One of the great questions about our solar system is: what was it like as it formed? We know that a protosolar nebula birthed the Sun and planets. And, we know planets in our solar system have slightly different orbital inclinations, probably due to some interesting dynamics in the birth crèche. Why is that? The answer may be in a slightly weird-looking protoplanetary disk circling the newborn star TW Hydrae.
This stellar infant lies just under 200 light-years away from Earth. Astronomers using Hubble Space Telescope (HST) data have been looking at the massive disk gas and dust circling that star. Now, HST has found something wonky about it: there are at least two, and possibly three “sub” disks there. And, there may be at least two planets, and possibly three forming there. Interactions between disks and planetary objects are warping the disks and casting shadows.
The system first drew attention in 2017 when astronomers saw a shadow sweeping across the face of the protoplanetary disk system surrounding the star. There appears to be an inner and a larger outer disk. The shadow didn’t come from a planet. Instead, it’s cast on the outer disk by the inner one. Interestingly, there’s at least one of those possible planets is pulling gas and dust from the disk toward itself.
But Wait! There’s Another Shadow!
Now, it looks like there’s an additional shadow. It might be from another disk inside the system, along with the possible planets. If so, then this gives astronomers a clue as to what our own solar system looked like more than 4.5 billion years ago. The second shadow showed up in observations obtained on June 6, 2021, as part of a multi-year program designed to track the shadows in circumstellar disks. Astronomer John Debes at Space Telescope Science Institute compared the TW Hydrae disk to Hubble observations made several years ago.
“We found out that the shadow had done something completely different,” he said. “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 explanation invokes two misaligned disks that are each casting shadows. The first observation missed them because the pair are pretty close together. However, in the years between observations, they’ve now separated and split into two shadows. Apparently, the two misaligned disks are that way thanks to the gravitational pull of two planets in slightly different orbital planes. “We’ve never really seen this before on a protoplanetary disk. It makes the system much more complex than we originally thought,” said Debes.
About Newborn Star TW Hydrae
This newborn star is an interesting infant system. It’s called a “T Tauri” star. That’s because it’s part of a class of variables that are no older than 10 million years. Astronomers consider it a “pre-main-sequence” star. (Meaning it hasn’t started nuclear fusion or formed a radiative zone). Many of this stellar class reside near giant molecular clouds. They’re typically the youngest stars and can be quite active.
TW Hydrae fits the category really well. It’s about 80 percent of the mass of the Sun and is slightly larger than our star. Astronomers think it’s around 8 to 10 million years old. The protoplanetary disk region surrounding it is feeding the newborn star and apparently also forming planets. TW Hydrae one member of a larger collection of young stars called the “TW Hydrae Association.”
Racecar Planets in the Protoplanetary Disk
The disk structures inside TW Hydrae’s protoplanetary disk could be related to the action of planets lapping each other as they orbit. “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,” 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. That’s about equal to the orbital period for a planet at that distance.
These two inner disks are inclined about five to seven degrees relative to the plane of the outer disk. 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.
Future Studies
Obviously, given how closely this mimics what astronomers suspect happened in our own solar system, HST and other telescopes, such as the Atacama Large Millimeter Array, will continue to study the TW Hydrae system. The outer disk that the shadows are falling on may extend out quite far away. Some scientists suggest it could stretch out as far as several times the radius of our solar system’s Kuiper belt. Interestingly, this larger disk seems to have a gap at twice Pluto’s average distance from the Sun. That could be evidence for a third planet in the system.
Close in to the star, it would be very difficult to see any smaller, inner planets. That’s because they’d be lost in the glare. In addition, the system is quite dusty, making it more difficult to spot planets. However, it’s possible that the JWST could study the system in search of more details in the disk. In addition, ESA’s Gaia space observatory could do some long-term measurements of the system to find a wobble in the star if Jupiter-mass planets are tugging on it. The Atacama Large Millimeter Array in Chile has observed this system, providing very clear radio views of the protoplanetary disk around TW Hydrae. Future studies should reveal more details of this interesting newborn star and its developing planetary system.
For More Information
Newborn Star Surrounded By Planet-Forming Disks at Different Angles
The Surprising Evolution of the Shadow on the TW Hya Disk (PDF)
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