Evidence of Things Not Seen

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Science  01 Jun 2012:
Vol. 336, Issue 6085, pp. 1121-1122
DOI: 10.1126/science.1222590

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The inventory of known extrasolar planets (planets orbiting stars other than our Sun) has grown explosively in the past 3 years. The explosion relies on detecting the small but distinct decrease in the flux of light from a star caused by the passage of a planet across the stellar disk, an event known as a transit (see the figure). NASA's Kepler telescope, with a photometric precision of ∼20 parts per million, has identified more than 2300 extrasolar planetary candidates (1). For comparison, radial velocity surveys, which identify planets by the orbital velocity they impart to their host stars, have found only about a thousand planets since 1995. For the brightest host stars, one can obtain the high-quality spectra that are needed for radial velocity measurements to verify or exclude the planetary nature of the transiting system. Unfortunately, the bulk of the transit host stars are too dim to obtain radial velocities. Enter transit timing variations (TTVs) in the length of a candidate extrasolar planet's year. Since the advent of Kepler, it has become common to use TTVs to verify the planetary nature of transiting objects in multiplanet systems (2, 3). Recently, TTVs were used to infer the presence of an unseen planetary companion, but with only weak constraints on the orbital period and companion mass (4). On page 1133 of this week's issue, Nesvorný et al. (5) take the next step by identifying and characterizing unseen (nontransiting) planets using TTVs.