176 light years from Earth, in the constellation Hydra (the Water Serpent), lies a small young star and what appears to be a brand new planet. Recently spotted by the Hubble Space Telescope, TW Hydrae is breaking records and disrupting current theories on planetary formation, because not only is the star apparently too young to have formed planets, but the planet itself is about twelve billion kilometers from the star — twice as far out as Pluto.
Of the thousands of exoplanet candidates that have been discovered so far, most are found by noting tiny changes in their star’s brightness or movement. These exoplanets tend to be extremely close to their star’s since that is when those methods are most sensitive. TW Hydrae’s planet, however, was found when astronomers noticed a large gap in its protoplanetary disk. The disk itself has a diameter of about 66 billion kilometers, and Hubble Space Telescope (HST) images of the disk showed that it has a very obvious gap some three billion kilometers thick. This gap is mostly likely carved out by a new planet, whose gravity either draws material in or slingshots it out into a new orbit. The data at the moment suggest a fairly wide range of possible sizes for this strange world, at between six and twenty-eight times more massive than the Earth, which puts it somewhere between the Rocky Planet and Gas Giant categories we’re familiar with back in our own Solar System.
Current theories on the formation of planets describe a very slow process in which grains of dust stick together to form pebbles, and these pebbles sometimes stick together when they collide. Eventually they collect enough mass together in one place to begin attracting more material inwards, and the planet begins to grow until all nearby material is used up. The problem posed by TW Hydrae is that the star is only eight million years old. This process is supposed to take tens of millions of years under normal circumstances, and so far out where the material is spread so thinly and moving so slowly, it should take even longer. The low mass of the star (55% that of the Sun) slows down matters even more.
“It’s so intriguing to see a system like this,” said John Debes of the Space Telescope Science Institute in Baltimore, who leads the research team that identified the gap. “This is the lowest-mass star for which we’ve observed a gap so far out.”
One possible explanation is an alternative planetary formation theory, which suggests that sections of a disk become gravitationally unstable and collapse inwards. A process like this could form planets in only a few thousand years.
The image was taken with the HST’s Near Infrared Camera and Multi-Object Spectrometer in near-infrared light, and compared with older images taken with other Hubble instruments at visible wavelengths, confirming that the gap is real structural feature, and not just an optical effect. Although the raw image is monochrome, it has been coloured here to make faint variations in brightness more visible.
This work was published on June 14 in The Astrophysical Journal.