308 research outputs found
The Kepler-19 System: A Thick-envelope Super-Earth with Two Neptune-mass Companions Characterized Using Radial Velocities and Transit Timing Variations
We report a detailed characterization of the Kepler-19 system. This star was previously known to host a transiting planet with a period of 9.29 days, a radius of 2.2 R_β, and an upper limit on the mass of 20 M_β. The presence of a second, non-transiting planet was inferred from the transit time variations (TTVs) of Kepler-19b over eight quarters of Kepler photometry, although neither the mass nor period could be determined. By combining new TTVs measurements from all the Kepler quarters and 91 high-precision radial velocities obtained with the HARPS-N spectrograph, using dynamical simulations we obtained a mass of 8.4 Β± 1.6 M β for Kepler-19b. From the same data, assuming system coplanarity, we determined an orbital period of 28.7 days and a mass of 13.1 Β± 2.7 M_β for Kepler-19c and discovered a Neptune-like planet with a mass of 20.3 Β± 3.4 M_β on a 63-day orbit. By comparing dynamical simulations with non-interacting Keplerian orbits, we concluded that neglecting interactions between planets may lead to systematic errors that can hamper the precision in the orbital parameters when the data set spans several years. With a density of 4.32 Β± 0.87 g cm^(β3) (0.78 Β± 0.16 Ο_β) Kepler-19b belongs to the group of planets with a rocky core and a significant fraction of volatiles, in opposition to low-density planets characterized only by transit time variations and an increasing number of rocky planets with Earth-like density. Kepler-19 joins the small number of systems that reconcile transit timing variation and radial velocity measurements
Kepler-21b: A Rocky Planet Around a V = 8.25 Magnitude Star
HD 179070, aka Kepler-21, is a V = 8.25 F6IV star and the brightest exoplanet host discovered by Kepler. An early detailed analysis by Howell et al. of the first 13 months (Q0βQ5) of Kepler light curves revealed transits of a planetary companion, Kepler-21b, with a radius of about 1.60 Β± 0.04 Rβ and an orbital period of about 2.7857 days. However, they could not determine the mass of the planet from the initial radial velocity (RV) observations with Keck-HIRES, and were only able to impose a 2Ο upper limit of 10 Mβ. Here, we present results from the analysis of 82 new RV observations of this system obtained with HARPS-N, together with the existing 14 HIRES data points. We detect the Doppler signal of Kepler-21b with a RV semiamplitude K = 2.00 Β± 0.65 m s^(-1), which corresponds to a planetary mass of 5.1 Β± 1.7 Mβ. We also measure an improved radius for the planet of 1.639 +0.019/β0.015 Rβ, in agreement with the radius reported by Howell et al. We conclude that Kepler-21b, with a density of 6.4 Β± 2.1 g cm^(-3), belongs to the population of small, βΎ6 Mβ planets with iron and magnesium silicate interiors, which have lost the majority of their envelope volatiles via stellar winds or gravitational escape. The RV analysis presented in this paper serves as an example of the type of analysis that will be necessary to confirm the masses of TESS small planet candidates
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The Occurrence Rate of Small Planets Around Small Stars
We use the optical and near-infrared photometry from the Kepler Input Catalog to provide improved estimates of the stellar characteristics of the smallest stars in the Kepler target list. We find 3897 dwarfs with temperatures below 4000 K, including 64 planet candidate host stars orbited by 95 transiting planet candidates. We refit the transit events in the Kepler light curves for these planet candidates and combine the revised planet/star radius ratios with our improved stellar radii to revise the radii of the planet candidates orbiting the cool target stars. We then compare the number of observed planet candidates to the number of stars around which such planets could have been detected in order to estimate the planet occurrence rate around cool stars. We find that the occurrence rate of 0.5-4 R β planets with orbital periods shorter than 50 days is planets per star. The occurrence rate of Earth-size (0.5-1.4 R β) planets is constant across the temperature range of our sample at Earth-size planets per star, but the occurrence of 1.4-4 R β planets decreases significantly at cooler temperatures. Our sample includes two Earth-size planet candidates in the habitable zone, allowing us to estimate that the mean number of Earth-size planets in the habitable zone is planets per cool star. Our 95% confidence lower limit on the occurrence rate of Earth-size planets in the habitable zones of cool stars is 0.04 planets per star. With 95% confidence, the nearest transiting Earth-size planet in the habitable zone of a cool star is within 21 pc. Moreover, the nearest non-transiting planet in the habitable zone is within 5 pc with 95% confidence.Astronom
Predicting the Yield of Small Transiting Exoplanets around Mid-M and Ultra-Cool Dwarfs in the Nancy Grace Roman Space Telescope Galactic Bulge Time Domain Survey
We simulate the yield of small (0.5-4.0 R) transiting exoplanets
around single mid-M and ultra-cool dwarfs (UCDs) in the Nancy Grace Roman Space
Telescope Galactic Bulge Time Domain Survey. We consider multiple approaches
for simulating M3-T9 sources within the survey fields, including scaling local
space densities and using Galactic stellar population synthesis models. These
approaches independently predict 100,000 single mid-M dwarfs and UCDs
brighter than a Roman F146 magnitude of 21 that are within the survey fields.
