Validation of planet detections

Before a candidate detection can be considered to be a validated planet, a rigorous validation process must be executed to ensure that it is not due to some other phenomenon (Borucki et al. 2003). Public release of false positives would ultimately discredit any mission results. Therefore, to be considered a validated planet, the detection must meet several requirements:

1. The total statistical significance (SNR) of the superimposed transits must exceed 7a. This requirement prevents false positives produced by statistical noise when 8 x 1011 statistical tests are carried out on 105 stars for orbital periods from 1-700 days.

2. At least three transits must be observed that demonstrate a period constant to 10 ppm. This test is independent of the previous test and demonstrates the presence of a highly periodic process. It essentially rules out mistaking stellar phenomenon for transits. (Exceptions must be made for planets showing timing variations caused by mutual perturbations.)

3. The duration, depth, and shape of the light curve must be consistent. The duration must be constant over all transits and consistent with Kepler's laws based on the orbital period. The depth must be consistent over all transits. A weaker requirement is that the shape must be consistent with a "U" shape of a planetary transit, rather than a "V" shape of a grazing eclipse of a binary star. Low-amplitude transits are likely to be too noisy to make this distinction.

4. The position of the centroid of the target star determined outside of the transits must be the same as that of the differential transit signal. If there is a significant change in position, the cause of the signal is likely to be an eclipsing star in the background.

5. Radial velocity measurements must be conducted to demonstrate that the target star is not an eclipsing binary with the period of the transits.

6. High-precision radial velocity measurements must be made to measure the mass of the companion or provide an upper limit that is consistent with that of a small planet.

7. High spatial-resolution measurements must be made of the area immediately surrounding the target star to demonstrate that there is no background star in the aperture capable of producing a false positive signal.

Requirements #1 and #2 insure that statistical fluctuations in the data series will produce less than one false positive for the entire mission, which is critical for the case of a null result. Requirements #3 through #7 greatly reduce the probability that unrelated physical phenomena could be mistaken for a pattern of planetary transits.

Other checks are also possible. For example, if future instrumentation on HST and JWST has sufficient precision to detect the color changes during the transit, a measured color change consistent with the differential limb darkening expected of the target star would strengthen the validation (Borucki & Summers 1984). A shape, depth, or color change substantially different than expected would point to the possibility of a very close background star that differed in spectral type or reddening.

Giant planets like 51 Pegasi, with orbits of less than seven days, are also detected by the periodic phase modulation of their reflected light without requiring a transit (Borucki et al. 1997). For the short-period giant planets that do transit, the planetary albedo can be calculated. Information on the scattering properties of the planet's atmosphere can also be derived from the phase curves (Marley et al. 1999; Seager et al. 2000; Sudarsky et al. 2000).

Ground-based Doppler spectroscopy and/or space-based astrometry with SIM can be used to measure the larger planetary masses and can distinguish between a planet and a brown dwarf. These complementary methods can also detect additional massive companions in the systems to better define the structure of each planetary system. The density of any giant planet detected by both photometry and either radial velocity or astrometry can be calculated. Determination of the planet size, mass, semi-major axis, and stellar properties provides the properties critical for the validation and development of theoretical models of planetary structure.

White dwarf stars are about the size of the Earth and might be expected to produce a transit signal of similar magnitude. However, because of the gravitational lensing caused by their large, but compact, mass, the transits actually result in an increase in brightness (Sahu & Gilliland 2003) and are thereby readily distinguished from those of a planet.

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