Optical-to-near-infrared Simultaneous Observations for the Hot Uranus GJ3470b: A Hint of a Cloud-free Atmosphere

We present optical (g', R-c, and I-c) to near-infrared (J) simultaneous photometric observations for a primary transit of GJ3470b, a Uranus-mass transiting planet around a nearby M dwarf, by using the 50 cm MITSuME telescope and the 188 cm telescope, both at the Okayama Astrophysical Observatory. From these data, we derive the planetary mass, radius, and density as 14.1 +/- 1.3M(circle plus), 4.32(-0.10)(+0.21) R-circle plus, and 0.94 +/- 0.12 g cm(-3), respectively, thus confirming the low density that was reported by Demory et al. based on the Spitzer/IRAC 4.5 mu m photometry (0.72(-0.12)(+0.13) g cm(-3)). Although the planetary radius is about 10% smaller than that reported by Demory et al., this difference does not alter their conclusion that the planet possesses a hydrogen-rich envelope whose mass is approximately 10% of the planetary total mass. On the other hand, we find that the planet-to-star radius ratio (R-p/R-s) in the J band (0.07577(-0.00075)(+0.00072)) is smaller than that in the I-c (0.0802 +/- 0.0013) and 4.5 mu m (0.07806(-0.00054)(+0.00052)) bands by 5.8% +/- 2.0% and 2.9% +/- 1.1%, respectively. A plausible explanation for the differences is that the planetary atmospheric opacity varies with wavelength due to absorption and/or scattering by atmospheric molecules. Although the significance of the observed R-p/R-s variations is low, if confirmed, this fact would suggest that GJ3470b does not have a thick cloud layer in the atmosphere. This property would offer a wealth of opportunity for future transmission-spectroscopic observations of this planet to search for certain molecular features, such as H2O, CH4, and CO, without being prevented by clouds.