This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Exoplanetary systems seem to have more tricks up their sleeves than we can imagine. Take for example the system TW Hydrae, some 192 light years away.
This is an extremely youthful stellar environment, clocking in at about 8 to 10 million years old. The central star (TW Hydrae) is estimated to be about 80% the mass of our modern Sun and is still surrounded by a disk of proto-planetary gas and dust, seen almost face-on from our vantage point.
Although this system has been studied by the Hubble Space Telescope for almost two decades, it wasn't until 2005 that data from the Space Telescope Imaging Spectrograph (STIS) showed a hint of a strangely asymmetric brightness in the disk. This prompted Debes et al. to gather data from multiple years to try to piece together the story.
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Remarkably, over a period of some 16 years the TW Hydrae disk showed a clear cycle of changes entirely incompatible with any normal orbital motion. In addition to established features, like dark gaps or 'lanes' that hint at the presence of forming planets, a darkened pie-slice segment of the disk seems to rotate around the star as if it's a single, solid structure.
Dark 'segment' moves by 20 degrees from 2015 to 2016. Credit: NASA, ESA, and J. Debes (STScI))
The scale of this feature is enormous. The Hubble instrument and image processing blocks light from within 1 billion miles of the central star, and the disk is imaged to a distance of at least 10 billion miles further out.
The current best-bet hypothesis is that what we're seeing is a bit of an optical illusion. A Jupiter-class planet orbiting within 100 million miles of the star (inside the blanked out part of the image) would perturb the proto-planetary disk through its gravitational pull. This perturbation can cause the inner part of the disk to warp, or mis-align with the plane of the larger disk (a fact that may be corroborated by new data from the ALMA observatory).
That warped, wobbling inner disk can cast a shadow across the outer disk. If it wobbles (precesses) with a 16 year period it would neatly match what we see happening from Earth. Here's a handy cartoon illustration of the proposed configuration.
What may be happening in this system. Credit: NASA, ESA, and A. Feild (STScI))
While this is currently only a working hypothesis, it does seem like an elegant and plausible solution to a very dramatic observation. If correct it also suggests that we might end up seeing more examples of this phenomenon in other young planetary systems - offering a new probe of the inner workings of their formation.
Finally, here's an animation of the shadow model - as it would appear without all the noise and confusion of the real data:
Video Credit: NASA, ESA, and J. Debes (STScI)
