How the Hubble telescope follows the shadow play around the planet-forming disk.

This artist’s concept is based on Hubble Space Telescope images of gas-and-dust disks around the young star TW Hydrae. These images show shadows sweeping across the disks, which are suggested to be from slightly inclined inner disks blocking starlight from reaching the outer disk. The suggestion is that these shadows are being cast due to the gravitational pull of unseen planets warping the disk structure, causing the disks to be slightly inclined to each other. This concept provides a unique insight into the formation of planets in terms of their gravitational effect on the disk structure, suggesting further evidence for planet formation.

The young star TW Hydrae has been under observation by scientists using NASA’s Hubble Space Telescope, and what they’ve found is truly remarkable. Instead of the usual gas and dust disk surrounding the star, there is an inner disk with a slight inclination relative to the outer disk, causing it to cast a shadow. This is likely due to an unseen planet’s gravity, which is pulling dust and gas into its inclined orbit. These findings are quite incredible, as if TW Hydrae were playing a game of ‘shadow puppets’ with scientists watching in awe.

The incredible discovery of a second, shadow-like presence between the observations stored in Hubble’s MAST archive, which was found to be playing a game of peek-a-boo, could indicate the presence of yet another disk nestled in the system. This finding would suggest that there could exist a pair of planets in the process of being formed. It is likely that this new discovery will help astronomers to gain greater insight into the formation process of planets and information about the conditions necessary for their creation. Furthermore, by learning more about this new discovery, it is hoped that more can be uncovered about the other disks around the system and if they too are part of the planet formation process.

TW Hydrae is a star system that is estimated to be less than 10 million years old and resides about 200 light-years away. In its infancy, our solar system may have been similar to the TW Hydrae system, some 4.6 billion years ago. This makes the TW Hydrae system an excellent target to observe how planetary systems form and evolve, as it is tilted nearly face-on in relation to Earth’s view. Recently, John Debes of AURA/STScI compared the TW Hydrae disk to Hubble observations taken several years ago and confirmed the second shadow in the circumstellar disk. This discovery was made as part of a multi-year program designed to track and study these shadows in order to learn more about planetary formation and evolution.

The study published in Astrophysical Journal, which reveals new findings about a protoplanetary disk – a rotating disk of gas and dust surrounding a young star that is believed to be a precursor to the formation of planets. The study’s principal investigator and lead author, Debes, explains that the team originally thought that the data they had gathered was incorrect because it did not match their expectations. After further analysis, they discovered that there were two misaligned disks casting shadows, which they had not noticed before. These disks were so close to each other in the earlier observation that they appeared as a single disk.

The team determined that the misaligned disks were likely caused by the gravitational pull of two planets in slightly different orbital planes. This discovery adds complexity to the system and provides a new understanding of the protoplanetary disk’s architecture. The Hubble telescope is helping to piece together a more comprehensive view of the system, providing new insights into the formation of planets.

The Comparison images from the Hubble Space Telescope, taken several years apart, have uncovered two eerie shadows moving counterclockwise across a gas-and-dust disk encircling the young star TW Hydrae. The disks are tilted face-on to Earth and so give astronomers a bird’s-eye view of what’s happening around the star. The left image, taken in 2016, shows just one shadow at the 11:00 o’clock position.

This shadow is cast by an inner disk that is slightly inclined to the outer disk and so blocks starlight. In 2021, the picture on the left shows a second shadow that has emerged from yet another nested disk at the 7:00 o’clock position. The original inner disk is marked in this later view. Both shadows rotate around the star at different rates indicating that two unseen planets have pulled dust into their orbits, making them slightly inclined to each other. This is a visible-light photo taken with the Space Telescope Imaging Spectrograph and artificial color has been added to enhance details.

The two disks detected by astronomers could be proxies for two planets orbiting the same star. The planets would be lapping each other as they whirl around their parent star, just like two vinyl phonograph records spinning at slightly different speeds, with their labels occasionally matching up before one overtakes the other. According to Debes, it means that the two planets must be fairly close to one another, since any significant difference in their speed would have been detected earlier. Furthermore, the shadow of the planets indicates that they are located roughly at the same distance as Jupiter is from our Sun, and have an orbital period of around 15 years. This is exactly what is expected of any planet found at that distance from its star.

The TW Hydrae system is composed of two inner disks that are inclined at an angle of five to seven degrees relative to the plane of the outer disk. This inclination is similar to the range of orbital inclinations found in our Solar System, making the architecture of the system comparable to that of our own. According to Debes, this indicates that it has a solar system style architecture. The outer disk is estimated to extend to several times the radius of the Kuiper Belt in our Solar System, and contains a curious gap at twice Pluto’s average distance from the Sun.

This could be evidence for a third planet in the system, however it would be difficult to detect due to its light being obscured and dimmed by both dust in the system and the glare of the star. The TW Hydrae data was collected using Hubble’s Space Telescope Imaging Spectrograph, and the James Webb Space Telescope’s infrared vision may be able to show more detail in the shadows. With its longer orbital periods, it may take years for ESA’s Gaia space observatory to measure a wobble in the star if Jupiter-mass planets are tugging on it, but it is worth pursuing nonetheless.