My research interests concern the origin and formation of planets. The big questions that drive my research are “how do solar systems like our own form,” “how do planetary systems form generally,” and “how unique is our solar system in the context of planet formation?” I use big telescopes, special optics, and computer algorithms to study the orbital properties and photometric variability of low mass embedded companions: peering inside planetary cradles to observe infant exoplanets growing inside circumstellar disks. I also use smaller telescopes to observe young and low mass stars, leveraging a plethora of methods to understand the growth of pre-main-sequence stars and the accretion process. I love observational astronomy, and most of my work involves direct imaging, simulations of direct imaging, and the aggregation of direct imaging results. There’s a visceral satisfaction in taking a picture of something hundreds of light years away (especially if that something is a planet) and using that picture to understand something deeper about the physics of the universe.
In jargon-y, technical terms, I use a handful of direct imaging detection methods to observe accreting protoplanets embedded within circumstellar disks. I focus on transition disks and the planets that form within them. Transition disks are circumstellar disks which exhibit significant material and morphological evolution when compared to younger protoplanetary disks. Transition disks boast wide gaps and striking spiral arms indicative of planet formation, and observations of these systems can reveal information about the composition of planetary building blocks, and the dynamical evolution of planet formation). Important high contrast observations over the past two decades continue to overturn our understanding of the planet formation process. The image to the right is a schematic (drawn by the wonderful Elijah Spiro, NASA) of the star formation process, the byproduct of which is a circumstellar disk and eventually a solar system.
There are a lot of contentious claims about planet formation and direct imaging, plenty of competing theories, and so many open questions, which is what makes the field so exciting. These kinds of studies also require methods and instruments which exist on the bleeding edge of observational and computational innovation. It’s an exciting time to be an astronomer interested in these kinds of questions!
Giant Accreting Protoplanets
Starlight obscures planets, which are much dimmer than their host stars. This is rather frustrating, because one of the ways we can characterize the atmospheres exoplanets (looking for important molecules, like water or methane), or understand the planet formation process, is by observing light from the planet itself.
Our ability to resolve two sources of light is dependent on their angular separation (theta, which is dependent on the distance between the two sources and the distance between those sources and the observer), the size of your telescope (D), the wavelength of the photons of light the telescope collects from the system (lambda). Even if you can create a telescope which can resolve two sources, if one is brighter than the other, the signal of the dimmer source will get buried under the signal of the brighter source. In nearly all of these regards, the physical limitations of the universe are stacked against astronomers from Earth spatially resolving and distinguishing distant, dim, and close in exoplanets (i.e. exo-Earths or habitable planet analogs) from images of their host stars. That doesn’t mean directly observing exoplanets is impossible, however.
A good analogy to help understand this problem goes something like: trying to image a Jupiter sized planet around a sun-like star is like trying to take a picture of a firefly next to a lighthouse from across the continental USA.
This has been accomplished! Astronomers to date have been able to take direct images of widely separated, massive planets by using big telescopes, clever algorithms, and specialized optics. Giant, widely separated planets (more massive than Jupiter, but on Neptune sized orbits) are rare when compared to close in, less massive worlds. This means direct imaging has only produced a handful of results when compared with other detection methods. Meme pages like “Exoplanet Memes for Habitable Teens” make this abundantly clear. Still, direct imaging is the only way to fully characterize exoplanet properties (like atmospheric composition), and observe populations of exoplanets that other methods are not sensitive to. Advancements in technology will allow astronomers in the near future to directly image more common exoplanets, and gather more detections. How do astronomers currently circumvent these limitations and directly observe these rare, massive, and widely separated exoplanets?
We expect that younger planets will glow brighter than older planets, because planets are imparted with thermal during their formation (which is why the Earth’s core is molten) and radiate this energy away over time, cooling down. A younger planet has had less time to radiate away that energy, and will be hotter, and therefore brighter. This is good news, because it means that young planets should be easier to see when compared to their bright stars, right?
Yes and no. Many impressive direct imaging results (like the discovery of Beta Pic b) leverage this fact, but planets radiate most of their energy at infrared wavelengths (like you and I do), and longer wavelengths require larger telescopes to resolve clearly (keeping theta constant, a larger lambda means a larger D). Until larger telescopes are built (and they are being built) we have to be a bit clever about where we look for exoplanets.
Planets form from circumstellar disks, which are the “leftovers” of the star formation process that settle into rotation around a protostar. These “cradles” of planet formation swaddle young planets which grow within the disk. We know circumstellar disks host planets because we can observe the dynamical footprint of planets carving rings and spirals into the disks and because we’ve been able to discover exoplanets within circumstellar disks (more on that in a moment). Dust within the disk coagulates to form pebbles, which form rocks, boulders, planetesimals, and so on. Gas giant planets, like Jupiter, likely begin as icy, rocky cores which grow massive enough to pull tons of hydrogen gas from the disk into their atmospheres.
