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 ⊕ , 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 ( 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 ( ) is smaller than that in the I c (0.0802 ± 0.0013) and 4.5 μm ( ) 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 H 2 O, CH 4 , and CO, without being prevented by clouds.