NASA’s Hubble Telescope observes shadows in planetary construction yard

Scientists observing the young star TW Hydrae with NASA’s Hubble Space Telescope have detected a fascinating phenomenon akin to “shadow puppets”. A shadow moving across a vast gas-and-dust disk surrounding the star was first observed in 2017, but it wasn’t caused by a planet. Rather, it was traced to an inner disk slightly askew from a much larger outer disk, resulting in a shadow effect. This inner disk could be orbiting an unseen planet with its gravity pulling dust and gas into its inclined orbit.

Recently, scientists have discovered another shadow, which appeared like a game of peek-a-boo, in the Hubble Space Telescope’s MAST archive. This could indicate the existence of yet another disk nestled within the system, suggesting that a pair of planets is under construction.

TW Hydrae, which is situated approximately 200 light-years away, is less than 10 million years old, and its system might have been similar to our solar system’s infancy, 4.6 billion years ago. As the TW Hydrae system is almost face-on from Earth’s view, it provides a prime target for investigating planetary construction yards.

On June 6, 2021, scientists discovered the second shadow as part of a multi-year project designed to monitor circumstellar disk shadows. John Debes of AURA/STScI for the European Space Agency at the Space Telescope Science Institute in Baltimore, Maryland, compared current observations of the TW Hydrae disk with Hubble data collected several years ago.

According to the study published in The Astrophysical Journal, John Debes, the principal investigator and lead author of the research, stated that they were surprised by what they discovered. “We found out that the shadow had done something completely different,” he said. Initially, when Debes examined the data, he thought something went wrong with the observation since it was not what he expected. It took some time for him and his colleagues to come up with an explanation.

The most plausible explanation is that there are two misaligned disks causing the shadows, which were so close to each other in the previous observation that they were overlooked. However, with time, they have drifted apart, resulting in two distinct shadows. Debes stated that this phenomenon has never been observed before on a protoplanetary disk, and it has made the system far more complicated than previously believed.

Comparison images from the Hubble Space Telescope, taken several years apart, have uncovered two eerie shadows moving counterclockwise across a gas-and-dust disk encircling the young star TW Hydrae. The disks are tilted face-on to Earth and so give astronomers a bird’s-eye view of what’s happening around the star. The left image, taken in 2016, shows just one shadow [A] at the 11:00 o’clock position. This shadow is cast by an inner disk that is slightly inclined to the outer disk and so blocks starlight. The picture on the left shows a second shadow that emerged from yet another nested disk [C] at the 7:00 o’clock position, as photographed in 2021. The original inner disk is marked [B] in this later view. The shadows rotate around the star at different rates like the hands on a clock. They are evidence for two unseen planets that have pulled dust into their orbits. This makes them slightly inclined to each other. This is a visible-light photo taken with the Space Telescope Imaging Spectrograph. Artificial color, to enhance details, has been added. Credit: NASA, ESA, STScI, John Debes (AURA/STScI for ESA)IMAGE PROCESSING: Joseph DePasquale (STScI)

The gravitational pull of two planets in slightly different orbital planes is the simplest explanation for the misaligned disks causing the shadows. The Hubble Space Telescope is working to provide a comprehensive view of the system’s architecture.

The disks are possibly serving as proxies for planets that are orbiting the star in a manner similar to two phonograph records spinning at different speeds. The proximity of the two planets means that they need to be fairly close to each other, which would have been noticed in earlier observations if one was moving significantly faster than the other.

These planets are estimated to be located at a distance from the star equivalent to Jupiter’s distance from our Sun, and the shadows complete one rotation around the star every 15 years. The inclination of the inner disks is similar to that observed in the orbital inclinations inside our solar system.

The outer disk, on which the shadows are falling, may extend several times the radius of our solar system’s Kuiper belt, and it displays a noticeable gap at twice Pluto’s average distance from the Sun. This gap may be an indication of a third planet in the system.

Detecting inner planets is difficult because their light is lost in the glare of the star, and dust in the system dims their reflected light. ESA’s Gaia space observatory may detect a wobble in the star if Jupiter-mass planets are tugging on it, but it would take years due to the long orbital periods.

Data from Hubble’s Space Telescope Imaging Spectrograph were used in this study. The James Webb Space Telescope’s infrared vision may provide more detailed information on the shadows.

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