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.
- Mystery of Interstellar Visitor ‘Oumuamua Gets Trickier in Scientific American that discusses reasons why the hydrogen comet theory is in question.