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

In that event,astronomers were able to observe the icy comet long before it struck the solar system's biggest planet.

VOA: special.2009.08.25

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

But the object that struck Jupiter in July is estimated to have been much smaller and harder to see than comet Shoemaker-Levy.

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

Since then, scientists around the world have been working to identify substances gathered from Comet Wild-Two.

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

On February fourteenth, Stardust will approach to within two hundred kilometers of comet Temple 1.

VOA: special.2011.02.09

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

VOA: special.2009.09.30

Many scientists dismiss the comet theory.

VOA: special.2009.01.20