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Register a new userThe California-Kepler Survey V. Peas in a Pod: Planets in a Kepler Multi-planet System are Similar in Size and Regularly Spaced
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We have established precise planet radii, semimajor axes, incident stellar fluxes, and stellar masses for 909 planets in 355 multi-planet systems discovered by Kepler. In this sample, we find that planets within a single multi-planet system have correlated sizes: each planet is more likely to be the size of its neighbor than a size drawn at random from the distribution of observed planet sizes. In systems with three or more planets, the planets tend to have a regular spacing: the orbital period ratios of adjacent pairs of planets are correlated. Furthermore, the orbital period ratios are smaller in systems with smaller planets, suggesting that the patterns in planet sizes and spacing are linked through formation and/or subsequent orbital dynamics. Yet, we find that essentially no planets have orbital period ratios smaller than $1.2$, regardless of planet size. Using empirical mass-radius relationships, we estimate the mutual Hill separations of planet pairs. We find that $93%$ of the planet pairs are at least 10 mutual Hill radii apart, and that a spacing of $sim20$ mutual Hill radii is most common. We also find that when comparing planet sizes, the outer planet is larger in $65 pm 0.4%$ of cases, and the typical ratio of the outer to inner planet size is positively correlated with the temperature difference between the planets. This could be the result of photo-evaporation.
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The California-Kepler Survey (CKS) catalog contains precise stellar and planetary properties for the Kepler planet candidates, including systems with multiple detected transiting planets (multis) and systems with just one detected transiting planet (singles, although additional planets could exist). We compared the stellar and planetary properties of the multis and singles in a homogenous subset of the full CKS-Gaia catalog. We found that sub-Neptune sized singles and multis do not differ in their stellar properties or planet radii. In particular: (1.) The distributions of stellar properties $M_star$, [Fe/H], and $vmathrm{sin}i$ for the Kepler sub Neptune-sized singles and multis are statistically indistinguishable. (2.) The radius distributions of the sub-Neptune sized singles and multis with $P > 3$ days are indistinguishable, and both have a valley at $sim1.8~R_oplus$. However, there are significantly more detected short-period ($P < 3$ days), sub-Neptune sized singles than multis. The similarity of the host star properties, planet radii, and radius valley for singles and multis suggests a common origin. The similar radius valley, which is likely sculpted by photo-evaporation from the host star within the first 100 Myr, suggests that planets in both singles and multis spend much of the first 100 Myr near their present, close-in locations. One explanation that is consistent with the similar fundamental properties of singles and multis is that many of the singles are members of multi-planet systems that underwent planet-planet scattering.
The size of a planet is an observable property directly connected to the physics of its formation and evolution. We used precise radius measurements from the California-Kepler Survey (CKS) to study the size distribution of 2025 $textit{Kepler}$ planets in fine detail. We detect a factor of $geq$2 deficit in the occurrence rate distribution at 1.5-2.0 R$_{oplus}$. This gap splits the population of close-in ($P$ < 100 d) small planets into two size regimes: R$_P$ < 1.5 R$_{oplus}$ and R$_P$ = 2.0-3.0 R$_{oplus}$, with few planets in between. Planets in these two regimes have nearly the same intrinsic frequency based on occurrence measurements that account for planet detection efficiencies. The paucity of planets between 1.5 and 2.0 R$_{oplus}$ supports the emerging picture that close-in planets smaller than Neptune are composed of rocky cores measuring 1.5 R$_{oplus}$ or smaller with varying amounts of low-density gas that determine their total sizes.
