Despite the detection of more than 100 extrasolar planets through their effect on the radial velocity of the stars they orbit, no extrasolar planet has yet been seen directly by its emitted or reflected light. Detections by spectroscopic  techniques have so far been unsuccessful while photometric detection  requires accuracies which are beyond that which can currently be achieved.

We have suggested that planets orbiting close to their star (hot Jupiters) could be detected by the polarization of the light scattered from their atmospheres. While the resulting polarization in the combined light of the planet and star is small (10^-5 to 10^-6), polarization measurments can in principle be made with very high sensitivity since polarimetry is a differential measurement and is not limited by the stability of the earth's atmosphere as photometry is.

We have designed a stellar polarimeter which should be capable of achieving the required sensitivity. With such an instrument on a 4m or larger telescope we should be able to detect the polarization  signature from the planets of bright hot Jupiter systems such as Tau Boo, Upsilon And or 51 Peg.

The polarimeter has been built at University of Hertfordshire in the UK in collaboration with the Australian Centre for Astrobiology at Macquarie University.

Polarimeter Features:

- Photoelastic modulator operating at 20kHz. The high modulation speed
  makes system insensitive to seeing and tracking drifts which are on
  slower timescales.

- Wollaston prism as analyser and beam splitter.

- Detectors are thermoelectrically cooled avalanche photodiodes (APDs) with
  analogue signal processing.

   -   High quantum efficiency  (70%)
   -   Linear up to very high photon rates
   -   Frequency response DC-70kHz
   -   Low noise 10^-15 WHz^-1/2

- Synchronous detection of 2nd harmonic signal (40kHz) to measure Stokes Q and U

- DC channel to measure Stokes I.