Planet Earth as an example in the search for life on distant planets
How do you detect life on a planet light years away? During her PhD research, astronomer Willeke Mulder worked on an instrument to detect such signals – culminating in an experiment from a hot air balloon.
Mulder worked in the optical lab, analysed data and models, and, as the icing on the cake, took measurements from a hot air balloon. All in the hope of one day being able to detect life on other planets. ‘The question I am trying to answer is: “What would Earth look like at the distance of an exoplanet?”’ says Mulder. ‘You need very good instruments to study that.’
That is why she investigated which signals of life are measurable to begin with. You might think of oxygen in the atmosphere, or oceans and green vegetation on the surface. But Mulder looks at light.
Recognising life with light
Central to her research is polarisation: a property of light that changes through interaction with matter. ‘Circular polarisation hardly occurs in the universe,’ the astronomer explains. ‘It arises from a property shared by all the building blocks of life as we know it: homochirality. This means that molecules such as sugars or DNA only exist in a right-handed or left-handed orientation. When light interacts with such molecules, it subsequently exhibits circular polarisation. So if we see that reflected on another planet, we’re onto something.’
An experiment at high altitude
The PhD candidate summarises her project in three clear steps: build an instrument and observe the Earth’s circular polarisation from a distance. Create a model. Place that model at the distance of an exoplanet. ‘Then you know what a signal of life on another planet might look like. But that is, of course, easier said than done,’ she admits.
Together with her supervisors, she compared various options for observing the Earth. A ski lift was suggested and rejected. Too unstable, according to Mulder. You could hang the instrument under a helicopter, but then you can't access it during the measurement.
Ultimately, the researchers decided that a hot air balloon was the best option. ‘That was quite an adventure to arrange,’ she laughs. ‘The tricky part is that hot air balloons only take off early in the morning or late in the evening, whereas we need a lot of light. All in all, the final flight was without a doubt, literally and figuratively, the highlight of my PhD!’
From measurement to model
The experiment was a success, Mulder explains. ‘We looked at trees and plants. We saw signals in the spring, but not in autumn. That is exactly what you expect, because in the autumn the green leaves turn orange and red. In doing so, they lose the chlorophyll that causes circular polarisation.’
The results show that the model works. That is an important step in the field, because a model of circular polarisation on Earth-like planets was still missing. ‘Unfortunately, it is not finished yet,’ says the PhD candidate. ‘To model the entire Earth, you must include all solar angles and viewing angles. That is a simple experiment, but we were unlucky and had no more sunny days.’
The next step: the ISS
‘As a scientist, there are always more things you want to do, but I think we have achieved a great deal,’ concludes Mulder. ‘Ultimately, the goal is to place this instrument on the International Space Station (ISS) to observe the Earth. Those negotiations have even started already, so who knows, that might soon become a reality. That would be fantastic.’
Frans Snik and Ignas Snellen were the PhD supervisors of this research, Daphne Stam was closely involved. Mulder defends her thesis titled 'Tracing life through light: Towards detecting life on exoplanets with spectroscopy and spectropolarimetry' on April 2.