Planetpol polarimetry of the exoplanet systems 55 Cnc and tau Boo

We present very sensitive polarimetry of 55 Cnc and tau Boo in an attempt to detect the partially polarised reflected light from the planets orbiting these two stars. 55 Cnc is orbited by a hot Neptune planet (55 Cnc e) at 0.038 AU, a hot Jupiter planet (55 Cnc b) at 0.11 AU, and at least 3 more distant planets. The polarisation of this system is very stable, showing no sign of the periodic variations that would be expected if a short period planet were detected. The measured standard deviation of the night averaged Stokes Q/I and U/I parameters is 2.2x10^{-6}. We derive upper limits on the geometric albedo, A_G and planetary radius using Monte Carlo multiple scattering simulations of a simple model atmosphere. We assume Rayleigh-like scattering (scaled by the maximum polarisation, p_m at 90 degrees) and pressure insensitive extinction. Atmospheres in which multiple scattering plays only a small role have an almost linear relation between polarisation and A_G. In this case, the 4 sigma upper limits is A_G<0.13(R/1.2 R_{Jup})^{-2}p_m^{-1} for 55 Cnc e. This suggests that 55 Cnc e is relatively small and not a pure H-He planet. The data do not give a useful upper limit for 55 Cnc b. tau Boo is orbited by an unusually massive hot Jupiter planet. The standard deviation in the night averaged Stokes Q/I and U/I polarisation parameters is 5.1x10^{-6}. The 4 sigma upper limit is A_G<0.37(R/1.2 R_{Jup})^{-2}p_m^{-1} for tau Boo b. This extends the similar upper limits reported previously for this planet to longer wavelengths. The larger scatter in the tau Boo data may be due to the spot activity detected photometrically by the MOST satellite. These results contrast markedly with the recent claim of a 3 sigma detection of a periodic polarisation signal from HD189733 with amplitude P=2x10^{-4}, attributed to the planet HD189733 b.