Max Planck Institute for Astronomy - Annual Report 2007

34 II. Highlights
II.4 An Exoplanet’s First Calibrated Spectrum
After an extrasolar planet was indirectly proven to exist
in 1995, astronomers immediately sought to discover
other such heavenly bodies and then to analyze them
spectroscopically in order to ascertain their atmospheric
composition. Due to the high contrast between a star's
brightness and that of its planet, as well as the small
distance between them, this undertaking is extremely
difficult however and is, almost in all cases, beyond current
technical capabilities. A team of astronomers under
Mark Swain from the Jet Propulsion Laboratory and
Jeroen Bouwman from the MPIA have now been able
to capture a mid-infrared spectrum from the exoplanet
HD 209458b. It is the first spectrum ever of any planet to
supply the radiation flow in absolute units. These data
are already allowing for careful conclusions regarding
the atmospheres of these “Hot Jupiters”.
Because the images of the star and its planets practically
fall together, their spectra are also overlaid. They must
be separated from each other with the help of tricky techniques
in order to obtain information about the planets.
To do this there are methods which can be used to search
for so-called transit planets; these are found in systems
which, by chance, are observed from a viewpoint exactly
at the edge of the planet’s orbital plane, so that the planet
transits in front of the star and later disappears behind it.
This geometry allows for three observation modes.
Fig. II.4.1: Scheme of the Secondary Occultation Method.
Spectra are taken while the planet is on its way to transiting
behind the star und turns its hot side toward earth. These spectra
contain the light of both star and planet. Spectra are then taken
Star + Planet
Combines Spectrum
Star
Eclipse Spectrum
Isolating a Planet's Spectrum
Spectroscopic Methods for Exoplanets
First, the Reflection Method: A planet (or its atmosphere)
reflects the star’s light. A telescope captures both the direct
light of the star as well as the light reflected from the
planet. When the planet disappears behind the star, the
telescope captures only the star’s light. Subtracting the
pure stellar spectrum from the spectrum of the combined
light of the star and the planet leaves the planet's spectrum.
This can contain absorption lines from elements in
the planetary atmosphere. However, no experiments to
date of this type have been able to provide unambiguous
results.
Second, the Transmission Method: If the planet
passes in front of the star, a portion of the star’s light
traverses the planet’s atmosphere; this light contains
spectroscopic information about the chemical composition
of its gaseous shell. In order to extract this from
the full spectrum, two spectral series are taken: one of
them briefly before the planet transits in front of the
star (the primary occultation) – it contains the star’s
radiation as well as possible thermal emission from the
planet’s night side; the second series is taken during the
transit phase. In the difference spectrum derived from
these two spectra, absorption lines of elements in the
planetary atmosphere may appear. In this manner, two
American astronomers were able to prove the existence
of sodium as well as water and methane in the atmo-
of the star spectrum alone while the planet transits behind the
star. The difference between the two spectra leaves the planet’s
spectrum. (Diagram: Na s a)
Planet
Planet Spectrum