Turbulent Disks are Never Stable: Fragmentation and Turbulence-Promoted Planet Formation

A fundamental assumption in our understanding of disks is that when the Toomre Q>>1, the disk is stable against fragmentation into self-gravitating objects (and so cannot form planets via direct collapse). But if disks are turbulent, this neglects a spectrum of stochastic density fluctuations that can produce rare, high-density mass concentrations. Here, we use a recently-developed analytic framework to predict the statistics of these fluctuations, i.e. the rate of fragmentation and mass spectrum of fragments formed in a turbulent Keplerian disk. Turbulent disks are never completely stable: we calculate the (always finite) probability of forming self-gravitating structures via stochastic turbulent density fluctuations in such disks. Modest sub-sonic turbulence above Mach number ~0.1 can produce a few stochastic fragmentation or 'direct collapse' events over ~Myr timescales, even if Q>>1 and cooling is slow (t_cool>>t_orbit). In trans-sonic turbulence this extends to Q~100. We derive the true Q-criterion needed to suppress such events, which scales exponentially with Mach number. We specify to turbulence driven by MRI, convection, or spiral waves, and derive equivalent criteria in terms of Q and the cooling time. Cooling times >~50*t_dyn may be required to completely suppress fragmentation. These gravoturbulent events produce mass spectra peaked near ~M_disk*(Q*M_disk/M_star)^2 (rocky-to-giant planet masses, increasing with distance from the star). We apply this to protoplanetary disk models and show that even minimum mass solar nebulae could experience stochastic collapse events, provided a source of turbulence.Comment: 15 pages, 5 figures (+appendix), accepted to ApJ (added clarifications and discussion to match accepted version

Kepler Planet Detection Metrics: Pixel-Level Transit Injection Tests of Pipeline Detection Efficiency for Data Release 25

This document describes the results of the fourth pixel-level transit injection experiment, which was designed to measure the detection efficiency of both the Kepler pipeline (Jenkins 2002, 2010; Jenkins et al. 2017) and the Robovetter (Coughlin 2017). Previous transit injection experiments are described in Christiansen et al. (2013, 2015a,b, 2016).In order to calculate planet occurrence rates using a given Kepler planet catalogue, produced with a given version of the Kepler pipeline, we need to know the detection efficiency of that pipeline. This can be empirically determined by injecting a suite of simulated transit signals into the Kepler data, processing the data through the pipeline, and examining the distribution of successfully recovered transits. This document describes the results for the pixel-level transit injection experiment performed to accompany the final Q1-Q17 Data Release 25 (DR25) catalogue (Thompson et al. 2017)of the Kepler Objects of Interest. The catalogue was generated using the SOC pipeline version 9.3 and the DR25 Robovetter acting on the uniformly processed Q1-Q17 DR25 light curves (Thompson et al. 2016a) and assuming the Q1-Q17 DR25 Kepler stellar properties (Mathur et al. 2017)

Kepler planet occurrence rates for mid-type M dwarfs as a function of spectral type

Previous studies of planet occurrence rates largely relied on photometric stellar characterizations. In this paper, we present planet occurrence rates for mid-type M dwarfs using spectroscopy, parallaxes, and photometry to determine stellar characteristics. Our spectroscopic observations have allowed us to constrain spectral type, temperatures, and, in some cases, metallicities for 337 out of 561 probable mid-type M dwarfs in the primary Kepler field. We use a random forest classifier to assign a spectral type to the remaining 224 stars. Combining our data with Gaia parallaxes, we compute precise (~3%) stellar radii and masses, which we use to update planet parameters and occurrence rates for Kepler mid-type M dwarfs. Within the Kepler field, there are seven M3 V to M5 V stars that host 13 confirmed planets between 0.5 and 2.5 Earth radii and at orbital periods between 0.5 and 10 days. For this population, we compute a planet occurrence rate of ${1.19}_{-0.49}^{+0.70}$ planets per star. For M3 V, M4 V, and M5 V, we compute planet occurrence rates of ${0.86}_{-0.68}^{+1.32}$, ${1.36}_{-1.02}^{+2.30}$, and ${3.07}_{-2.49}^{+5.49}$ planets per star, respectively.Published versio

