The Hubble Space Telescope has made multiple significant discoveries about planet formation and protoplanetary disks around young stars. These discoveries have shed light on the mechanisms that lead to the formation of planets and planetary systems. TW Hydrae, a newborn star, is playing “shadow puppets” with scientists using NASA’s Hubble Space Telescope to observe it.
Astronomers discovered a shadow racing across the face of a massive pancake-shaped gas-and-dust disk encircling the red dwarf star in 2017. The shadow is formed by an inner disk that is slightly tilted relative to the much larger outer disk, causing it to cast a shadow. One possibility is that the gravity of an invisible planet pulls dust and gas into the planet’s tilted orbit.
In just a few years, a second shadow, playing peek-a-boo, has arisen between observations held in Hubble’s MAST archive. This could be from another disk hidden somewhere inside the system. The two disks are most likely indications of a pair of planets in the making.
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.
John Debes
TW Hydrae is less than 10 million years old and is located approximately 200 light-years away. Our solar system may have resembled the TW Hydrae system in its infancy 4.6 billion years ago. The TW Hydrae system is an ideal target for gaining a bull’s-eye view of a planetary construction yard because it is virtually face-on to our view from Earth.
The second shadow was identified during observations on June 6, 2021, as part of a multi-year initiative to track shadows in circumstellar disks. At the Space Telescope Science Institute in Baltimore, Maryland, John Debes of AURA/STScI for the European Space Agency compared the TW Hydrae disk to Hubble images obtained several years ago.
“We found out that the shadow had done something completely different,” said Debes, main investigator and lead author of the study published in The Astrophysical Journal. “When I first saw the data, I thought something had gone wrong with the observation because it wasn’t what I expected. I was perplexed at first, and all of my collaborators were wondering what was going on. We really had to scratch our heads and it took us a while to actually figure out an explanation.”
The team’s best solution is that there are two misaligned disks casting shadows. They were so close in the previous inspection that they were overlooked. They’ve now separated and split into two shadows throughout time. “We’ve never seen anything like this before on a protoplanetary disk. It makes the system far more complex than we anticipated,” he explained.
The mismatched disks are most likely created by the gravitational force of two planets in slightly different orbital planes. Hubble is building together a comprehensive view of the system’s architecture.
The disks may be proxies for planets that are lapping each other as they whirl around the star. It’s sort of like spinning two vinyl phonograph records at slightly different speeds. Sometimes 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 race cars that are close to each other, but one slowly overtakes and laps the other,” said Debes.
The suspected planets are in an area around the size of Jupiter from our Sun. Furthermore, the shadows complete one rotation around the star every 15 years – the orbital period expected at that distance from the star. Furthermore, the two inner disks are tilted by around 5 to 7 degrees relative to the plane of the outer disk. This corresponds to the range of orbital inclinations inside our solar system. “This is right in line with typical solar system style architecture,” Debes stated.
The outer disk on which the shadows fall may stretch several times the radius of our solar system’s Kuiper belt. This bigger disk features an unusual gap at twice the typical distance between Pluto and the Sun. This could be proof for the presence of a third planet in the system.
Any inner planets would be difficult to identify since their light would be obscured by the star’s brightness. Furthermore, dust in the system would reduce the reflected light. The ESA’s Gaia space observatory may be able to detect a wobble in the star if Jupiter-mass planets are pushing on it, but because to the long orbital periods, this would take years.
Hubble’s Space Telescope Imaging Spectrograph provided the TW Hydrae data. The infrared vision of the James Webb Space Telescope may also be able to see the shadows in greater detail. The Hubble Space Telescope is a NASA-ESA international collaboration project. The telescope is managed by NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Hubble science operations are managed by the Space Telescope Science Institute (STScI) in Baltimore. The Association of Universities for Research in Astronomy in Washington, D.C. operates STScI for NASA.
