In 2017, astronomers reported the discovery of a shadow stretching across the surface of a massive, flat-shaped disk of gas and dust surrounding a red dwarf star. The shadow is not 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, just a few years later, a second shadow has appeared among the observations stored in Hubble’s MAST archive—playing a peeping game. 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 is doing something completely different,” said Debes, principal investigator and lead author of the study, published in The 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 coworkers were like what was going on? We really had to scratch our heads and it took a while to find 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.

Comparative images taken by the Hubble Space Telescope several years apart have revealed two startling shadows moving counterclockwise across the gas dust disk surrounding the young star TW Hydrae.

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 suggests 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 that are close together but one is slowly overtaking the other,” Debes said.

The predicted planets are located in a region related to Jupiter’s distance from our Sun. And the shadows make a revolution around the star approximately every 15 years – this is the orbital period that can be expected this far from the star.

Additionally, these two inner discs are inclined 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 matches exactly with the typical architecture of the solar system,” Debes said.

The outer disk on which shadows fall may extend to several times the radius of our solar system’s Kuiper belt. This larger disk has an incredible gap at twice the average distance of Pluto from the Sun. This could be evidence of the existence of a third planet in the system.

Any inner planets will be difficult to detect because their light will be lost in the star’s glow. Also, the dust in the system will reduce the reflected light. ESA’s Gaia space observatory can measure star wobble if Jupiter-massive planets tug on it, but given their long orbital periods this will take years.

TW Hydrae data is from the Hubble Space Telescope Imaging Spectrograph. The infrared vision of the James Webb Space Telescope can also show shadows in greater detail.

The Hubble Space Telescope is an international collaborative project between NASA and ESA. The telescope is operated by NASA’s Goddard Space Flight Center in Greenbelt, Maryland. The Space Telescope Science Institute (STScI) in Baltimore runs Hubble’s science operations. STScI is administered by the Association of Astronomical Research Universities for NASA in Washington, DC.