ETH Zurich Professor Sasha P. Quanz is researching future missions of exoplanets that will seek life outside our solar system.

In June 2021, a senior committee advising the European Space Agency (ESA) on the planning of the 2035-2050 science program identified the characteristics of temperate terrestrial exoplanets – Earth-like planets orbiting stars other than the Sun – as one of the main scientific priorities for a future L-Class mission (The largest mission class there is within ESA). The main motivation is to determine empirically the frequency of habitable planets with conditions that allow liquid water to exist on their surface – a prerequisite for the prosperity of life as we know it. In addition, if properly designed, such a space mission can also identify so-called biosignatures in exoplanet atmospheres, which may be a direct indication of extraterrestrial biological activity. The LIFE initiative, which presented the initial ESA White Paper1proposes to answer these scientific questions by building a space zero interferometer operating at medium infrared wavelengths, which will analyze the inherent heat emissions of exoplanets.

In November 2021, the US National Academies published their consensus study Report “Pathways to Discoveries in Astronomy and Astrophysics for 2020”, identifying the most compelling scientific goals and presenting an ambitious program of terrestrial and space activities for future investment. This report recommends the development of a large space UV / optical / near-infrared telescope with a 6-meter main mirror as the next leading-class mission for the 2040s *. Also here the main motivation is the search for habitable and potentially habitable exoplanets. However, instead of studying the thermal emissions of exoplanets, this NASA-led mission will discover and study exoplanets in reflected light, which is light from host stars reflected back from exoplanets.

Exoplanetary missions: Complementing, not competing

The major international astrophysical and space science communities strongly support both recommendations, which support the general consensus that the search for life outside the solar system should be a priority for basic natural science in the 21st century. While there is no doubt that NASA will indeed be tasked with fulfilling their flagship mission of reflected light, the European counterpart is still competing with another scientific topic: close infrared tracking of current Gaia’s mission. However, it must be realized that the possible combination of two major exoplanet missions, a NASA-led reflected light mission and an ESA-led thermal infrared mission flying at approximately the same time, is a unique opportunity. This provides an opportunity to transform the search for life outside the solar system, a centuries-old question of humanity, from a vision to a scientific experiment. As with sending multiple rovers and spacecraft to our neighboring planets, Mars and Venus, to gain a fuller understanding of their formation and evolution, as routinely done by leading space agencies, the launch of two complementary and highly synergistic missions to study temperate terrestrial exoplanets is useful and scientifically motivated. These missions are not in competition, they complement and support each other.

FIG. 1: The observation of temperate terrestrial exoplanets in reflected light or heat radiation requires access to different wavelength regimes and therefore requires different technologies and missions. Both approaches provide access to additional information about the properties of objects and their life prospects. See 4 for a recent summary of exoplanetary biosignatures.

The whole is greater than the sum of the parts

In fact, a combination of data from the two missions may be needed to provide the strongest empirical support that some worlds there are similar to ours (see Figure 1). Analyzing terrestrial exoplanets at UV / optical / infrared wavelengths (~ 0.3 – 2.0 micrometers) in reflected light allows scientists to search for molecular oxygen, water and methane in their atmospheres, as these molecules absorb light in parts from this wavelength range and therefore leave an imprint in the reflection spectrum. Oxygen and methane are of particular interest here, as they can also be seen in the Earth’s atmosphere, and because their origin is mainly exclusively biological, they are considered important biosignatures. In addition, surface oceans or large areas of surface vegetation can be deciphered by reflected light. Liquid water will cause a strong peak of intensity for certain angles of reflection (the so-called “ocean glow effect”) and plants as we know them on Earth reflect not only green light effectively, but even more light with wavelengths longer than 0.7 micrometers, leaving a strong and distinctive signature (the so-called “red edge of vegetation”).

The heat radiation of an Earth-like exoplanet (~ 3 – 20 micrometers) contains even stronger absorption bands of interesting atmospheric molecules, including ozone (which is a direct by-product of molecular oxygen), water and methane, as well as nitrous oxide, ammonia, phosphine and isoprene. While nitric oxide is another biosignature in the Earth’s atmosphere, ammonia, phosphine and isoprene may be of biological origin in an atmosphere that is significantly different from that on Earth. In addition, measuring the inherent heat emissions of an terrestrial exoplanet provides direct access to its effective temperature and radius – two key parameters of interest in estimating potential habitability. Finally, heat emission encodes detailed information about the pressure-temperature profile of the planet’s atmosphere – so these parameters change as a function of altitude. As such, it can also limit the temperature and pressure on the surface of the object and help us understand whether liquid water may be present.

Undoubtedly, both types of missions will be transformative for our understanding of the exoplanetary environment. But despite their unprecedented scale and sensitivity, the outermost worlds will have extremely weak signals that could challenge firm conclusions about the habitability of sites and the unequivocal discovery of biosignatures if data from a single mission is available. It is in these cases that the addition of characterization methods pays off, which significantly increases the diagnostic power.

Guided by science, not politics

This will be crucial if we are to conduct a scientific experiment and test hypotheses about the frequency of inhabited – or inhabited – worlds: finding strong empirical support for the existence of exoplanet biological activity will be a breakthrough result that could change our understanding for our place in the universe forever. What if the first few temperate terrestrial exoplanets seem uninhabitable? Is this bad luck or are inhabitable worlds rare? To answer this question in a statistically stable way, we need to evaluate at least 30-50 objects well enough. Combining and using the insights from the two missions will be extremely powerful in this context and will significantly increase the credibility of our assessment in cases where data from just one mission would be ambiguous. For more than half of its 4.5 billion years of evolution, the Earth’s atmosphere has shown signs of biological activity in the form of atmospheric biosignature gases that were strong enough to be detected remotely at a distance of 20 parsecs with missions such as those described above. . Given our current understanding of the frequency of exoplanets, we can expect to find at least 30-50 terrestrial exoplanets with a temperate climate within a radius of 20 parsecs from the Sun, and if none of them show life-sustaining conditions, it really it would be a very strong indication that life as we know it is rare in our space neighborhood.

There is no doubt that national and international tax agencies operating with taxpayers’ money must make informed and careful decisions about how and where to invest their funding. We have come a long way in discovering and studying exoplanets over the last 25 years. Today we have the knowledge and resources to develop technology for space missions that can look for signatures of life outside the solar system. These missions should not be considered as the next mission within a single agency program. Executed as a joint endeavor inspired by the same vision, these missions have the potential to revolutionize our understanding of life itself. Actively seeking cooperation between agencies, calling for international partnerships and embracing a truly global perspective on the unique opportunity that awaits us and awaits its use would allow humanity to cope empirically by the middle of this century, one of the the biggest deep questions ever asked: are we alone in the universe?

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* In terms of wavelength range, this mission is similar to the 2.4-meter Hubble Space Telescope, but in terms of mirror size it is similar to the 6.5-meter James Webb Space Telescope.


1 Quanz et al. 2021, doi: 10.1007 / s10686-021-09791-z



4 Schwitterman et al. 2018, 10.1089 / ast.2017.1729

Please note that this article will also appear in the tenth edition of ours quarterly publication.

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The exoplanet missions set to revolutionise science

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