Since the discovery of the first extrasolar giant planets around Sun-like stars(1,2), evolving observational capabilities have brought us closer to the detection of true Earth analogues. The size of an exoplanet can be determined when it periodically passes in front of (transits) its parent star, causing a decrease in starlight proportional to its radius. The smallest exoplanet hitherto discovered(3) has a radius 1.42 times that of the Earth's radius (R-circle plus), and hence has 2.9 times its volume. Here we report the discovery of two planets, one Earth-sized (1.03 R-circle plus) and the other smaller than the Earth (0.87 R-circle plus), orbiting the star Kepler-20, which is already known to host three other, larger, transiting planets(4). The gravitational pull of the new planets on the parent star is too small to measure with current instrumentation. We apply a statistical method to show that the likelihood of the planetary interpretation of the transit signals is more than three orders of magnitude larger than that of the alternative hypothesis that the signals result from an eclipsing binary star. Theoretical considerations imply that these planets are rocky, with a composition of iron and silicate. The outer planet could have developed a thick water vapour atmosphere.

Most Sun-like stars in the Galaxy reside in gravitationally bound pairs of stars(1,2) (binaries). Although long anticipated(3-8), the existence of a 'circumbinary planet' orbiting such a pair of normal stars was not definitively established until the discovery(9) of the planet transiting (that is, passing in front of) Kepler-16. Questions remained, however, about the prevalence of circumbinary planets and their range of orbital and physical properties. Here we report two additional transiting circumbinary planets: Kepler-34 (AB)b and Kepler-35 (AB)b, referred to here as Kepler-34 b and Kepler-35 b, respectively. Each is a low-density gas-giant planet on an orbit closely aligned with that of its parent stars. Kepler-34 b orbits two Sun-like stars every 289 days, whereas Kepler-35 b orbits a pair of smaller stars (89% and 81% of the Sun's mass) every 131 days. The planets experience large multi-periodic variations in incident stellar radiation arising from the orbital motion of the stars. The observed rate of circumbinary planets in our sample implies that more than similar to 1% of close binary stars have giant planets in nearly coplanar orbits, yielding a Galactic population of at least several million.

We report the detection of three transiting planets around a Sun-like star, which we designate Kepler-18. The transit signals were detected in photometric data from the Kepler satellite, and were confirmed to arise from planets using a combination of large transit-timing variations (TTVs), radial velocity variations, Warm-Spitzer observations, and statistical analysis of false-positive probabilities. The Kepler-18 star has a mass of 0.97M(circle dot), a radius of 1.1R(circle dot), an effective temperature of 5345 K, and an iron abundance of [Fe/H] = +0.19. The planets have orbital periods of approximately 3.5, 7.6, and 14.9 days. The innermost planet "b" is a "super-Earth" with a mass of 6.9 +/- 3.4M(circle plus), a radius of 2.00 +/- 0.10R(circle plus), and a mean density of 4.9 +/- 2.4 g cm(3). The two outer planets "c" and "d" are both low-density Neptune-mass planets. Kepler-18c has a mass of 17.3 +/- 1.9 M-circle plus, a radius of 5.49 +/- 0.26R(circle plus), and a mean density of 0.59 +/- 0.07 g cm(3), while Kepler-18d has a mass of 16.4 +/- 1.4 M-circle plus, a radius of 6.98 +/- 0.33 R-circle plus and a mean density of 0.27 +/- 0.03 g cm(.)(3) Kepler-18c and Kepler-18d have orbital periods near a 2:1 mean-motion resonance, leading to large and readily detected TTVs.

The architectures of multiple planet systems can provide valuable constraints on models of planet formation, including orbital migration, and excitation of orbital eccentricities and inclinations. NASA's Kepler mission has identified 1235 transiting planet candidates. The method of transit timing variations (TTVs) has already confirmed seven planets in two planetary systems. We perform a transit timing analysis of the Kepler planet candidates. We find that at least similar to 11% of planet candidates currently suitable for TTV analysis show evidence suggestive of TTVs, representing at least similar to 65 TTV candidates. In all cases, the time span of observations must increase for TTVs to provide strong constraints on planet masses and/or orbits, as expected based on N-body integrations of multiple transiting planet candidate systems (assuming circular and coplanar orbits). We find the fraction of planet candidates showing TTVs in this data set does not vary significantly with the number of transiting planet candidates per star, suggesting significant mutual inclinations and that many stars with a single transiting planet should host additional non-transiting planets. We anticipate that Kepler could confirm (or reject) at least similar to 12 systems with multiple transiting planet candidates via TTVs. Thus, TTVs will provide a powerful tool for confirming transiting planets and characterizing the orbital dynamics of low-mass planets. If Kepler observations were extended to at least seven years, then TTVs would provide much more precise constraints on the dynamics of systems with multiple transiting planets and would become sensitive to planets with orbital periods extending into the habitable zone of solar-type stars.

The Kepler mission has recently announced the discovery of Kepler-10 b, the smallest exoplanet discovered to date and the first rocky planet found by the spacecraft. A second, 45 day period transit-like signal present in the photometry from the first eight months of data could not be confirmed as being caused by a planet at the time of that announcement. Here we apply the light curve modeling technique known as BLENDER to explore the possibility that the signal might be due to an astrophysical false positive (blend). To aid in this analysis we report the observation of two transits with the Spitzer Space Telescope at 4.5 mu m. When combined, they yield a transit depth of 344 +/- 85 ppm that is consistent with the depth in the Kepler passband (376 +/- 9 ppm, ignoring limb darkening), which rules out blends with an eclipsing binary of a significantly different color than the target. Using these observations along with other constraints from high-resolution imaging and spectroscopy, we are able to exclude the vast majority of possible false positives. We assess the likelihood of the remaining blends, and arrive conservatively at a false alarm rate of 1.6 x 10(-5) that is small enough to validate the candidate as a planet (designated Kepler-10 c) with a very high level of confidence. The radius of this object is measured to be R-p = 2.227(-0.057)(+ 0.052) R-circle plus (in which the error includes the uncertainty in the stellar properties), but currently available radial-velocity measurements only place an upper limit on its mass of about 20 M-circle plus. Kepler-10 c represents another example (with Kepler-9 d and Kepler-11 g) of statistical "validation" of a transiting exoplanet, as opposed to the usual "confirmation" that can take place when the Doppler signal is detected or transit timing variations are measured. It is anticipated that many of Kepler's smaller candidates will receive a similar treatment since dynamical confirmation may be difficult or impractical with the sensitivity of current instrumentation.

About one-third of the similar to 1200 transiting planet candidates detected in the first four months of Kepler data are members of multiple candidate systems. There are 115 target stars with two candidate transiting planets, 45 with three, 8 with four, and 1 each with five and six. We characterize the dynamical properties of these candidate multi-planet systems. The distribution of observed period ratios shows that the vast majority of candidate pairs are neither in nor near low-order mean-motion resonances. Nonetheless, there are small but statistically significant excesses of candidate pairs both in resonance and spaced slightly too far apart to be in resonance, particularly near the 2:1 resonance. We find that virtually all candidate systems are stable, as tested by numerical integrations that assume a nominal mass-radius relationship. Several considerations strongly suggest that the vast majority of these multi-candidate systems are true planetary systems. Using the observed multiplicity frequencies, we find that a single population of planetary systems that matches the higher multiplicities underpredicts the number of singly transiting systems. We provide constraints on the true multiplicity and mutual inclination distribution of the multi-candidate systems, revealing a population of systems with multiple super-Earth-size and Neptune-size planets with low to moderate mutual inclinations.