Neil deGrasse Tyson just rained all over Baker Mayfield's alien parade ... saying the Cleveland Browns quarterback most likely saw a meteor -- NOT a UFO.

Booooooooo.

Remember, Mayfield tweeted with "almost 100%" certainty he and his wife saw a UFO Wednesday night ... saying they saw a "very bright ball of light going straight down out of the sky."

But, Tyson -- certified science nerd -- says Baker needs to come back down the earth ... and suggests the QB brushes up on his meteor knowledge.

"Okay, so what he should do is go look at what meteors look like and most of them look like simple shooting stars," Tyson said Friday on FS1's 'The Herd' with Colin Cowherd.