Our universe is so capricious that sometimes it likes to play hide and seek. In 2017, astronomers were surprised to see a massive shadow stretching across the disk of dust and gas surrounding the nearby young star TW Hydrae. Shadow launches an inner disk of dust and gas that is slightly inclined to the plane of the outer disk. The shadow is clearly visible only because the system is tilted to face Earth, allowing astronomers to get a bird’s eye view of the disk as the shadow sweeps around the disk like a clockwork hand.

However, the watch has two hands (hour and minute) that rotate at different speeds. And as it turns out, so is TW Hydrae. Astronomers used Hubble to find a second shadow emanating from another inner disk tilted toward the two outer disks. So the system looks increasingly complex, with at least three nested discs slightly inclined relative to each other. Disks are a proxy for invisible planets around the star. Each planet gravitationally pulls matter near the star and deforms what would have been a perfectly flat, flat-shaped disk if there were no planets. This is not surprising, because the planets in our solar system have orbital planes whose inclinations differ from each other by several degrees. TW Hydrae gives astronomers a glimpse of what our solar system might have looked like in its formative years.

Young star TW Hydrae plays the role of “shadow puppets” as scientists observe it with NASA’s Hubble Space Telescope.

In 2017, astronomers reported that they discovered a shadow passing over the surface of a huge pancake-shaped disk of gas and dust surrounding a red dwarf star. The shadow is not coming from the planet, but from the inner disk, which is slightly inclined relative to the much larger outer disk, causing it to cast a shadow. One explanation is that the gravity of an unseen planet pulls dust and gas into the planet’s inclined orbit.

Now, only a few years later, a second shadow has appeared, playing a peek game among the observations stored in the Hubble MAST archive. It could be from another drive in the system. These two disks probably point to a pair of planets under construction.

TW Hydrae is more than 10 million years old and is about 200 light-years away. In its infancy, our Solar System may be similar to the TW Hydrae system about 4.6 billion years ago. The TW Hydrae system is tilted almost face-down to our view from Earth, making it the optimal target for obtaining a direct view of the planetary construction site.

The second shadow was discovered during observations on June 6, 2021, as part of a multi-year program designed to track shadows in discs surrounding stars. John Debes of the European Space Agency’s AURA/STScI at the Space Telescope Science Institute in Baltimore, Maryland, compared the TW Hydrae disk with Hubble observations made several years ago.

“We found that the shadow did something completely different,” said Debes, the study’s principal investigator and lead author. Astrophysical Journal. “When I first looked at the data, I thought there was something wrong with the observation because it wasn’t what I expected. At first I was confused and all my colleagues were thinking: What’s going on ? We really had to scratch our heads and it took us a while to really come up with an explanation.”

The best solution the team came up with is two misaligned disks that cast shadows. During the previous observation, they were so close to each other that they were not noticed. Over time, they parted and split into two shadows. “We’ve never seen this before in a protoplanetary disk. This makes the system much more complex than we first thought,” he said.

The simplest explanation is that the disc misalignment is probably due to the gravitational pull of two planets in slightly different orbital planes. Hubble collects a holistic view of the system’s architecture.

The disks can be representative for planets intersecting each other in the orbit of the star. It’s like spinning two record plates at slightly different speeds. Sometimes the labels overlap, but then one precedes the other.

“This shows that the two planets must be pretty close together. If one were moving much faster than the other, it would have been noticed in previous observations. It’s like two race cars standing side by side, but one slowly overtakes the other,” Debes said.