Are mirror planets opaque?

Abstract

Over the last few years, many close orbiting ($\sim 0.05$ A.U.) large mass planets ($\sim M_{J}$) of nearby stars have been discovered. Their existence has been inferred from tiny Doppler shifts in the light from the star and in one case a transit has been observed. Because ordinary planets are not expected to be able to form this close to ordinary stars due to the high temperatures, it has been speculated that the close-in large planets are in fact exotic heavenly bodies made of mirror matter. We show that the accretion of ordinary matter onto the mirror planet (from e.g.the solar wind from the host star) should make the mirror planet opaque to ordinary radiation with an effective radius ($R_p$) large enough to explain the measured size of the transiting close-in extrasolar planet, HD209458b. Furthermore we obtain the rough prediction that $R_{p} \propto \sqrt{T_s\over M_p}$ (where $T_s$, is the surface temperature of the ordinary matter in the mirror planet and $M_p$ is the mass of the mirror planet) which will be tested in the near future as more transiting planets are found. We also show that the mirror world interpretation of the close-in extra solar planets explains the low albedo of $τ$ Boo b because the large estimated mass of $τ$ Boo b ($\sim 7M_J$) implies a small effective radius of $R_p \approx 0.5R_J$ for $τ$ Boo.