Postdoc Spotlight: Astronomer Tri Astraatmadja

Tri Astraatmadja observing remotely at DTM, using internet connection to communicate with the telescope operator at the Las Campanas Observatory, Chile, and to connect to one of the computers that control the camera. Photo by Ucu Agustin
Tri Astraatmadja, using internet connection to communicate with the telescope operator at Las Campanas Observatory, Chile, and to connect to one of the computers that control the camera. Photo by Ucu Agustin.
Wednesday, November 22, 2017 

Astronomers use different methods to detect exoplanets. But astrometry, which requires exceptionally precise measurements of the positions of stars in the sky, is particularly difficult. While thousands of exoplanets have been discovered by Doppler spectroscopy and transit photometry, none have been discovered by astrometry. Tri Astraatmadja, who joined DTM as a postdoctoral fellow in 2016, works with DTM astronomers in an effort to discover the first confirmable exoplanet using the astrometric method.

In this Postdoc Spotlight, Astraatmadja tells us how a space enthusiast kid from Indonesia now contributes to the Carnegie Astrometric Planet Search (CAPS) project.

Tri Astraatmadja works with the CAPS project, led by DTM Staff Scientists Alan Boss and Alycia Weinberger. In addition to developing the data analysis pipeline, Astraatmadja also periodically takes new data using the Irénée du Pont Telescope in Chile. Photo by Roberto Molar Candanosa.

What do you do?

At DTM I'm working with Alan Boss and Alycia Weinberger, trying to discover planets orbiting other stars using astrometry, the oldest branch of astronomy. Astrometry is basically the measurements of the position of celestial objects. If we do these measurements repeatedly and with high accuracy, we'll find that stars exhibit motions as well. We see these motions because of a star's own motion and because Earth is revolving around the Sun. These components are called respectively the stellar proper motion and stellar parallax. Measuring them can tell us a great deal about our galaxy.

Stellar proper motions tell us about the dynamics of our galaxy. Parallaxes greatly help us in inferring distances to the stars, and knowing distances to the stars allows us to learn about the intrinsic properties of the stars, such as their true brightness. If we measure these components accurately, there might still be movements of the stars that are not taken into account, and these movements could be due to the gravitational pull of an exoplanet orbiting that star. Of course these movements are very small, and thus the challenge is to measure everything accurately.

What's so exciting about astrometry?

Astronomers have discovered lots of exoplanets using other methods, but so far no planet has been discovered using astrometry. Many have tried and claimed to have seen these stellar wobbles due the gravitational pull of exoplanets, but later have found that weird instrumental effects were not completely taken into account.

I'm a part of the Carnegie Astrometric Planet Search program (CAPS project), which has been using the 2.5 meter du Pont telescope at Las Campanas Observatory in Chile to measure the position of about 100 nearby stars since 2007. I'm doing this remotely, but it's a lot of fun and exciting since this is the first time I am an observational astronomer!

The challenge of ground-based astrometry is that starlight bends as it passes through Earth's atmosphere. Imagine you're at the bottom of a pool and you have to (accurately!) measure the position of stuff located above the pool. This is probably too much of an exaggeration because water has different properties than the atmosphere, but both are basically fluid, so that illustrates the challenge of ground-based astrometry. It is this great technical challenge of this project, and addressing these type of challenges is very exciting for me! I was skeptical at first about astrometric exoplanet detection using a ground-based telescope. But then I thought about how these problems can be addressed, read papers on similar projects, and thought this looked like an awesome project with challenging problems I think I can solve.

The astrometric method measures the positions of stars with high accuracy as they move across the sky. Animation by NASA's Jet Propulsion Laboratory, available at

Why is astrometry important?

Exoplanets have been found with various methods, but these methods are particularly sensitive to exoplanets with short period orbits. Astrometry, however, is particularly suitable to detect exoplanets with long period orbits. Detecting long period exoplanets allows us to learn a great deal about their occurrences and how they form, giving us a better picture of how planetary systems form. So there is a window to discovery (so to speak) here that hasn't been exploited due to past failures.

Also, astronomers have tried to detect exoplanets with astrometry since the 19th century. Many had claimed astrometric discovery of exoplanets but later got disproved, and this particular method to detect exoplanet unfortunately fell into disrepute. I believe it is important to show that this method can work, as long as we have the data and are careful with the analysis.

The CAPS project is a marathon, not a sprint, where endurance is needed because astrometric detection of exoplanets is at least a decade-long project. This is a nice reminder that some scientific efforts are long-term projects. It is like a "Time for the Stars" (to quote Robert Heinlein) project.

