Rens Waters (astronomer at Radboud University) has always been fascinated by sulphur: ‘I have long been curious about how sulphur gets into planets. Sulphur is one of the six chemical elements that are essential for life on earth. In addition, the amount of sulphur in a planet says something about its formation history.’
As part of his master's degree programme in Astronomy at Radboud University, Jesse Polman was assigned by Waters and second supervisor Michiel Min (SRON Leiden) to look at the detectability of sulphur in the atmospheres of planets that are similar to Jupiter, but much hotter. These are known as 'hot Jupiters': gas planets that orbit close to their star and therefore have very short orbital periods. Polman: ‘We started looking at sulphur because it is relevant to planet formation, but it has not been studied very much. Researchers more often look at carbon compounds or oxygen.’ Polman set out to investigate under what conditions you could detect sulphur dioxide in the atmospheres of exoplanets. He discovered that a strong signal of sulphur dioxide could be detected in the atmospheres of hot Jupiters with a certain chemical composition. He predicted that such planets could be effectively studied with the JWST.
In the months after Polman's thesis was published, it became clear that sulphur dioxide had indeed been detected in the atmosphere of one of the first hot Jupiter exoplanets targeted by the James Webb Space Telescope. Waters: ‘That was great, his predictions were immediately confirmed. And then we wanted to know: is this a coincidence, or is this the case with more planets?’
Waters then enlisted undergraduate Olaf Renes, who set out to see which of the 5,000 currently known exoplanets have the right conditions to have sulphur dioxide in their atmospheres. To do so, he analysed NASA's database of all exoplanets. ‘First I filtered for hot Jupiters, which left about 500. After I looked at temperature, shape of the orbit and metal content, among other things, three planets remained that could contain sulphur and could be viewed with the JWST.’
Proposal to use JWST
With those three planets and a proposal, a team of astronomers including Renes and Polman applied for observation time at the JWST. Waters: ‘Our expectations were tempered, it is a hugely competitive field. But last month, we were told we could use the JWST for 35 hours. That's fantastic.’
Renes: "I could hardly believe the news, I was so excited. I thought to myself: Is this really possible? Unbelievable!" Polman: ‘I was in my apartment checking my in-box all the time to see if our proposal had been approved. When the email from NASA arrived, I was very pleased. When I started this study, I did not expect this to happen.’
Beginning in July, the 35 hours of observing time will be divided among the three exoplanets to look for sulphur dioxide in their atmospheres. After the results are in, the team will continue its work. Waters: ‘After some time we will know whether Jesse's predictions apply to other planets, and in this way we will learn more and more about how planets are formed. The JWST really is a gold mine for scientists, it is a huge discovery machine. It is very special that we are allowed to use this remarkable instrument.’