Caleb K. Harada

NSF Graduate Research Fellow

UC Berkeley, Department of Astronomy

See my work portrait photo

About Me

Hello! I'm a second-year astrophysics Ph.D. student and National Science Foundation Graduate Research Fellow at the University of California, Berkeley. I'm currently taking courses and researching extrasolar planets with Professor Courtney Dressing. Click here to learn more about my research! Prior to coming to Berkeley, I was an undergraduate at the University of Maryland, College Park, where I earned a B.S. in Physics and a B.S. in Astronomy with High Honors in 2020. My undergraduate thesis, supervised by Professor Eliza Kempton, focused on modeling high-resolution thermal emission spectra of cloudy hot Jupiters using 3D GCMs (see here).

In addition to exoplanet research, I'm also passionate about teaching science and promoting equity and diversity in astronomy through DEI work. When I'm not thinking about science, I enjoy hiking, birdwatching, and exploring the SF Bay Area.

My pronouns are they/them (or he/him).

Me standing in front of the Shane 3m telescope at Lick Observatory

~ Research ~

I use computers and data from telescopes to study planets orbiting stars beyond our Solar System.
To see a complete list of my work, check out my CV here.

Sections:
artistic depiction of exoplanets

TESS Multiplanet Systems


What can solar systems with multiple planets tell us about how planets form and evolve?

Multiplanet systems are highly valuable for studying the physics governing planet formation and evolution, but many of the multiplanet systems discovered prior to the TESS mission orbit stars too faint for detailed follow-up observations. With new TESS Cycle 3 observations of relatively bright stars known to host at least one planet, we can discover new transiting planets and expand the sample of long-period planets by detecting single-transit events. I'm currently developing a software package known as TATER (the "Tess trAnsiT fittER") that will identify new planets in multiplanet systems in TESS data using the transit least squares method, and constrain system parameters by comparing the data to transit models in a Bayesian framework. We will measure TATER's search completeness with transit injection and recovery tests, then analyze our entire sample of TESS targets with known planets. From our catalog of newly detected planets, we will able to identify promising targets for atmospheric characterization and use newly estimated planet masses and densities to investigate system formation and architecture.

Relevant papers: Turtelboom et al. (2021) (poster abstract)

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artistic depiction of an exoplanet with clouds

Clouds in Exoplanet Atmospheres


How does the dynamical nature of clouds in exoplanet atmospheres affect high-resolution emission spectra?

Hot Jupiters are perhaps the most well-studied class of exoplanets, yet the physical properties which determine their observables (e.g., high-resolution thermal emission spectra) are not fully understood. Clouds likely play an important role in shaping emission spectra because of their ability to scatter and absorb radiation. However, the chemical composition and vertical extent (among other properties) of clouds in hot Jupiter atmospheres remain poorly constrained, and several different approaches to modeling clouds have been developed spanning a wide range of physics. By comparing observations to simulated spectra from different cloud models, we can better understand the properties of clouds in hot Jupiter atmospheres.

Relevant papers: Harada et al. (2021), Roman et al. (2021), Roman & Rauscher (2019)

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artistic depiction of an exoplanet with escaping atmosphere

Atmospheric Escape from Hot Jupiters


How can we use metastable helium to understand the role of atmospheric escape in planet evolution?

Exoplanets with large gaseous envelopes which are subject to intense stellar UV radiation may lose significant portions of their atmospheres to hydrodynamic escape. On short enough timescales, atmospheric escapes can be an important evolutionary process for irradiated gas-rich planets, and could help explain trends in the observed population of exoplanets (e.g., the photoevaporation valley). Evidence for atmospheric escape has traditionally come from observations of excess hydrogen Ly-α absorption in exoplanet transmission spectra. However, recent studies of the 10830 Å line of metastable helium have begun to mature as an alternative way of probing atmospheric escape. Unlike Ly-α, this IR helium spectral feature is unaffected by the ISM and geocoronal emission, and is observable from ground-based observatories.

Relevant papers: Vissapragada et al. (2020), Oklopčić et al. (2018)

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All image credits: NASA

~ Miscellaneous Science ~

Here are some random animations and side projects that I've worked on. If you'd like to use any of these for your own demonstrations or talks, kindly remember to credit me!

Sections:

General circulation model of a cloudy hot Jupiter

This animation of a simulated hot Jupiter was originally published in Harada et al. (2021) to complement a suite of simulated high-resolution emission spectra.

