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Register a new userThe discovery and legacy of Keplers multi-transiting planetary systems
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We revisit the discovery and implications of the first candidate systems to contain multiple transiting exoplanets. These systems were discovered using data from the Kepler space telescope. The initial paper, presenting five systems (Steffen et al. 2010), was posted online at the time the project released the first catalog of Kepler planet candidates. The first extensive analysis of the observed population of multis was presented in a follow-up paper published the following year (Lissauer et al. 2011a). Multiply-transiting systems allow us to answer a variety of important questions related to the formation and dynamical evolution of planetary systems. These two papers addressed a wide array of topics including: the distribution of orbital period ratios, planet size ratios, system architectures, mean-motion resonance, orbital eccentricities, planet validation and confirmation, and the identification of different planet populations. They set the stage for many subsequent, detailed studies by other groups. Intensive studies of individual multiplanet systems provided some of Keplers most important exoplanet discoveries. As we examine the scientific impact of the first of these systems, we also present some history of the people and circumstances surrounding their discoveries.
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We report on the orbital architectures of Kepler systems having multiple planet candidates identified in the analysis of data from the first six quarters of Kepler data and reported by Batalha et al. (2013). These data show 899 transiting planet candidates in 365 multiple-planet systems and provide a powerful means to study the statistical properties of planetary systems. Using a generic mass-radius relationship, we find that only two pairs of planets in these candidate systems (out of 761 pairs total) appear to be on Hill-unstable orbits, indicating ~96% of the candidate planetary systems are correctly interpreted as true systems. We find that planet pairs show little statistical preference to be near mean-motion resonances. We identify an asymmetry in the distribution of period ratios near first-order resonances (e.g., 2:1, 3:2), with an excess of planet pairs lying wide of resonance and relatively few lying narrow of resonance. Finally, based upon the transit duration ratios of adjacent planets in each system, we find that the interior planet tends to have a smaller transit impact parameter than the exterior planet does. This finding suggests that the mode of the mutual inclinations of planetary orbital planes is in the range 1.0-2.2 degrees, for the packed systems of small planets probed by these observations.
In an effort to measure the masses of planets discovered by the NASA {it K2} mission, we have conducted precise Doppler observations of five stars with transiting planets. We present the results of a joint analysis of these new data and previously published Doppler data. The first star, an M dwarf known as K2-3 or EPIC~201367065, has three transiting planets (b, with radius $2.1~R_{oplus}$; c, $1.7~R_{oplus}$; and d, $1.5~R_{oplus}$). Our analysis leads to the mass constraints: $M_{b}=8.1^{+2.0}_{-1.9}~M_{oplus}$ and $M_{c}$ < $ 4.2~M_{oplus}$~(95%~conf.). The mass of planet d is poorly constrained because its orbital period is close to the stellar rotation period, making it difficult to disentangle the planetary signal from spurious Doppler shifts due to stellar activity. The second star, a G dwarf known as K2-19 or EPIC~201505350, has two planets (b, $7.7~R_{oplus}$; and c, $4.9~R_{oplus}$) in a 3:2 mean-motion resonance, as well as a shorter-period planet (d, $1.1~R_{oplus}$). We find $M_{b}$= $28.5^{+5.4}_{-5.0} ~M_{oplus}$, $M_{c}$= $25.6^{+7.1}_{-7.1} ~M_{oplus}$ and $M_{d}$ < $14.0~M_{oplus} $~(95%~conf.). The third star, a G dwarf known as K2-24 or EPIC~203771098, hosts two transiting planets (b, $5.7~R_{oplus}$; and c, $7.8~R_{oplus}$) with orbital periods in a nearly 2:1 ratio. We find $M_{b}$= $19.8^{+4.5}_{-4.4} ~M_{oplus}$ and $M_{c}$ = $26.0^{+5.8}_{-6.1}~M_{oplus}$.....
The photoevaporation model is one of the leading explanations for the evolution of small, close-in planets and the origin of the radius-valley. However, without planet mass measurements, it is challenging to test the photoevaporation scenario. Even if masses are available for individual planets, the host stars unknown EUV/X-ray history makes it difficult to assess the role of photoevaporation. We show that systems with multiple transiting planets are the best in which to rigorously test the photoevaporation model. By scaling one planet to another in a multi-transiting system, the host stars uncertain EUV/X-ray history can be negated. By focusing on systems that contain planets that straddle the radius-valley, one can estimate the minimum-masses of planets above the radius-valley (and thus are assumed to have retained a voluminous hydrogen/helium envelope). This minimum-mass is estimated by assuming that the planet below the radius-valley entirely lost its initial hydrogen/helium envelope, then calculating how massive any planet above the valley needs to be to retain its envelope. We apply this method to 104 planets above the radius gap in 73 systems for which precise enough radii measurements are available. We find excellent agreement with the photoevaporation model. Only two planets (Kepler - 100c & 142c) appear to be inconsistent, suggesting they had a different formation history or followed a different evolutionary pathway to the bulk of the population. Our method can be used to identify TESS systems that warrant radial-velocity follow-up to further test the photoevaporation model.
We perform a detailed study of six transiting planetary systems with relatively bright stars close enough to affect observations of these systems. Light curves are analysed taking into account the contaminating light and its uncertainty. We present and apply a method to correct the velocity amplitudes of the host stars for the presence of contaminating light. We determine the physical properties of six systems (WASP-20, WASP-70, WASP-8, WASP-76, WASP-2 and WASP-131) accounting for contaminating light. In the case of WASP-20 the measured physical properties are very different for the three scenarios considered (ignoring binarity, planet transits brighter star, and planet transits fainter star). In the other five cases our results are very similar to those obtained neglecting contaminating light. We use our results to determine the mean correction factors to planet radius, $langle X_Rrangle$, mass, $langle X_Mrangle$, and density, $langle X_rhorangle$, caused by nearby objects. We find $langle X_Rrangle=1.009pm0.045$, which is smaller than literature values because we were able to reject the possibility that the planet orbits the fainter star in all but one case. We find $langle X_Mrangle=1.031pm0.019$, which is larger than $langle X_Rrangle$ because of the strength of the effect of contaminating light on the radial velocity measurements of the host star. We find $langle X_rhorangle=0.995pm 0.046$: the small size of this correction is due to two effects: the corrections on planet radius and mass partially cancel; and some nearby stars are close enough to contaminate the light curves of the system but not radial velocities of the host star. We conclude that binarity of planet host stars is important for the small number of transiting hot Jupiters with a very bright and close nearby star, but it has only a small effect on population-level studies of these objects.
We calculate and analyze the distribution of period ratios observed in systems of Kepler exoplanet candidates including studies of both adjacent planet pairs and all planet pairs. These distributions account for both the geometrical bias against detecting more distant planets and the effects of incompleteness due to planets missed by the data reduction pipeline. In addition to some of the known features near first-order mean-motion resonances (MMR), there is a significant excess of planet pairs with period ratios near 2.2. The statistical significance of this feature is assessed using Monte Carlo simulation. We also investigate the distribution of period ratios near first-order MMR and compare different quantities used to measure this distribution. We find that beyond period ratios of ~2.5, the distribution of all period ratios follows a power-law with an exponent -1.26 +/- 0.05. We discuss implications that these results may have on the formation and dynamical evolution of Kepler-like planetary systems---systems of sub-Neptune/super-Earth planets with relatively short orbital periods.
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