Over the past few decades, thousands of planets have been discovered outside our Solar System, called exoplanets. Astronomers estimate that most stars host one and often multiple planets. The Milky Way alone has hundreds of billions of stars. You do the math. The discoveries of exoplanets have shown that the classical architecture of our Solar System is not standard; a rich variety of systems have been discovered. For example, giant planets which complete an orbit around their host star in only a few days or planetary systems with many tightly packed rocky planets. Also new classes of planets have been discovered: so-called Super-Earths and Sub-Neptunes.
We are now faced with the task of explaining this richness in planetary diversity. Moreover, the discovery that planetary systems are so common made the question if life can exist somewhere else in the Universe more intriguing than ever before. SRON’s exoplanet research focuses on the characterisation of the interiors and atmospheres of exoplanets. Which gases can we find? What does this tell us about the chemistry in the atmospheres? We use the chemical properties of the atmosphere as a probe for the planet’s evolutionary history and formation mechanism.
We work on these questions through modelling of atmospheres and interiors, writing software, and analysing data from space telescopes. We are also engaged in the development of those telescopes, such as PLATO and Ariel.
What planets are out there?
Until the mid-1990s, we knew only of the planets within our own Solar System. Astronomers had long predicted the presence of exoplanets, and it was expected that these planetary systems would resemble our own, with small rocky planets closer to the star and large gaseous worlds located further out. However, the detection of 51 Peg b, a planet of similar size to Jupiter but in a three-day orbit around its host star, broke this mould. We have since discovered nearly 6,000 exoplanets and found that a wide range of system architectures exist. Indeed, none of the discovered exoplanetary systems could be considered comparable to ours. In the future, this may change. For instance, ESA’s PLATO mission, due for launch in 2026, will find thousands of exoplanets and specifically search for terrestrial analogues. SRON performed the cold vacuum validation and characterisation of 11 of the 26 cameras and we will be heavily involved in the analysis of PLATO data once it has launched.
How do planets form and evolve?
So planetary systems are diverse. But why? How did these architectures come to be? After the formation of a star, there is leftover gas and dust. It is from this protoplanetary disk that planets form as these dust grains collide and coalesce. Where a planet forms affects the material available to it: lighter elements are expelled from the inner disk by stellar irradiation and the thermal gradients in the disk impact the chemistry. So does the formation process leave a chemical trail that we can follow by characterising the atmospheres of fully-formed exoplanets? That remains to be seen.
One stumbling block is that once the planet has formed, it continues to change. For instance, it can migrate through the protoplanetary disk, changing its orbit and accreting further material as it does so. The constant irradiation by the host star can also strip away atmospheric gases, particularly light ones such as hydrogen and helium, changing the overall composition. Modelling this photoevaporation helps to understand the final fates of young planets, or to predict the original state of mature ones, thus helping to build-up an understanding of the life cycle of planets.
Hence, to even attempt to understand if the formation process can be unveiled by studying exoplanetary atmospheres, we need to study large, diverse populations of planets. Planets of all sizes and ages, in a variety of different orbits around various types of stars. With ground-based telescopes and space missions such as Hubble and James Webb, this effort is already being undertaken. And in 2029, ESA’s Ariel mission will launch with the aim of characterising a thousand exo-atmospheres. SRON is responsible for determining the exoplanets that could be studied by Ariel. Additionally, scientists from SRON and other institutes in The Netherlands are strongly involved in performance simulation and scientific data analysis for the mission.
Are there potentially habitable exoplanets?
Detecting and characterising Earth-like planets is a crucial step in understanding our place in the Universe. However, if we want to characterise rocky exoplanets in the habitable zone, there are still significant steps to be taken in terms of technology development. At SRON we are currently developing the key technologies needed for this endeavour, paving the way for telescopes like the Extremely Large Telescope, Habitable Worlds Observatory and LIFE which will all be capable of searching for and characterising potentially habitable planets. In the coming decades, we hope to answer the question that humankind has been asking for centuries when staring at the night sky; is someone staring back?
