In 1967, young Irish student Jocelyn Bell detected a strange signal in the records of the radio telescope she was working with. The signal, a series of short pulses repeating every 1.3 seconds, appeared every day at the same time according to star time rather than civilian time, indicating that it was coming from outer space. However, the 1.3 second interval between pulses was much, much shorter and more regular than that of other known pulsed sources. Nothing similar had ever been detected coming from the sky, and of course, by her own admission, the thought crossed her mind: Had she detected signs of an alien civilization? Although neither she nor her mentor mentioned this idea in public, the press was full of green men trying to communicate with us.

A few months later, astronomers Thomas Gold and Fred Hoyle identified the origin of these signals, or at least offered a non-alien explanation for their origin: neutron stars. The existence of these objects had been predicted in 1933 by Walter Baade and Fritz Zwitcky as the final state in the evolution of massive stars. These objects emit radiation along their magnetic poles that, due to their rapid rotation, we see only intermittently, like the light from a lighthouse on the coast.

A great example of this is Oumuamua, that strange elongated object that wandered through our solar system a few years ago. Oumuamua appeared in the sky as a reddish, pulsating, very faint blob. It was not a normal object. It was 400 meters long and 10 times less wide and was spinning at high speed. Those who first observed it, astronomers from the University of Hawaii, gave it its name, which means “messenger from far away that arrives first” in Hawaiian. In addition to its shape, the strangest thing about it was its trajectory and its great speed, which increased as it moved away from the Sun, contradicting Kepler’s second law. This behavior is also observed in comets due to the phenomenon known as outgassing: the heat of the Sun melts its ices and produces gases that, when expelled, act as propulsion, creating the beautiful tails that characterize them. Oumuamua, however, had no tail.

A renowned Harvard astrophysicist, Avi Loeb, argued that Oumuamua was an alien artifact. In particular, a light sail propelled by solar wind radiation. However, scientists at the University of Arizona concluded, after analyzing it in detail, that it could be a fragment of a dwarf planet or asteroid similar to Pluto, ejected into interstellar space by an impact. Pluto also has a red tint due to the radiation that processes its surface methane into hydrocarbons and could accelerate like comets thanks to the outgassing of nitrogen ice, very abundant in this type of object. This outgassing would, however, be invisible. Again, this may not be the correct or complete explanation, but it makes it clear that there are perfectly plausible natural explanations for Oumuamua.

Tabby’s star is another good example of this. Discovered by Tabitha Boyajian in 2011, this star showed crazy variations in brightness that had never been seen before and for which no logical explanation could be found. After some hypotheses were discarded, some scientists suggested the presence of alien megastructures similar to those known as Dyson spheres or swarms, which surround the star to capture as much radiation as possible. This interpretation was based on the fact that no known natural phenomena easily explained the observations and had quite a run. The largest radio telescopes around the world pointed in the direction of Tabby and, of course, the SETI antennas. Since then, new data have revealed that the dimming does not affect all colors equally as we would expect in an opaque structure, and yet it is very similar to the dimming produced by cosmic dust, which lets through more red light than blue. Much remains to be explained, but current hypotheses point to dust from a recently destroyed planet or satellite as the most likely cause.

As this all sounds a bit of a spoilsport, let’s also look at some of the indications that still have no satisfactory natural explanations. Many will have heard of the WOW signal. This was a strange spike in radio emission detected in August 1977 by Big Ear, the Ohio State University radio telescope. No simple explanation for this signal has emerged, but neither has a similar signal ever been seen again. Or has it? On April 29, 2019, the Parkes radio telescope was pointing toward Proxima Centauri, our nearest stellar neighbor, when it detected a spike of emission at a frequency of 982.002 MHz that drifted toward higher frequencies for three hours.

This signal went unnoticed for more than a year, until a student, working on the Breakthrough Listen extraterrestrial life search project, reanalyzed the data. There are many radio sources in the sky, mostly man-made noise, but Breakthrough Listen has detailed protocols for filtering them out. This signal escaped the filters because of its frequency motion and because it is only seen when the telescope is pointed at Proxima and not when it is pointed elsewhere. This is especially exciting because Proxima Centauri has a planetary system that includes an Earth-like mass planet that is at the right distance from Proxima for liquid water to exist on its surface. If the increased frequency indicates a signal approach, are we looking at an invading fleet of Proximans? Well, there are very good reasons to think not. For example, Proxima Cen is a dangerously active red dwarf with multiple flares and coronal mass ejections, so a planetary atmosphere is unlikely to survive its outbursts. In addition, the signals are spaced at regular frequency intervals and these appear to be multiples of frequencies commonly used in the oscillators of our electronic devices. This again tips the scales to the dull side, even though the specific source of the interference could not be identified. On the other hand, that the signal is not detected when the telescope is not pointed at Proxima Centauri is certainly odd.

The fact is that the observation of the universe always surprises us with new phenomena and, although the alien idea always bursts in, in every case a natural culprit is found. Does this mean that we will never be able to conclude the existence of extraterrestrials based on any observation? When will we ever be able to believe that a new clue is an unequivocal detection of extraterrestrial life? Believe me, if it is extraterrestrials, the evidence will mount. Most scientists want it to be extraterrestrials, more than anyone else, and they will be the first to jump for joy when the most likely interpretation of the data is that. Right now it is not, but millions of dollars, hours of work and a lot of effort are devoted to try to make it so and to be able to announce, finally, that we are not the only intelligent inhabitants in all of space-time.