About Me

Please visit my new website at skgrunblatt.github.io, this website is no longer up to date!

I'm a graduate student at the Institute for Astronomy at the University of Hawaii working with Daniel Huber and Eric Gaidos on finding and understanding giant planets orbiting giant stars. Previously, I worked with Stuart Jefferies on understanding the subsurface structure of the Sun and Andrew Howard on a new method to describe stellar variability when measuring the masses of exoplanets..

I'm interested in understanding the mysteries behind stellar and planetary evolution, and how they affect our past and future. In my free time, I enjoy making music, attempting to surf, and spreading my enthusiasm about astronomy to others.

  • Curriculum Vitae

  • Publications

  • GitHub

  • Twitter



Planet Occurrence Around Evolved Stars

In 2016, I began a search for giant planets orbiting giant stars using data from the NASA K2 Mission. So far, our search has identified two new planets, K2-97b and K2-132b, whose discoveries we published here and here. Both of these planets are gas giants, 1.3 times the size and half the mass of Jupiter, orbiting their host stars approximately every 9 days. These targets were identified and accurately and precisely characterized with asteroseismology, the study of oscillations of stars. We find that among stars that are 3 to 8 times the size of our Sun, it seems that planets larger than the size of Jupiter at orbital periods of 10 days or less are significantly more common than such planets are around main sequence, Sunlike stars. With the launch of TESS in early 2018, we will soon be sensitive to these types of systems all over the sky, revolutionizing our understanding of planetary evolution and tidal dynamics. These two poorly understood processes are crucial to life everywhere. Check back in a few months for more updates!

Eccentric Giants Orbiting Giants

Studies have predicted that as main sequence stars evolve into red giants, any long-period, massive planets on eccentric orbits should be pulled closer and into more circular orbits. This should create a population of short-period, moderately eccentric giant planets orbiting giant stars. In 2018, analysis of radial velocity measurements from the Keck-I telescope revealed that both planets discovered by our survey of giant planets orbiting giant stars were on moderately eccentric orbits, suggesting that they may have migrated to their current locations from longer period, more eccentric orbits. A similar Kepler planetary system revealed a slightly more eccentric, longer-period planet orbiting a less evolved star, indicative of an evolutionary sequence.

Comparing the populations of all known planets with measured eccentricities reveals that close-in giant planets seem to be significantly more eccentric around giant stars than dwarf stars. Additional information about this study can be found by reading the paper here or by watching the Aloha Brief below.

Planet Re-Inflation

A large fraction of gas giant planets with temperatures above 1000 K are much larger than 1.2 times the size of Jupiter, the maximum size models predict for a self-gravitating sphere of hydrogen and helium. Though these inflated giant planets have been known for over twenty years, the mechanism responsible for their inflation remains unclear. However, if we were to observe an inflated planet receiving a moderate amount of radiation from a red giant host star, such that the planet would have been too cool to inflate until the host star became a red giant, this would provide evidence for a planet inflation mechanism where the stellar irradiation deposited into the planet's interior causes it to expand. In December 2016, I discovered K2-97b, an inflated planet orbiting a red giant star every 8.4 days, and discovered its cosmic twin a year later. These planets' incident flux history indicate they were too cool to inflate until their host stars became red giants, and thus provide the first evidence that planets may be inflated directly by incident stellar radiation rather than by delayed loss of heat from formation. Assuming that gas planets begin their lives inflated due to heat from formation, K2-97b and K2-132b are likely the first known planets to be currently re-inflated. For more information, check out the original discovery papers: K2-97b: A (Re-?)Inflated Planet Orbiting a Red Giant Star and Seeing Double with K2: Testing Re-inflation with Two Remarkably Similar Planets around Red Giant Branch Stars.

This plot shows the radius evolution of K2-132b over time, where the rainbow lines correspond to different evolutionary models. At early times, the planet is hot and inflated from its formation, but quickly cools on the main sequence. Then, once the host star evolves off the main sequence, the planet can once again grow in size, earning its re-inflated status.

New Methods For Analyzing Stellar Variability

In 2015, I published a paper with Andrew Howard and Raphaelle Haywood entitled Determining the Mass of Kepler-78b with Nonparametric Gaussian Process Estimation. In this paper, we used radial velocity observations of nearby stars to detect a Earth-sized planet. In order to measure a planet's mass from the radial velocity data from a star, it is often necessary to first characterize the background radial velocity noise due to the star or other non-astrophysical factors. In order to better characterize this predominant stellar activity signal, we can test different statistical models to remove the stellar signal and extract the planetary signal with greater precision. We used a Gaussian Process regression, a nonparametric statistical time-series analysis technique, and tuned parameters describing general characteristics of the data, to describe and remove the predominant stellar signal. Using this technique, we were able to achieve provide a more robust technique for detecting planets via radial velocity measurements. I extended this technique to account for stellar oscillation and granulation in photometric lightcurves in my 2017 publication.

This plot shows the radial velocity measurements of Kepler-78 taken by both the HARPS-N and HIRES instruments on the TNG and Keck telescopes, respectively. A quasiperiodic Gaussian process model has been used to describe stellar activity in both datasets, with a common rotation period shared between the models. Removing the stellar activity signal using a Gaussian process method produces a more robust mass measurement for the small lava world Kepler-78b. Read more about this technique in our publication here!

The gray data in this plot shows the power spectral density of the lightcurve of K2-132. Power due to granulation variability can be seen on the left side of the plot, whereas asteroseismic oscillations can be seen to the right near 250 microHertz. We use a Gaussian process method based on the Python package celerite to construct a model of the lightcurve as a sum of simple harmonic oscillators in the time domain, to reproduce the overall frequency domain structure seen in our data. For more info, check out our paper here!


Teaching/Outreach: Philosophy and Materials

In addition to doing research, I am passionate about getting involved with my community in order to spread astronomy knowledge further into the general public. I believe that the most influential sceintific discoveries were made possible by the embrace of those ideas by the public.

In summer 2017, I participated in the HI-STAR Program at UH--Manoa, a program designed to help high school students get more involved in astronomy research. In the fall and winter, I also mentored three of these high school students on regional science fair projects. All three students made it to the state competition, and are now working to publish their results in professional journals. My students presenting their HI-STAR research project. I think we definitely had the coolest title.

HISTAR Lightcurve Analysis Tutorial

I have also developed multiple lab experioments and class activities for the UH undergraduate astronomy lab class. As part of the Professional Development Program through ISEE at UC Santa Cruz, I was able to design two different lab courses which were included in the 2016 and 2017 curricula for the UH astronomy laboratory. Explaining the transit method of searching for exoplanets to one of the UH undergrad astronomy majors.

Astro 300L Fourier Transform Demo

I've also written a couple of blog posts about how to perform scientific analysis in Python. These posts are designed to be understandable for an undergraduate science major. I also enjoy talking to the general public about astronomy and science in general!

Scientific Coding with Python: Performing a Least Squares Fit to Data

Scientific Coding with Python: Making a Power Spectrum of the Sun

After giving a talk about my favorite exoplanets and doing a bit of solar observing at the MDA summer camp on Oahu's north shore.