The radio signals could be coming from another object in the telescope’s field of view of Ross 128, but astronomers haven’t observed many other celestial bodies there.

They could have been emitted by a passing satellite, a common occurrence in stellar observations. But the astronomers say they have never seen satellites release signals of this nature.

The signals could be the product of some kind of interaction between the star and an orbiting planet. The Arecibo Observatory astronomers believe the interplay between closely orbiting planets and the star’s magnetic field could produce tiny changes in its radio emissions. The Arecibo Observatory actively studies seven red dwarfs, two of which have known planets. But so far, no planets have been discovered around Ross 128.

The signals could be the result of radio frequency interference, a common culprit in the search for extraterrestrial life. The source could be something as small as a cell phone. “There are lots and lots of ways for our technology here on Earth to leak into our radio telescope observations,” says Andrew Siemion, the director of the Berkeley SETI Research Center who leads the center’s Breakthrough Listen Initiative, a project aimed at finding evidence of extraterrestrial civilizations.

But the Arecibo astronomers don’t think that’s it. “We believe that the signals are not local radio frequency interferences since they are unique to Ross 128 and observations of other stars immediately before and after did not show anything similar,” Abel Méndez, the director of the Planetary Habitability Laboratory at the Arecibo Observatory, wrote last week.

Which brings us, finally, to what astronomers believe is the least likely explanation: aliens. “That’s a remote possibility,” Siemion said. “But at this point, it is indeed a possibility.”

Astronomers must first rule out the aforementioned theories, all of which they say are far more plausible. “The recurrent aliens hypothesis is at the bottom of [the list of] many other better explanations,” Méndez wrote in his post.

So, what’s next? Astronomers must spend more time observing Ross 128. The Arecibo Observatory scanned the star again on Sunday, and will announce any new findings this week. The Breakthrough Listen team, acting on the Arecibo astronomers’ request for help, observed Ross 128 on Sunday using the Green Bank radio telescope in West Virginia. Siemion said there was no evidence of the emission Arecibo found back in May, but added that their findings are not conclusive.

Astronomers could try to come up with a theoretical model that could replicate the mysterious radio signals, or attempt to observe Ross 128 with optical telescopes.

Red dwarfs like Ross 128 are particularly interesting targets in the search for extraterrestrial life. Their small size and luminosity make it easier for telescopes to spot objects that could be planets as they pass in front of them. Data from the Kepler Space Telescope, the premiere exoplanet-hunting mission, suggest many red dwarfs may have planets, some of which could be orbiting in the habitable zone, where temperatures are just right for liquid water to pool on the surface. Last year, astronomers discovered a planet orbiting in the habitable zone of Proxima Centauri, a red dwarf and the closest star to Earth, located about four light-years away.

For now, astronomers are just as in the dark as the rest of us about Ross 128. On Monday, Méndez tweeted a link to a survey that asked users to pick which source they believe explains the mysterious phenomenon.