Title: Illusion and Reality in the Atmospheres of Exoplanets,
Abstract: The atmospheres of exoplanets reveal all their properties beyond mass,
radius, and orbit. Based on bulk densities, we know that exoplanets larger than
1.5 Earth radii must have gaseous envelopes, hence atmospheres. We discuss
contemporary techniques for characterization of exoplanetary atmospheres. The
measurements are difficult, because - even in current favorable cases - the
signals can be as small as 0.001-percent of the host star's flux. Consequently,
some early results have been illusory, and not confirmed by subsequent
investigations. Prominent illusions to date include polarized scattered light,
temperature inversions, and the existence of carbon planets. The field moves
from the first tentative and often incorrect conclusions, converging to the
reality of exoplanetary atmospheres. That reality is revealed using transits
for close-in exoplanets, and direct imaging for young or massive exoplanets in
distant orbits. Several atomic and molecular constituents have now been
robustly detected in exoplanets as small as Neptune. In our current
observations, the effects of clouds and haze appear ubiquitous. Topics at the
current frontier include the measurement of heavy element abundances in giant
planets, detection of carbon-based molecules, measurement of atmospheric
temperature profiles, definition of heat circulation efficiencies for tidally
locked planets, and the push to detect and characterize the atmospheres of
super-Earths. Future observatories for this quest include the James Webb Space
Telescope, and the new generation of Extremely Large Telescopes on the ground.
On a more distant horizon, NASA's concepts for the HabEx and LUVOIR missions
could extend the study of exoplanetary atmospheres to true twins of Earth.