Analyzing extraterrestrial atmospheres
We now know that in the universe tens of billions of planets orbit stars other than our Sun. A good number of those exoplanets might resemble Earth. Already a good number of Earth-sized and ’super-Earth’ sized planets have been discovered, some of them in the habitable zone around their host star. With thousands of planets already discovered and the increasing interest and investments from the community, astronomers expect to find a twin sister of the Earth within decades.
To answer the big question if life can exist on one or more of those exoplanets, we need to be able to chart the characteristics of the planet’s atmosphere. This might very well be possible in the coming years. We can detect the exoplanet through the starlight that its atmosphere reflects and through the radiation that the planet emits itself. This also provides us with a lot of information about the composition of its atmosphere and potential habitability.
Over the past few decaded thousands of planets have been discovered outside our Solar System. These planets show that the classical architecture of our Solar System is not standard; a rich variety of systems has come up, with extremely close in giant planets or planets on extremely wide orbits. Also, a new class of planets has been discovered; the so-called super Earths. These planets, of which we have none in the Solar System, are significantly larger than the Earth, significantly smaller than Neptune, and appear the be the most common type of planet.
We are now faced with the task to explore and explain this richness in the planet diversity. Moreover, the discovery that planetary systems are so common made the question if life can exist somewhere else in the Universe more actual than ever. This realization has lead to the founding of the exoplanet programme at SRON. Within this programme the focus lies on the characterization of the atmospheres of exoplanets. Which gasses can we find? What does this tell us about the chemistry in the atmospheres? Can we use the chemical properties of the atmosphere as a probe for the planet's evolutionary history and formation mechanism?
The way to answer these questions is currently best done through transit spectroscopy. When a planet passes in front of its host star, part of the light is blocked by the planet atmosphere giving rise to a dip in the light we receive on Earth. Different molecules block different wavelengths of light. So when measuring the dip in the lightcurve for different colors gives us a spectroscopic fingerprint of the molecular content of the atmosphere.
We develop retrieval algorithms, focusing on physical processes, to robustly constrain the atmospheric properties of the exoplanets. One important aspect, which is currently hampering the analysis of exoplanet spectra, is the formation of clouds. By using knowledge of the physics of cloud formation together with a detailed treatment of cloud radiative transfer we aim to get around this problem.
Future ESA missions, like PLATO or the current M4 competitor ARIEL, are the cornerstones of the research into cloud characteristics and atmosphere composition. Also the James Web Space Telescope, with a large Dutch contribution to the MIRI instrument, is among the key tools for this research area.
With current and near future technology, we can access the atmospheres of large planets down to maybe the super Earth size. If we want to characterize Earth-like exoplanets in the so-called habitable zone, there are still significant steps to be taken in terms of technology development. At SRON we are currently setting up a technological path towards a Dutch contribution to the search for and characterization of habitable exoplanets. SRON is also part of the 'Are we alone?' programme, a collaboration between different institutes and Universities. In the coming decades we hope to answer the question human kind is asking themselves already for centuries when staring at the night sky; is someone staring back?