An outline of the paper: Optical-to-near-infrared Simultaneous Observations for the Hot Uranus GJ3470b: A Hint of a Cloud-free Atmosphere by Fukui et al.
The discovery of low-mass planets orbiting nearby low-mass stars have recently allowed us to study their atmospheres. However, despite being within the technical capabilities of current instruments, the detection of atmospheric features in the atmospheres of low-mass planets has not been particularly fruitful. The classic examples is the super-Earth GJ 1214b, a planet with a mass ~7 times that of Earth, which so far has been found by most studies to display a featureless transmission spectrum over both optical near-IR wavelengths (see previous posts). Another example is GJ 436b where no firm molecular feature has been detected, most likely due to instrumental systematics and stellar activity (see here for a summary).
In the pursuit of finding atmospheric features in planets less massive than hot Jupiters, Fukui and his team of Japanese collaborators have been observing GJ 3470b, a Uranus-mass transiting planet, which orbits an M-star, and is thought to have an extended, largely hydrogen-dominated atmosphere capable of displaying large spectral features. Fukui and his team used two different telescopes to simultaneously obtain observations in both optical (g’, Rc and Ic) and near-IR (J) wavelengths. The strength of doing simultaneous observations is that the effects of stellar activity, such as passing star spots, will not influence the observations significantly, as the star spots which might be present will be observed at all wavelengths simultaneously. Had the observations been done weeks or months apart, it would be necessary to know how the brightness of the host star had changed in the interval, because what matters in transit spectroscopy is the relative depth variations (for a refresher on Transmission Spectroscopy see here).
Fukui and his team detect an increase in the planet-to-star radius ratio in the optical (Ic) compared to the near-IR (J) and Spitzer 4.5 μm wavelengths, which they claim could be a detection of an atmosphere and is unlikely to be due to systematic effects or to stellar activity. They do, however, caution that the statistical significance of their detection is low.
If GJ 3470b is in fact not obscured by clouds then it will be an ideal target for looking at molecular features such as water, methane and CO in the atmospheres of low-mass planets. It is worth noting that these observations were done using a 50 cm and a 180 cm telescope which goes to show that smaller telescopes can still play a vital role in the characterisation of exoplanet atmospheres in an era where 8-10 m telescopes are more commonplace. A larger telescopes will undoubtedly be able to rule out this tentative detection as suggested by the authors.
Feature Image: © National Astronomical Observatory of Japan.