Alien world’s shadow SEEN BY HUBBLE
The planet is too close to its star to be detected directly
A shadow has been spotted moving across the gas and dust disc of a young star in a new study of archived Hubble Space Telescope data, hinting at the presence of an inner planet. The discovery could be the start of a new way to observe young planets orbiting close to their stars.
The star in question, TW Hydrae, is only eight million years old and is still surrounded by a protoplanetary disc, a dense region of gas and dust. Researchers analysed 18 years’ worth of Hubble images and found a dark patch moving clockwise around the disc, 16 billion km from the star – this is equivalent to three times the distance between Pluto and the Sun.
What surprised the researchers was that the patch completed an orbit in just 16 years. At that distance it should take centuries to complete one rotation, indicating that the blotch was not physically part of the disc. It might, they say, be a shadow, cast on the outer disc by an inner section that migrated out of the main plane.
“The most plausible scenario is the gravitational influence of an unseen planet, which is pulling material out of the plane of the disc and twisting the inner disc,” says John Debes from the Space Telescope Science Institute in Baltimore. “The misaligned disc is inside the planet’s orbit.”
The planet is estimated to be Jupiter-mass and around 160 million km from the star, a similar orbital distance to that of Earth. Currently Hubble can only directly image planets that are more than 1.5 billion km from their stars, around the same distance as Saturn’s orbit. However, if such twisted discs are common in the Galaxy, then it could allow the telescope to examine regions much closer in to stars.
“What is surprising is that we can learn something about an unseen part of the disc by studying the disc’s outer region and by measuring the motion, location and behaviour of a shadow,” says Debes. “This study shows us that even these large discs, whose inner regions are unobservable, are still dynamic or changing in detectable ways we didn’t imagine.” http://hubblesite.org/