Hubble Space Telescope casts shadows on young stars

Hubble Space Telescope casts shadows on young stars

Researchers say the shadows indicate that two entirely new planets are forming in the system.

About 200 light-years away, we find the very young star TW Hydrae. This nearby star is less than 10 million years old and has been in the sights of astronomers for several years. That’s because TW Hydrae is known to contain what’s called a protoplanetary disk. A disk of gas and dust in which planets are forming. With the help of Hubble, researchers have now detected two strange shadows moving across this massive disk. And that’s interesting. Because these shadows may be the result of two daylight-seeing planets in the system.

Astronomers have already detected the first shadow in 2017. It is not a planet that is casting this shadow, but an inner ring. This inner ring is tilted slightly against the larger outer disk, blocking out the starlight. Now a second shadow has formed in just a few years, which has baffled astronomers.

Two Hubble images, taken several years later. In the left image, there is a single shadow in the boxed area. Two shades are highlighted on the right. Image: NASA, ESA, J.Debes STScI

The second shadow was detected in June 2021 as part of a multi-year program designed to study shadows in debris disks around stars. The researchers then compared the most recent observations of the TW Hydrae disk with Hubble observations from several years ago. “When I first looked at the data, I thought something had gone wrong,” said researcher John Debs. “I was shocked. We really scratched our heads and it took us a while to find an explanation.”

a permit
The best explanation the researchers have come up with so far is that shadows are cast in the system by two skewed loops, they write in Astrophysical Journal. The detected shadows likely originate from slightly tilted inner rings that prevent starlight from reaching the outer disk and thus cast a shadow. The fact that Shadow Two didn’t appear in 2017 may have something to do with the two episodes’ close alignment at the time. As a result, there seemed to be only one shadow. Over time, the rings separated, leaving two different shades. Shadows rotate around the star at different speeds, like the hands of a clock. “We’ve never seen this on a protoplanetary disk,” Debs says. “It makes the system much more complex than we originally thought.”

The researchers suspect that these “skewed” rings are evidence of two planets under construction. They are pulled together slightly by the gravitational force of the unseen planets distorting the disk structure. “We think the two planets are very close to each other,” says Debs. “If one of them was moving much faster than the other, this would have been observed in previous observations. It is like two racing cars close together, but one is slowly overtaking the other.”

In this illustration, we have an oblique view of three concentric rings of dust and gas. In the center is the bright white glow of the central star. The red rings are tilted toward each other, casting dark shadows on the outer disk. Image: NASA. ESA, L. Hustak (STScI)

The suspected planets are very far from TW Hydrae, compared to Jupiter’s distance from our sun. The shadows complete one orbit around the star about every 15 years – an expected orbital period for planets at this distance from their parent star. In addition, the two inner rings are inclined by approximately five to seven degrees to the plane of the outer disk. This is also in line with what we see in our own solar system.

More planets
Overall, the results indicate the emergence of two new planets around TW Hydrae. And it may not stop there. Researchers have discovered a strange hole on the outer disk (where the shadows fall), about twice the average distance from the dwarf planet Pluto to our sun. This means that another planet may form in the system. It is difficult to say whether TW Hydrae also hosts closer planets. Inner planets are difficult to detect because the light they reflect is lost in the star’s glow. Moreover, it is often hidden behind built-in fabric curtains. The Gaia space telescope may be able to see the star “wobble” as it is affected by the massive body’s gravity. But given the long orbital times, that could take years.

Astronomers will continue to closely monitor the intriguing TW Hydrae system. This system can also reveal many secrets about our solar system. For example, researchers suspect that our solar system was very similar to the TW Hydrae about 4.6 billion years ago. Moreover, it is positioned in such a way that we can look at it directly from the ground. This allows us to perfectly follow the evolution of the system and the formation of planets.

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