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Modelling the atmospheres of exoplanets

‘Twenty-five years ago scientists still expected that planets in other solar systems would look like the ones we have in our own system. But we were in for some big surprises’, says astrophysicist Antonija Oklopčić. Oklopčić is an expert in modelling the atmospheres of planets in other solar systems, which are known as exoplanets. She has recently left her prestigious NASA fellowship at Harvard University to join the University of Amsterdam’s Anton Pannekoek Institute for Astronomy as a MacGillavry fellow.

This is her first faculty position, and Oklopčić comments she is ‘very happy and excited’ that she managed to get it. ‘Dutch astronomy is globally regarded to be very strong. There are many excellent Dutch astronomers and they participate in big European projects like the Extremely Large Telescope and the European Space Agency. So it is a great opportunity to be part of this community.’ 

Oklopčić’s research field is still quite new, which is one of the reasons she finds it so exciting. ‘It was only after the development of precise spectrographs and dedicated space telescopes, such as the Kepler mission, that astronomers started to find planets outside of our solar system in large numbers.’

These exoplanets are often very different from the ones in our own system. ‘For instance, our solar system lacks planets with a size in between that of the Earth and Neptune. But planets of such a size are incredibly common around other stars,’ Oklopčić explains. ‘Many exoplanets also orbit their stars very closely, making their climates extreme. There are for example planets we call “ultra-hot Jupiters”, with temperatures of a few thousand degrees Kelvin. This means they are as hot as some stars.’

Antonija Oklopčić
Antonija Oklopčić

Million questions

Although her area of interest may sound as a niche topic, Oklopčić has always had a strong drive to understand how the entire universe works. Growing up in Croatia she liked science, space and astronomy, and was determined to become a physicist since she was thirteen or fourteen years old.

Throughout her Physics studies at the University of Zagreb, she did research projects that focused on cosmology, the early universe and galaxy evolution. Oklopčić: ‘It is not one single question that drives me. All topics of astronomy are fascinating and beautiful to me. There are still a million questions to answer.’ 

The work she did during her PhD at the California Institute of Technology in the United States also reflects her broad interest. While there, Oklopčić wrote a series of papers on various astrophysical topics. ‘It might be different in the Netherlands, but in the US it is quite common to explore various topics during your PhD programme,’ she explains. ‘But the common thread throughout my PhD was the interaction between light and matter in different astrophysical media. And how we could use spectroscopy to study different astrophysical objects.’

Galaxy evolution & atmospheres

One of her PhD projects had to do with galaxy evolution, and especially with a period when star formation in galaxies was at its peak, which occurred 10 billion years ago. ‘I used simulations to study why galaxies in that epoch looked much clumpier than the beautiful disc shape that many galaxies have today,’ says Oklopčić.

‘These galaxies had much more gas in them, which fragmented into clumps. The key question was how these clumps affect the evolution of their host galaxy. I found that the clumps disperse quite rapidly, after about twenty million years, which means that they do not live long enough to have a significant impact on the galaxy as a whole.’ 

Another of her projects concerned the extreme environment of exoplanet’s atmospheres. ‘I studied how a particular type of light scattering in the atmosphere of planets - called Raman scattering - leaves signatures behind in the spectra of planets that we observe. I analysed what these signatures could tell us about the composition of atmospheres and whether there are clouds or whether it’s clear.’

In 2017 Oklopčić moved to Harvard University as a postdoctoral fellow, and in 2019 she was awarded a NASA Hubble Fellowship at the same university. Here she continued to study the atmospheres of exoplanets. More specifically, she studied the process of atmospheric escape on various exoplanets, including a planet called WASP-107b. Oklopčić: ‘WASP-107b is a “warm Saturn”. In terms of size it is similar to Saturn, but it is much closer to its star.

Due to high levels of radiation, its upper atmosphere can get heated to very high temperatures and, as a result, evaporate and escape from the planet. My colleagues and I were the first to find evidence of gas escaping from WASP-107b by tracing helium atoms in its upper atmosphere.’

Closing the gender gap

The MacGillavry Fellowship at the University of Amsterdam was established to attract highly talented  female scientists from all over the world. Special tenure programmes like these are one of the ways in which universities are trying to close the gender gap, which is still a problem in many scientific fields, including astrophysics.

Oklopčić mentions a study that was published in scientific journal PLOS One a few years ago: ‘In this study, the researchers extrapolated the speed with which the gap closes for different fields of science. For astronomy it will take another 130 years before the male-female ratio will be equal. That is awful! If we would just sit around and wait, even my great grandchildren will experience this gender gap when they start their careers. I think that is unacceptable. We need to do something about it, it is not going to happen on its own.’

When asked about her own experiences of being treated differently as a woman in science, Oklopčić shares an example of what she experienced during her studies. ‘When I did really well on a test in Croatia, a professor would act surprised and shout out: “Oh, look at that. A girl has the highest mark! You should be ashamed of yourselves, boys.”

I got upset and tired about things like that, because it happened so often. Nowadays I would not keep silent but say something. The lack of women is still noticeable in my field, although it depends on the career stage. The student population is close to achieving gender parity, but it gets worse at more senior levels.’

Oklopčić is a member of a couple of social network groups designed for women in astrophysics. ‘Many institutions have mentoring groups for women in science, and I’ve participated in several of such programmes throughout my career, including meetings with other MacGillavry fellows. These meetings offer a great opportunity for networking. In these groups we don’t just talk about “being a woman in science”. Mostly we talk about the usual academic issues, like writing grant proposals and mentoring students.’

Future plans

The next couple of years at the UvA will be exciting for Oklopčić, since the Extremely Large Telescope (ELT) is currently being built in Chile and the James Webb telescope will be launched in 2021.

‘Many telescopes that we now use to study exoplanets were built before exoplanets were discovered. So they were not designed with this purpose in mind. The new ELT and James Webb Telescope will be real game changers for our field, since these telescopes will be much bigger than the ones we use now. In the next couple of years, I will focus on developing 3D models of exoplanet atmospheres to interpret the data that we’ll get from these new observatories.’

CV

2006-2011: MSc in Physics, University of Zagreb, Croatia
2011-2014: Fulbright International Science and Technology Fellowship
2011-2017: PhD in Astrophysics, California Institute of Technology, United States
2017-2019: Fellowship at the Institute for Theory and Computation, Center for Astrophysics, Harvard University, United States
2019-2020: NASA Hubble Fellowship Program Sagan fellow, Center for Astrophysics, Harvard University, Unites States
2020-present: Assistant Professor and MacGillavry fellow, Anton Pannekoek Institute for Astronomy, University of Amsterdam, the Netherlands