EXOPLANETS

Artist’s impression of the deep blue planet HD 189733b This illustration shows HD 189733b, a huge gas giant that orbits very close to its host star HD 189733. The planet’s atmosphere is scorching with a temperature of over 1000 degrees Celsius, and it possibly rains glass, sideways, in howling 7000 kilometre-per-hour winds. At a distance of 63 light-years from us, this turbulent alien world is one of the nearest exoplanets to Earth that can be seen crossing the face of its star. By observing this planet before, during, and after it disappeared behind its host star during orbit, astronomers were able to deduce that HD 189733b is a deep, azure blue — reminiscent of Earth’s colour as seen from space. © NASA, ESA, M. Kornmesser

Probing the atmosphere of exoplanets

Exoplanets orbit a star other than our own sun, and over recent decades more than 4,000 have been detected across many different planetary systems. As Principal Investigator of the ExoplANETS A project, Dr Pierre-Olivier Lagage is working to characterise the atmosphere of these exoplanets that lie far beyond our own solar system.

Over 4,000 exoplanets have been

detected over the last few decades, including some within a few parsecs (around 3.26 light years) of the Earth, while others are much further away. As the Principal Investigator of the ExoplANETS A project, Dr Pierre-Olivier Lagage is now looking to gain deeper insights into the atmosphere around exoplanets. “We are interested in characterising the atmosphere of exoplanets, which is a new avenue of research,” he says. An exoplanet can be initially detected using a variety of methods, one of which is called exoplanet transit. “When a planet passes between us and the star we are looking at, there is a faint decrease in the brightness that we can see from Earth. This is because the planet is preventing some of the light of the star from coming to us; this is what we call transit,” explains Dr Lagage. “This is one way to detect an exoplanet, and if that exoplanet has an atmosphere, the molecules in the atmosphere can also absorb some of the light coming from the star in specific wavelength bands, especially in the infrared.”

Spectroscopic observations

By performing spectroscopic observations in the infrared at this point, researchers can see some of the features of these molecules in the atmosphere, as they stop part of the light emanating from the star. This represents one way to observe the atmosphere of an exoplanet, while Dr Lagage says it’s also possible to gain further insights at the point when an exoplanet passes behind its star. “When the exoplanet is behind the star, we only see the starlight, and we can draw comparisons between before and after to learn about the emission from the exoplanet,” he explains.

Of the more than 4,000 exoplanets that have been detected, spectroscopic information is available on around 100, but with only a limited wavelength coverage. Characterising the atmosphere of an exoplanet is a major challenge. “It’s much more difficult to characterise the atmosphere of an exoplanet than it is to detect it in the first place,” stresses Dr Lagage. This is an issue at the core of the project’s research, in which the key objective is to develop novel methods to analyse and interpret the available data. Indeed, one major challenge

In addition to looking at distant stars and galaxies, the NASA/ESA/CSA James Webb Space Telescope will probe the atmosphere of exoplanets. Credit: Northrup Grumman, © ESA/Hubble.

Still from Hubblecast 102: Taking the fingerprints of exoplanets. © ESA/Hubble, NASA.

in characterising the atmosphere of an exoplanet is that systematic noises from the instruments and telescopes can be higher than the actual signal of interest. “We are developing methods to remove the systematics with a data driven method,” says Dr Lagage.

Cataloguing star properties

The project’s agenda also includes research into the stars around which these exoplanets orbit, with Dr Lagage’s colleagues looking to catalogue their properties. The star itself exerts an important influence on the evolution of the exoplanet, a topic that the ExoplANETS A team is exploring. “There is irradiation, there is also the possibility of star-planet interactions, through magnetic field, tidal forces, wind from the star, which it is important to study,” says Dr Lagage.

Most of the spectroscopic data on exoplanets has been gathered by the long-serving Hubble Space Telescope (HST), but the James Webb Space Telescope (JWST) is set to be launched later this year as its successor, and Dr Lagage is busy preparing. “We have lab measurements about the systematics of the detector used for one of the instruments for the JWST, the MIRI instrument. We can then produce realistic simulations of JWST observations and test our new method of systematics removal, to be ready when we get the first observations from the JWST,” he outlines. “The JWST will be a game changer in the field.”

