Skip to main content

Exoplanet atmospheres as indicators of life: From hot gas giants to Earth-like planets

Periodic Reporting for period 3 - EXOPLANETBIO (Exoplanet atmospheres as indicators of life: From hot gas giants to Earth-like planets)

Reporting period: 2019-09-01 to 2021-02-28

Placing our solar system in the context of other planetary systems is one of the central objectives driving the study of extrasolar planets – planets that orbit other stars than our Sun. One of the most fascinating questions in modern science is whether other life-bearing planets exist. In recent years, we have learned that planet are very common, but also very complex and diverse. Already the planets in our solar system, the gas giants have hydrogen dominated atmospheres with complex systems of clouds and hazes with powerful zonal atmospheric flows and storms. The rocky planets have a variety of secondary atmospheres, such as a super-rotating CO2-based atmosphere supporting opaque sulphuric acid clouds in the case of Venus, an N2-based partially transparent atmosphere with a large abundance of biotic oxygen in the case of Earth, and a teneous CO2 atmosphere in the case of Mars.

This picture is further amplified by the discovery of several new classes of planets around other stars. These include hot Jupiters, gas giants planets in very close-in orbits, a class of super-Earths (or mini-Neptunes) with properties in between rocky and ice-giant planets, and super-Jupiters – very massive planets on very wide orbits. Detailed studies of large numbers of different types of exoplanets are required to properly understand planet atmospheres and their evolutionary histories. Only then will we be in a position to unambiguously identify biomarker gases that point to biological activity. This is the main driver behind the enormous surge in exoplanet atmospheric research, and this ERC project.

In this project we use an innovative spectroscopic technique to study the atmospheres of extrasolar planets, utilizing a soon to be available instrument CRIRES+ on the Very Large Telescope of the European Southern Observatory in Chile. We aim to make an inventory of planet spin rates as function of planet mass and age, start probe the atmospheres of super-Earths and mini-Neptunes and try to solve for their bulk compositions, understand the thermal structures and atmospheric compositions of hot Jupiters and probe their isotope-ratios. The project will serve as a stepping stone in developing high-dispersion spectroscopic techniques for studying Earth-like planets with the future European Extremely Large Telescope.
During the first part of this ERC project, we already made some major scientific advancements, and developed and refined techniques utilizing high-dispersion spectroscopy for characterizing extrasolar planets. These include several firsts:

For the first time, we have detected helium in the exosphere of an extrasolar planet from the ground. It gives a unique insight into the evaporation of atmospheres for planets very close to their host stars. We have repeated this now for several other planets. Atmospheric evaporation plays an important role in the evolution of planetary atmospheres, also for the early Earth and for other planets in our Solar System. Planets that orbit very distant stars are the only way to see these atmospheric escape processes in action.

Waiting on the completion of the CRIRES+ spectrograph by ESO, we have used a range of other instruments to probe exoplanet atmospheres. In this way we measured for the first time water vapor from the ground in planet HD189733b. Also, we devised a new method using medium-resolution spectrographs, called molecule mapping, that can specifically target different types of molecules in exoplanet atmospheres. This has resulted in very interesting results for two planets. Using these new techniques, we show that this can also be used for the soon-to-be-launched James Webb Space Telescope. We show that we can target the possible Earth-like planet Proxima-b and determine whether it has an atmosphere. It would be a major step towards the characterization of this enigmatic world.

In another first, we measured the mass of a young gas-giant exoplanet for the first time, combining data from two astrometric space satellites, HIPPARCOS, that retired already more than two decades ago, and Gaia – an ESA flagship space telescopes that is currently operational. This important, pioneering measurement confirms the mass expected from theoretical considerations, explained by the way we think planets form. It is a forebode of the many great things to come from Gaia in the coming years, in particular in the field of exoplanets.

One of the great prospects of the high-dispersion spectroscopy method is the detection of molecular isotopologues in exoplanet atmospheres. Isotopologues are molecules with the same chemical properties, with a slightly different mass. These can give great and unique insights in the formation and evolutionary history of the planets they are detected in. For example, the fraction of deuterium (heavy water) is significantly higher in the Earth oceans than elsewhere in the solar neighborhood, which currently points to an origin from bodies in the asteroid belt.

We have calculated for the first time how well one can measure isotopologues, including deuterium in exoplanet atmospheres, using the Very Large Telescope, but also the future European Extremely Large Telescope (ELT). It shows that deuterium, if Proxima b is water rich, will be within reach of the ELT – a very exciting prospect.

As in the isotopologue project, the ultimate value of any observation we perform is heavily dependent on the quality and availability of proper models and simulations. Therefore, we have been working hard to produce a highly flexible radiative transfer modeling code for exoplanet atmospheres that includes high-dispersion spectroscopy, which is now submitted for publication and ready to use.
We are now ready for the next big step in the project, which will rely on the new CRIRES+ instrument on the Very Large Telescope of the European Southern Observatory in Chile. We now know exactly what planet to target and how to get the first isotopologue measurements. This will provide further information on how well we can extend this to smaller and cooler planets, possibly even measuring deuterium to hydrogen ratios.

Another great prospect is the measurements of new molecules, such as methane, H3+, ammonia, and/or even oxygen. How far will we be able to push this towards smaller and cooler planets?

Already very exciting results have been obtained on atmospheric escape and the accompanying velocity fields of extended atmospheres. We will soon learn why there are such great differences from planet to planet and system to system, and what are the main driving mechanisms behind these escape processes.

We are also pushing for interferometric measurements, another part of the ERC project. First test observations are in taken with GRAVITY, and more observations have been proposed for the new MATISSE instrument – for which we have Guaranteed Time Observations.

One other great prospect is the measurement of molecular absorption from clouded or hazy planets. The hazes have prevented any measurement yet, but the high dispersion spectroscopy technique will be very sensitive to molecular gas very high-up in the planet atmospheres, possibly probing above the clouds and hazes.
Strong helium absorption from a slowly evaporating exoplanet (Nortmann et al., Science, 2018)