Probing the connection between a stars metallicity and the presence and properties of any associated planets offers an observational link between conditions during the epoch of planet formation and mature planetary systems. We explore this connection by analyzing the metallicities of Kepler target stars and the subset of stars found to host transiting planets. After correcting for survey incompleteness, we measure planet occurrence: the number of planets per 100 stars with a given metallicity $M$. Planet occurrence correlates with metallicity for some, but not all, planet sizes and orbital periods. For warm super-Earths having $P = 10-100$ days and $R_P = 1.0-1.7~R_E$, planet occurrence is nearly constant over metallicities spanning $-$0.4 dex to +0.4 dex. We find 20 warm super-Earths per 100 stars, regardless of metallicity. In contrast, the occurrence of warm sub-Neptunes ($R_P = 1.7-4.0~R_E$) doubles over that same metallicity interval, from 20 to 40 planets per 100 stars. We model the distribution of planets as $d f propto 10^{beta M} d M$, where $beta$ characterizes the strength of any metallicity correlation. This correlation steepens with decreasing orbital period and increasing planet size. For warm super-Earths $beta = -0.3^{+0.2}_{-0.2}$, while for hot Jupiters $beta = +3.4^{+0.9}_{-0.8}$. High metallicities in protoplanetary disks may increase the mass of the largest rocky cores or the speed at which they are assembled, enhancing the production of planets larger than 1.7 $R_E$. The association between high metallicity and short-period planets may reflect disk density profiles that facilitate the inward migration of solids or higher rates of planet-planet scattering.
We report the latest Planet Hunter results, including PH2 b, a Jupiter-size (R_PL = 10.12 pm 0.56 R_E) planet orbiting in the habitable zone of a solar-type star. PH2 b was elevated from candidate status when a series of false positive tests yielded a 99.9% confidence level that transit events detected around the star KIC 12735740 had a planetary origin. Planet Hunter volunteers have also discovered 42 new planet candidates in the Kepler public archive data, of which 33 have at least three transits recorded. Most of these transit candidates have orbital periods longer than 100 days and 20 are potentially located in the habitable zones of their host stars. Nine candidates were detected with only two transit events and the prospective periods are longer than 400 days. The photometric models suggest that these objects have radii that range between Neptune to Jupiter. These detections nearly double the number of gas giant planet candidates orbiting at habitable zone distances. We conducted spectroscopic observations for nine of the brighter targets to improve the stellar parameters and we obtained adaptive optics imaging for four of the stars to search for blended background or foreground stars that could confuse our photometric modeling. We present an iterative analysis method to derive the stellar and planet properties and uncertainties by combining the available spectroscopic parameters, stellar evolution models, and transiting light curve parameters, weighted by the measurement errors. Planet Hunters is a citizen science project that crowd-sources the assessment of NASA Kepler light curves. The discovery of these 43 planet candidates demonstrates the success of citizen scientists at identifying planet candidates, even in longer period orbits with only two or three transit events.
The vast majority of the 4700 confirmed planets and planet candidates discovered by the Kepler mission were first found by the Kepler pipeline. In the pipeline, after a transit signal is found, all data points associated with those transits are removed, creating a Swiss cheese-like light curve full of holes, which is then used for subsequent transit searches. These holes could render an additional planet undetectable (or lost). We examine a sample of 114 stars with $3+$ confirmed planets to evaluate the effect of this Swiss cheesing. A simulation determines that the probability that a transiting planet is lost due to the transit masking is low, but non-negligible, reaching a plateau at $sim3.3%$ lost in the period range of $P=400-500$ days. We then model all planet transits and subtract out the transit signals for each star, restoring the in-transit data points, and use the Kepler pipeline to search the transit-subtracted (i.e., transit-cleaned) light curves. However, the pipeline did not discover any credible new transit signals. This demonstrates the validity and robustness of the Kepler pipelines choice to use transit masking over transit subtraction. However, a follow-up visual search through all the transit-subtracted data, which allows for easier visual identification of new transits, revealed the existence of a new, Neptune-sized exoplanet (Kepler-150 f) and a potential single transit of a likely false positive (Kepler-208). Kepler-150 f ($P=637.2$ days, $R_{rm{P}}=3.64^{+0.52}_{-0.39}$ R$_{oplus}$) is confirmed with $>99.998%$ confidence using a combination of the planet multiplicity argument, a false positive probability analysis, and a transit duration analysis.
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