The space visitor that came from another star

The very first known Interstellar Object (ISO) to visit our Solar System was a rocky, elongated interloper with a slightly reddish hue. Spotted in 2017, it was dubbed with a Hawaiian name and was almost 10 times as long as it was wide, an extremely unusual shape for objects in our Solar System. Astronomers hoped it could provide some clues into how other star systems form and function.

For hundreds of millions of years longer than I've been around, this bizarre guest has been wandering through our home Milky Way galaxy, not bothering to settle down in any star system, until it came across ours. After the space traveler was discovered, hundreds and probably even thousands of telescopes all over the world, including ESO's Very Large Telescope in Chile, sprang into action. They started measuring the object's orbit, its color, and brightness, and that's when it became apparent that the space rock had an unusual orbit. Some astronomers suggested that 'Oumuamua was emitting hydrogen it had picked up during its journey between stars, which was a simple explanation for the mystery that had evoked lots of outlandish theories.

When this bizarre guest was first spotted upon entering the Solar System, scientists were not sure what its nature was. Some claimed it was a comet, while others disagreed, saying it didn't have the features typical for comets, such as a visible long tail and a coma (a cloud of gases surrounding the nucleus of a comet). Plus, its shape was different from that of other comets. The only thing that made 'Oumuamua more comet-like was the way it accelerated as it went away from the Sun. The unusual object started to slow down on its way out, but in a strange way, as if not only gravity was in play. There seemed to be something creating a force to counter this gravity, but unfortunately, this theory didn't quite fit either.

The problem is that comets usually have large quantities of water ice on their surface, and as the Sun heats this ice, it gets ejected as jets of gas. Those jets act as many rocket boosters, but 'Oumuamua not only had no tail whatsoever, but it was also too small to capture enough solar energy to support this kind of activity. Of course, this mysterious space visitor caused some more outlandish theories. For example, some people started to claim that 'Oumuamua could be a spacecraft sent by a civilization living in another star system. However, scientists found a better explanation. A comet that is traveling between stars gets cooked by cosmic radiation. These rays penetrate thick layers of ice, converting up to 25 percent of water molecules into molecules of hydrogen. Then, this trapped hydrogen gets released when some star warms the comet. The effects of this process are almost invisible, which might be the reason why we didn't see a spectacular tail accompanying 'Oumuamua.

At the same time, the potential comet was so small that this could produce enough force to power its acceleration. As for the amount of ice released as 'Oumuamua was coming closer to the Sun, it was likely too small for astronomers to spot.

Now, even though 'Oumuamua was the first known interstellar object to enter our Solar System, it wasn't the last. In August 2019, Comet 2I/Borisov visited us, becoming the second ISO astronomers managed to spot. Now, no one can argue that there simply must be more visitors from far away star systems than those two. ISOs are rare, but our Solar System is pretty old, it must have been capturing some interstellar travelers over the millions and millions of years of its existence, even though they never stayed for very long.

One study has taken a closer look at interstellar objects, and they concluded that these space travelers might be caught not in solar orbits, but in near Earth orbits. The astronomers working on this project even go as far as to claim that there might be a lot of ISOs in orbit around Earth. Now, finding tiny objects in the vastness of the cosmos is extremely tricky. Think about it: with images of distant galaxies and nebulae, it's like looking for a needle in a haystack.