TPF-I SWG Report - Exoplanet Exploration Program - NASA

C HAPTER 2
trivial in the absence of any other information on the observed planet. For an Earth-like planet there are
some atmospheric windows that can be used in most of the cases, especially between 8 and 11 µm. This
window would, however, become opaque at high H 2 O partial pressure (e.g., the inner part of the HZ
where a lot of water is vaporized) and at high CO 2 pressure (e.g., a very young Earth or the outer part of
the HZ).
Let us look at the case of the three known terrestrial planets, Venus, Earth and Mars. For Mars, the
temperature deduced from the shape of the infrared spectrum is a good approximation to the surface
temperature, except in the CO 2 band. On Earth, the infrared spectrum is a mixture of surface and cloud
emission, the latter occurring at lower temperature. The temperature given by the envelope of the
spectrum is thus slightly lower, by about 10 K on average, than the average surface temperature. In the
extreme case of Venus, the spectrum envelope gives a temperature of 277 K, much lower than the 740 K
of the surface. The reason for this discrepancy comes from the fact that the atmosphere of Venus is
completely opaque below 60 km because of the permanent cloud cover and the absorption continuum,
induced at high pressure by CO 2 –CO 2 collisions.
With low-resolution spectral observations, it is difficult to determine if the lower atmosphere contributes
to the spectrum and therefore, if the temperature reflects the surface conditions. The accuracy of the
radius and temperature determination will depend on the quality of the fit (and thus on the sensitivity and
resolution of the spectrum), the precision of the Sun–star distance, and also the distribution of brightness
temperatures over the planetary surface.
Finally, if the effective temperature is measured in the infrared, then the visible albedo can be inferred,
using F star (1 – A) = 4σT eff 4 .
2.3.2 Orbital Flux Variation
The variation or constancy of infrared flux with orbital position (i.e., with phase angle) provides us some
information about the surface of the planet. One approach is to note that the orbital flux variation in the
infrared can distinguish planets with and without an atmosphere (Selsis et al. 2003, Gaidos and Williams
2004). A strong variation of the thermal flux with phase angle can be consistent with the absence of an
atmosphere, because here we are looking at a rocky surface with low thermal inertia; and therefore, a
strong day–night temperature variation. Examples are Mercury and the Moon. In such a case one has to
readjust the inferred radius estimate of the planet by taking the viewing geometry of the system into
account.
The opposite case, when the apparent effective temperature is constant along the orbit, implies a large
thermal inertia from, for example an ocean, and/or a rapid circulation of incident energy through large
scale atmospheric motions.
Therefore, habitable planets are potentially distinguishable from airless or Mars-like planets by the
amplitude of the observed variations of effective temperature, however since Venus and Earth are roughly
similar in this way, additional spectroscopy is needed to separate such cases.
An exception to the above cases is υ Andromedae b, a tidally locked hot Jupiter with an observed day–
night temperature difference of about 1400 K (Harrington et al. 2006). Here, unlike Venus, the massive
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