A unique method reveals the influence of a planet within the stellar disk around TW Hydrae.
Visualization of shadow traveling around the disk surrounding star TW Hydrae. The black circle in the center shows the region blocked by the coronograph. NASA / Space Telescope Science Institute
A fascinating bit of astronomical sleuthing spanning more than a decade may have unveiled an exoplanet orbiting the star TW Hydrae.
The hypnotic view, gathered over 18 years by the Hubble's Space Telescope's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) and later using the Space Telescope Imaging Spectrograph suggests that a shadow cast along the dust disk circles the young star every 16 years.
“This is the first well-characterized shadow that has clear and well-measured motion across many epochs,” says John Debes (Space Telescope Science Institute). “A few other shadows have been reported for disks, but the origin and nature of the shadows is poorly known and based on one or two images."
"The discovery will accelerate the progress of looking at these kinds of disks more frequently so as to see how common moving shadows are,” he adds.
The Shadow Knows
Astronomers think the shadow isn't cast by the exoplanet itself but by an inner disk of dust and gas that's tilted relative to the orbital plane.
The asymmetrical brightness of the dust disk that surrounds TW Hydrae was first noticed back in 2005, but the lack of further observations prevented firm conclusions. Debes and colleagues dug back in the archives, compiling observations of the irregular disk taken back to 1998. They also asked Hubble to take new images, one in 2015 and one in 2016, using a coronagraph to block the star's light and get a better look at its disk.
All of the images, spanning 18 years in all, used the NICMOS camera, which was installed during the second Hubble servicing mission, STS-82, in 1997. The observations showed that the shadow was moving, circling the star at a rate of 22.7° per year, equivalent to a 16-year period. That's fast — especially for a shadow with such a long cast, spanning 50 to 150 astronomical units (a.u.). (Neptune's average distance from the Sun, for comparison, is 30 a.u.)
Nothing so far from the star should move that swiftly, so it was immediately clear that the observations revealed not an object, but a shadow of something in the inner disk. The cause? A Jupiter-sized world (out of sight behind the coronograph) only 1 a.u. from its host star. Though the world itself remains unseen, it betrays its presence by pulling on the inner disk, tilting it and misaligning it to the outer disk. The tilted inner disk casts a shadow that precesses, or wobbles, around the star every 16 Earth years.
Observations from the Atacama Large Millimeter/submillimeter Array (ALMA), based in the Atacama desert in Chile, support the existence of an inner dust disk. ALMA revealed a suspicious gap just less than 1 a.u. from the host star, right around where the suspected planet should be.
The Strange System of TW Hydrae
Located just 194 light-years from Earth in the southern constellation Hydra, the Sea Serpent, TW Hydrae is an 11th-magnitude, T Tauri-type star that's just 8 million years old. We happen to have a nearly face-on view of the main disk surrounding TW Hydrae, tilted just seven degrees from our line of sight.
The find pioneers a new technique in the hunt for exoplanets. Catching the shadow cast by an inner disk allows us to reveal newborn planets that wouldn't be seen otherwise. Coronagraphs and adaptive optics are key technologies for direct-imaging telescopes, which aim to resolve exoplanets and protoplanetary disks.
Future missions may shed more light on the system. James Webb Space Telescope could, for example, track the shadow of the TW Hydrae disk at different wavelengths, and missions currently in space such as European Space Agency's Gaia may witness the central star's wobble caused by the exoplanet's gravitational tug.
“It's just amazing whenever something that spans billions of miles across changes in our lifetime,” says Debes. “It's exciting to think that the shadow was present in data taken when I was getting ready to graduate college, and the most recent data was taken just 18 years later.”
Indeed, astronomy induces humility, as we see changes in early proto-solar disks millions of years old take place over a human lifetime.
Congrats to the Space Telescope institute team, on a fine piece of astronomical detective work.