Limits to Transits of the Neptune-Mass Planet Orbiting GJ 581

We have monitored the Neptune-mass, exoplanet-hosting M dwarf GJ 581 with the 1 m Swope Telescope at Las Campanas Observatory over two predicted transit epochs. A neutral density filter centered at 550 nm was used during the first epoch, yielding 6.33 hr of continuous light-curve coverage with an average photometric precision of 1.6 mmag and a cadence of 2.85 minutes. The second epoch was monitored in the B band over 5.85 hr, with an average photometric precision of 1.2 mmag and a 4.28 minute cadence. No transits are apparent on either night, indicating that the orbital inclination is less than 88.1d for all planets with radii larger than 0.38R_Nep (=1.48 R_⊕). Because planets with the most likely interior compositions will have radii larger than 1.55 R_⊕, we place an inclination limit of 88.1d for the system. The corresponding minimum mass of the exoplanet GJ 581b remains 0.97M_Nep (=16.6 R_⊕).

Based on observations obtained with the 1 m Swope Telescope at Las Campanas Observatory, which is operated by the Carnegie Institution of Washington.