One argument against the comet theory is that a comet would have left a large hole in the Earth's surface.

VOA: special.2009.01.20

Suppose the planet were hit by a comet-- this happened--something like this happened sixty-five million years ago, so it could happen again.

假设这个星球被一颗彗星击中-,这样的事，大约六千五百万年前,发生过，所以它还可能再发生。

耶鲁公开课 - 金融市场课程节选

Nanodiamonds are produced under only a very high temperature and high pressure event, such as a comet striking Earth.

VOA: special.2009.01.20

Scientists at the Goddard Space Flight Center in Greenbelt,Maryland, found the substance glycine in material brought back to Earth from a comet.

VOA: special.2009.09.30

A comet is a huge ball of frozen gas and dust that often releases a long trail of material as it nears the sun.

VOA: special.2009.09.30

But it is the first time an amino acid has been discovered on a comet.

VOA: special.2009.09.30

But scientists think it was either a comet or rock from space.

VOA: special.2009.08.25

Stardust passed through a cloud of material surrounding comet Wild Two in January of two thousand four.

VOA: special.2009.09.30

A specially designed collector gathered dust particles from the comet and stored them on the spacecraft.

VOA: special.2009.09.30