NASA: New planet forming far from its star 'shouldn't exist'

A small planet is being born about 176 light-years away from Earth in the Hydra constellation, and according to current planet formation theories, it shouldn't even exist.

NASA's Hubble Space Telescope spotted a gap in the swirling protoplanetary disk of dust and gas around the red dwarf TW Hydrae, likely caused by a planet "under construction.

The suspected extrasolar planet is orbiting 7.5 billion miles away from its star -- roughly twice the distance Pluto is from our sun -- the farthest distance from a host star a planet has ever been observed. The planet is estimated to be relatively small, at 6 to 28 times more massive than Earth

Planets are thought to form over tens of millions of years as dust and debris collide to form larger rocks, and eventually planets.

A planet 7.5 billion miles from its star should take more than 200 times longer to form than Jupiter did at its distance from the sun because of its much slower orbital speed and the more sparse material in the disk. Jupiter is 500 million miles from the sun and it formed in about 10 million years.

But TW Hydrae is only 8 million years old, making it an unlikely star to host a planet, according to this theory. Adding to the mystery, TW Hydrae is only about 55 percent as massive as our sun.

Further, observations from the Atacama Large Millimeter Array show that larger dust grains are not present beyond about 5.5 billion miles from the star, just short of the gap.

"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," said said John Debes of the Space Telescope Science Institute, lead researcher on the study.

"It's so intriguing to see a system like this," said Debes. "This is the lowest-mass star for which we've observed a gap so far out." The team's findings are published in the Astrophysical Journal.