Hubble catches shadows on planet-forming disk around infant star

Hubble has been watching the 10 million-year-old red dwarf TW Hydrae for years, and researchers have identified shadows on its planetary disk likely caused by a pair of growing planets.
John Loeffler
Gas and dust disks around the young star TW Hydrae
Gas and dust disks around the young star TW Hydrae

NASA, AURA/STScI for ESA, Leah Hustak (STScl)  

The young red dwarf star TW Hydrae has been in astronomers' sights for years thanks to its planetary disk giving us a ring-side seat to the earliest stages of a star system's development, and now Hubble has spotted peculiar shadows playing across the disk — likely due to evolving proto-exoplanets in the disk.

NASA and European Space Agency (ESA) astronomers have been studying TW Hydrae for many years, and starting in 2017, a shadow has been seen sweeping across the disk. This shadow isn't from an exoplanet, but from an inner ring in the disk tilted at an incline so that it casts a shadow from the star's light onto the ring behind it.

In a new study published this week in The Astrophysical Journal, however, a second shadow has now been identified, and ESA researchers believe that it might be evidence of a pair of unseen exoplanets forming in different orbital planes.

"We found out that the shadow had done something completely different," John Debes, an ESA researcher and the principal investigator and lead author of the new study, said in a statement. "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."

Now, what they believe is happening in the TW Hydrae system is that there are two rings in the disk tilted at different angles to one another. When Debes looked at data from 2017, he and his colleagues believe that only one shadow appeared because the two rings in the disk were closely aligned, so that their shadows were interpreted as a single shadow. But, years later, the disks slipped out of alignment, casting different shadows that researchers were not expecting.

"We've never really seen this before on a protoplanetary disk," Debes said. "It makes the system much more complex than we originally thought."

The likely culprits are a pair of proto-exoplanets forming in orbit on slightly different planes from each other, and their gravity is pulling gas and dust out of the disk and into the tilted rings. The way the shadows are playing against the ring also gives some insight into how the planets are forming and their characteristics.

"It does suggest that the two planets have to be fairly close to each other," Debes said. "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."

What we know about the suspect protoplanets

Hubble catches shadows on planet-forming disk around infant star
The shadows over TW Hydrae's planetary disk

The TW Hydrae system is very young, with the red dwarf star in its center only about 10 million years old. The system is also notable for its angle relative to Earth. We are positioned in a way that we look nearly straight down at the system, giving us a bullseye-like view of the system, perfect for observing the entire system's evolution as a whole.

The suspect planets would be located at a distance from TW Hydrae at roughly the distance of Jupiter and Saturn in our own system, with the shadows completing a rotation around the disk in about 15 years, the orbital period expected for planets at this distance from their star.

The five-to-seven-degree inclination of the rings relative to the plane of the main disk around the star is also in line with what we see in our own system, Debes said: "This is right in line with typical solar system style architecture."

The main disk around TW Hydrae which the shadows are playing across is quite massive, stretching several times farther than the radius of the Kuiper belt in our solar system. The researchers have also spotted a third mysterious gap in the disk at about twice the average distance of Pluto from the sun, evidence of yet another planet orbiting at that distance from TW Hydrae.

While there are some key differences in the formation of our solar system, ours might have started out very much in the same manner as TW Hydrae, making it a very compelling target for study. As for rocky inner planets, any protoplanets closer to the star are largely obscured by the glare of its light, as well as the concentration of dust around the young star.

And, since we are observing the system from a nearly top-down perspective, it's not likely that the planetary transit method would be able to spot them like it has many other exoplanets. The "wobble" method — where slight perturbances in the rotation of the star caused by Jupiter-sized exoplanets can be detected — might reveal more details about the unseen exo-protoplanets in the system, but this might be many years away, given the long rotational periods involved.