Extrasolar Planet Challenges Planet Formation Theories

A strange new extrasolar planet is challenging conventional theories on planet formation, forming in an extremely short period of time at an incredible distance from its parent star.

The planet, although not confirmed, is theorized to be the cause of a strange gap observed by NASA's Hubble Space Telescope in the protoplanetary disk of gas, rock, and dust surrounding star TW Hydrae, located 176 light-years away in the Hydra constellation. The protoplanetary disk is leftover debris from the star's formation, and is what eventually coalesces into planets, asteroids, and other solar objects.

"If we can actually confirm that there's a planet there, we can connect its characteristics to measurements of the gap properties," said team leader John Debes from the Space Telescope Science Institute in Baltimore, Md. "That might add to planet formation theories as to how you can actually form a planet very far out. There's definitely a gap structure. We think it's probably a planet given the fact that the gap is sharp and circular."

Most astronomers agree that planets take tens of millions of years to form. TW Hydrae, the parent star, however, only eight million years old. The extrasolar planet, at a distance of 7.5 billion miles from TW Hydrae (or twice the distance from the Sun to Pluto), would theoretically take 200 times longer than Jupiter took to form due to its distance from its host star.

One proposed theory suggests that the protoplanetary disk can become gravitationally unstable at a point, and collapse into itself, possibly creating a planet in as little as a few thousand years.

There are still many mysteries surrounding the formation of the disk's gap. TW Hydrae is a small star, weighing in at only 55 percent of the Sun's mass, and the protoplanetary disk surrounding the star is filled with incredibly small particles, ones smaller than what scientists are used to seeing in planet formation.

"Typically, you need pebbles before you can have a planet. So, if there is a planet and there is no dust larger than a grain of sand farther out, that would be a huge challenge to traditional planet-formation models," Debes said.

You can read the published findings in The Astrophysical Journal.