ASTRO Workshop Brings Researchers together to Discuss Exoplanet Exploration

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Contributed by: Ian Czekala, Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), Postdoctoral Fellow, Stanford University

My research focuses on understanding young stars and their protoplanetary disks during the planet formation epoch. For a number of reasons, I was particularly excited about attending the Statistical and Applied Mathematics Institute (SAMSI) Hierarchical Bayesian Modeling of Exoplanet Populations, October 17-28. The main reason was that my previous experiences at SAMSI have always been so positive. For example, I first learned about many of the topics and techniques that I use on a regular basis in my research through a similar SAMSI workshop in 2013. Three years later, I was again eager to learn new analysis methods from the statistical expertise gathered at SAMSI. Although two weeks may seem like a long time for a workshop, I knew that the close-knit environment would foster collaboration, catalyze many new projects, and make the conference pass by way too quickly. The following is a brief account of the conference with highlights of aspects that I found particularly interesting, by no way is this a complete or unbiased survey of all that transpired!

First, allow me to explain the context for our workshop. In August, 80 researchers from the fields of exoplanets, gravitational waves, and statistics converged upon Research Triangle Park to kick off the year-long SAMSI program on Statistical, Mathematical and Computational Methods for Astronomy. At the Opening Workshop for this program, we explored ideas and statistical techniques common to these fields and brainstormed interesting projects to work on over the next year. We splintered into five “working groups,” each focused on a particular topic or technique. I joined Working Group IV – Astrophysical Populations, which was focused on hierarchical Bayesian inference of exoplanet populations. Each working group has maintained momentum through weekly teleconferences, and most groups will have a workshop at SAMSI at some point during the academic year. The year-long program will be capped by a “transition” workshop in May 2017.

Angie Wolfgang, a National Science Foundation Fellow at Penn State University and Eric Ford, a professor, also at Penn State, were the main organizers of the Astrophysical Populations workshop. We had about 20 participants split equally between astrophysics and statistics. Our first morning was spent discussing our research interests and what we hoped to accomplish over the next two weeks. Two major groups evolved from this discussion. The first was centered on exploring the mass-radius relationship of exoplanets from photometric transit and radial velocity datasets. The second was focused on spectroscopic techniques to characterize stars and measure their radial velocity. Although our workshop was nominally about exoplanets, it turns out that a proper understanding of stars is fundamental to detecting and understanding the exoplanets that orbit them.

Understanding the Planet Mass-Radius Relationship…
In the past decade, astronomers have transitioned from knowing of the existence of only a handful of exoplanets to discovering a vast collection of several thousand. Most planets have been discovered by the Kepler Mission, which finds planets by measuring the dip in light as a planet transits its host star. It is most informative about a planet’s radius. For a select subset of these planets, precise radial velocity monitoring yields the masses of the planets as well. Because we are necessarily operating at the detection limit of our telescopes when studying small planets, it is very important to utilize proper statistical analysis lest our interpretation be biased.  The fundamental unknown that links a planet’s mass and radius is the planets composition, and so with a proper statistical framework we might hope to infer how planet composition varies amongst the thousands of known exoplanets, telling us something deep about the planet formation process in general.

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Leslie Rogers, an Assistant Professor in the Department of Astronomy and Astrophysics at the University of Chicago, speaks about planet composition distribution.

Angie Wolfgang, Bo Ning, a Ph.D. candidate in the Department of Statistics at N.C. State University, and Sujit Ghosh, SAMSI Deputy Director, explored using Bernstein Polynomials to model the planet mass-radius relationship non-parametrically, and showed promising results that included measurement uncertainties. Leslie Rogers, an Assistant Professor in the Department of Astronomy and Astrophysics at the University of Chicago, talked about the planet composition distribution. In addition, she also discussed how to link physically motivated models of planet composition to data and determine if this composition changes as a function of planet formation mechanism. Kaisey Mandel, a Postdoctroal Fellow at the Harvard-Smithsonian Center for Astrophysics, worked on understanding selection effects as they apply to exoplanet surveys. This was his focus since he is also interested in selection effects of Type Ia supernovae surveys.

