hubble
January 10, 2017

Strange shadow reveals new previously unknown planet

Hunting for planets close to other stars is a difficult proposition. They appear so tiny and faint to us, it’s difficult to spot them.

NASA astronomers recently found signs of one potential planet in a nearby system in a particularly odd way: by noticing a shadow moving across the face of a massive gas-and-dust disk encircling a young star.

Interestingly, the planetary body itself doesn't appear to be casting the shadow, according to a NASA press release. The shadow appears to have formed by a planet pulling on material close to the star, bending the interior of the disk. The warped, misaligned inside disk is throwing its shadow across the outer disk.

The shadow was discovered in the stellar system TW Hydrae, situated 192 light-years away in the constellation Hydra. The star is about 8 million years old and somewhat smaller than our sun. The team found the shadow anomaly while combing through18 years’ worth of archived data from the 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,” said team member John Debes of the Space Telescope Science Institute in Baltimore. “That gives us hope that this shadow phenomenon may be fairly common in young stellar systems.”

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These images, taken a year apart by NASA's Hubble Space Telescope, reveal a shadow moving counterclockwise around a gas-and-dust disk encircling the young star TW Hydrae. Credit: NASA, ESA and J. Debes (STScl)

The team's first hint was brightness in the disk that shifted with location. Scientists using Hubble’s Space Telescope Imaging Spectrograph (STIS) first mentioned the event in 2005, but they had just one group of observations, and could not make a conclusive determination on the source of the odd feature.

Combing the archive, researchers assembled six pictures from several different times. As time passed, the structure seemed to move in counter-clockwise manner around the disk, until, in 2016, it was in the same location as it was in pictures taken in 2000.

The researchers concluded that the relatively short period meant the feature was moving much too quickly to be contained within the disk.

“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.”

The researchers determined that the source producing the shadow must be deep within the 41-billion-mile-wide disk, so near to the star it can't be photographed by any telescope, including Hubble.

Using the Atacama Large Millimeter Array (ALMA) telescope in Chile, the researchers found another possible feature: a warp in the inner disk. To the research team, this was a major key.

“The most plausible scenario is the gravitational influence of an unseen planet, which is pulling material out of the plane of the disk and twisting the inner disk,” Debes explained. “The misaligned disk is inside the planet’s orbit.”

The new discovery isn’t just a unique phenomenon; it also offers a way to hunt for planets and find out what is happening very near a star in a disk of gas and dust, an area not visible via direct imaging.

“What is surprising is that we can learn something about an unseen part of the disk by studying the disk’s outer region and by measuring the motion, location, and behavior of a shadow,” Debes said. “This study shows us that even these large disks, whose inner regions are unobservable, are still dynamic, or changing in detectable ways which we didn’t imagine.”

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Image credit: NASA