by Calen Henderson
The plan was two weeks. But two became three and weeks stretched to months as I toiled to tailor my application to the requirements of the NSF Graduate Research Opportunities Worldwide (GROW) fellowship. Then in April of this year, several months after submitting the final version, I received the congratulatory email informing me that I’d been awarded the fellowship and would be spending the summer working with a team of my collaborators in Cheongju, South Korea. My name is Calen Henderson, I’m a senior-level PhD student who studies exoplanets, or planets that orbit other stars, and below I’m going to describe an extraordinary summer in the life of an astrophysicist.
First, some background. Prior to 1989, humans knew about the existence of nine planets. They all resided in our Solar System, and Pluto had not yet been demoted. Then, in 1989, our knowledge of other worlds, and with it our own worldview, began to change. To date, astronomers have discovered over one thousand exoplanets that collectively provide insight into how planets form, how common it is for a star to have one or more planets orbiting it (extremely common, it turns out, with our Milky Way Galaxy alone estimated to harbor over 100 billion planets, or one planet per star, on average), and specifically what kinds of planets and planetary architectures are out there. Such findings are the result of using a variety of exoplanet discovery techniques, including transit, radial velocity, and pulsar timing. I specialize in one known as gravitational microlensing, and this method forms the backbone of my PhD research.
Normally when you look at a star, whether with your naked eyes or through a telescope, its brightness remains relatively constant as time marches forward. But, all objects in space are moving, with stars passing in front of and behind other stars, and every so often the alignment of the Earth and two distant stars will be so nearly collinear (all three in a straight line) that the gravity of the middle star will bend some of the light rays emitted from the background star so that they reach Earth rather than continuing to traverse through empty space. We see the manifestation of this alignment as a brightening of the background star, and if the intervening star has a planet in orbit around it, it is possible to see additional amplification of the light of the background star due to the gravity of the planet.
Such an occurrence is rare and unpredictable, and so requires monitoring tens of millions of stars to detect a few thousand microlensing events per year. A single microlensing event refers to a star that has been magnified by the gravity of an intervening star that may or may not host one or more planets. Ultimately this process leads to the discovery of just a handful of planets each year. But wait! There’s less! Not only is detecting a possible planetary signature in a microlensing event an unlikely situation, but sifting through the mountains of data taken by the dozens of telescopes around the world to accurately and precisely determine the physical parameters of the planetary system—how massive the planet is, how far away it is from its host star, et cetera—can often take years. Furthermore, only a small number of people have both the knowledge and the access to the vast amounts of computing power necessary to do so.
So far we’ve learned that microlensing is infrequent, difficult, and not very popular. These factors, coupled with the fact that microlensing has not yet discovered as many planets as certain other techniques, often leads to other astronomers viewing microlensing as the “Whose Line Is It Anyway” of exoplanets—the science is made up and the planets don’t really matter. In fact, such an assessment couldn’t be further from the truth. Microlensing is unique in that is has unparalleled sensitivity to low-mass planets orbiting their host stars at large distances. Whereas other discovery techniques can easily find a Jupiter-mass planet orbiting a star at the distance at which Mercury orbits the Sun, only microlensing can easily find planets with the mass of Earth at the distance of Earth. This is integral for our understanding of how common architectures like our own Solar System are as well as how planets are formed.
Re-enter our hero. One of the premier teams of astronomers in the world with the expertise and ability to analyze microlensing events and look for planets is based at Chungbuk National University in Cheongju, South Korea. They are led by Dr. Cheongho Han, who obtained his PhD in astronomy from OSU and with whom I have closely collaborated for the past two years. Through the NSF GROW fellowship, I was able to spend the summer working with Dr. Han and his group to learn how to analyze microlensing events.
The primary office in which Dr. Han, his graduate students, and I worked is probably the closest thing to Hollywood’s portrayal of science that I’ve ever witnessed. In this “War Room” there are computers lining two of the walls, with a third hosting a bank of clocks to help keep track of the current time for telescopes at a variety of longitudes. The center of the room is dominated by four huge monitors—the kind measured in feet, not inches—each connected to a different computing cluster, all of which are housed in a separate building due to the loud whirring of the fans that cool them as they radiate extreme amounts of heat while performing billions and billions of calculations.
What makes his group’s approach so unique and powerful is its comprehensiveness. Every day we would analyze all ongoing microlensing events in search of anomalies in the magnified brightness of the microlensed star due to the existence of a planet orbiting the intervening star. We would subsequently circulate our findings to the teams of astronomers in charge of the telescopes to encourage more or fewer observations, depending on our estimation of the likelihood of the existence of a planet. Any spare time was spent reanalyzing older microlensing events—perhaps there were additional or improved data, or maybe someone had insight into a different solution that would better describe the data.
In order to competently perform these tasks, there were several skills I needed to learn. To efficaciously utilize the computing clusters, Dr. Han’s team helped me become proficient in what is called parallel computation. Rather than using a single computer to solve a problem you use several in concert, farming out different tasks to each machine to most efficiently complete the computational task. I also brushed up on my complex analysis background. It turns out that microlensing equations can be more readily solved if you use imaginary numbers, a formalism that is the foundation of the best analysis codes in the world. Most importantly, however, was being able to analyze a multitude of microlensing events and the experience gained from it. When a star becomes microlensed by another, intervening star, there is a myriad of ways that the resulting magnification the light of that background star experiences can change as a function of time. In many cases, knowing a lot of math and having a lot of computers at your disposal can only get you so far—being able to intuit a good guess at the solution right off the bat is your most valuable skill.
While Dr. Han and his graduate students imparted a wealth of astrophysics, computer science, and higher-level math to me, some of the most meaningful things I took away had nothing to do with microlensing. They were unremitting in their hospitality and eagerness to help me explore Korean cuisine and culture. Their patience was admirable, particularly since I was the visitor who was able to speak only a few phrases of Korean here and there. And their desire to teach was paralleled only by the vast trove of knowledge, both within and external to science, upon which they were able to draw. I find it appropriate and comforting that my search for other worlds comes hand in hand with learning more about my own.
About Calen Henderson
Hey! I enjoy playing classical piano music, running, and traveling. I spend my spare time working on a PhD in astronomy at The Ohio State University, trying to discover and characterize exoplanets and gain insight into planet formation. As an Eagle Scout, outreach is also a big component of my life, and I love giving shows at our newly-renovated digital planetarium!