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.
Reference:
Misra & Meadows (2014), “Discriminating Between Cloudy,
Hazy and Clearsky Exoplanets Using Refraction”, arXiv:1409.7072 [astro-ph.EP]