Q&A With Dr. Keivan Stassun
Dr. Keivan Stassun is a Professor of Physics and Astronomy at Vanderbilt University. He is one of the directors of the recently launched TESS (Transiting Exoplanet Survey Satellite) mission. The mission is one of multiple exoplanet-hunting programs. Dr. Stassun’s team was tasked with finding which stars the satellite was going to measure using the transit method (explanation below). Previously, Dr. Stassun’s work was focused on binary star systems; specifically, he looked for the effects that the stars had on each other.
Explain the Transit method in layman’s termsThe main technique that Astronomers use to find planets around other stars involves measuring the brightness of the star over and over again, looking for a sign that the star has gotten dimmer. What that dimming tells us is that something is orbiting that star and is blocking some of the light that the star is emitting.
What is the TESS mission?
It’s a small telescope that was launched a few months ago that will look at around 250,000 star systems. The ultimate purpose of our mission is to find little planets, like earth, orbiting stars that are relatively close to ours. TESS differs from similar missions in its scale; We are focusing on many stars that are all relatively nearby. To do this, we have a much wider, yet shallower, view of space, whereas other missions go deeper into space with a narrower field of view. TESS is also able to monitor the atmospheric composition of exoplanets surrounding particularly bright stars.
How can you analyze the atmosphere of an exoplanet?
When a planet moves in front of its star, the body of the planet blocks out some of the light coming from the star, which is the dimming we record with the transit method. If, on top of that, the planet has an atmosphere, not only will we be able to measure the dimming, but–and this requires exquisitely sensitive measurement–we can detect the chemical makeup of the planet’s atmosphere by analyzing how the star’s light passes through this atmosphere.
Where do you see TESS taking us?
Well, if an exoplanet is really close, that opens the door for us to potentially take a trip over.
What were some of your biggest challenges?
My team was in charge of picking which region of space we would measure in order to get the best possible star systems. There were about 20 million eligible stars and we could only choose a quarter of a million. So, we had to create a robust way to maximize the amount of small, bright, and nearby stars that we could measure. We had to come up with work-around, redundancies, and techniques; It was very complex science-wise and gave us many headaches.
Why are you equipped to be a director of TESS?
The biggest part of my work up until TESS was focused on binary star systems. The effect that binary stars have on each other is very similar to the effect that exoplanets have on stars, only on a much larger scale. Being an expert on these effects allowed me to transition seamlessly into working on TESS.
How successful has the TESS mission been so far?
[smirking] I’m not allowed to tell you.
Transcript
Why don’t you introduce yourself for the record?
My name is Keivan Stassun, professor of Physics and Astronomy
Could you explain TESS at a middle school level?
Sure. There are two main techniques–well the main technique that astronomers use to find planets around other stars involves measuring the brightness of the star over and over again, looking for a sign that the star has gotten dimmer. What that dimming tells us is that something is orbiting that star and is blocking some of the light that the star is emitting. And so the Tess mission is a small telescope that we launched a few months ago that will do that, looking at a quarter of a million stars, over and over again, looking for that dimming, and we’ll hopefully find all the planets that we can; the planets we’re mainly interested in finding are smaller planets like Earth.
How does TESS differ from other Transit method projects?
That main difference is that TESS is looking for stars across the entire sky and what that allows the mission to od is target especially nearby bright stars, which, if we find earth’s around them will be really exciting. Because they are nearby and bright, we’d be able to analyze the chemical makeup of their atmospheres; which you could only do when studying a planet around a really bright star. If a star is really close, that opens the door for potential space travel.
So is it more stars than another transit project?
In terms of number of stars, it’s not more. One could say that other missions go deeper into space with a narrower field of view. For example, the Kepler mission looked at a comparable number of stars; but that telescope spent all of its time staring in one direction. When you’re staring in one direction, you have a lot of stars that are all farther and farther away from each other– you’re going deeper into space. So the vast majority of stars studied in the Kepler mission were distant and dim, whereas we are focusing on many stars that are all relatively bright and nearby. To do this, we have a much wider, yet shallower, view of space.
Could you touch on planetary atmospheric analysis a bit more?
Yeah. So When a planet moves in front of its star, the body of the planet blocks out some of the light coming from the star, which is the dimming we record with the transit method. If, on top of that, the planet has a sliver of an atmosphere, like Earth does, not only will we be able to measure the dimming, but–and this requires exquisitely sensitive measurement–we can detect the chemical imprint on the light spectrum of the planet’s atmosphere by analyzing how the star’s light passes through this atmosphere. You can measure what’s in the atmosphere by the absorption spectrum. If you have water in the atmosphere, for example,
That’d be a promising sign
Ineed; you’ll see the slight additional absorption due to water in the atmosphere.
What were some of the biggest problems?
For technical reasons, we are limited to a quarter of a million stars for which we can download the data. There are obviously more than a quarter of a million stars that are nearby and bright in the sky. There were about 20 million eligible stars and we could only choose a quarter of a million. And my team was in charge of picking which region of space we would measure in order to get the best possible star systems. We wanted to pick small stars, which are easier for planet detection because, f you imagine a little earth, it’s gonna get swamped in the light of a giant star. Dimming is easier to detect when it’s a smaller star. We had to come up with work-around, redundancies, and techniques; It was very complex science-wise and gave us many headaches. And only about ⅔ of the stars we chose are going to be eligible once the data is analyzed, so we really had to be very strategic about which stars we chose.
I know the mission launched only a few months ago, but how’s it going?
[smirking] I’m not allowed to tell you. The results should be available sometime in the spring but [still chuckling] as of yet I am bound to secrecy.
So what sort of work did you do before TESS?
The biggest part of my work up until TESS was focused on binary star systems. Specifically, i used binary stars in order to make the most accurate possible measurement about the fundamental properties of stars. For example, stars that are twice the mass of our sun have this temperature, this luminosity, this diameter–those relationships, those dependencies rely on fundamental benchmarks from which we can extrapolate. It turns out that binary star systems that periodically eclipse one another are really good benchmarks for this purpose. By virtue of the fact that they eclipse one another, we can measure their size; by virtue of their movement around each other, they are doppler shifting; by virtue of their mass, they are causing gravitational shifts
So it’s kind of like all of the methods for finding exoplanets
Exactly. The effect that binary stars have on each other is very similar to the effect that exoplanets have on stars, only on a much larger scale. Except that both are huge and luminous. So, as you surmised, by developing this deep expertise of eclipsing binary stars, being an expert on these effects allowed me to transition seamlessly into working on TESS.
Obviously TESS is taking up most of your time, but are there any other projects on your radar?
Yeah, actually a new project is a center that we just got started at Vanderbilt. It’s the center for Autism and Innovation; it’s focused on better recognizing the talents and capabilities of autistic peoples in a science and engineering context. The way I think about it–I know these things because my older son is autistic–most autism research is primarily educational or medicinal, yet few institutions cater to many autistic people’s scientific and technical aptitudes. I wanted to create a new third vertex that wasn;t more important than those other two things, but something that comes at autism from the standpoint of engagement and enlisting talents for science and engineering.