Planets

The search for planets orbiting other stars in our galaxy (exoplanets/extrasolar planets) is a fascinating and rapidly growing field of research. Most of the >200 exoplanets discovered so far have been found by indirect observation methods, eg by radial velocity ("stellar wobble") and transit methods. There are several recent claims for directly imaged very young planets (<10 million years old), but the exact evolutionary status of these objects is controversial. To date, no-one has directly seen an old, mature bona-fide exoplanet like the Jovian gas giants in our solar system.

What makes directly observing planets so difficult is their proximity to their parent star. Even the closest stars are several light years away - so when looking for exoplanets we are trying to resolve very small angles on the sky. Now add to this the fact that the star produces a billion times more light than the planet, making it near enough impossible to see the planet against the star's glare.

In contrast white dwarfs are excellent targets for direct imaging searches for planets since they are up to 10,000 fainter than their main sequence progenitors, giving a significant gain on the brightness problem.

Also the orbits of planets outside the red giant envelope are expected to widen considerably, making them far easier to resolve. Even when we cannot resolve a companion to a white dwarf, detection is still possible by looking for excess infra-red (IR) emission, since the brightness contrast between white dwarfs, planets and brown dwarfs is strongest at IR wavelengths. In addition, sometimes excess IR emission in white dwarfs can be explained by the presence of a circumstellar dust disk. The origins of these disks are still unclear but could be evidence for the remains of a planetary system, for example an asteroid that strayed too close to the white dwarf and was ripped apart, leaving a dust disk behind.

At Leicester University we are actively involved in the search for both resolved and unresolved companions and circumstellar dust disks around white dwarfs.

For further information about our current research, please see the Extrasolar Planets pages.

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