Hubble follows shadow play around planet-forming disc
In 2017 astronomers reported discovering a shadow sweeping across the face of a vast pancake-shaped disc of gas and dust surrounding the red dwarf star TW Hydrae. 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 its 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 MAST archive of the NASA/ESA Hubble Space Telescope. This could be from yet another disc nestled inside the system. The two discs are likely evidence of a pair of planets under construction.
TW Hydrae is less than 10 million years old and resides about 200 light-years away. In its infancy, some 4.6 billion years ago, our Solar System may have resembled the TW Hydrae system. Because the TW Hydrae system is tilted nearly face-on as seen from Earth, it is an optimum target for getting a bird’s-eye view of a planetary construction yard.
The second shadow was discovered in observations obtained on 6 June 2021, as part of a multi-year programme designed to track the shadows in circumstellar discs. John Debes of AURA/STScI for the European Space Agency at the Space Telescope Science Institute in Baltimore, Maryland, compared these latest observations of the TW Hydrae disc to Hubble observations made several years ago.
“We found out that the shadow had done something completely different,” said Debes, who is principal investigator and lead author of the study published in The Astrophysical Journal. “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.”
“We hatched a theory of what might be causing the changing shadows,” added Rebecca Nealon, a member of the science team at the University of Warwick in the United Kingdom. “But to test this we had to run sophisticated models where we varied the number of discs and their orientations to try to reproduce Hubble’s observations.”
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,” said Debes.
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 records at slightly different speeds. Sometimes the 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 racing cars that are close to each other, but one slowly overtakes and laps the other,” said Debes.
The suspected planets are located in a region roughly the distance of Jupiter from our Sun. And the shadows complete one rotation around the star about every 15 years — the orbital period that would be expected at that distance from the star.
Also, these two inner discs are inclined by about five to seven degrees relative to the plane of the outer disc. This is comparable to the range of orbital inclinations inside our Solar System. “This is right in line with typical Solar System-style architecture,” 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.
Any inner planets would be difficult to detect because their light would be lost in the glare of the star. Also, dust in the system would dim their reflected light. ESA’s Gaia space observatory may be able to measure a wobble in the star if Jupiter-mass planets are tugging on it, but this would take years given the long orbital periods.
The TW Hydrae data are from Hubble’s Space Telescope Imaging Spectrograph. The NASA/ESA/CSA James Webb Space Telescope’s infrared vision may also be able to show the shadows in more detail.