This accretion process, by which a giant planet gains mass, causes the accreting molecular hydrogen gas to heat up, splitting the H2 moleculues into atomic hydrogen. Atomic hydrogen, when heated, shines brightly at specific colors (or lines) of light, which correspond with electron transitions between the various electron energy states within the atom. An accreting planet will shine brightly at these colors, bright enough in comparison to its host star to be detectable by the current generation of telescopes. Importantly, heated hydrogen gas shines brightly in visible light, which doesn’t require an exceedingly large telescope to resolve. By taking an image in a hydrogen line filter (which captures signal from the star, the planet, and the planet’s accretion), and then a nearby, boring filter (capturing only the starlight) we can subtract the two images, leaving us with an image of the planet and the accretion signal, and no more pesky starlight.
Another limiting factor is our ability to resolve sources through the distortion of Earth’s atmosphere. In order to combat this, astronomers use deform-able mirrors to cancel out the distortion of the atmosphere. This technique is referred to as “adaptive-optics” (AO). The Magellan telescope, the source of GAPlanetS data, boasts the Magellan Adaptive Optics (MagAO) system. There’s a great online resource on adaptive optics written by a professor at RIT here. I’ve included some cool explainer images in the gallery below as well.
I remove starlight and spurious disk signals from adaptive optics images in a process referred to as “PSF subtraction.” I use angular differential imaging (ADI), leveraging the rotational diversity of a time series of observations and spectral differential imaging (SDI), leveraging the excess luminosity exhibited by objects undergoing accretion to remove starlight and disk structure from images via the python implementation of the fancy sounding Karhunen-Loeve Image Processing (pyKLIP) algorithm. Another analogy: I essentially use a form of facial recognition to create a model of the host’s starlight, which I subtract from images to reveal growing planets. I do this work under the tutelage of Dr. Kate Follette, who is the PI of the Giant Accreting Protoplanet Survey (GAPlanetS).
After uncovering planet signal using adaptive optics, angular and spectral differential imaging, and the KLIP principle component analysis algorithm, I use a pyKLIP forward modeling technique to determine the position and brightness of planets. This allows for a more nuanced understanding of their properties, evolutionary history, and how they shape the environment of their host transition disks.
Q: “What is your website logo?”
A: Good question! An artifact of the KLIP PSF subtraction process is “self-subtraction” which occurs because the modeled PSF is constructed from a finite series of images (good explanation here). So when you reveal a planet using KLIP or a similar ADI algorithm, the signal of the planet is this emblematic little butterfly thing, with the planet light surrounded by two self-subtraction lobes. My website logo in particular is one of the results of my undergraduate thesis research.
Simulating Exozodi Yield
During the summer of 2020 (amid other things) I worked remotely with Dmitry Savranski in the Space Imaging and Optical Systems Lab (SIOS Lab) at Cornell University. I used EXOSIMS, a direct imaging simulation suite developed by SIOS lab, to predict the yield of the miniaturized Distributed Occulting Telescope (mDOT), a novel cubesat proposed by some very smart people at Stanford University which will demonstrate the effectiveness of an occulting star shade in negating starlight to observe exoplanets and circumstellar dust. You can view slides from a presentation I gave on this research here.
Transition Disk Database
The Transition Disk Database (TDD) is an aggregation of transition disk morphology and properties. I presented the results of this literature review at the 235th meeting of the American Astronomical Society (see my poster here).
Very Low Mass Variability
I observed very low mass (VLM) stars using the Half-Degree-Imager on the WIYN 0.9m telescope atop Kitt Peak National Observatory on the Tohono O’odham Nation. I presented some of this research with my collaborator Lena Treiber to the Five College Astronomy Department (who paid for my trip to KPNO), you can see our poster and some very beautiful images of the Taurus star forming cloud below. I kept an online journal during the spring semester, which you can read here.
Teaching Assistance and Star Clusters
I work as a TA for many of the astronomy course offerings at Amherst College (ASTR 112: Alien Worlds, ASTR 337: Observational Techniques I, and ASTR 341: Obs. Tech. II). I set up and operate the 11in Cassegrain telescopes atop the New Science Center’s observatory, working under one of the coolest people I know, Sarah Betti. We’ve gotten some pretty good data, given our local weather conditions and elevation, which I’ve used to fit isochrones to open clusters. We plan on making an Amherst College Observatory calendar sometime soon.