MAY 18, 2023 6:00 AM PDT

Hubble Observes Shadows from Misaligned Planetary Disks

Approximately 200 light-years from Earth resides the red dwarf (K dwarf) star, TW Hydrae, which is estimated to be less than 10 million years old, meaning TW Hydrae is in its infancy, much like our own solar system was 4.6 billion years ago. The difference is K dwarfs have an approximate lifespan of 50 billion years, compared to our own Sun of 10 billion years, making TW Hydrae even younger relative to when our own Sun was only 10 million years old.

Given its extremely young age, the TW Hydrae system has massive disks of gas and dust encircling it as new planets are still being clumped together and formed. These disks, and more specifically the shadows they cast on their star, is what an international team of researchers examined in a recent study published in The Astrophysical Journal using data from NASA’s Hubble Space Telescope.

Artist's illustration of misaligned disks within the TW Hydrae system based on data from NASA's Hubble Space Telescope. (Credit: ARTWORK: NASA, AURA/STScI for ESA, Leah Hustak (STScI)) 

This recent study builds off a 2017 finding where astronomers first observed a massive shadow traversing TW Hydrae. Such transits are normally attributed to exoplanets passing in front of their star, and the astronomers determined that this was likely from the inner disk of gas and dust where planets could be slowly forming.

In this most recent study, astronomers claim to have observed a second shadow in June 2021 that they hypothesize could be from a second inner disk of planet-forming gas and dust encircling the system.

"We found out that the shadow had done something completely different," said Dr. John Debes, who is principal investigator of Hubble’s STIS instrument, and lead author of the study. "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 research team determined that the separate shadows are likely caused by two separate misaligned disks of gas and dust that might have been parallel to each other during the first observation in 2017. Since then, the two disks have likely separated, possibly from the gravitational pull of two separate planets, allowing for this most recent observation to take place.

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

According to Dr. Peter Plavchan, who is a professor of physics and astronomy at George Mason University and a co-author on the study, the team hasn’t observed direct evidence of planets, but says if planets are present, they are likely no bigger than Jupiter.

Based on Kepler’s Third Law of Planetary Motion, more inward planets orbit faster than outer planets, which means the disks are also rotating at different speeds. A good analogy is two vinyl records spinning at different speeds, meaning their labels might match up from time to time, but are not aligned most of the time. In this case, the labels are likely planets slowly being constructed within the disks.

"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 Dr. Debes.

Despite the misalignment, the inclination of the two inner disks from both observations differ only about five to seven degrees from the outer disk within the system which is much farther out, and Dr. Debes notes this as being “right in line with typical solar system style architecture.”

What new discoveries will astronomers make about planet-forming disks in the coming years and decades? Only time will tell, and this is why we science!

Sources: Wikipedia, NASA, The Astrophysical Journal, NASA (1), NASA (2), George Mason University, NASA (3)

As always, keep doing science & keep looking up!

About the Author
Master's (MA/MS/Other)
Laurence Tognetti is a six-year USAF Veteran who earned both a BSc and MSc from the School of Earth and Space Exploration at Arizona State University. Laurence is extremely passionate about outer space and science communication, and is the author of “Outer Solar System Moons: Your Personal 3D Journey”.
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