Resolves once there’s clear expert consensus on the matter, at least 80 years in the future. So the market must remain open at least until May 29, 2103, to allow time for more information to come out and be evaluated. After this, the market resolves as soon as there’s strong consensus (at least 95% of the relevant experts agree) one way or the other. The market does not resolve if there’s significant disagreement about who the relevant experts are or what their consensus is.

For the purposes of this market, an alien is any life form or autonomous descendant of life forms (such as intelligent machines) that originated outside of Earth, excluding any entities whose lineage can be traced back to life forms that did originate on Earth.* (So if some life form originated on Earth but made its way to space and continued to replicate, its descendants would not be considered aliens.)

An alien is considered to have “been to Earth” if it was within 100 kilometers of Earth’s surface (i.e., it crossed the Kármán line) at some point since Earth’s formation roughly 4.6 billion years ago.

If the alien visited Earth in some non-physical way or another way that we don’t currently understand, such that this definition of “been to Earth” is not applicable, this market resolves according to whatever future experts think is most accurate given the spirit of the question. The same is true if the definition of “alien” or the concept of "life" is insufficiently precise.

*One exception to this definition of alien: If all current life on Earth descended from life that originally formed elsewhere in the universe and then made its way to Earth (i.e., exogenesis), none of this life is considered to be alien.