Searching for planets in other solar systems is more like a detective game than something you actually see. Worlds, unlike the stars they orbit, do not shine, and the intense glow from their host stars often keeps them hidden in the glare. So astronomers have to look for other clues of their existence, such as looking for their influence on a star's wobble.
Now an analysis of 18 years of observations by the Hubble Space Telescope of a star (TW Hydrae), which lies 192 light-years from Earth, may have revealed a new way to search for exoplanets. A research team noticed a strange shadow sweeping across the face of the vast gas-and-dust disk surrounding the young star, and the best explanation available for the shadow is that it represents a new world.
"This is the very first disk where we have so many images over such a long period of time, therefore allowing us to see this interesting effect," said John Debes, lead author on the study, in a statement. "That gives us hope that this shadow phenomenon may be fairly common in young stellar systems."
What a shadow tells you
The detective story is more complicated than just looking at shadows of planets, however. The shadow is not actually being cast by the planet directly. Rather, the planet is having a gravitational influence on material near the star, thus warping the inner part of the disk. It's this twisted, odd-shaped inner disk that is casting its shadow across the surface of the outer disk. So there are a few steps involved to infer the existence of the planet, but astronomers don't know what else it could be.
The shape and nature of the shadow can reveal a surprising amount of information about the planet that ultimately creates it. Calculations suggest that this planet lies about 100 million miles from TW Hydrae, which is about the distance from Earth to our sun. The comparison to Earth probably ends there, though. This exoplanet is likely five times more massive than Jupiter, so it's huge. But even a planet of this size could not have been detected using conventional methods, since it sits so close to its star.
"What is surprising is that we can learn something about an unseen part of the disk by studying the disk's outer region and by measuring the motion, location, and behavior of a shadow," explained Debes. "This study shows us that even these large disks, whose inner regions are unobservable, are still dynamic, or changing in detectable ways which we didn't imagine."