TPF-I SWG Report - Exoplanet Exploration Program - NASA

E X O P L A N E T S CIENCE
wavelength region. The instrument design has to take the noise levels at both wavelength ends into
account as well as the photons emitted from the planet. Preliminary studies indicate that it will be equally
difficult to detect the water line at both features. To conclude this issue, considerable more detailed study
is needed.
2.4.5 Biomarkers and Their Evolution over Geological Timescales
Terrestrial planets found around other stars may be observed at different stages in their geological and
biological evolution. Earth's atmosphere has experienced dramatic evolution over 4.5 billion years, and
other planets may exhibit similar or greater evolution, and at different rates.
Studies (see e.g., Des Marais et al. 2002; Schindler and Kasting 2000; Kasting and Catling 2003; Selsis,
2000; Pavlov 2000; Traub and Jucks 2002; Segura et al. 2003; Kaltenegger et al. 2007; Meadows 2006)
are designed to guide the interpretation of an observed spectrum of such a planet by future instruments
that will characterize exoplanets. Figure 2-6 shows theoretical mid-infrared spectra of the Earth at six
epochs during its geological evolution (Kaltenegger et al. 2007). The epochs are chosen to represent
major developmental stages of the Earth and life on Earth. If an extrasolar planet is found with a
corresponding spectrum, we will have good evidence for characterizing its evolutionary state, its
habitability, and the degree to which it shows signs of life. The oxygen and ozone absorption features
could have been used to indicate the presence of biological activity on Earth anytime during the past 50%
of the age of the Solar System. The dark lines show a resolution of 25 in the IR, as proposed for the
Darwin/TPF-I mission.
2.5 Suitable Targets
Due to angular resolution and sensitivity constraints, the most suitable target stars around which TPF-I
can search for exoplanets are the stars nearest the Sun. However, the nearest stars encompass a large
variety of stellar types, classes, ages, and multiplicities (as well as distances); and so the set of all nearby
stars needs to be trimmed or culled, for both scientific reasons and engineering/observational constraints.
The current scientific source selection criteria, defined at a workshop held at the Naval Research
Observatory in Washington, DC, in Nov. 2004, are summarized in this section.
2.5.1 Science Criteria
The identification of candidate scientific targets for TPF-I should begin with a complete list of nearby
stars. The master list selected is the Hipparcos catalogue (ESA 1997) of nearby stars, out to a distance
cutoff of 30 pc. This list does not include every star within 30 pc because dim stars at the larger distances
do not fall within the Hipparcos sensitivity limit, implying that the list grows less and less complete as
stellar temperatures decrease. However, for the primary targets of interest, “solar-like” F, G, and K stars,
the list should be fairly complete.
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