A sizable group of people worked on translating hierarchical sampling code into the new language STAN. In particular, Megan Shabram, a Postdoctoral Fellow with NASA’s Kepler Mission and Joe Catanzarite, a SOC Scientific Programmer with NASA’s Kepler team, produced an open-source Jupyter notebook that implemented planet occurrence rate calculations in PySTAN.

Central to many of our problems discussed at this workshop was the topic of “emulation” or “uncertainty quantification,” which is actually the primary topic of Working Group I – Uncertainty Quantification and Astrophysical Emulation. Bekki Dawson, an Assistant Professor in the Penn State Department of Astronomy and Astrophysics, and Assistant Professor, Anirban Mondal of the Mathematics, Applies Mathematics and Statistics Department at Case Western Reserve University, worked on developing astrophysical emulators for planet formation models, so that more accurate (and computationally expensive) models could be used in hierarchical Bayesian inference to understand the formation of super-Earths and mini-Neptunes. Related to this problem, Jessi Cisewski, Assistant Professor in Yale’s Department of Statistics, made several informative presentations on Approximate Bayesian Computing (ABC) to solve inference problems where it is difficult to write down a likelihood function.

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David Stenning, one of two SAMSI Postdoctoral fellows at the workshop, presented talks on techniques using astrostatistics to improve exoplanet analysis.

Hierarchical Spectroscopic Inference with Time Series Stellar Spectroscopy…
A large group of astronomers and statisticians worked on techniques to improve radial velocity precision, with the hopes of finding planets with the mass of earth and below. Eric Ford, Jessi Cisewski, David Stenning and David Jones, Postdoctoral Fellows at SAMSI, Robert Wolpert, a Professor of Statistical Science and the Environment at Duke Univesity; Tom Loredo, a Senior Research Associate in Astronomy at Cornell; Ben Montet, a Postdoctoral Researcher from the University of Chicago and I worked on radial velocity fitting using mock spectral datasets with known statistical characteristics. These datasets are comprised of real stellar spectra of the sun to which have been added planets (the signal of interest) and star spots (a confounding signal). We examined interesting principal component analysis with the hope of isolating the orbiting planet from stellar activity. During this period, we were also treated to two presentations by the SAMSI postdocs David Stenning and David Jones about using Gaussian processes to correlate stellar activity indicators with radial velocity jitter and using diffusion mapping to understand stellar variability.

By the end of the workshop, we were all knee-deep in immersive projects that we had started just 10 days prior – we were reluctant to leave!  The collaborative working environment, with daily updates of what we had accomplished certainly fueled an exciting work schedule, since everyone was motivated to complete new ideas to share with the group. By the end of the workshop, several of us remarked that in fact two weeks was not a long enough period for us to get anything done – we were all so dedicated to the research, we wanted to stay! To cap it all off, we were treated to a tasty “special presentation” by Tom Loredo, who shared with us how chocolate is made.

These researchers will collaborate over the next several months on this continued analysis of exoplanet discovery.

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Workshop participants were treated to a chocolate tasting from Tom Loredo, a Senior Research Associate in Astronomy at Cornell. Loredo is a hobbyist chocolatier and candymaker and his confections were enjoyed by the group.

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E&O Undergraduate Astrostatistics Workshop: A Stellar Learning Experience

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Contributed by: Rachel Matheson, Mathematics Undergraduate Student, Vassar College – Poughkeepsie, NY

As a math major at a liberal arts school, choosing my classes for the next semester always feels like a lot is at stake. I want to take physics, neuroscience, astronomy and biology, but I also want to take social sciences and humanities. Dipping in to the Statistical and Applied Mathematical Sciences Institute’s (SAMSI) Undergraduate Workshops gives me a chance to experience the different flavors of applied math and statistics without the commitment of a class. I was therefore extremely delighted to be invited to come back to SAMSI this October for a two-day undergraduate workshop focused on Statistical, Mathematical and Computational Methods for Astronomy (ASTRO).

