Neptune was struck by a giant comet about two centuries ago, according to new research. The find adds to a growing body of evidence that cometary collisions with gas giant planets may be more common than astronomers thought.

On rocky planets with thin atmospheres, such as Mercury (pictures) and Mars, it's easy to count the craters made by impacts dating back millions of years, giving scientists a rough estimate of how frequently space rocks collide with other worlds.

But the gas giants—Jupiter, Saturn, Uranus, and Neptune—are planets that are almost all roiling atmosphere with just tiny cores of rock, making it much harder to find evidence of past impacts.

In 1994 the comet Shoemaker-Levy 9 broke apart and smashed into Jupiter, and several space probes, including Voyager 2, Galileo, and the Hubble Space Telescope, were able to document the rare event.

Using what they'd learned about chemicals left in Jupiter's atmosphere after the comet crash, scientists from the French observatory LESIA and the Max Planck Institutes in Germany analyzed the composition of Neptune's atmosphere with the European research satellite Herschel.

The data show that the amount of carbon monoxide in Neptune's upper atmosphere is higher than in the planet's lower atmosphere. Since gas would normally thin out as it reaches higher atmospheric layers, the extra gas had to come from some outside source, the scientists say.

A comet, which carries carbon monoxide in its icy tail, is the "main explanation," said study co-author Paul Hartogh of the Max Planck Institute for Solar System Research.

Comet Also Smashed Saturn?

The researchers estimate the comet that struck Neptune was 1.2 miles (2 kilometers) wide, which would have made its impact very powerful.

The first fragment of Shoemaker-Levy 9 was just half the size of the unnamed Neptune comet. When the fragment struck Jupiter, it released energy equal to 225,000 megatons of TNT and created a plume 621 miles (1,000 kilometers) high.

In addition, a similar technique used earlier this year found evidence for a comet crash on Saturn 300 years ago.

Adding the Neptune find to the relatively recent Saturn and Jupiter smashups implies that comet collisions might be more frequent than astronomers had thought.

A 1997 study estimated that comets about a mile (1.6 kilometers) wide hit Jupiter once every 6,000 years. And mathematical models for Neptune have suggested that a major comet hits the gas giant planet every 8,000 years, Hartogh said.

Based on the old calculations, he said, "statistically, it's not so probable" that Neptune, Jupiter, and Staurn would each suffer a major impact event within a 300-year period—suggesting a new estimate may be in order.

"Maybe there are more impacts than we think."