In the star system TW Hydrae, 176 light-years from Earth, a team of astronomers using Hubble telescope has found strong evidence of a planet forming around it at a distance so great, it shouldn't be possible according to current theories.
Around the TW Hydrae, a red dwarf star with 55% the mass of our Sun, there is a very large protoplanetary disk 66 billion kilometers wide. At 12 billion kilometers from the star, there is a 3 billion kilometers wide gap in the disk. This strongly suggests there is a planet being formed, picking up tiny pieces of dust and rocks as it orbits the star, leaving the trail which can be seen as a gap in the disk. The planet is estimated to be 6 to 28 times heavier than Earth, making it relatively small planet.
Out of around 900 detected extrasolar planets, this one is the farthest from its star, orbiting TW Hydrae at a distance twice as long as Pluto's distance from the Sun. At this distance, formation of a planet should be very slow compared to distances near the star. For example, Jupiter is thought to had formed in 10 million years, orbiting the Sun at 800 million kilometers. A planet 12 billion kilometers from the star should take considerably longer because of its slow orbit and because at these distances in TW Hydrae's protoplanetary disk, grains of dust larger than grain of sand are not present. Yet, the evidence is strong that there is truly a planet forming in the disk's gap. What's more, the star is only 8 million years old, meaning of course that the planet itself is younger than this. According to current theories, a star this young is unlikely to have planets forming around it.
The team led by John Debes of the Space Telescope Science Institute in Baltimore, USA, has observed the star in near-infrared light, using Near Infrared Camera and Multi-Object Spectrometer (NICMOS) and then compared the images with data already collected by Hubble such as optical and spectroscopic observations from Hubble's Space Telescope Imaging Spectrograph (STIS). The gap is seen at all vawelenghts, indicating it is real and not an illusion caused by the instruments or scattered light.
"It's so intriguing to see a system like this," said John Debes. "This is the lowest-mass star for which we've observed a gap so far out. If we can actually confirm that there's a planet there, we can connect its characteristics to measurements of the gap properties. That might add to planet formation theories as to how you can actually form a planet very far out."
There is another theory on how planets form which suggests that a portion of a plotoplanetary disk can become gravitationally unstable and collapse on itelf, thus allowing the planet to form more quickly. This process would supposedly only take a few thousand years.