These images were taken a year apart by the Hubble Space Telescope. They reveal a shadow moving counterclockwise around a gas-and-dust disk encircling the young star TW Hydrae. The two images at the top show an uneven brightness across the disk. In the bottom pair of images, which have have been subjected to enhanced image processing, the darkening becomes even more apparent. The dimmer areas of the disk are caused by a shadow spreading across the outer disk. The dotted lines approximate the shadow’s coverage, and the long arrows show how far the shadow has moved in a year (from 2015 to 2016), roughly 20 degrees. Using Hubble archival data, astronomers have determined that the shadow completes a rotation around the central star every 16 years. The feature is certainly a shadow because dust and gas in the disk do not orbit the star nearly that quickly. The, the feature must not be caused by a part of the physical disk. The shadow might be caused by the gravitational effect of an unseen planet orbiting close to the star. Such a planet could disturb material from the main disk, creating a warped inner disk, and the twisted disk might block light from the star, casting a shadow onto the disk’s outer region.
Image Credits: NASA / ESA