I am an assistant professor at Northwestern in CIERA and the Physics and Astronomy department. I take images of faint exoplanets by removing the glare of their host stars using techniques such as coronagraphy, spectroscopy, and interferometry so that we can trace out their orbits and measure the properites of their atmospheres. I help develop new instruments with these capabilities and machine learning/signal processing techniques to pull out faint exoplanet signals from the data. I also am intersted in software development, especially for astronomy. I lead the BOBA Group at Norhtwestern.
I grew up in Dublin, Ohio, got my B.A. in Physics with a Minor in Computer Science from Cornell University, and my PhD in Astrophysics from UC Berkeley. I was previously a 51 Pegasi b postdoc at Caltech
Office
1800 Sherman, 8059Social Media
Optical Interferometry with GRAVITY
GRAVITY point in top-right plot with 200x zoom
I am working with the GRAVITY instrument, an optical interferometer that can combine light from the four 8-meter VLTs. With its ~100 meter baselines, we are capable of reaching down to 10 μas astrometric precision. We made the first ever detection of an exoplanet with this technique on HR 8799 e, achieving ten times better astrometric precision than ever before.
I am part of the ExoGRAVITY team, a multi-year VLTI large program to monitor and characterize all of the currently known directly imaged exoplanets. I am leading the astrometric effort to reveal the detailed dynamical environments of these exoplanetary systems and measure dynamical masses to constrain their formation pathways. Some early highlights include the first direct detection of a radial-velocity discovered planet (Nowak et al. 2020) and dynamically measuring its mass by measuring its perturbation on the orbit of the outer planet in the system (Lacour, Wang, et al. (2021)). We also used it to study the nature of protoplanets (see Imaging Protoplanets section below).
High Resolution Spectroscopy of Exoplanets
I am part of the team working on the Keck Planet Imager and Characterizer (KPIC), a series of upgrades to the Keck II adaptive optics system and instrument suite to enable high-resolution (R~35,000) spectra of young Jovian-sized planets. At this spectral resolution, we can resolve individual spectral lines in their atmospheres, allowing us to measure the Doppler shift of the planet, molecular abundances, and planetary spin.
In 2022, we just installed several new hardware upgrades as part of KPIC Phase II, which will further enhance the capabilities of the instrument such as characterizing closer-in exoplanets and obtaining L-band spectra. For KPIC Phase II, I designing the atmospheric dispersion corrector.
Imaging Newborn Protoplanets
Read Wang et al. (2020) and Wang et al. (2021)
I used new technologies at Keck (better coronagraph to suppress starlight, infrared pyramid wavefront sensor for better atmospheric turbulence correction) and VLTI/GRAVITY (milliarcsecond-level spatial resolution) to study the two protoplanets orbiting PDS 70 (PDS 70 b and c). Unlike all other exoplanets, these two planets are still in the process of forming, so the true nature of these protoplanets were uncertain.
On the Keck data, I worked with Bin Ren and Nicole Wallack to subtract out the protoplanetary disk from the images so we can make unbiased measurements of their brightnesses and positions. I teamed up with Sivan Ginzburg and Peter Gao to infer the bulk properties of these planets. We found that their masses should be between 1-4 Jupiter masses (making them some of the lowest mass planets imaged to date), and they are accreting so much dust that they are shrouded by dust making it difficult to see deep into the planets' atmospheres. Our work was publisehd in this paper and featured in press releases at Keck Observatory and at the Heising-Simons Foundation.
Using VLTI/GRAVITY, we placed dynamical limits on the mass of PDS 70 b to be lower than 10 Jupiter masses, found spectroscopic evidence that we are seeing emission from the dust-shrouded atmospheres of these protoplanets, and made the first sub-au observations into a circumplanetary environment, constraining any bright circumplanary disk around PDS 70 b to be smaller than 0.3 au. Check out our paper here.
The Gemini Planet Imager Exoplanet Survey (GPIES)
I am part of the team working on the Gemini Planet Imager (GPI), a near-infrared extreme-AO system that will directly image and take spectra of young Jovian-sized planets and the debris disks in which they form.
I led the development of the software infrastructure to automate the data processing for the GPI Exoplanet Survey (see our JATIS paper). My main contributions are building the GPIES Data Cruncher (poster pdf), an automatic data reduction system for our survey, and pyKLIP (Bitbucket repo), a PSF subtraction library written in python.
The software infrastructure I developed allowed us to discover GPI's first exoplanet!
Previously, I examined how well we could measure the star that is blocked by the coronagraph mask, in particular to measure the location and spectrum of the occulted star. This work was presented in a poster and conference proceeding at the 2014 SPIE Astronomical Instrumentation conference.
I also worked on observations of AU Microscopii (AU Mic). In an ApJ Letter, we showed that the SE and NW sides of the disk are asymmetric and half different vertical extents.
Precise Astrometry and Orbits of Exoplanets
I developed a new technique to obtain more precise astrometry of directly imaged exoplanets. This technique is based off the mathematical framework developed by Pueyo (2016) which allows us to forward model the signature of an exoplanet through the data reduction process, giving us a more accurate planet model. In addition, I developed a Bayesian framework that properly accounts for the correlated nature of the noise in the data with Gaussian process regression. You can read more about the technique in this AJ paper.
