does harbor water in the form of ice hidden deep under the crust.)
Breakthrough Listen’s data comes from observations by the Green Bank Telescope, a steerable radio telescope in West Virginia. The telescope was prepared to detect a signal, if it existed, similar to the radio waves coming from a cellphone. For eight hours in December, the GBT observed ‘Oumuamua across four bands of radio waves. Breakthrough Listen usually observes targets in one radio band, Enriquez said, but for ‘Oumuamua, they widened the search parameters to include as many frequencies as existing technology allowed.
“As in any other SETI experiment, we have no prior knowledge of which frequency any civilization might be sending any kind of signal,” he noted. “So the idea is because we don’t know, basically we need to search all of the available frequencies.”
In addition, GBT rotated between four receivers every 30 minutes so that each one had a shot at observing ‘Oumuamua as the object completed one full spin, which takes about seven hours and 20 minutes. This swapping allowed astronomers to study every part of the object.
“Imagine there’s only one single antenna pointing in one single direction and you have these rotations,” Enriquez said. “It’s like a lighthouse. You need to wait until the lighthouse hits your direction.”
Enriquez said Breakthrough Listen currently has no plans for follow-up observations, but the team will continue to examine the results of the observations. Now that they’ve looked for continuous radio signals in the data, they’ll look next for pulsating signals.
When I asked whether they’re ready to call it—move along, nothing SETI to see here—Enriquez said such certainty isn’t quite possible, thanks to the nature of SETI experiments in general. Astronomers are limited not by the extent of their search, but by the capacity of current technology. The GBT and other radio telescopes are not able to study ‘Oumuamua in every frequency.
“We were not able to observe at other frequencies, so we don’t know if, for instance, that it might have been a signal that is lower than the frequency that we observed or higher than the frequency that we observed,” Enriquez said. “And unfortunately, that’s kind of the end of the experiment.”
Rumblings about ‘Oumuamua being a possible target for SETI observations started soon after its discovery in October, which was the first time humanity had spotted an object of its kind in our solar system. The more scientists learned about ‘Oumuamua, the weirder it seemed. The object appeared to be an asteroid and not a comet—the kind of object scientists had predicted would be most likely to become ejected from its own solar system, travel through interstellar space, and get deposited into ours. The shape of ‘Oumuamua—extremely elongated, like a cigar—was unlike anything they had seen, and would be difficult to create through natural, known processes of the universe.
In early December, Avi Loeb, the chair of Harvard’s astronomy department and an advisers on the Breakthrough Listen Initiative, took those rumblings to Milner. Loeb suggested ‘Oumuamua could be an artificial probe dispatched by an alien civilization into the cosmos. Milner was intrigued, and within hours the Breakthrough Listen team was preparing an plan for observations using the GBT.
At the time of Milner’s decision, the object was about two astronomical units (AU) from Earth, or about twice the distance between the Earth and the sun from our planet. At the time of this writing, the asteroid was about 2.23 AU from Earth, according to Karen Meech, an astronomer at the University of Hawaii Institute for Astronomy whose team discovered ‘Oumuamua.
As ‘Oumuamua speeds away from us, observations by even the most powerful telescopes are becoming more difficult. But the study of interstellar objects is just beginning. Scientists predict that as more telescopes like the one that detected ‘Oumuamua come online and search the skies, they will find more interstellar objects floating in the solar system—and more targets for the search for extraterrestrial life. Thanks to the data they got from ‘Oumuamua, they’ll be more prepared for the next one. “If you look more, everywhere, I think chances are that eventually you will find something,” Milner told me last month.
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