Postdoc Jaehan Bae: Protoplanetary Disks From Scratch

Tuesday, July 31, 2018 

In 2015, the Atacama Large Millimiter Array telescope in Chile unveiled the first ever image of a protoplanetary disk. Everyone was shocked. Before that iconic ALMA picture of the young star HL Tauri and its protoplanetary disk was released, few astronomers had thought that such a picture could be imaged. One of these astronomers was Jaehan Bae, now a Rubin Postdoctoral Fellow at DTM. The feeling he got by looking at that picture was truly sensational, Bae says, pretty similar to how he felt the first time he saw the Milky Way in his native South Korea as part of a school field trip.

In this postdoc spotlight, Bae discusses his approach to studying planet formation using computer simulations of the disks in which they form.

What do you do?

I'm interested in planet formation. Things can be studied in various ways. You can do lab experiments, for example, you can run computer simulations, or you can observe things. The problem with planet formation is that we cannot really make planets in laboratories, but it is also really difficult to observe planets forming. So people usually do numerical simulations to make planets like Earth and those in our Solar System.

Numerical simulations are computer simulations where you basically let a computer solve equations that you want to solve and then see what happens over time. That's what I do, but strictly speaking I am not doing planet formation simulations. Instead, I simulate the environments in which planets form and grow. I am looking at disks.

If you think about the Solar System, the Sun is at the center, and we had a disk from which the System's planets were formed. Beyond the Solar System, we have lots of young disks where we think planets are forming. We are observing those disks, and by looking at that, I'm trying to understand what's going on in the disk. For example, I want to know whether planets are forming in the disk, and then I'm trying to explain those observations using computer simulations.

What do you mean by "environment"?

Disks are 99 percent gas—that's Hydrogen and Helium—and only 1 percent of the disk material is solid particles. So, we are simulating fluid dynamics. I'm mostly looking at density of a disk, or how the disk's density changes. For example, when you sail on a boat, it will generate a wake. Far from the boat you can see waves generated. It's a similar thing when a planet orbits around a star because it will generate some signatures, like wakes. By looking at those signatures, I am trying to infer how massive that planet should be to generate those signatures in the environment of the disk.

An animated figure showing the density distribution of a protoplanetary disk in which a planet creates ringed substructures similar to the ones seen in observations. Credit: Roberto Molar Candanosa, Carnegie DTM.

Why do you do this type of research?

I was interested in astronomy since I was very young, but as for this particular topic about protoplanetary disks, I took a bunch of different undergrad astronomy classes, and this topic of protoplanetary disks was the most interesting. I took classes in galaxies, stars, and everything else, but I liked planets. I have always been wondering how the Earth formed in the very first place and how life emerged on Earth. I believe that if we can answer the question of how planets form, we might solve some problems such as how life originates on a planet. So all along I have been interested in how planets form and how life is originated.

How can this particular research on disks help you and others tackle that question of planet formation and life origins?

Looking at planets forming in disks is similar to looking at human beings being born. We basically are looking at newborns in a hospital. It's the time of the birth of a planet, and by looking at those planets, we can learn so many things, such as what material the planets accrete onto their body, how rapidly they obtain their mass, and where in the disks they form. For example, Earth is at 1 Astronomical Unit (AU) from the Sun and Jupiter is at 5 AU from the Sun: That is, five times the distance between the Sun and the Earth. But we don't know whether they were born at that particular place. They could've been born somewhere else, at 10 or 15 AU and then just migrated in. We don't really know, so by looking at planets forming in situ, we can better understand how Solar System planets formed at the very beginning of the Solar System's history.

When was the first time you were interested in astronomy?

I have been interested in astronomy in general for a long time. I am from Incheon, a big city about 30 minutes away from Seoul, the capital of South Korea. Incheon is pretty big, and it has lots of tall buildings, as well as light and air pollution. So it was always hard to see stars at night. I could count the stars with my hands. But in elementary school, my class went out for a couple of days at a suburban area with no air pollution and no light pollution. I was something like 10 years old, and we were in a forest with rivers and trees. I still remember the scene I saw at night in the sky. I saw the milky way, tons of stars. It was just amazing—something I never forgot. That's how I got interested in stars and astronomy.

There was also certainly an influential class I took on Solar System dynamics. It covered planet formation and evolution. One specific thing that I learned to do from that class was computer simulations. It was very simple at the beginning, something like writing a code that would add up from 1 to 10 or compute the area of a triangle—very basic things in coding. At the end of the semester we learned how to simulate the Solar System body, all the planets and the Sun. That was one big thing, because that's what I'm doing now, and it was in that class as an undergrad in South Korea when I first learned to do computer simulations.

How do you think your field will advance in the next few years?

In the next few years, I believe we will be able to directly image forming planets in protoplanetary disks, not just the signatures imprinted on disks by those planets. So far, there has only been one recent, but very convincing, detection of a young, forming planet. By detecting planets together with the disk structures they create, we can have a much better idea of planet formation processes and interaction with their environments.

When you talk to people outside of your field, are there things they struggle to understand about what you do?

People often ask me if I think there is life outside of our planet in the Solar System and in exoplanets. That's the thing a lot of people ask me when I tell them I am an astronomer. And I often answer yes. I do believe there will be some intelligent life in the universe. We know there are so many exoplanets right now. We have discovered nearly 4,000 exoplanets now, and we know that almost every single star should have at least one planet, so there are tons of planets. The chances we might discover extrasolar intelligent life are low, of course, but I am optimistic with that.

What have you been working on lately?

I recently was part of a study on a particular disk. There were three dark rings in the disk. Naively, one would think that there are planets orbiting within the dark rings, opening those gaps. But of course there are other theories that can produce those structures. As I mentioned before, almost all of a disk's mass—99 percent—is Hydrogen and Helium gas and only 1 percent is solid. People have focused on observations on solely the solid particles. It's a lot easier to observe emissions coming from particles than gas, for technical reasons. But again, that's only 1 percent of the disk's mass. So although it's a lot harder to observe gas, we tried to see how the gas was distributed in the disk, and we found that it's exactly what it looks like in dust. So by confirming that gas and dust are distributed the same way in the disk, we could rule out some other possibilities that could generate those dark gaps.

An artist’s impression of protoplanets forming around a young star, courtesy of NRAO/AUI/NSF; S. Dagnello

What are you going to be working on next?

We are expanding the size of the sample. That study was just for one object, just one disk. But there are many other objects with similar features, with dark and bright rings. We are interested in other disks as well, and we actually have some preliminary results on a few other disks where we see a similar thing where gas and dust are distributed in the same way. Eventually, we want to detect signatures coming directly from the planets themselves. Again, we are looking at disk structures and inferring the presence of the planets in those disks, but the next step will be detecting those planets. And actually, Carnegie telescopes can help with that. We have the Magellan telescopes in Chile, which can detect certain hydrogen line emissions. And then the Giant Magellan Telescope, which will be one of a few instruments in the world that can also do that.

—Roberto Molar Candanosa