A shadow spotted by NASA's Hubble telescope sweeping across the face of a vast pancake-shaped gas-and-dust disk surrounding a young star may point to a new planet located 192 light-years away, scientists say.
Although the planet itself is not casting the shadow, it is doing some heavy lifting by gravitationally pulling on material near the star and warping the inner part of the disk, researchers said.
The twisted, misaligned inner disk is casting its shadow across the surface of the outer disk.
Astronomers led by John Debes of the Space Telescope Science Institute in in the US said this scenario is the most plausible explanation for the shadow they spotted in the stellar system TW Hydrae, located 192 light-years away in the constellation Hydra, also known as the Female Water Snake.
The star is roughly 8 million years old and slightly less massive than our Sun.
The researchers uncovered the phenomenon while analysing 18 years' worth of archival observations taken by NASA's Hubble Space Telescope.
"This is the very first disk where we have so many images over such a long period of time, therefore allowing us to see this interesting effect," Debes said.
"That gives us hope that this shadow phenomenon may be fairly common in young stellar systems," he said.
Debes' first clue to the phenomenon was a brightness in the disk that changed with position. Astronomers using Hubble's Space Telescope Imaging Spectrograph (STIS) first noted this brightness asymmetry in 2005.
However, they had only one set of observations and could not make a definitive determination about the nature of the mystery feature.
Searching the archive, researchers put together six images from several different epochs.
The observations were made by STIS and by Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS).
STIS is equipped with a coronagraph that blocks starlight to within about 1 billion miles from the star, allowing Hubble to look as close to the star as Saturn is to our sun.
Over time, the structure appeared to move in counterclockwise fashion around the disk, until in 2016, it was in the same position as it was in images taken in 2000.
This 16-year period puzzled the researchers. They originally thought the feature was part of the disk, but the short period meant that the feature was moving way too fast to be physically in the disk.
Under the laws of gravity, disks rotate at glacial speeds. The outermost parts of the TW Hydrae disk would take centuries to complete one rotation.
"The fact that I saw the same motion over 10 billion miles from the star was pretty significant, and told me that I was seeing something that was imprinted on the outer disk rather than something that was happening directly in the disk itself," Debes said.
"The best explanation is that the feature is a shadow moving across the surface of the disk," he said.
