Direct imaging probes the outer architecture of planetary systems beyond about 10 AU and
complements indirect methods for finding planets which rely on transits, radial velocities, microlensing, and astrometric motion— all sensitive to companions at smaller separations.
Deep adaptive optics imaging surveys provide statistical constraints on planets at large orbital distances and
can be used to directly test models of planet formation and migration.
When planets are found, spectroscopy can then be used to characterize their atmospheres and examine physical properties like temperature,
surface gravity, composition, and cloud structure.
My research spans a variety of topics related to the formation, evolution, architectures, and atmospheres of giant planets and is motivated by few basic questions:
- How do planets form and dynamically evolve over time?
- What do the atmospheres of giant planets look like over a broad range of masses?
- How do the atmospheres of planets change as they cool over time?
- How common are planets on wide orbits and how does this occurrence rate vary with orbital distance, planet mass, and host star mass?
- What is the relationship between giant planets and their higher-mass brown dwarf counterparts?
- What technical and strategic steps need to be taken to reach even lower masses with direct imaging?
I use a multipronged approach to answer these questions by carrying out large direct imaging surveys to
discover new planets and measure their statistical properties, performing more targeted individual studies of the atmospheric and
orbital properties of known systems, and studying the immediate environments and formation routes of young planets
through their circumplanetary accretion disks. Some of these ongoing programs are described in more detail below.
Searching for Planets with Direct Imaging
I am leading several programs to search for giant planets with direct imaging.
The Planets Around Low-Mass Stars (PALMS) survey is the largest of these and focuses on gas giants
orbiting young M dwarfs to
study the influence of stellar host mass on planet frequency.
M dwarfs comprise about 75% of all stars and are probably the most common sites of planet formation.
They also have the most favorable planet-star contrasts of any spectral class which makes them especially amenable to high-contrast imaging.
PALMS is one of the largest direct imaging surveys to date and has
resulted in numerous discoveries including new brown dwarfs and giant planets with remarkable atmospheric properties.
Spectroscopy of Exoplanets
Spectroscopy probes the atmospheres of exoplanets to reveal their temperature, chemical composition, surface gravity, and cloud properties.
Young giant planets have extraordinarily red spectra and lack methane absorption at temperatures where it
is seen in brown dwarfs.
Because of their low surface gravities, planets appear to retain photospheric clouds at low temperatures
and have strong disequilibrium carbon chemistry.
One of my goals is to map the spectra of
giant planets spanning large ranges in mass and age to study the evolution
of planetary atmospheres over time.
This includes characterizing the youngest planets soon after they formed,
juvenile planets as they undergo large changes in cloud dynamics and structure,
and mature planets at cold temperatures approaching that of Jupiter.
Statistical Properties of Exoplanets
The frequency and mass-period distribution of exoplanets provide important clues about planet formation mechanisms and migration pathways.
A major theme in my research is to measure these properties for planets at wide orbital distances and examine
whether these properties change over time.
Any differences would suggest that planetary systems undergo dynamical evolution,
perhaps from planet-planet scattering events or Kozai-Lidov oscillations with a third body.
Other fundamental relationships I am interested in include occurrence rates as a function of stellar host mass
and the connection between the overlapping brown dwarf and giant planet companion mass functions.
Accreting Protoplanets and Circumplanetary Disks
Giant planets gain mass by accreting gas through
circumplanetary accretion disks like miniature analogs of young protostars.
These subdisks regulate angular momentrum transport onto protoplanets and are
intimately tied to their growth and evolution.
Circumplanetary disks are probably ubiquitous but have been difficult to directly study.
I am leading multi-wavelength campaigns spanning the
optical with HST out to sub-mm wavelengths with ALMA to investigate the
frequency, diversity, and evolution of circumplanetary disks around young directly imaged planets.
New Targets for Direct Imaging
Planets cool and grow fainter over time so the best way to image planets is to
search around young stars. Young moving groups have emerged as optimal targets because of their proximity (<100 pc) and youth (10-120 Myr).
I am carrying out a large search for new members of these groups in preparation for future direct imaging planet surveys.
Much of this work is being done at small telescopes both to confirm membership by measuring radial velocities and to remove close visual
binaries with adaptive optics imaging prior to deeper observations at larger facilities.
Looking forward, a new era of telescopes with unprecedented sensitivities and imaging capabilities is quickly approaching.
The James Webb Space Telescope, WFIRST, and 30-meter class telescopes on the ground will probe even deeper mass limits and closer separations,
guaranteeing that direct imaging will play a dominant role in astronomy for the foreseeable future.
Ultimately, one of the driving goals of my own research and the field in general is to directly study
the atmospheres of Super Earths and rocky planets like our own.