When a planet transits in front of its host star, a tiny fraction of the starlight passes through the planet’s atmosphere and carries with it signatures of the planet’s atmospheric constituents. This can allow the planet’s atmosphere to be characterised using an observational technique known as transmission spectroscopy. However, the atmospheres of planets can be cloudy, hazy or clear-sky (i.e. free of clouds and hazes). The presence of clouds or hazes can obscure the lower layers of the atmosphere and make the planet less desirable for characterisation. As a result, it is worth identifying whether a planet has clear skies before a large amount of telescope time is dedicated to characterising its atmosphere.
Misra & Meadows (2014) propose a method to readily distinguish cloudy, hazy and clear-sky planets. This involves measuring the amount of starlight being refracted through the atmospheres of transiting planets using upcoming large collecting area space and ground-based telescopes such as the James Webb Space Telescope (JWST) and the European Extremely Large Telescope (E-ELT). The refraction of starlight by a planet’s atmosphere can lead to an increase of flux both prior to ingress (i.e. before the start of a transit) and subsequent to egress (i.e. after the end of a transit).
The presence of a global cloud or haze coverage tends to obscure layers of a planet’s atmosphere that refract light. As a result, the detection of refracted light pre-ingress and post-egress would strongly suggest the absence of a global cloud or haze layer, making the planet a promising candidate for follow-up observations to characterise its atmosphere. In the models, the atmospheric pressure cut-offs are at 1 mbar (hazy case), 0.1 bars (cloudy case) and 1 bar (clear-sky case). A higher pressure cut-off indicates a greater depth of measurable atmosphere.
Results from the study show detecting refracted light requires less than 10 hours of total observing time for Jupiter-sized planets with JWST and for Super-Earths/Mini-Neptunes with E-ELT. Since the increase in flux due to refraction prior to ingress and subsequent to egress can be readily detected for clear-sky planets, it can quickly identify whether a planet is a good candidate for extended follow-up observations. Characterising a planet’s atmosphere is a very time consuming process, making it important to select good candidates (i.e. clear-sky planets) prior to characterisation.
Misra & Meadows (2014), “Discriminating Between Cloudy, Hazy and Clearsky Exoplanets Using Refraction”, arXiv:1409.7072 [astro-ph.EP]