An interstellar object that passed through our solar system in 2018. Researchers believe it to be solid chunk of hydrogen that was formed in a gas cloud 40MM years ago. However, if does not have the usual features of a comet (e.g. a coma) and it’s accelerating. An alternative explanation is that it’s extraterrestrial in origin.
Read Mystery of Interstellar Visitor ‘Oumuamua Gets Trickier in Scientific American that discusses reasons why the hydrogen comet theory is in question.
See also:
Links to this note
-
The idea that life originated on another planet and was transported to Earth. It’s difficult to prove or disprove in a similar way to the Silurian hypothesis, evidence is difficult to obtain—artifacts from civilizations don’t last that long, evidence of life is not visible on any likely originating planet within our solar system.