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 ± 1.3 M _⊕, 4.32^{+0.21}_{-0.10} R _⊕, 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 μm photometry (0.72^{+0.13}_{-0.12} 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^{+0.00072}_{-0.00075}) is smaller than that in the I _c (0.0802 ± 0.0013) and 4.5 μm (0.07806^{+0.00052}_{-0.00054}) 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.