Free Astronomy Magazine July-August 2023

24 JULY-AUGUST 2023 ASTRO PUBLISHING C omparison images from the NASA/ESA Hubble Space Telescope, taken several years apart, have uncovered two eerie shad- ows moving counterclockwise across a disc of gas and dust encircling the young star TW Hydrae. The discs are tilted face- on as seen from 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 [A] at the 11 o’clock position. This shadow is cast by an inner disc that is slightly inclined to the outer disc and so blocks starlight. The picture on the right shows a second shadow that emerged from yet another nested disc at the 7 o’clock position, as photographed in 2021. What was originally the inner disc is marked [B] in this later view. The shadows rotate around the star at different rates like the hands on a clock. They are evidence for two unseen planets that have pulled dust into their orbits. This makes them slightly inclined to each other. This is a visible-light photo taken with the Space Telescope Imaging Spectrograph. Artificial colour has been added to enhance details. [NASA, ESA, J. Debes STScI] ! The simplest explanation is that the misaligned discs are likely caused by the gravitational pull of two planets in slightly different orbital planes. Hubble is piecing together a holistic view of the architecture of the sys- tem. The discs may be proxies for planets that are lapping each other as they whirl around the star. It’s sort of like spinning two vinyl records at slightly different speeds. Sometimes the 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 no- ticed in earlier observations. It’s like two racing cars that are close to each other, but one slowly overtakes and laps the other,” said Debes. The sus- pected planets are located in a re- gion roughly the distance of Jupiter from our Sun. And the shadows com- plete one rotation around the star about every 15 years — the orbital period that would be expected at that distance from the star. Also, these two inner discs are in- clined by about five to seven degrees relative to the plane of the outer disc. This is comparable to the range of orbital inclinations inside our So- lar System. “This is right in line with typical Solar System-style architec- ture,” said Debes. The outer disc that the shadows are falling on may extend as far as sev- eral times the radius of our Solar Sys- tem’s Kuiper belt. This larger disc has a curious gap at twice Pluto’s aver- age distance from the Sun. This might be evidence for a third planet in the system. Any inner planets would be difficult to detect because their light would be lost in the glare of the star. Also, dust in the system would dim their reflected light. ESA’s Gaia space ob- servatory may be able to measure a wobble in the star if Jupiter-mass planets are tugging on it, but this would take years given the long or- bital periods. The TW Hydrae data are from Hub- ble’s Space Telescope Imaging Spec- trograph. The NASA/ESA/CSA James Webb Space Telescope’s infrared vi- sion may also be able to show the shadows in more detail.