Opinion There are interstellar intruders in our solar system. But are they aliens?
ATLAS was deemed to move faster than Oumuamua or Borisov — other recent ‘trespassers’ — and was clearly unbound to the solar system
Since its discovery in July, 3I/ATLAS has fascinated researchers as a confirmed interstellar object, likely originating from a distant, unidentified stellar system in the Milky Way. (Image: NASA) A prowler alert has been sounded by astronomers. An object has arrived, it seems, from somewhere in interstellar space, outside the solar system. It came closest to the Earth last December before swinging away in a different direction than it came from. A few astronomers have even raised an alarm that the object may actually be a spaceship controlled by aliens. Apparently, they have us earthlings in their crosshairs. And this is not the first time. There have been three such sightings of interstellar intruders in the last five years. The internet has naturally exploded with memes of invading extra-terrestrials. The theme of infiltrators that dominates our collective psyche these days has thus acquired a deep space twist.
The first trespasser was discovered in November 2017, when a set of telescopes in Hawaii designed to detect near-Earth objects found something unusual. An object was moving way too fast to be kept a slave to the Sun’s gravity and was seen in the wrong part of the sky. It was also on a path that was sharply inclined to the plane in which most objects in the solar system operate. After swinging around the Sun, it was flung out in a path that was unlike that of a typical orbiting comet. It was heading towards somewhere outside the solar system, having come from deep interstellar space. Astronomers named it “Oumuamua”, which in the Hawaiian language means “distant messenger”.
Its shape was also rather strange. Judging from the variation of its brightness with time as it tumbled through space, astronomers first thought Oumuamua had the shape of a long cigar, then concluded it was more like a pancake. Its jerky movement was reminiscent of comets, which often get pushed by jets of vapour created by the heat of sunlight on icy comets. But Oumuamua didn’t have a fuzzy halo around it as comets usually do. Astronomers surmised that it could be a cometary object from a distant planetary system, and had lost much of its gas during its long journey through interstellar space.
How long has it been hurtling through space? The fact that it was still bright made some astronomers hypothesise that Oumuamua was mostly made of nitrogen. Some objects at the edge of our own solar system, like Pluto, are nitrogenous. Maybe Oumuamua was a piece of one such peripheral icy object in a distant system, created in a collision and hurled into deep space as a result. Theorists tried to estimate its age by considering the ill effects of dangerously energetic particles in space (cosmic rays) impinging on the object. According to them, Oumuamua was rather “young”, having been catapulted from its star system not earlier than two billion years ago. Had it been moving in deep space for a longer time than this, it would have lost its nitrogen-bearing outer layer.
Some scientists thought otherwise. The probability of such a nitrogen-bearing object was too low, according to them. On the contrary, they claimed that Oumuamua’s weird shape, movement, and trajectory could be explained if it was piloted by aliens. But there was no radio signal that astronomers could detect from it, as would have been expected in the case of its being a spaceship. Unfortunately, there was no time to gather more data as Oumuamua was already on its way out.
Then, a Russian amateur astronomer, Gennadiy Borisov, discovered a second encroaching object in August 2019. It was also moving faster than objects bound within the solar system, and its trajectory clearly pointed towards its origin being in interstellar space. “Borisov” was more or less spherical in shape, with a cometary halo around it, and astronomers gathered from the analysis of its light that although the object contained a rather large amount of carbon monoxide and carbon dioxide, it did carry some water ice as well. The proportion was rare but not outright impossible for a comet-like object.
During the Covid lockdown, in March and April 2020, astronomers took a peek at it with the Hubble Space Telescope and saw Borisov being fragmented, with a piece breaking off. This is typical of comets, which are nothing but balls of ice. When sunlight falls on them, the ensuing heat often makes chunks of ice fall apart from the main body.
The old discussion of aliens came back to haunt the scientific sphere after the discovery of the third object in July 2025. Dubbed “ATLAS” after the name of the array of telescopes with which it was spotted, it was deemed to move faster than Oumuamua or Borisov and clearly unbound to the solar system. It went past the Earth on December 19 — missing it by a whopping 270 million km, more than twice the distance between the Earth and Sun. In other words, ATLAS was “behind” the Sun during the closest encounter with the Earth. Next March, it is going to travel past Jupiter, before dashing out of the solar system.
Although it doesn’t have the signature tail of a comet, it is surrounded by a comet-like glow. The spectrum of its light indicates that ATLAS has a larger amount of carbon dioxide compared to water than typical comets in our solar system do. It implies that ATLAS originated in a region of a distant planetary system where the temperature is lower than -200 degrees Celsius, where dry ice can dominate over water ice. Its eerie greenish glow of ATLAS, due to the presence of nickel vapour, has been the subject of much animated discussions, although it is not unusual in the world of comets.
What has, however, cranked up the rumours involving aliens is the sighting of a tail in a direction (towards the Sun) opposite to what is usually found in comets. The claim is that such “anti-tails” cannot but be the evidence of a spaceship doing some suspicious maneuvers. But it turns out that such anti-tails are not impossible to explain in terms of natural phenomena, and have been occasionally sighted in comets.
For most astronomers, though, such intruders from deep space allow them to study distant planetary systems without having to travel far. It gives them a chance to test the theories of comet formation, and the extent of variations in their compositions, as well as probe the environment outside our solar system. These interstellar objects need not be a cause for alarm, but are rather welcome visitors for our better understanding of the cosmos.
The writer is Visiting Professor, IISER Mohali was earlier an astrophysicist at the Raman Research Institute, Bengaluru
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