The odd interstellar object that zoomed through the inner solar system last fall might be an alien spacecraft with a light sail, a new study suggests.
Indeed, an artificial origin would explain quite nicely the many weird characteristics of that cosmic visitor, which is known as 'Oumuamua, study co-author Avi Loeb said.
And Loeb said his colleagues shouldn't dismiss this hypothesis out of hand just because it invokes intelligent aliens. ['Oumuamua: Solar System's 1st Interstellar Visitor Explained in Photos]
"That is a prejudice that we shouldn't have," Loeb, who chairs the astronomy department at Harvard University and directs the Institute for Theory and Computation (ITC) at the Harvard-Smithsonian Center for Astrophysics, told Space.com. "Science should be open-minded."
The weirdness of 'Oumuamua
'Oumuamua was discovered on Oct. 19, 2017, by astronomers using the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) in Hawaii. (The object's name means "messenger from afar" in Hawaiian.)
'Oumuamua's trajectory indicated that it's not from around here, making it the first confirmed interstellar object observed in our solar system. And 'Oumuamua is unique in other ways as well — in its shape, for example.
Observations of 'Oumuamua indicate that object is extremely long and narrow, measuring perhaps 1,300 feet long by 130 feet wide (400 by 40 meters). No other known cosmic object exhibits such a needle-like form.
Nobody knows for sure what 'Oumuamua looks like or how big it is. Astronomers weren't able to capture a resolved image of the object, and estimates of the visitor's size are based on its brightness (and a working knowledge of the reflectivity of comets and asteroids).
It's been tough to nail down what 'Oumuamua actually is, too.
Scientists initially regarded the object as a comet, but they reclassified it as an asteroid after observations failed to reveal a tail or fuzzy coma. But earlier this year, a team of researchers once again branded 'Oumuamua as a comet after noticing that it displays nongravitational acceleration. That's motion that can't be attributed to the tug of the sun, planets or other solar system objects and therefore (according to the authors of that previous study) probably resulted from cometary outgassing.
That brings us to the new study, which Loeb conducted with lead author Shmuel Bialy, a postdoctoral fellow at the ITC. Bialy and Loeb don't think the comet explanation holds up, because astronomers still haven't spotted a tail or coma around 'Oumuamua.
In addition, outgassing would have altered 'Oumuamua's rotation period, an effect that "would have been easy to identify," Loeb said. "But we haven't seen such a change."
There's another possible natural explanation, he added. If 'Oumuamua is just a fragment of a larger object, the observed motion could be from the "kick" that knocked that fragment loose. But such a kick would be a one-time thing, whereas 'Oumuamua has shown repeated nongravitational acceleration, Loeb said.
So, Bialy and Loeb came up with a new hypothesis: Sunlight may be responsible. In the new study, the duo determined that solar radiation pressure could cause the observed nongravitational movement if 'Oumuamua is just 0.3 to 0.9 millimeters thick.
What kind of structure could be so gossamer-thin? Bialy and Loeb suggested one candidate: a light sail, which is designed to harness the momentum of photons as a propulsive force. Humanity has already demonstrated this tech in space; Japan's IKAROS probe successfully trekked to Venus' neighborhood in 2010. (NASA's NanoSail-D2 and The Planetary Society's LightSail craft zipped around Earth in 2011 and 2015, respectively, but their missions primarily involved proving out sail deployment.)
Bialy and Loeb further calculated that a light-sail 'Oumuamua could probably survive a long interstellar trek. Given the concentration of gas and dust in the interstellar medium and the speeds at which the object would encounter these deep-space flecks, the researchers determined that 'Oumuamua could make it at least 16,000 light-years from its home system (whatever that may be — 'Oumuamua's provenance remains a mystery).
"This is good news for SETI," Loeb said, referring to the search for extraterrestrial intelligence.
"This is not speculation," he added. "It's evidence-based; we have an anomaly in the data. The anomaly is that the orbit deviates from a Keplerian orbit — deviates from what gravity alone would do. And the [viable] explanations for that do not exist, other than the one that we are proposing." [13 Ways to Hunt Intelligent Aliens]
Space junk or spy craft?
To be clear, Bialy and Loeb aren't claiming in this study that 'Oumuamua is definitely an alien spacecraft. But they do think this is a real possibility.
The object could be a piece of alien space junk, a defunct sail craft that has accidentally found its way to us. Or it may be an active reconnaissance craft sent here to check out our solar system, the researchers wrote in the study.
There are reasons to favor the latter explanation, Loeb said. For starters, 'Oumuamua's very discovery was something of an anomaly. If the visitor is a member of a random population, stumbling across it the way we did suggests that every star in the Milky Way ejects into interstellar space 1,000 trillion such objects over its lifetime, Loeb said. (These numbers are based on calculations performed by Loeb and his colleagues a decade ago.) Our own solar system doesn't shed so many 'Oumuamua-like objects, he added.
In addition, 'Oumuamua's motion is roughly coincident with the average motion of material in our patch of the Milky Way galaxy — a kinematic space known as the local standard of rest. Indeed, that's why the object's system of origin has been so hard to pin down.
"If I created an instrument whose identity I would like to hide, I would put it at the local standard of rest," Loeb said.
Astronomers have used the Green Bank Telescope in West Virginia and the Allen Telescope Array in Northern California to listen for any possible pings coming from 'Oumuamua. To date, the object has remained silent.
Getting an answer
'Oumuamua is cruising toward the outer solar system so fast that we'll never catch up to it using traditional chemical rockets. But that doesn't mean we could never get an up-close look at the object.
If we figure out how to accelerate tiny, sail-equipped robotic probes to tremendous speeds using lasers, we could eventually launch a mission to 'Oumuamua, Loeb said. Projects such as Breakthrough Starshot aim to launch such craft in the next few decades. (Loeb chairs Breakthrough Starshot's advisory board.)
Regardless of whether we ever manage to chase 'Oumuamua down, we should take a lesson from its visitation and be ready to mount an all-out observational blitz the next time such an object passes through our neighborhood, Loeb said.
And such a detection could come relatively soon. The powerful Large Synoptic Survey Telescope, which is scheduled to come online in the early 2020s in the Chilean Andes, may well turn up another 'Oumuamua or two, Loeb said.
The example provided by 'Oumuamua should also inspire astronomers to take a harder look at other intriguing objects that are in view right now, said Douglas Vakoch, president of the San Francisco-based nonprofit METI (Messaging Extraterrestrial Intelligence).
As an example, he pointed to KIC 8462852, also known as Tabby's Star or Boyajian's Star, after the researcher who noticed its bizarre and dramatic dimming events a few years ago.
"It's exciting to imagine that 'Oumuamua is an alien solar sail darting through our solar system, but we're lacking the one critical feature that SETI scientists demand. We can’t replicate the observations, because 'Oumuamua is already beyond the reach of our best telescopes," Vakoch told Space.com via email.
"Astronomers have emphasized the importance of gathering more data as they've tried to understand the weird dimming of KIC 8462852, without positing the existence of Dyson Spheres," he added. "But there's one critical difference between Boyajian's Star and 'Oumuamua. One of them is still visible, and the other has disappeared."
The new study will be published next week in The Astrophysical Journal Letters. You can read the paper for free at the online preprint site arXiv.org.
Mike Wall's book about the search for alien life, "Out There," will be published on Nov. 13 by Grand Central Publishing. Follow him on Twitter @michaeldwall. Follow us @Spacedotcom or Facebook. Originally published on Space.com.