The holy grail in studying the atmosphere of exoplanets is to find evidence of life beyond our own solar system, through the detection of biomarkers. While it will not be possible to observe the atmosphere of an Earth-like planet orbiting a sun-like star in the near or mid future, the launch of the JWST will open up new possibilities. “Thanks to the large mirror of the JWST (6.5 m), we hope to be able to study the atmosphere of temperate Earth-size planets orbiting around a dwarf star,” says Dr Lagage. As the French co-PI of the MIRI instrument, Dr Lagage is also playing a major role in conducting observations during the time they have been allocated. “I am coordinating the observations of exoplanets to be done in the framework of the time that we have been allocated. One of my favourite targets is the TRAPPIST 1-b exoplanet, an Earthsize exoplanet which was discovered in 2016,” he outlines. The focus here is more on investigating the physical and chemical processes at work in an atmosphere than detecting biomarkers, and Dr Lagage says this research is breaking new ground; such an investigation is a pre-requisite in searching for biomarkers.

“With the detection of expolanets, we can also study atmospheres which are very different from those that we are used to studying in our own solar system,” he explains. “We can test, model and see chemical and physical processes that we cannot study with a planet in our solar system.”

A number of these exoplanets are in orbit very close to their own star, so they are heated by its radiation to a very high temperature. It is thought that the Earth also went through a phase of being very

hot earlier in its history, so Dr Lagage says studying exoplanets is a way of effectively looking into the past of our own planet. “Studying such exoplanets is a way to study how the Earth was in its early stages,” he says.

There are also other very interesting exoplanets with features that we do not find in the Solar System. Indeed, a high degree of diversity has been seen in the exoplanets

The next step after JWST will be brought by the ESA Ariel mission, which is set to be launched in 2029, and is entirely devoted to characterising the atmosphere of exoplanets. The aim is to study 1,000 exoplanets and gather statistical information about their atmosphere, and Dr Lagage says this is an exciting time in exoplanet research. “While the study of exoplanet atmospheres is still in its infancy,

it will considerably expand in the next two decades and the exoplANETS A project has been a key step in preparing Europe for the emerging challenges that we will face in the domain,” he concludes.

With the detection of exoplanets, we can study atmospheres which are very different from those

that we are used to studying in our own solar system. We can test, model and see chemical and physical processes that

we cannot study with a planet in our solar system.

that have been observed so far. “We have detected what we call inflated exoplanets, which have a radius much higher than Jupiter, but with the same mass,” continues Dr Lagage. “We have detected what we call super-earths – which have a mass between that of the Earth and Neptune. In fact, these are the most abundant exoplanets in the galaxy.”

With observations from the JWST, Dr Lagage says it will also be possible to make more deductions about the atmosphere of an exoplanet. “The existing HST has only a very narrow spectral range, which is sufficient to detect certain features in the atmosphere, such as H2O. But if we want to know if there are other chemical species in the atmosphere, such as CO, CO2, CH4, NH3, we need to have a wider wavelength coverage, which will be provided by the JWST,” he explains.

Artist view of the ARIEL space mission. ESA/STFC RAL Space/UCL/UK Space Agency/ ATG Medialab © ESA/Hubble.

Exoplanet Atmosphere New Emission Transmission Spectra Analysis Project Objectives

To establish new knowledge on the atmosphere of exoplanets by exploiting archived space data. To establish new insight on the influence of the star on the planet atmosphere. To disseminate knowledge in terms of science products, public outreach and educational resources.

Project Funding

Exoplanets-A has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No GA 776403.

Project Partners

The whole chain from data to new knowledge dissemination is covered by the partners. MPIA Heidelberg is developing novel data reduction tools. SRON and UCL are developing the methods to retrieve the atmosphere parameters for the reduced data. Leicester University, Wien University and INTA are producing the catalog with the properties of the host star. UCL and CEA are developing the modelisation of exoplanet atmospheres and star-planet interactions. CEA and INTA are in charge of the knowledge server.

Contact Details

Principal Investigator, Pierre-Olivier Lagage CEA Paris-Saclay Department of Astrophysics - Instrumentation, Modelisation (AIM) Bâtiment 709 91191 Gif-Sur-Yvette, France T: +33 67 673 8723 E: pierre-olivier.lagage@cea.fr W: http://exoplanet-atmosphere.eu

Pierre-Olivier Lagage

Pierre-Olivier Lagage is a senior researcher at CEA Paris-Saclay, France. He is deeply involved in two space missions which will be a game changer for exoplanets studies: the James Webb Space Telescope and Ariel. He is coordinating the ExoplANETS-A H2020 project which is an excellent preparation to the scientific exploitation of these missions.