TPF-I SWG Report - Exoplanet Exploration Program - NASA

C HAPTER 5
30
25
5 M earth
3 M earth
All (1014)
TPF-C [75 - 100%] (26)
TPF-C [50 - 75%] (24)
TPF-C [25 - 50%] (27)
TPF-C [0 - 25%] (17)
TPF-I [90%] (240)
Series7
10 M earth
SIM (100 stars)
Distance / pc
20
15
2 M earth
10
Sun @ 10 pc
5
1 M earth
0
301 605
0 50 100 150 200
Projected radius of Inner Habitable Zone / mas
Figure 5-22. Comparison of TPF-I, TPF-C, and SIM Missions. The colored circles represent those stars
for which an Earth-sized planet could be detected in the Habitable Zone by the TPF-C mission. The color
indicates the completeness of the measurement, as indicated in the legend. For example, the yellow
symbols show stars for which there is a probability of between 50 and 75% that a given Earth-sized
planet in the habitable zone is detected. There are 24 such stars. The completeness diminishes with
distance and proximity of the habitable zone to the star. The circles with a heavy outline denote the 240
stars that can be surveyed for Earth-sized planets in the habitable zone with 90% completeness by the
TPF-I mission. The blue contours represent the planet mass sensitivity of the SIM mission for an Earthequivalent
orbit. The astrometric signature is proportional to the mass of the planet and the projected orbit
radius, and inversely proportional to the mass of the parent star. For a star lying on the 3-Earth-mass
contour, there is a 50% probability that a given 3-Earth-mass planet will be detected.
The spectroscopy observations are made in exactly the same manner as for detection and orbit
determination: that is, the array is rotated continuously about the line of sight to the star and
measurements are made “on-the-fly.” Many rotations may be needed to accumulate the necessary
integration time. Efficiency (integration time/mission time) is assumed to be 75%.
Figure 5-23 shows how the SNR in a 0.5-μm wide channel varies with wavelength, for an integration time
of 29 days on an Earth-sized planet at 15 pc. Much of the variation in SNR (Fig. 5-23b) can be attributed
to the variation in signal photon rate from the planet (modeled as a 265-K black body as shown in Fig. 5-
23a). The SNR is reduced at short wavelengths by the increase in stellar leakage, since the width of the
null is proportional to the wavelength. At long wavelengths the SNR is depressed by the increase in
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