Planetpol polarimetry of the exoplanet systems 55 Cnc and τ Boo ⋆

We present very sensitive polarimetry of 55 Cnc and τ 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 nightly averaged Stokes Q/I and U/I parameters is 2.2×10. We derive upper limits on the geometric albedo, AG and planetary radius using Monte Carlo multiple scattering simulations of a simple model atmosphere. We assume Rayleigh-like scattering and polarisation behaviour (scaled by the maximum polarisation, pm at 90 ◦) and pressure insensitive extinction. Atmospheres in which multiple scattering plays only a small role have an almost linear relation between polarisation and AG. In this case, the 4σ upper limit is AG < 0.13(R/1.2RJup) p m for 55 Cnc e. This is most easily explained if 55 Cnc e is relatively small, like GJ436b, and therefore not a pure H-He planet. The data do not provide a useful upper limit for 55 Cnc b. τ Boo is orbited by an unusually massive hot Jupiter planet. The data show a standard deviation in the night to night average Stokes Q/I and U/I polarisation parameters of 5.1×10. The 4σ upper limit is AG < 0.37(R/1.2RJup) p m for τ Boo b, adopting the fairly well established orbital inclination i∼40◦. This extends the similar upper limits reported previously for this planet to longer wavelengths. The fact that the τ Boo data show more scatter, despite the smaller photon noise for this bright star, may be due to the spot activity detected photometrically by the MOST satellite. These results contrast markedly with the recent claim of a 3σ detection of a periodic polarisation signal from HD189733 with amplitude P= 2× 10, attributed to the planet HD189733 b.