A Framework for Prioritizing the TESS Planetary Candidates Most Amenable to Atmospheric Characterization

A key legacy of the recently launched the Transiting Exoplanet Survey Satellite (TESS) mission will be to provide the astronomical community with many of the best transiting exoplanet targets for atmospheric characterization. However, time is of the essence to take full advantage of this opportunity. The James Webb Space Telescope (JWST), although delayed, will still complete its nominal five year mission on a timeline that motivates rapid identification, confirmation, and mass measurement of the top atmospheric characterization targets from TESS. Beyond JWST, future dedicated missions for atmospheric studies such as the Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) require the discovery and confirmation of several hundred additional sub-Jovian size planets (R_p < 10 R⊕) orbiting bright stars, beyond those known today, to ensure a successful statistical census of exoplanet atmospheres. Ground-based extremely large telescopes (ELTs) will also contribute to surveying the atmospheres of the transiting planets discovered by TESS. Here we present a set of two straightforward analytic metrics, quantifying the expected signal-to-noise in transmission and thermal emission spectroscopy for a given planet, that will allow the top atmospheric characterization targets to be readily identified among the TESS planet candidates. Targets that meet our proposed threshold values for these metrics would be encouraged for rapid follow-up and confirmation via radial velocity mass measurements. Based on the catalog of simulated TESS detections by Sullivan et al., we determine appropriate cutoff values of the metrics, such that the TESS mission will ultimately yield a sample of ~300 high-quality atmospheric characterization targets across a range of planet size bins, extending down to Earth-size, potentially habitable worlds

Mapping out the time-evolution of exoplanet processes

There are many competing theories and models describing the formation, migration and evolution of exoplanet systems. As both the precision with which we can characterize exoplanets and their host stars, and the number of systems for which we can make such a characterization increase, we begin to see pathways forward for validating these theories. In this white paper we identify predicted, observable correlations that are accessible in the near future, particularly trends in exoplanet populations, radii, orbits and atmospheres with host star age. By compiling a statistically significant sample of well-characterized exoplanets with precisely measured ages, we should be able to begin identifying the dominant processes governing the time-evolution of exoplanet systems.Comment: Astro2020 white pape

TESS Discovery of an Ultra-short-period Planet around the Nearby M Dwarf LHS 3844

Data from the newly commissioned Transiting Exoplanet Survey Satellite has revealed a "hot Earth" around LHS 3844, an M dwarf located 15 pc away. The planet has a radius of 1.303 ± 0.022 R⊕ and orbits the star every 11 hr. Although the existence of an atmosphere around such a strongly irradiated planet is questionable, the star is bright enough (I = 11.9, K = 9.1) for this possibility to be investigated with transit and occultation spectroscopy. The star's brightness and the planet's short period will also facilitate the measurement of the planet's mass through Doppler spectroscopy

TESS Discovery of a Transiting Super-Earth in the pi Mensae System

We report the detection of a transiting planet around π Men (HD 39091), using data from the Transiting Exoplanet Survey Satellite (TESS). The solar-type host star is unusually bright (V = 5.7) and was already known to host a Jovian planet on a highly eccentric, 5.7 yr orbit. The newly discovered planet has a size of 2.04 ± 0.05 R⊕ and an orbital period of 6.27 days. Radial-velocity data from the High-Accuracy Radial-velocity Planet Searcher and Anglo-Australian Telescope/University College London Echelle Spectrograph archives also displays a 6.27 day periodicity, confirming the existence of the planet and leading to a mass determination of 4.82 ± 0.85 M⊕. The star's proximity and brightness will facilitate further investigations, such as atmospheric spectroscopy, asteroseismology, the Rossiter–McLaughlin effect, astrometry, and direct imaging