The time has come for the James Webb Space Telescope to take exoplanet astronomy to the outer reaches. European researchers have been doing a lot of groundwork in preparation for this moment.
BY MICHAEL ALLEN
Since its 25 December 2021 launch aboard an Ariane 5 rocket from French Guiana and following 30 years in the making, the James Webb Space Telescope (JWST) is the Christmas Day gift to astronomers that keeps on giving.
Like many astronomers in Europe, Pierre-Olivier Lagage, an astrophysicist at the Paris-based French Alternative Energies and Atomic Energy Commission (CEA), has been preparing for JWST for years.
A joint project with NASA, the Canadian Space Agency (CSA) and the European Space Agency (ESA), JWST started beaming back its first images of the cosmos in July 2022 after arriving at its vantage point 1.5 million kilometres from the Earth and unfurling its distinctive giant sunshield.
A worthy successor to the iconic Hubble Space Telescope, the €10 billion JWST has big scientific goals. These include the study of the early universe shortly after the Big Bang, galaxies and star formation, black holes, our own solar system and the search for the building blocks of life in the universe.
Scientific goldmine
‘An exoplanet is a planet orbiting a star other than the Sun,’ Lagage said.
Lagage is the principal investigator of the H2020-funded ‘Exoplanet Atmosphere New Emission Transmission Spectra Analysis’ or ExoplANETS A project. He and his colleagues developed a data tool to exploit the wealth of existing spectroscopic data from previous missions to study exoplanets.
From a standing start, exoplanet astronomy has discovered thousands of exoplanets in the past 20 years. Now JWST’s spectroscopy instrumentation offers an unprecedented opportunity to study exoplanets for the chemical signatures of life in their atmosphere.
Spectroscopy of transiting exoplanets is one of the main techniques in exoplanet astronomy. When an orbiting planet moves in front of its star relative to the observer, the spectrum of light from the star changes as it passes through the planet’s atmosphere. When the changes in the light are detected, they indicate the chemical composition of the planet’s atmosphere and whether it is likely to support life or not.
The Exoplanets A tool uses data analysis to enable astronomers to characterise a broad range of exoplanet atmospheres. Astronomers using JWST are likely to find this useful to help their own observations by indicating which information is likely to be useful, and what is likely to be noise.
One drawback of spectroscopic observations is that while they are a goldmine of information, the signal is mixed up with a lot of noise. Useless information not related to the exoplanet’s atmosphere can obscure the valuable data in the observation.
Systematic noise
This is because the signal created by the planetary atmosphere is tiny compared with the rest of the light coming from the star, according to Lagage. ‘So, you have to develop tools to remove this systematic noise and get the right signal,’ he said.
The Exoplanets A project goes further. In order to model the atmosphere of an exoplanet, you also need to have a good understanding of its host star. To assist with this, the project created a database of the properties of stars with exoplanets. It was made with archived data from the ESA’s XMM-Newton and Gaia space observatory.
JWST’s initial exoplanet observations were of hot gas giant planet WASP-39b, described as a ‘hot Jupiter’. It orbits a Sun-like star 700 light-years away. Last month, using spectroscopy, JWST made the first confirmed observation of carbon dioxide in an exoplanet.
Caught in a TRAPPIST-1
The ESCAPE (Exploring Shortcuts for the Characterization of the Atmospheres of Planets similar to Earth) project has also been looking for shortcuts to help characterise the atmospheres of Earth-like exoplanets.
Martin Turbet, an astrophysicist at the French National Centre for Scientific Research (CNRS) and principal investigator on the H2020-funded ESCAPE project, said that this required exploring novel observation techniques, using different ground- and space-based telescopes.
For example, the astronomers have been developing new methods to calculate the density of the planets orbiting TRAPPIST-1, an ultra-cool red dwarf star around 40 light years from our solar system.
Discovered initially in 2000, it was later announced in 2017 that the TRAPPIST-1 star hosts seven small exoplanets, orbiting in tight formation, at least some of which may be habitable.
To calculate the density of a planet, you need to know its radius and mass. Sizing the planet can be done using spectroscopic observations. Mass can be calculated by observing the effect of the planet’s gravitational pull on its host star.
Weighing exoplanets
‘This is the classical way to measure the weight of a planet,’ said Turbet. ‘But in the case of the TRAPPIST-1 planets, the mass of the planets is so small that the classical technique doesn’t work.’
However, the TRAPPIST-1 system is peculiar because the seven planets all orbit very close to each other and exert strong gravitational forces on each other, he said.
This effects their orbits and means that they don’t pass, or transit, in front of their host star at fixed time points.
Measuring the deviations in these transit timings enabled the researchers to assess the strength of the gravitational forces between the planets and evaluate their masses, said Turbet.
Thanks to this this technique, they say they are now able to make the most accurate predictions so far of the water content of the seven known planets in the TRAPPIST-1 system.
The observations, and mass, density and water content calculations were made using ground-based telescopes – such as the SPECULOOS telescope at the European Southern Observatory (ESO) in Chile, space-telescopes, and novel theoretical work.
Turbet said that the JWST and the planned Extremely Large Telescope (ELT) could be able to detect potential signs of life, known as biomarkers, in exoplanet atmospheres.
He cautioned, however, that these ‘cannot be used as definitive proof that there is life on the planet’. This is because recent work has shown that biomarkers, such as oxygen, can be formed without life.
Reflected starlight
Turbet and his colleagues have also been investigating a newer spectroscopy technique, known as reflected light spectroscopy. Rather than analysing how a star’s light changes as a planet passes in front of it, this method looks at how light from the star is reflected by the planet’s atmosphere.
The models of water content and planetary atmospheres will also assist observations from the JWST, Turbet said. They will enable astronomers to plan their observations to maximise the collection of data of genuine interest.
That being said, exoplanet research isn’t just about looking for alien life. Exoplanets could also provide us with information on the history of Earth and how its atmosphere developed, according to Lagage.
‘What I am most interested in is the atmosphere of super-Earth and Earth-sized exoplanets,’ he said.
Research in this article was funded via the EU’s Marie Skłodowska-Curie Actions (MSCA) and was originally published in Horizon, the EU Research and Innovation Magazine.
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