One of the big questions about our solar system is: What was it like when it formed? We know that a protosolar nebula gave birth to the sun and planets. And we know that planets in our solar system have slightly different orbital inclinations, likely due to some interesting dynamics in the nativity scene. Why this? The answer may lie in a slightly odd-looking protoplanetary disk orbiting newborn star TW Hydrae.
This star child is nearly 200 light-years from Earth. Astronomers using Hubble Space Telescope (HST) data have been looking at the massive disk gas and dust orbiting this star. Now, HST has found something unusual about it: there are at least two, and possibly three, “sub” drives. And there can be at least two planets and possibly three forming there. Interactions between disks and planetary objects deform the disks and cast shadows.
The system first attracted attention in 2017 when astronomers saw a shadow sweep across the face of the protoplanetary disk system surrounding the star. There appears to be an inner disc and a larger outer disc. The shadow didn’t come from a planet. Instead, it is thrown from the inner disk to the outer disk. Interestingly, at least one of these possible planets pulls gas and dust from the disk towards itself.
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These images, taken a year apart by the NASA/ESA Hubble Space Telescope, show a shadow traveling counterclockwise around a disk of gas and dust orbiting young newborn star TW Hydrae. The shadow may be caused by the gravitational pull of an invisible planet orbiting close to the star. The planet pulls up material from the main disk, creating a warped inner disk. The twisted disk blocks the star’s light and casts a shadow on the disk’s outer region.
But wait! There’s another shadow!
Now it looks like there is an extra shadow. It could be from another disk within 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 appeared in observations made on June 6, 2021 as part of a multi-year program to track shadows in circumstellar disks. Space Telescope Science Institute astronomer John Debes compared the TW Hydrae disk to Hubble observations made a few years ago.
“We found that the shadow had done something very 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 expected. At first I was stunned and all my co-workers said, what’s going on? We really had to scratch our heads and it took us a while to actually come up with an explanation.”
The explanation refers to two misaligned discs, each casting shadows. The first observation missed her because the pair are fairly close together. However, in the years between observations, they have separated and split into two shadows. Apparently, the two misaligned disks are like that, thanks to the gravitational pull of two planets in slightly different orbital planes. “We’ve never seen this before on a protoplanetary disk. This makes the system much more complex than we originally thought,” said Debes.
About the newborn star TW Hydrae
This newborn star is an interesting infant system. It is called the “T Tauri” star. That’s because it’s part of a class of variables no older than 10 million years. Astronomers consider it a “pre-main sequence” star. (That is, it did not start nuclear fusion or form a radiation zone). Many of this class of stars are found near giant molecular clouds. They are usually the youngest stars and can be quite active.
TW Hydrae fits the category really well. It accounts for about 80 percent of the Sun’s mass and is slightly larger than our star. Astronomers estimate it is around 8 to 10 million years old. The protoplanetary disc region surrounding it feeds the newborn star and apparently also forms planets. TW Hydrae is a member of a larger collection of young stars called the TW Hydrae Association.
Racing car planets in the protoplanetary disc
The disk structures in the TW Hydrae protoplanetary disk may be related to the effect of planets overlapping each other as they orbit. “It suggests that the two planets must be fairly close together,” Debes said. “If one had moved much faster than the other, earlier observations would have picked it up. It’s like two race cars driving close together, but one slowly overtakes and laps the other,” said Debes.
The suspected planets are in a region roughly the distance of Jupiter from our sun. And the shadows complete one revolution around the star about every 15 years. This corresponds roughly to the orbital period of a planet at this distance.
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 in our solar system. “This corresponds exactly to typical solar system-style architecture,” Debes said.
Given how closely this mimics what astronomers suspect is 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 disc on which the shadows fall can stretch quite a bit. Some scientists suggest that it may extend several times the radius of our solar system’s Kuiper Belt. Interestingly, this larger disc appears to have a gap twice the average distance of Pluto from the Sun. This could be an indication of a third planet in the system.
ALMA’s image of the planetary disk around newborn star TW Hydrae. It shows the classic rings and gaps that signify planets forming in this system.
It would be very difficult to see smaller, inner planets near the star. This is because they would be lost in the glare. Also, the system is quite dusty, making it difficult to spot planets. However, it is possible that the JWST will examine the system looking for more details on the hard drive. In addition, ESA’s Gaia space observatory could take some long-term measurements of the system to find wobbles in the star when Jupiter-mass planets pull on it. The Atacama Large Millimeter Array in Chile observed this system and provided very clear radio images of the protoplanetary disk around TW Hydrae. Future studies should reveal more details of this interesting newborn star and its evolving planetary system.
For more informations
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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