Do you remember the first time you realized you wanted to be an astronomer?

My family is quite astronomy friendly, and I was introduced to astronomy at an early age. In 1983, when I was two years old, we went to observe the total solar eclipse that passed through Java, Indonesia. The 18-year dictatorial Suharto regime that ruled Indonesia at that time tried to scare people away from observing the eclipse, so our family expedition was probably as much about defying the regime as about introducing the wonders of the universe to the kids.

That trip was so memorable. My dad always keeps our family photo from the trip in his wallet. Then in 1986 we went to the outskirt of Jakarta, Indonesia to observe Comet Halley. We didn't get to see the comet, but the Jakarta Planetarium and the Amateur Astronomy Club invited a really old guy who saw the comet in 1910. That was really awesome.

Despite these experiences, I don't think I ever thought about pursuing a career in astronomy or studying it in college. I realized early-on that astronomy is not about making nice colorful photos of celestial objects, but you have to be good at math and physics—and I was not really good at those subjects. I was thinking of becoming a comic artist because I can draw things and write stories rather well, but this started to change when I was in high school.

Full confession here: I wasn't really a big fan of this going-to-school thing, so I often skipped school and went to the British Council Library instead. In the VHS section they had Cosmos, this documentary series presented by Carl Sagan. I watched it there in the library. Perhaps it was because of Carl Sagan's soothing voice, his way of presenting ideas, and the breadth of topics presented (this is a series about the whole universe, after all) that I was hypnotized, awestruck, and just felt like I wanted to know more! I then looked for more information on where to study astronomy (at a technical university perceived to be the best and most prestigious in Indonesia) and how to go there (apply for the national university entrance exam and be the best 10% among the applicants). The exam consisted not only of math and physics, but also chemistry and biology, so I went back to school, made amends with my teachers, and took extra lessons after school and during school holidays.

I was 16 then when I realized that my path in life is the path to the stars, and so I started to forge my way to it. Maybe I was too naive, but then again I watched a lot of Star Trek at that time.

Irénée du Pont Telescope at Carnegie's Last Campanas Observatory, February 1, 2008. Photo by Krystof Ulaczyk via CC license.

How did you first hear about DTM?

First time I heard about DTM was when we learned about dark matter in college. Yup, you guessed it right, Vera Rubin's name kept popping out in the materials that we had to study, so I read her papers, her biography, and that's how I knew about DTM. The second time I heard about DTM was when my college friend—an exoplanet aficionado—showed me papers written by Paul Butler. I wasn't really an exoplanet guy at that time but rather a Milky Way person (my copy of Galactic Dynamics has signatures of James Binney and Scott Tremaine), but I thought that the fact that Paul Butler was also a DTM scientists was awesome. Many years later, my friend Berry Holl told me about DTM and the CAPSCam project. I have known Berry since we studied together in Leiden for our master degrees and then for the Gaia astrometric mission.

So far it has been great at DTM! I am impressed that DTM gives a lot of freedom for scientists to pursue areas and methods that they think are the best to address the problems at hand.

What research do you hope to pursue at your next job?

I think I'm pretty good at data analysis and big data, so I'm open to any project that involves statistical inference, machine learning, and analysis of huge amounts of data.

What's your dream job?

In 20 years I hope to be a part of a collaborative research project that ends up revealing a huge knowledge that changes our perspective of the universe, with good data analysis that make the conclusions compelling and make people say, "wowzers, there's a formidable analysis right there,'' when they read the paper. That's probably my dream job: Being in a place where I can confidently be able to make conclusions from incomplete observations.

Teaching is also something that I want to do, as a way of sharing to a future generation of scientists what we know so far and where we are heading. I don't think I will ever get a free round-trip ticket to Stockholm, but that doesn't really matter because I'm not in this for that kind of thing.

Anything else you want to share?

In my free time, I do analog photography. In fact, I know how to develop black and white films! I just love pre-digital things... Another thing that I do with my free time is writing popular articles on recent astronomical discoveries. I'm a regular columnist at langitselatan (it means "the southern sky" in Bahasa Indonesia), a web-based community of amateur and professional astronomers that regularly do astronomy outreach in various parts of Indonesia. The community is partnered with various international astronomy outreach organization, such as the Universe Awareness and the Office of Astronomy Outreach of the International Astronomical Union. I have been with langitselatan since its inception about 10 years ago, and I am proud to be a part of its growth.


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