A Direct Imaging Search for Substellar and Planetary Mass Companions around White Dwarfs

Even though the radial velocity technique has detected almost all of the 269 extrasolar planets discovered to date, this method does not directly observe the light from the planet. The ability to directly detect this light would allow spectra of extrasolar planets to be obtained, providing information about their information and evolution through the investigation of their composition and structure. To date, none of the extrasolar planets found using the radial velocity technique have been directly imaged, as these faint companions are too close to their bright parent stars. White dwarfs are intrinsically faint objects and can be upto 10,000 times less luminous than their main sequence progenitors, substantially increasing the probability of directly imaging an extrasolar planet in orbit around them. The Degenerate Objects around Degenerate Objects (DODO) survey aims to obtain a direct image of an extrasolar planet in a wide orbit around a white dwarf. By acquiring J band images of 26 equatorial and northern hemisphere white dwarfs a year or two apart, common proper motion companions to the white dwarfs can be identified. The discovery of such a system could supply new information on the frequency and mass distribution of extrasolar planets around intermediate mass main sequence starts and confirm whether these companions can survive the final stages of stellar evolution. In addition, the direct detection of an extrasolar planet in orbit around a white dwarf would allow the spectroscopic investigation of planets much older than any previously found. Using the 24 white dwarfs in the DODO survey within 20pc, the frequency of substellar companions with effective temperatures > 500K and projected physical separations from the white dwarf between 60-200AU is estimated to be <5%. For the same range of projected physical separations, the frequency of substellar companions with masses >10Mjup is estimated to be<9%.