If Earth were an exoplanet
Researchers from the Institute for Particle Physics and Astrophysics at ETH Zurich and the Department of Astrophysics at the University of Zurich asked if the potential future LIFE space mission could detect evidence of a habitable and inhabited Earth – and the answer is affirmative.
The branch of exoplanet research that deals with planetary habitability wrestles with dauntingly big questions. What forms of life could inhabit other planets? What constitutes an unambiguous biosignature, that is, evidence of life, and how can it be detected reliably? And, coming to think about it, what classifies as life? In the face of such complexity, a sensible approach is to learn as much as possible from the one known habitable world – our own.
In a paper just published in The Astrophysical Journal, researchers from ETH Zurich and the University of Zurich looked at how well a potential future space mission known as external page Large Interferometer for Exoplanets (LIFE) could characterise Earth in terms of its habitability. LIFE is conceived to study terrestrial exoplanets found in the habitable zone of their host stars by measuring the planets' mid-infrared (MIR) spectral emissions, which are known to give access to relevant atmospheric and surface information. The team concludes that LIFE would successfully identify Earth as a habitable planet, finding signatures of crucial atmospheric species and detecting the planet's temperate climate as well as surface conditions allowing for liquid water.
Earth seen from afar
Two questions lie at the heart of the paper: first, if LIFE observed Earth, what kind of MIR spectra would it acquire? As with any exoplanet, LIFE would be so far from Earth that the planet would look like a featureless spot, a single pixel on a digital image. The spectra would then be spatial and temporal averages determined by which views of the planet (also called observing geometries) LIFE would catch and for how long. If these averaged spectra were then analysed to retrieve information about the Earth's atmosphere and surface conditions, how would the outcomes depend on factors such as viewing geometry and seasonal variations? What makes the present investigation unique is that the team tested the capability of LIFE on real instead of simulated spectra: they used Earth remote sensing climate data from NASA's Atmospheric Infrared Sounder aboard the Aqua satellite to build MIR emission spectra like those that could be acquired in future exoplanet observations. The researchers considered three observing geometries – the two views from the Poles and an additional equatorial view – and focussed on data taken in the months of January and July to account for the largest seasonal changes.
First author Dr Jean-Noël Mettler, who recently completed his PhD under the supervision of co-authors Professor Sascha Quanz at ETH Zurich and Professor Ravit Helled at the University of Zurich, worked on the MIR emission spectra and can trace back the steps that led to this study, which was indeed preceded by two related publications in 2020 and 2023. "In the first paper we analysed satellite data to see if we could infer evidence of seasonality on Earth from thermal emission data. In the second paper we assumed that Earth is far away and studied the temporal variability of its thermal spectrum for full-disc observing geometries," Mettler explains. "Now we used the spectra we studied in the second paper as input for an actual simulation of what LIFE could tell us about a terrestrial exoplanet."
Looking for life with LIFE
The paper's main outcome is encouraging: if LIFE were to observe (exo)planet Earth, it would find quantitative evidence for a temperate, habitable world. The team found detectable levels of atmospheric gases CO2, H2O, O3 and CH4, and surface conditions favourable to the presence of liquid water. Detecting O3 and CH4 is especially important, as these gases are produced by Earth's biosphere – they're known as atmospheric biosignatures. These results turn out to be essentially unaffected by the viewing geometry: this is good news, because the exact observing geometry will likely be unknown in future observations of terrestrial exoplanets.
The reconstructed abundances of atmospheric gases are also independent of the considered season, which is primarily good news. However, this finding also suggests that a mission like LIFE – at least with the resolution and signal-to-noise ratio considered in the paper – may not be able to detect an exoplanet's atmospheric seasonality, intended as the small seasonal variation of some molecular biosignatures.
The long game
The idea of using an infrared nulling interferometer to characterise exoplanets goes back to the seventies. Both ESA and NASA pursued investigations in this direction in the nineties and the early two-thousands: ESA's Darwin and NASA's TPF-I mission concepts, which were developed between 1996 and 2007 and were designed to operate in the mid-infrared regime, may be viewed as the closest ancestors to LIFE. Crucially, some of the technological components required for a space-based nulling interferometer are much more mature now than was the case seventeen years ago.
