The TW Hydrae star system has been caught playing a game of shadow puppets with NASA’s Hubble Space Telescope. The young star, which is less than 10 million years old and located around 200 light-years away, has been observed by astronomers who have discovered two shadows sweeping across the face of a vast pancake-shaped gas-and-dust disk surrounding the red dwarf star. The first shadow was discovered in 2017 and was caused by an inner disk slightly inclined relative to the much larger outer disk. The second shadow was discovered in observations obtained on June 6, 2021, and could be from yet another disk nestled inside the system. The two disks are likely evidence of a pair of planets under construction.
The TW Hydrae system is tilted nearly face-on to our view from Earth, making it an optimum target for getting a bull’s-eye-view of a planetary construction yard. John Debes of AURA/STScI for the European Space Agency at the Space Telescope Science Institute in Baltimore, Maryland, compared the TW Hydrae disk to Hubble observations made several years ago and discovered that the second shadow had done something completely different. The best solution the team came up with is that there are two misaligned disks casting shadows, likely caused by the gravitational pull of two planets in slightly different orbital planes.
The disks may be proxies for planets that are lapping each other as they whirl around the star. It’s sort of like spinning two vinyl phonograph records at slightly different speeds. Sometimes labels will match up, but then one gets ahead of the other. The suspected planets are located in a region roughly the distance of Jupiter from our Sun, and the shadows complete one rotation around the star about every 15 years – the orbital period that would be expected at that distance from the star.
The outer disk that the shadows are falling on may extend as far as several times the radius of our solar system’s Kuiper belt. This larger disk has a curious gap at twice Pluto’s average 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. ESA’s Gaia space observatory 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 orbital periods.
The TW Hydrae data are from Hubble’s Space Telescope Imaging Spectrograph. The James Webb Space Telescope’s infrared vision may also be able to show the shadows in more detail. The Hubble Space Telescope is a project of international cooperation between NASA and ESA, managed by NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and conducted by the Space Telescope Science Institute (STScI) in Baltimore. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.