SAMSI Workshops – Full of Information…

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Jessi Cisewski, an Assistant Professor in Yale University’s Department of Statistics, conducts a lecture on Approximate Bayesian Computation in Astrostatistics. This talk was one of several performed during the workshop.

Though the workshop was brief, it was packed with interesting lectures and hands-on activities. After a yummy breakfast at the hotel, we shuttled off to SAMSI’s campus and were greeted by SAMSI Deputy Director, Sujit Ghosh, who delivered his opening remarks to our group. From there, we quickly transitioned into a lecture from Jessi Cisewski, an Assistant Professor in Yale University’s Department of Statistics, on Approximate Bayesian Computation in Astrostatistics. The lecture was very enjoyable and informative – it served as a reflection and an extension of what I have been learning in my probability class, applied to the stellar initial mass function.  Bekki Dawson, an Assistant Professor from Penn State University’s Department of Astronomy and Astrophysics, then dazzled my mind with stellar facts during her lecture titled Time Domain Challenges for Exoplanets. I was surprised to learn that this is an area where technology is good and up-to-date but we still don’t have the statistical methods to interpret noise in the data properly in order to detect exoplanets similar to Earth.

“SAMSI serves largely as a space for me to feel motivated about my pursuit of applied math and connect with people who feel just as excited as I do about it.”

After a short break, we delved back in to a tutorial on R led by SAMSI Post-Doctorate fellows David Jones, David Stenning and Hyungsuk Tak. This was a helpful overview to lead up to the intensive, hands-on workshop of modeling Gaussian processes. Line-by-line comments in the R code kept me from feeling lost as the lecture sped on, deep into the mathematics and emulator needed in order to make this model run. I could easily go back and gain understanding as post-doctorate fellows stood throughout the room ready to help at an arm’s wave. It felt like a really positive learning environment, despite the high speed at which the material was presented.

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On Day 2 of the Undergraduate Workshop, students got to visit the Morehead Planetarium on the campus of UNC-Chapel Hill. The students enjoyed two presentations on the universe and the existence of blackholes.

Opportunities and Guidance from those who have done it…

One of my favorite aspects of coming to SAMSI is being able to talk to the post-doctorate fellows, SAMSI faculty, and my peers, about anything from career path choices to, quite literally, the stars in the sky. The panel on career opportunities led by some of the graduate fellows was a wealth of information for nervous undergraduates to seek advice from those who have “made it,” as well as to start conversations to continue later on. I ended up eating dinner with two post-doctorate fellows, who advised me on everything from which classes to take to not worrying too much.

Leaving with a new sense of purpose…

After a visit to the Morehead Planetarium, I felt sad to be leaving almost as quickly as it began. It is always so reassuring to talk to people who are pursuing what I am interested in, not to mention truly inspiring and exciting. SAMSI serves largely as a space for me to feel motivated about my pursuit of applied math and connect with people who feel just as excited as I do about it. It forges what may be a 2-day community, but that community gets to live on through email and LinkedIn. I am too glad to have had the opportunity to experience SAMSI as a community and as a learning space – it excels at being both!

To see more about what happened at this workshop, visit: ww.samsi.info/astro-undergrad. To see past and upcoming workshops in our ASTRO Program, visit: www.samsi.info/astro.

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Students from the Undergraduate Workshop pose for a picture at the working sun dial located at the Morehead Planetarium on the campus of UNC-Chapel Hill. The students visited the planetarium as part of their workshop activities. The 2-day undergraduate workshop was part of the Education and Outreach for SAMSI’s Program on Statistical, Mathematical and Computational Methods for Astronomy (ASTRO).