WASHINGTON (AP) — Where did the Earth get its water?

It’s a mystery that got murkier today as some astronomers essentially ruled out one of the chief suspects: comets.

Over the past few months, the European Space Agency’s Rosetta space probe closely examined the type of comet that some scientists believed could have brought water to our planet 4 billion years ago. It found water, but the wrong kind. It contains more of a hydrogen isotope than water on Earth does.

The author of a study in the journal Science says it could be that asteroids brought water to Earth. But others disagree.

Many scientists have long believed that Earth had water when it first formed, but that it boiled off, so that the water on the planet now had to have come from an outside source.