Assuming planet occurrence statistics previously measured for early-to-mid M
dwarfs, we predict that the survey will discover 1347 small
transiting planets around these sources, each to a significance of 7.1
or greater. Significant departures from this prediction would test whether the
occurrence rates of small planets increase or decrease around mid-M dwarfs and
UCDs compared to early-M dwarfs. We predict the detection of 13
habitable, terrestrial planets (1.23 R) in the survey. However,
atmospheric characterization of these planets will be challenging with current
or near-future space telescope facilities due to the faintness of the host
stars. Nevertheless, accurate statistics for the occurrence of small planets
around mid-M dwarfs and UCDs will enable direct tests of predictions from
planet formation theories and will determine our understanding of planet
demographics around the objects at the bottom of the main sequence. This
understanding is critical given the prevalence of such objects in our Galaxy,
whose planets may therefore comprise the bulk of the galactic census of
exoplanets.Comment: 19 pages, 10 figures, accepted to A
The Kepler-454 System: A Small, Not-rocky Inner Planet, a Jovian World, and a Distant Companion
Kepler-454 (KOI-273) is a relatively bright (V = 11.69 mag), Sun-like star that hosts a transiting planet candidate in a 10.6 day orbit. From spectroscopy, we estimate the stellar temperature to be 5687 Β± 50 K, its metallicity to be [m/H] = 0.32 Β± 0.08, and the projected rotational velocity to be v sin i 10 years and mass >12.1 MJ. The 12 exoplanets with radii <2.7 R_β and precise mass measurements appear to fall into two populations, with those <1.6 R_β following an Earth-like composition curve and larger planets requiring a significant fraction of volatiles. With a density of 2.76 Β± 0.73 g cm^(β3), Kepler-454b lies near the mass transition between these two populations and requires the presence of volatiles and/or H/He gas
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The Occurrence of Potentially Habitable Planets Orbiting M Dwarfs Estimated From the Full Kepler Dataset and an Empirical Measurement of the Detection Sensitivity
We present an improved estimate of the occurrence rate of small planets orbiting small stars by searching the full four-year Kepler data set for transiting planets using our own planet detection pipeline and conducting transit injection and recovery simulations to empirically measure the search completeness of our pipeline. We identified 156 planet candidates, including one object that was not previously identified as a Kepler Object of Interest. We inspected all publicly available follow-up images, observing notes, and centroid analyses, and corrected for the likelihood of false positives. We evaluated the sensitivity of our detection pipeline on a star-by-star basis by injecting 2000 transit signals into the light curve of each target star. For periods shorter than 50 days, we find 0.56+0.06 β0.05 Earthsize planets (1 β 1.5 Rβ) and 0.46+0.07 β0.05 super-Earths (1.5 β 2 Rβ) per M dwarf. In total, we estimate a cumulative planet occurrence rate of 2.5 Β± 0.2 planets per M dwarf with radii 1 β 4 Rβ and periods shorter than 200 days. Within a conservatively defined habitable zone based on the moist greenhouse inner limit and maximum greenhouse outer limit, we estimate an occurrence rate of 0.16+0.17 β0.07 Earthsize planets and 0.12+0.10 β0.05 super-Earths per M dwarf habitable zone. Adopting the broader insolation boundaries of the recent Venus and early Mars limits yields a higher estimate of 0.24+0.18β0.08 Earth-size planets and 0.21+0.11 β0.06 super-Earths per M dwarf habitable zone. This suggests that the nearest potentially habitable non-transiting and transiting Earth-size planets are 2.6 Β± 0.4 pc and 10.6 +1.6 β1.8 pc away, respectively. If we include super-Earths, these distances diminish to 2.1Β±0.2 pc and 8.6 +0.7β0.8 pc.Astronom
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