The small radii, low-masses, and sheer numbers of M dwarfs provide a direct path to detecting and characterizing nearby, rocky, potentially habitable planets. The prime Kepler mission observed nearly 4000 M dwarfs and detected just under 160 planets. Statistical studies of this sample revealed that small, Earth-like planets are ubiquitous around low-mass stars with more than 2 planets per star in the 1-4 Earth radius range. However, detailed characterization of this population is difficult since the majority of the host stars are too faint for high precision RV follow-up and transit spectroscopy. The recent failure of 2 of Kepler's reaction wheels has resulted in a novel, revised observing strategy to gather photometry for stars in 16 fields around the ecliptic for ~80 days each. This new mission, called K2, enables the observation of many thousands more M dwarfs and provides the opportunity to discover small, Earth-like planets around low-mass stars much brighter than those observed by Kepler.
I am a primary collaborator in the K2 M Dwarf Program, a comprehensive plan to propose M dwarf targets in each K2 field, generate calibrated light curves and identify candidate planets, and obtain follow-up observations to validate and characterize new planetary system. K2 data has already yielded many new M dwarf planet discoveries, including cool, Earth-like planets in the habitable zone and several small planets orbiting bright stars that are amenable to RV follow-up and transit spectroscopy. Early in the K2 mission, our team discovered two multi-planet systems orbiting nearby, bright M dwarfs, K2-3bcd and K2-21bc, providing new targets for RV follow-up and exoplanet atmosphere studies. We continue to search each K2 campaign for new planets with great success (see my publication list for all results). These new planets fill in a poorly explored part of the planetary mass-radius-temperature diagram and provide key insight into the transition from Earth-like, silicate dominated planets to larger, volatile rich planets with substantial H/He atmospheres. They are also prime targets for early transit spectroscopy observations with the JWST to characterize their atmospheres.
The JWST M dwarf imaging program, the K2 M Dwarf Program, and other ongoing efforts (RV and microlensing surveys) will provide a comprehensive view of planets around low-mass stars. Studying both the small, close in and large, wide separation planets will place constraints on the dominant modes of planet formation across M dwarf disks. These observations could also reveal trends that point toward the best targets for future missions aimed at the characterization of nearby, habitable planets.
JHKL'-band imaging of the nearby, young, very low-mass binary NLTT 33370. The images were obtained using adpative optics with VLT/NACO and LBTI/LMIRCam in Feb., March, and April 2013. The separation of the components is ~75 mas, 1.25 AU at the distance of the binary. Over 2 months, the position angle changes by ~20 deg. [Image credit: Schlieder et al. 2014].
Direct Exoplanet Imaging and the Exoplanet/Brown Dwarf Connection
My experience with nearby, young stars led to collaboration in four large exoplanet imaging surveys to search for companions around stars more massive than M dwarfs: LBT/LEECH, VLT/SPHERE, Subaru/SEEDS, and VLT/NaCo-LP. I led or contributed to target selection and characterization for these surveys and continue to contribute to exoplanet host characterization. Highlights from these surveys include the discovery of a low-mass companion to a young high-mass star, strong constraints on the possibility of a fifth inner planet in the HR 8799 system, and the discovery of dynamic features in a nearby debris disk.
Brown dwarfs and directly imaged giant exoplanets likely have different formation histories. Yet, they exhibit some of the same physical properties as observed via their photometry and spectra. In particular, evidence for atmospheric clouds. Young, low-mass, brown dwarfs and directly imaged giant exoplanets are known to have peculiar properties that are attributed to low-surface gravity. The relationship between low surface gravity, dust, and clouds is poorly constrained and is the focus of current research. Recent discoveries of more easily observed young, brown dwarfs with planetary masses may lead to a deeper understanding of young, giant exoplanets. I have contributed to studies of brown dwarf variability which led to the first ever map of clouds on a brown dwarf and the first detection of variability in a planetary mass object. I also participate in studies to compare young brown dwarf spectra to atmospheric models.
Small, Cool Planets Around Small, Cool Stars with K2
Young stars exist near the Sun as members of moving groups, coeval associations of stars moving through the galaxy with common motion. These kinematic groups are believed to originate from regions of past star formation, often having kinematics and ages similar to nearby open clusters and OB associations (e.g. Sco-Cen). Classically, most known members of NYMGs were spectral type (SpTy) F,G, and K dwarfs, with few of SpTy M. Since low-mass stars are the dominant stellar component of the universe (>70%), the census of known members in young moving groups was probably incomplete. The under-sampling of low-mass members is a consequence of their low luminosity and wide distribution on the sky. This has led to the development of an all-sky survey to identify and characterize these under-sampled stars -- the CASTOFFS Survey.
The Cool Astrometrically Selected Targets Optimal For Follow-up Spectroscopy (CASTOFFS) Survey began in 2011 and has recently completed survey observations. More than 300 spectra of candidate young M dwarfs have been obtained and their bulk analysis is ongoing. Early results from the survey revealed several novel new systems, including a nearby, young triple system harboring a debris disk, a young, very low-mass benchmark binary. Targets from CASTOFFS are being considered in a expansion of the JWST imaging program described above and have been included in several exoplanet imaging surveys. Thus far, a few new binaries, a young brown dwarf companion, and two planetary mass companions (available here and here) have been discovered. Stay tuned for the first round of CASTOFFS survey results.
Phase folded light curves of K2-3bcd, super-Earths orbiting a nearby bright M dwarf discovered by K2. Best fit transit models are plotted as solid lines. All of the planets have estimated equilibrium temperatures <500K and planet d lies in the star's habitable zone [Image credit: Crossfield, Petigura, Schlieder et al. 2015].
Predicted contrast for the JWST NIRCam instrument with and without a coronagraph. The horizontal bars on the right designate the planet mass sensitivity limits for an M0 dwarf at different ages. Even at an age of 1 Gyr, Jupiter mass planets are detectable [Image credit: Krist et al. 2007].
Nearby Young Stars
Near-IR color-magnitude diagram of substellar objects. The isolated, young, planetary mass object PSO 318.5-22 occupies a region of color magnitude space similar to the directly imaged planets surrounding HR 8799 [Image credit: Liu et al. 2013].
Sub-Jupiter Mass Planets Around M Dwarfs with the JWST
The frequencies and demographics of giant planets on wide orbits around low-mass stars are poorly constrained. This is due mostly to the limited sensitivity of imaging cameras on large ground based telescopes equipped with adaptive optics. The evolution of our own solar system, where the migration of Jupiter is responsible for the current planet configuration, indicates that understanding this population of M dwarf planets is critical to understanding the evolution and habitability of the large population of Earth-like planets closer in.
Some success has been had imaging wide companions around young M dwarfs with many times the mass of Jupiter, but no lower-mass gas-giants have yet been detected. However, trends in long term radial velocity data and detections via gravitational microlensing have revealed a substantial population of Neptune to Jupiter mass planets on intermediate orbits (~1-10 AU). Directly imaging these planets requires instrumentation with greater sensitivity and a carefully constructed sample of stars. I lead a Guaranteed Time Observing (GTO) program to use NIRCam on the James Webb Space Telescope (JWST) to directly image these planets. The JWST is the only near-term facility with the necessary sensitivity and contrast to directly probe the M dwarf giant planet population detected via RVs and microlensing. Imaging of these low-mass gas-giants would be unprecedented and provide critical constraints on their frequency and the dominant modes of planet formation beyond M dwarf ice lines. A brief summary of our efforts is available here and the GTO program details are available here.