Planetpol polarimetry of the exoplanet systems 55 Cnc and τ Boo

We present very sensitive polarimetry of 55 Cnc and tau Boo in an attempt to detect the partially polarized 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 three more distant planets. The polarization 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 x 10(-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 and polarization behaviour ( scaled by the maximum polarization, 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 polarization and AG. In this case, the 4 sigma upper limit is A(G) < 0.13(R/1.2R(Jup))(-2)p(m)(-1) m for 55Cnce. This is most easily explained if 55 Cnc e is relatively small, like GJ 436b, and therefore not a pure H-He planet. The data do not provide a useful upper limit for 55 Cnc b. t 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 polarization parameters of 5.1 x 10(-6). The 4 sigma upper limit is AG < 0.37(R/ 1.2R(Jup))(-2)p(m)(-1) m for tau Boo b, adopting the fairly well established orbital inclination i similar to 40 degrees. This extends the similar upper limits reported previously for this planet to longer wavelengths. The fact that the tau 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 Microvariability & Oscillations of Stars (MOST) satellite. These results contrast markedly with the recent claim of a 3 sigma detection of a periodic polarization signal from HD 189733 with amplitude P = 2 x 10(-4), attributed to the planet HD 189733 b.