Figure 4.
In secondary eclipse, we measure the dayside
emission spectrum of the planet as its light is
blocked by the host star. Emission spectroscopy
is sensitive to the absolute chemical
abundances and the thermal structure.
In transit, we measure the transmission spectrum of
the planet as light from the host star is absorbed by
chemical species in the planet’s atmosphere. These data
are most sensitive to the relative chemical abundances
and the presence of cloud or haze particles.
due to the absorption by chemical species.
Also, measurements when transiting planets pass behind their host stars can reveal
their thermal emission and reflection spectra. Figure 4 illustrates the geometry of
transit spectroscopy observations and discusses what information can be deduced
from these observations.
Transit spectroscopy measurements have
been used to probe the atmospheres of
planets ranging from the hottest Jupitersize to moderate-temperature Neptune-size
planets and even warm super-Earths. These
measurements have been used to deduce
the presence of sodium, water, methane,
hazes, etc., in these planets’ atmospheres,
and also to constrain their thermal structure,
dynamics, and evaporation.
However, there are still many outstanding
questions about the fundamental nature of
exoplanet atmospheres despite the many
recent successful applications of the transit spectroscopy technique. Progress in this
area requires observations of more targets
and over a wider range of wavelengths than
has been obtained so far.
12
Observing Exoplanet Atmospheres
from the Ground
Just as the blurring effect of Earth’s atmosphere hinders direct imaging of exoplanets, the scintillation component of
atmospheric seeing limits the precision of
ground-based transit observations. That’s
why most transit spectroscopy observations have been done with space telescopes
like Hubble and Spitzer.
However, these telescopes have limitations.
Both are relatively small, and so achieving
the 1 part in 10,000 or better type of precision that is needed for this work can only
be done for planets orbiting very bright
host stars. Spitzer also no longer has spectroscopic capabilities, and Hubble’s spectrographs have limited wavelength coverage.
The large ground-based telescopes of today offer the potential for complementary
wavelength coverage, especially in the optical. They also have the reach to target interesting planets around fainter host stars.
However, the limitations imposed by Earth’s
atmosphere first need to be overcome.
GeminiFocus 2013 Year in Review
January2014