Hubble spies possible planet 'shadow'

The Hubble Space Telescope may have revealed an exoplanet in orbit around a young star by capturing a shadow moving across the star's dusty disc.

Hubble Space Telescope images show a shadow moving counterclockwise around the disc surrounding young star TW Hydrae. The two top images show an uneven brightness across the disc. The enhanced images below allowed astronomers to determine the reason for the changes in brightness. The dimmer areas of the disk, surrounded by the dotted line, show a shadow moving across the disc. The long arrows show how far the shadow has moved between 2015 and 2016.
Credits: NASA, ESA, and J. Debes (STScI)

Hubble Space Telescope observations may have revealed a young planet embedded deep in the dusty disc surrounding the star TW Hydrae, by spotting a shadow sweeping across the face of the disc.

TW Hydrae is about 192 lightyears away from Earth and is about eight million years old. Like most young stars, it is surrounded by a flat disc of cosmic gas and dust: the materials out of which planets may eventually grow.

In 2005, astronomers first noticed a brightness in the disc that changed its position over time, but they did not have enough observational data to work out the source of the phenomenon. John Debes of the Space Telescope Science Institute in Baltimore, Maryland, led a team of astronomers who looked at 18 years’ worth of observations of the star by the Hubble Space Telescope’s Imaging Spectrograph (STIS). STIS is able to block starlight to within about 1.6 billion km, meaning the astronomers could get a good look at the star close up.

They found that the feature moves counterclockwise around the disc and in 2016 arrived at the same position it was in images captured in 2000. Since circumstellar discs rotate much slower than would allow for this observation, the team decided that the phenomenon was not part of the disc itself.

A simulation showing the mysterious shadow sweeping across the disc surrounding TW Hydrae
NASA, ESA, and J. Debes (STScI)

"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 disc rather than something that was happening directly in the disc itself," Debes says. "The best explanation is that the feature is a shadow moving across the surface of the disc.”

Further research led the team to conclude that the shadow’s source must be embedded deep inside the disc, which is about 65 billion km wide, meaning it cannot be imaged directly by any telescope currently in operation.

Observations of TW Hydrae by the Atacama Large Millimeter Array (ALMA) in Chile suggested the inner disc may be warped, which led the team to consider the possibility of an orbiting planet whose gravitational influence is causing the warping to occur.

If it does exist, the planet is estimated to be about 160 million km from the star; about as close as Earth is from the Sun. It would also have to be about the size of Jupiter to be able to affect the shape of the disc in this way.

If the team’s theory is correct, it could offer a new method of detecting exoplanets.

”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 behavior of a shadow," Debes says. "This study shows us that even these large diskc, whose inner regions are unobservable, are still dynamic, or changing in detectable ways which we didn't imagine."

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