In 1967, the young Irish student Jocelyn Bell detected a strange signal in the records of the radio telescope with which he was working. The signal, a series of short pulses that repeated every 1.3 seconds, appeared each day at the same time according to star time and not civil time, indicating that it was coming from outer space. However, the interval of 1.3 seconds between pulses was much shorter and more regular than that of other known pulsating sources. She had never detected anything similar coming from the sky and of course, she admits it herself, the idea 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 filled with 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: the 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 only see intermittently, like the light from a lighthouse on the coast.
Sherlock Holmes, the famous fictional detective created by the British author Arthur Conan Doyle, used to say: “When you have eliminated the impossible, what remains, however improbable, must be the truth.” This character, famous for his use of deductive thinking, made a logically correct statement: the improbable outweighs the impossible in statistical terms. But it is exactly this illusion of logic that makes Sherlock’s argument so misleading, and by far the most common cause of false advertisements in the press. The universe is stranger than we can imagine and it is arrogant to assume that we know all the possible non-extraterrestrial phenomena that could lead to a particular observation.
A great example of this is Oumuamua, that strange elongated object that walked through our solar system a few years ago. Oumuamua appeared in the sky as a faint, throbbing, reddish spot. It was not a normal object. It was 400 meters long and 10 times less wide and it rotated at high speed. Those who first observed it, astronomers from the University of Hawaii, gave it its name, which means “messenger from afar who 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 the Kepler’s second law. This behavior is also observed in comets due to the phenomenon known as degassing: the sun’s heat melts their ice 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 the radiation of the solar wind. However, scientists from 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, expelled into interstellar space by an impact. Pluto also has a red tint due to radiation that processes its surface methane into hydrocarbons and could be accelerated like comets thanks to the outgassing of nitrogen ice, which is 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 star is another good example of this. Discovered by Tabitha Boyajian in 2011, this star showed some crazy variations in its brightness that had never been seen before and for which no logical explanation could be found. After discarding some hypotheses, 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 phenomenon easily explained the observations and it had a long way to go. The largest radio telescopes around the world pointed in Tabby’s direction and, of course, the antennas of the SETI. Since then, new data has 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 caused by cosmic dust, which lets through more red light than blue. There are still many things to explain, but current hypotheses point to dust from a recently destroyed planet or satellite as the most likely cause.
As all this sounds a bit spoilsport, we are also going to see some indications that do not yet have satisfactory natural explanations. Many will have heard of the wow sign. This was a strange peak of radio emission detected in August 1977 by the Bigear, The Ohio State University Radio Telescope. No simple explanation for this signal has emerged, but a similar signal has never been seen since. Or if? On April 29, 2019, the radio telescope Parks was pointing towards Proxima Centauriour closest stellar neighbor, when it detected a peak of emission at a frequency of 982.002 MHz that moved towards higher frequencies for three hours.
This signal went unnoticed for more than a year, until a student working on the search for extraterrestrial life project Breakthrough Listen, 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 thanks to its movement in frequency since it is only seen when the telescope points towards Proxima and not when it points elsewhere. This is especially exciting because Proxima Centauri has a planetary system that includes a planet of similar mass to Earth that is just the right distance from Proxima for liquid water to exist on its surface. If the increase in frequency indicates an approach of the signal, are we seeing 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, making it unlikely that a planetary atmosphere would survive its outbursts. Furthermore, the signals are spaced at regular frequency intervals and these appear to be multiples of frequencies commonly used in oscillators in our electronic devices. This tips the scales back to the boring 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 strange.
The fact is that the observation of the universe always surprises us with new phenomena and, although the alien idea always breaks out, in all cases a natural responsible party is found. Does this mean that we can never conclude the existence of aliens based on any observation? When can we believe that a new clue is an unequivocal detection of extraterrestrial life? Believe me, if it’s about aliens, the evidence will mount. Most scientists want them to be aliens, more than anyone else, and they will be the first to glee when that is the most likely interpretation of the data. Right now it is not, but millions of dollars, hours of work and a lot of effort are dedicated to trying to make it so and finally being able to announce that we are not the only intelligent inhabitants in all of space-time.
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