The paper authored by Mettler, Konrad, Quanz and Helled has an illustrious predecessor too. In October 1993, astronomer Carl Sagan and his colleagues published an article titled external page "A search for life on Earth from the Galileo spacecraft": taking advantage of Galileo's close Earth flyby as it journeyed towards Jupiter, the team analysed data acquired by the spacecraft's instrumentation and detected atmospheric oxygen and methane, together with a sharp absorption feature in the infrared region of the emission spectrum attributable to the presence of vegetation. Sagan and his co-authors considered this evidence to be "strongly suggestive of life on Earth." The paper became an inspiration and a landmark control experiment for exoplanet research.
A computational challenge
If one core ingredient of the paper is given by the MIR spectra, the other is the so-called atmospheric retrieval routine. This is the computational framework that takes Earth's emission spectra as input and outputs an atmospheric characterisation of the planet's atmosphere that includes a pressure-temperature profile and value intervals for the abundance of atmospheric gases. First author Björn Konrad, who is a doctoral student in Quanz's Exoplanets and Habitability Group, worked on the retrieval routine and on the characterisation of its accuracy and robustness. Comparing the retrieved information with so-called ground truths – that is, abundances of atmospheric gases and a pressure-temperature structure extracted from existing satellite data – allowed the team to assess the limitations of the retrieval framework and its inherent biases. For example, it turns out that the retrieved values of surface pressure were systematically underestimated, which led to a noticeable but not critical overestimation of all gas abundances.
"The retrieval routine is computationally demanding, so the complexity of the forward model used for the retrieval is limited," Konrad notes. The forward model encompasses a set of parameters for the chosen model atmosphere – here taken to be a one-dimensional column – and a parametrised pressure-temperature profile; it also simulates radiative transfer, that is, how the exoplanet's (model) atmosphere emits and absorbs electromagnetic radiation. Konrad is now tackling two of the main shortcomings of the forward model, which for this paper neglected the presence of clouds and assumed vertically constant gas abundances. The latter assumption is especially crude for water, which is known to decrease strongly in abundance with decreasing pressure. "The whole exoplanet community is gearing up for increasing amounts of data that will be coming in the future from smaller and smaller planets," says Quanz, "and there are atmospheric features that matter for all terrestrial exoplanets – the water abundance profile is one of them." This is why Konrad and Quanz look forward to following up on the results presented in the paper. Indeed, a deeper understanding of the impact of some assumptions and simplifications on the output of exoplanet atmospheric retrieval frameworks will have a reach well beyond the LIFE mission.
The hardware for the software
The Large Interferometer for Exoplanets (LIFE), which is supported by the National Centre of Competence in Research external page PlanetS, is a mission concept that would rely on a formation of flying 'collector telescopes' with a 'combiner spacecraft' at their center to realise a nulling interferometric scheme at mid-infrared wavelengths – that is, cancel out the light signal coming from the host star of an observed terrestrial exoplanet through destructive interference. In 2020, the LIFE team kicked off the Nulling Interferometric Cryogenic Experiment for LIFE (NICE), which aims at building a laboratory-based testbed to demonstrate nulling interferometry with set requirements in terms of starlight suppression and stability under challenging cryogenic conditions.
This year will mark the start of a new LIFE collaboration with Professors Jérôme Faist and Rachel Grange, both in the Department of Physics at ETH Zurich, that will proceed in parallel with the NICE project and will explore the potential of integrated optics for the LIFE concept. Indeed, the ability to replace bulky mirrors and other optical elements prone to misalignment with photonic chips would lower both the payload and the complexity of the system – but there are many open questions to tackle, which is why the new collaboration will have three dedicated doctoral positions with shared supervision.
Reference
Mettler, J.-N., Konrad, B.S., Quanz, S.P. & Helled, R. Earth as an Exoplanet. III. Using Empirical Thermal Emission Spectra as Input for Atmospheric Retrieval of an Earth-Twin Exoplanet. ApJ 963, 24 (2024). external page DOI: 10.3847/1538-4357/ad198b
Further reading
Mettler, J.-N., Quanz, S.P. & Helled, R. Earth as an Exoplanet. I. Time Variable Thermal Emission Using Spatially Resolved Moderate Imaging Spectroradiometer Data. AJ 160, 246 (2020). external page DOI 10.3847/1538-3881/abbc15
Mettler, J.-N., Quanz, S.P., Helled, R., Olson, S.L., & Schwieterman, E.W. Earth as an Exoplanet. II. Earth's Time-variable Thermal Emission and Its Atmospheric Seasonality of Bioindicators. ApJ 946, 82 (2023). external page DOI 10.3847/1538-4357/acbe3c