Hubble Might Have Seen The Shadow Of A Planet Forming

TW Hydrae with the shadow visible in the top-left side. NASA, ESA, and J. Debes (STScI)

About 192 light-years from Earth, there’s a very young stellar system that astronomers think is currently in the process of forming planets, and now new observations by Hubble seem to provide more evidence for the idea. The space telescope has spotted a moving shadow that is believed to be linked to a massive object, most likely a planet, forming around the star.

Hubble has been studying TW Hydrae for 18 years, a system that is estimated to be 8 million years old. Its planets are too faint to be seen by our current instruments, but we have witnessed gaps in the disk of material that surrounds the star, a clear indication that planets are forming.

While looking at archive images of TW Hydrae, a team from the Space Telescope Institute in Baltimore spotted a curious shadow moving around it. The shadow is not cast by the planet itself, but it's possible the newly forming object is twisting the inner part of the disk and blocking the star’s light.

"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," John Debes, who led the research, said in a statement. "That gives us hope that this shadow phenomenon may be fairly common in young stellar systems."

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This animation shows the shadow caused by the planet moving around the star. NASA, ESA, and J. Debes (STScI)

The research, which was presented last week at the American Astronomical Society’s 229th meeting in Texas, solved a mystery that has lasted over 10 years. Debes first spotted the anomaly in the disk's light coloration in 2005. The shadow is located 16 billion kilometers (10 billion miles) from the star and has an orbital period of 16 years, too fast to be a physical feature of 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 added. "The best explanation is that the feature is a shadow moving across the surface of the disk."

The team says that the tilt in the disk caused by the planet is too close to the star, which is slightly less massive than our Sun, for Hubble to observe it.

"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."

Future telescopes, like the James Webb Space Telescope, might have a better chance at directly imaging more minute details of the system.

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