2013ApJ...770...95F

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_–0.10^+0.21 R_⊕, and 0.94±0.12 g/cm3, respectively, thus confirming the low density that was reported by Demory et al. based on the Spitzer/IRAC 4.5 µm photometry (0.72_–0.12^+0.13 g/cm3). 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 µm (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_svariations 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.