Dutch research is among the best in the world. That’s why every month KIJK editor Laurien Onderwater introduces you to a world of our soils in this column. This time: Ignas Snellen measures what the atmospheres of exoplanets are made of.

He always knew: when he grows up, he will become an astronomer. Ignas Snellen, born in 1970 in Heeze in Brabant, was browsing an astronomy book at the age of eight. For his birthday, he got a telescope, with which he spent several nights in the park, staring at the universe. But what Snellen, professor of observational astrophysics at Leiden University, did not dare to dream about, was that he would receive the highest Dutch prize in science.

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I met the astronomer—formerly an avid reader of KIJK—in his office in the Jan Hendrik Oort (named after the famous astronomer) building at Leiden University. Sitting at a massive wooden desk that once belonged to Dane Ejnar Hertzsprung, another big name in astronomy. But of course I didn’t cycle to Snellen’s office to talk about other astronomers. I want to know more about his research on planets orbiting stars other than our sun, or the so-called exoplanets.

Snellen studied astronomy at Leiden University, where he completed his doctoral research in radio radiation. He then worked for several years at the English University of Cambridge and then at the Scottish University of Edinburgh. Seven years later, he returned to the familiar Leiden – and never left. In the meantime, he abandoned the study of radio emissions, because “little progress has been made”, and began working with exoplanets.

He is now recognized as a pioneer in this field. “There are a lot of research groups working on finding exoplanets, but my team and I are really working on what we can learn about these planets. What kind of climate is there? What gases are in the atmosphere? And can we say something about rotation?”

He and his team are trying to answer these questions and more. They do this, among other things, using the transit method. “When an exoplanet moves in front of its star, it blocks the star’s light a little bit. But light also leaks through the atmosphere of an exoplanet. Gases in the atmosphere absorb the different colors of starlight, causing them to change. We can measure the color change, which called spectrum.

For example, an astronomer identifies the gases that make up the atmosphere of an exoplanet. Sneelen emphasizes that it sounds simple, but it is very difficult. Especially because measurements with ground-based telescopes, such as the Very Large Telescope in Chile, are made through our atmosphere. This atmosphere disrupts the signals that Snellen and his team want to measure. “We have developed a calibration technique by which we filter this effect. This way we can distinguish the spectrum of the exoplanet from that of the Earth. For a while, we were the only research group in the world using this method. Many astronomers use it now.”

This is because the technology works very well. So good, in fact, that Snellen can measure the difference in the atmosphere on the day side (the side facing the star) and the evening side of an exoplanet. He was also able to determine the rotation and weather patterns of a number of planets.

“We are now only applying this method to relatively nearby gas giants, but not yet to much smaller, rocky planets that emit weaker signals.” This requires a more powerful telescope. One has been under construction for several years: the Extremely Large Telescope (ELT). In 2025, this world’s largest eye will be operated in Chile – a moment that Snellen is eagerly looking forward to. “ELT will give us access to spectra of Earth-like planets. This telescope will have a mirror with a diameter of more than 39 meters, which increases sharpness dramatically. It will soon be able to make measurements hundreds of times more accurate than VLT measurements.”

Then what is the plan? Collect as much data as possible from exoplanets? Snellen: “I see this research as a kind of playground, where we learn as much as we can about different types of exoplanets. That’s nice, but the ultimate goal is to understand where our solar system is in the universe. Planets that orbit the sun with us don’t look alike at all. To understand these differences , you need to look outside our system and check how common planets like Venus and Mars are. That will really teach us something about ourselves.”

My crazy little animal

More than enough research, and this requires a lot of money. One night of observation with a VLT can easily cost tens of thousands of euros. After that, the researcher spends another six months sifting through the data. Snellen was pleasantly surprised when he received a phone call from the director of the NWO (Netherlands Organization for Scientific Research). “I was walking with dogs when he called me and told me that Spinoza Prize (Editor, the most important scientific award in the Netherlands). On the way back, I thought: “Did he really say that?” “It’s a great honor, says Snellen, but he also admits it’s a double. “You always do this research with a team, but you get this award on your own. This feels a little crazy.”

For someone who himself developed a very accurate measurement method and was recently told that he won €2.5 million in research money, Snellen is pretty modest. Have you ever seen him on the Nobel Committee?

This episode of Underwater Interrogated is also on KIJK 9/2022, and can be ordered easily and quickly via the button below.

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