The color scale indicates the temperature at the vertical level in the atmosphere corresponding approximately to the IR photosphere pressure (∼26 mbar). The gray-shaded area shows the integrated optical depth of clouds above the IR photosphere, with more opaque regions corresponding to thicker clouds. The red (and blue) contours show the net positive (and negative) line-of-sight velocities (in increments of 1 km/s) caused by atmospheric winds and the planet’s rotation at the IR photosphere level. The black dot indicates the antistellar point and the black "x" indicates the substellar point. The subplot on the left-hand side shows the location of the planet in its orbit as seen from the perspective of a distant observer, indicated by the triangle.

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N-body visualization of the Kozai mechanism

Under certain conditions in a three-body system, a dynamical phenomenon known as the Kozai Mechanism can cause a distant third body to dramatically affect the orbit of an otherwise stable inner binary. On a timescale much longer than the orbital period, the perturbing third body drives a periodic exchange between the inner system's eccentricity and inclination. This process may be an important factor in planet evolution and migration, and may be a key factor in explaining how hot Jupiters form.

The first animation shows a three-body simulation from the rest-frame comoving with the system's center-of-mass, with two massive "stars" in orbit around each other and a test particle orbiting the inner, more massive star. The axes show the spatial scale in AU and the timescale is shown at the top. The masses and orbital parameters of all three bodies were selected to optimize viewability for the purpose of the demostration. The second animation shows the same simulation, only transformed into the rest-frame of the more massive central star. Here, it becomes apparent that the orbit of the test mass is perturbed by the outermost star and we can see the periodic changes in the inclination and eccentricity of its orbit (remember, this is a numerical simulation!).

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More N-body simulations

Here are a few more N-body simulations created with the same software used above. As before, the axes shown the spatial scale in AU and the timescale is shown at the top of each animation.

The first animation depicts the TRAPPIST-1 system, a distant solar system with at least seven terrestrial planets orbiting a cool M-dwarf sun. The second animation shows a hypothetical exoplanet system with two planets in stable orbits around a central binary star. (A real example of such a configuration is the circumbinary planet Kepler-16b.) The third animation shows a hypothetical planet on a chaotic trajectory through a binary star system. The planet is ultimately ejected from the system.

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Lagrange points in the Earth-Moon system

Another interesting result of the restricted three-body problem, in which two orbiting masses are much more massive than the third, is the existence of five Lagrange points. At each of the Lagrange points, the forces due to the gravity of the two larger bodies and the centrifugal pseudoforce exactly balance out. Therefore, in the co-rotating frame of the two massive bodies, a less massive object placed at any of the Lagrange points will remain stationary (note, however, that the stability of the Lagrange points requires more careful consideration of the Coriolis pseudoforce).

This animation shows the 3D surface of the effective potential felt by a massless particle in the rotating frame of the Earth-Moon system. The mathematical form of the effective potential is $$\phi_{eff}(\vec{r}) = -\frac{G M_1}{|\vec{r} - \vec{r}_1|} - \frac{G M_2}{|\vec{r} - \vec{r}_2|} -\frac{1}{2} \frac{G(M_1 + M_2)}{|\vec{r}_2 - \vec{r}_1|^3} |\vec{r}|^2$$ where $M_1$, $\vec{r}_1$ and $M_2$, $\vec{r}_2$ are the masses and positions of the Earth and Moon respectively and $\vec{r}$ is measured from the center of mass. The first two terms are the gravitational potential and the third term is the centrifugal pseudopotential. Here, the effective potential is plotted in log space for contrast, and a 2D contour plot of the potential is projected onto the bottom of the figure. The larger and smaller black circles represent the positions of the Earth and Moon, and the blue points show the five Lagrange points (i.e., where the gradient of the potential is zero).

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Evolution of a simple stellar population

Simple stellar populations (SSPs) are key compoments of models of galactic evolution and can be useful for visualizing the evolution of a coeval population of stars (e.g., a cluster). By definition, an SSP is a group of stars born at the same time with the same initial composition. The distribution of the stars' masses is defined by the initial mass function (IMF). As an SSP evolves over time, the most massive stars die off first, followed by less massive stars. This leads to the overall reddening and dimming of the population over time, as well as the subsequent chemical enrichment of the surrounding ISM by exploding high-mass stars.

gif of the evolution of a simulated SSP and HR diagram

This simulation shows the evolution of a solar-metallicity SSP assuming a Kroupa IMF with one million stars, along with an HR diagram of the stars within the population. The SSP, shown here for ages ranging from 100 Myr to 10 Gyr, was simulated using the ArtPop package and individual frames were compiled into a gif using FFmpeg. The spatial distribution of stars was modeled using a Sersic profile with a Sersic index of 0.8, effective radius of 250 pc, and ellipticity of 0.3. The observations were simulated in the R, V, and B filters for a 10-meter class telescope. Notice the apparent reddening and increase in apparent magnitude of the population as stars toward the high-mass end of the main sequence evolve and die off.

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