ʻOumuamua
The account
The first interstellar object ever seen supposedly had a strange shape and unexplained acceleration: an alien solar sail.
What investigators found
The artificial hypothesis comes from astronomer Avi Loeb, of Harvard. Most scientists favor a natural origin, with gases escaping from the object. Other interstellar visitors have been detected since.
The story
On October 19, 2017, the Pan-STARRS telescope in Hawaii detected an object moving too fast and on an orbit that did not loop around the Sun. For the first time, humanity was seeing a visitor from outside the Solar System. It was given the Hawaiian name ʻOumuamua, "messenger from afar arriving first."
The object had no tail like a comet, seemed extremely elongated or flattened, and, as it moved away from the Sun, received a small push that gravity could not explain. The astronomer Avi Loeb, of Harvard, suggested it might be an artificial light sail built by another civilization.
Most scientists favor a natural origin: in 2023, a study showed that the object's ice may have released hydrogen, producing the push without forming a visible tail. Other interstellar visitors have come since, and new telescopes should find many more.