Almost 9 years ago, the first object from another solar system was discovered passing through our solar system. Astronomers called it 1I/Oumuamua (or just Oumuamua). The recognition that this was a chunk of something from somewhere outside the Sun’s family of planets, asteroids etc. was big news. Advances in telescope technology are what gave us the discovery, allowing us to track a faint, fast moving object like Oumuamua as it entered and left the solar system.

Astronomers expected the existence of these kinds of interlopers for a long time. Comets are muddy snow mountains left over from the birth of a solar system. They can easily get kicked out of a star’s gravitational grip (either by other, passing stars or by gravitational interactions with giant planets within a solar system). Once blown into interstellar space, a wandering alien comet might pass close to our Sun.  If that happens the frozen mud ball can get pulled into a new trajectory that passes through our little corner of the galaxy. So, while astronomers expected to eventually find a comet from another solar system, the discovery of Oumuamua was still a delightful surprise and big news.

But Oumuamua made even bigger news after astronomer Avi Loeb went hard on the claim that it wasn’t a comet but an “interstellar artifact” i.e an alien spaceship. The researchers who study comets for a living were not impressed with Loeb’s arguments, however.  As of today, almost no one other than Prof Loeb thinks Oumuamua was a technosignature - a marker of extraterrestrial intelligence.

At some point I’ll write a longer post about Oumuamua as a technosignature but today the big news is another solar system interloper has been found. 

Astronomers recently announced they’d discovered 3I/ATLAS, a 10-kilometer object hurtling through space at 60 km/s (that's 134,000 mph).  Most important, ATLAS is moving on a path taking it through the solar system. It’s another alien visitor!  

Note the “3” in the name. 3I/ATLAS is actually the 3rd passing object we’ve found. The second, 2I/Borisov was discovered in 2019. Everyone agreed Borisov was a comet. Will ATLAS hold any surprises for us? Already it appears to be different in appearance from both Oumuamua and Borisov (but still seems comet-like). Stay tuned because this story will unfold over the next few months. 

Here is the important thing. 3I/ATLAS doesn’t have to be a spaceship for it to teach us really cool science. Every one one of these visitors has a story to tell about the birth of stars and planets (but still, a spaceship would be awesome).