Leicester scientist lends insight into Hubble ‘shadow play’ around planet-forming disc

Artwork: NASA, AURA/STScI for ESA, Leah Hustak (STScI)

A University of Leicester scientist has helped to uncover what could be a burgeoning planet hidden in the shadow of its sibling in a planetary ‘nursery’.

The young star TW Hydrae is playing ‘shadow puppets’ with scientists observing it with NASA’s Hubble Space Telescope. 

In 2017 astronomers reported discovering a shadow sweeping across the face of a vast pancake-shaped gas-and-dust disc surrounding the red dwarf star. This is a protoplanetary disc – a planetary ‘nursery’ from which worlds are formed.

The shadow isn’t from a planet, but from an inner disc slightly inclined relative to the much larger outer disc – causing it to cast a shadow. One explanation is that an unseen planet’s gravity is pulling dust and gas into the planet’s inclined orbit.  

Now, a second shadow – playing a game of peek-a-boo – has emerged in just a few years between observations stored in the Hubble’s MAST archive. This could be from yet another disc nestled inside the system. The two discs are likely evidence of a pair of planets under construction.

The University of Leicester contributed modelling work with colleagues at the University of Warwick to complement the observations in a new paper published in The Astrophysical Journal. They joined the team working with the Hubble Telescope after publishing previous simulations of warping protoplanetary discs with planets specifically motivated by the previous observations of TW Hydrae.

Professor Richard Alexander from the University of Leicester School of Physics and Astronomy said: “The shadows on TW Hydrae’s disc allow us to probe planet formation on smaller scales than we can observe directly.  The moving shadows tell us that the inner parts of the disc are misaligned, but we don’t yet understand why.  The most appealing explanation is that young planets on misaligned orbits have tilted the disc, but the question then is how those planets became misaligned in the first place. Watching how the shadows evolve over the next couple of years will give us a much more detailed picture of what’s going on in this system.”

TW Hydrae is less than 10 million years old and resides about 200 light-years away. In its infancy, our solar system may have resembled the T.W. Hydrae system, some 4.6 billion years ago. Because the TW Hydrae system is tilted nearly face-on to our view from Earth, it is an optimum target for getting a bull’s-eye-view of a planetary construction yard.

The second shadow was discovered in observations obtained on June 6, 2021, as part of a multi-year program designed to track the shadows in circumstellar discs. John Debes of the Space Telescope Science Institute in Baltimore, Maryland compared the TW Hydrae disc to Hubble observations made several years ago.

“We found out that the shadow had done something completely different.” said Debes. “When I first looked at the data, I thought something had gone wrong with the observation because it wasn’t what I was expecting. I was flummoxed at first, and all my collaborators were like: what is going on?  We really had to scratch our heads and it took us a while to actually figure out an explanation.” 

The best solution the team came up with is that there are two misaligned discs casting shadows.  They were so close to each other in the earlier observation they were missed. Over time they've now separated and split into two shadows. “We've never really seen this before on a protoplanetary disc. It makes the system much more complex than we originally thought,” he said.

The simplest explanation is that the misaligned discs are likely caused by the gravitational pull of two planets in slightly different orbital planes. Hubble is piecing together a holistic view of the architecture of the system. 

The discs may be proxies for planets that are lapping each other as they whirl around the star. It's sort of like spinning two vinyl phonograph records at slightly different speeds. Sometimes labels will match up but then one gets ahead of the other.

“It does suggest that the two planets have to be fairly close to each other.  If one was moving much faster than the other, this would have been noticed in earlier observations. It’s like two race cars that are close to each other, but one slowly overtakes and laps the other,” said Debes.

The outer disc that the shadows are falling on may extend as far as several times the radius of our solar system’s Kuiper belt. This larger disc has a curious gap at twice Pluto’s average distance from the Sun. This might be evidence for a third planet in the system.

The TW Hydrae data are from Hubble’s Space Telescope Imaging Spectrograph. The James Webb Space Telescope’s infrared vision may also be able show the shadows in more detail.