I demonstrated one milliarcsecond astrometry on β Pictoris b taken with the Gemini Planet Imager. I published a paper showing that I could fit a Keplerian orbit with well-behaved fit residuals. Our measured astrometry were 2-4 times more precise than previously published values that used same data. We also were able to improve some of the orbital parameters by a factor of 2. Using this method, we showed the probability of the planet transiting the star was ruled out at 10-σ confidence. However, the Hill sphere of the planet transited the star in 2017, and we monitored that transit to look for circumplanetary material.
I further extended my orbit characterization work when studying the HR 8799 planetary system. Even with 1-2 mas astrometry, we had short orbital arcs for the four planets, resulting in many degenerate orbital solutions. However, given that these planets are super Jupiters, many of the orbits were not dynamically stable. I took the orbit posteriors from MCMC orbit fits and performed rejection sampling on them by simulating all of them through the REBOUND N-body code and rejecting the unstable configurations. This allowed us to obtain ~5x improvement on the orbital parameters as well as place dynamical upper limits on the planet masses. You can read more about it in this AJ paper.
I am contining to monitor the orbits of these exoplanets as well as others. As we get enough data, I have started making movies of these exoplanets in motion (check them out!). I also contribute to orbitize!, an open-soruce Python package to fit orbits of imaged systems.
Big planets: OrBits & Atmospheres
BOBA Group members generally work on imaging gas giant planets, with the goal of studying their dynamical evolution through monitoring their orbits and their bulk properties through atmospehric spectroscopy. We image and characterize planets by pioneering novel instrumentation and algorithms in high-contrast imaging.
Postdocs
Dino Hsu
Dino works on high resolution spectroscopy of exoplanets and brown dwarafs, with focus on their spins, orbits, and atmospheric properites. He is currently analzying data from the Keck Planet Imager and Characterizer's spectroscopy survey of substellar companions in addition to developing the KPIC data reduction pipeline. He also studies how brown dwarfs and low-mass stars are distributed in our Galaxy.
Sarah Blunt
Sarah is a CIERA Fellow working on studying the orbits and atmospheres of young exoplanets through direct imaging and radial velocity techniques. Sarah works on robust data-driven methods to infer planetary properties from noisy radial velocity data of young stars. She also works on studying the orbits and atmospheres of directly imaged planets to understand their formation processes. Sarah is also the co-founder of the Code/Astro workshop and the lead developer for the orbitize! software package.
Graduate Students
Amanda Chavez
Amanda is an Astronomy PhD student at Northwestern studying the orbits of exoplanets using optical interferometry and GRAVITY. She is currently measuring the orbits of the four planets orbiting HR 8799 using the improved astrometric precision of GRAVITY. She is using the improved orbits to understand what we can infer about the resonances, coplanarity, and dynamical masses in the system. She is also working on understanding GRAVITY systematics.
Jonathan Roberts
Jonathan is an Astronomy PhD student at Northwestern working on adapting astrophysical models to gradient-based optimization techniques originally developed for machine learning. He is currently working on adapting the orbitize! orbit fitting code to JAX. He is generally interested in machine learning techniques and their applications with astronomy.
Jea Adams
Jea is a PhD student at Harvard University studying planet formation and the atmospheres of terrestrial planets. She works on transmission spectroscopy of small planets from JWST, high-resolution spectroscopy of small planets with future extremely large telescopes, signatures of planet formation with ALMA, and exoplanet imaging.
Pre-Graduate Students
Malachi Noel
Malachi Noel is a high school student working on various projects in exoplanet imaging. He reduced the archival data to produce this 17-year time-lapse of the orbit of beta Pic b. He also is working on characterizing the long-term performance of the Gemini Planet Imager atmospheric dispersion corrector.
Juan Guerrero
Juan recently finished his undergraduate studies as Vassar and is working on studying the newborn GQ Lup system. He is using new GRAVITY measurements of the position of GQ Lup B to refine its orbit and study how it formed.
Former Group Members
Saanika Choudhary
Saanika just finished her Master's degree at Northwestern. She was working on deep-learning algorithms for efficient orbital determination and characterizing how increased orbital coverage improves our inferences on orbital parameters.
Events and Demos
I am involved in several hands-on astronomy outreach efforts.
- I lead a crowdfunding campaign that successfully rasied funds to develop virtual reality demonstrations in astronomy. Stay tuned for more!
- I am working on creating new outreach demonstrations, such as a parabollic mirror that can be used to roast marshmallows by focusing sunlight (as seen in this picture).
- I helped start up Astro Night, a monthly public lecture and stargazing series hosted by the Berkeley Astronomy department.
Education Initiatives
Outside of astronomy, I have been involved with the Cornell Computer Reuse Association (CCRA) to refurbish old computers, donate them to education and community organizations, and expand computer literacy all around the world. I also helped start the Zambia Community Education Initiative (ZCEI), a 501(c)3 not-for-profit that aims to improve education in Zambia. If you are interested in these organizations, contact me!
