Twenty-seven years ago, at the University of Geneva, Michel Mayor and Didier Queloz – now a professor at ETH – discovered the first extrasolar planet orbiting a Sun-like star. Much has happened since that initial discovery: astronomers have now identified more than 5,000 exoplanets, many of a similar size to Earth, in over 3,700 different planetary systems. With only a tiny portion of the universe analysed so far, it certainly seems plausible to suggest that life might exist on other planets outside our solar system.
Yet, as any scientist will tell you, a plausible hypothesis is not the same as proof. This has led many researchers to wonder how we might be able to demonstrate the existence of life beyond our solar system. One promising approach is to analyse the atmosphere of exoplanets. By studying the absorption lines in a host star’s optical spectrum, scientists can determine which molecules are present in an exoplanet’s atmosphere, at least in the case of larger planets.
As well as hunting for signs of methane, carbon dioxide, oxygen or water vapour, they are also interested in identifying the combinations in which these substances occur. "Both methane and oxygen are present in the Earth’s atmosphere," says Sascha Quanz, Professor of Exoplanets and Habitability at ETH Zurich. "This is a chemical disequilibrium that wouldn’t exist without living organisms." In other words, life must have caused this imbalance. The discovery of such a disequilibrium in the atmosphere of an Earth-like exoplanet would be a strong indicator of the presence of life.
Ideally, of course, it would be better if we could capture direct images of exoplanets rather than observing them indirectly as they pass in front of their host star. This is easier said than done, however, because exoplanets are almost completely hidden by the glare of their parent stars. To tackle this problem, Quanz has teamed up with other researchers to develop an instrument for the Extremely Large Telescope (ELT). Construction of the ELT in Chile’s Atacama Desert is currently underway and once operational, the telescope’s 39-metre mirror will massively enhance the ability of astronomers to peer deeper into space. "With the ELT, we’ll then be able for the first time to capture direct images of an Earth-like planet orbiting a nearby star, because this new instrument will block out the light of that star," says Quanz.
One surprise after another
But where should researchers direct the search for life? And what signals should they be looking for? Some clues can be found in physical models, such as those developed by Judit Szulágyi, Assistant Professor of Computational Astrophysics, and her group. These models can be used to reconstruct how planets form over time from the initial, protoplanetary disc of dust and gas that swirls around a newly formed star, and they also help determine which objects are worthy of closer inspection via telescope. Szulágyi builds models that take into account a whole range of factors, including gravitational forces, magnetism, the motion of gas, and the way starlight interacts with the disc material. By calculating countless different combinations of these parameters, we can get some idea of the diversity of planetary worlds that might exist in the universe.
Yet experience shows time and again that nature often has more up its sleeve than the models predict. For example, the first exoplanets took the scientific community by surprise because astronomers had never suspected that giant planets the size of Jupiter could orbit so close to their host star. Researchers were equally intrigued by the existence of so-called super-Earths, which are rocky like Earth but about one-and-a-half times larger.
Szulágyi acknowledges that her models regularly turn out to be inaccurate and require recalculation, yet she remains upbeat: "It constantly pushes us to rethink our ideas about how planets form." One of the key questions Szulágyi hopes to answer with her models concerns the origin of water. "Life on Earth requires water," she says. "Hence our interest in places that show evidence of water." Such bodies can even be found within our own solar system, and astronomers are keen to find out more about them in the years ahead. They include Jupiter’s moon Europa, which likely hosts an ocean beneath its thick icy crust, and Saturn’s moon Enceladus, where scientists have observed fountains of ice particles erupting from the surface.
Entirely different worlds
Geology can also provide useful clues to the composition of alien worlds in other planetary systems. Paolo Sossi, Assistant Professor of Experimental Planetology, investigates the exotic minerals, liquids and gases that make up the interior and atmosphere of other planets. "We simulate a wide range of conditions in our experiments," he says. "They help us build up a picture of what’s happening on a planet’s surface and what’s going on inside it."
Our knowledge of the chemical composition of other planets is still sketchy, which makes Sossi’s task more challenging. "Examining the host star’s optical spectrum gives us an initial idea of a planet’s chemical make-up," says Sossi. "That provides the basis for understanding which elements are present and in what quantity." By combining information on the various planets’ mass and diameter with the results of modelling, scientists can then deduce how different elements are actually distributed throughout the planetary system around the star. Our own solar system is a useful reference, because 60 to 70 percent of all the star systems studied so far have a similar chemical composition. Sossi is therefore using numerical models to try and gain a better understanding of how Earth and its neighbouring planets were formed. This gives him the information he needs to reconstruct the masses, number and distribution of the planets around other stars.
Yet there are also stars that have an entirely different chemical composition to that of our Sun. For example, a star may contain more carbon and less oxygen, which might mean that the planets orbiting it are composed of different minerals than our Earth. "The predominant minerals on such carbon-rich planets could be silicon carbide and titanium carbide, or even diamonds," says Sossi. This, in turn, would have an impact on the planet’s atmosphere – for example, rain on such a planet might consist of drops of graphite instead of water.
A long-term vision
Ultimately, the success of our search for alien life depends on a combination of different factors. Telescope observations, lab experiments and numerical models are undoubtedly key elements in any research programme. But we will also need intelligent algorithms that can glean as much scientific information as possible from vast quantities of data, as well as instruments that provide the precise data researchers need. "Instrument development is a top priority for planet researchers like me," says Quanz. "As researchers, we need to understand how instruments work in order to know what kind of information we can get from them."
A long-term perspective is also essential, which is why Quanz is already thinking a step ahead. He is in charge of an international initiative that aims to make major headway in the search for alien life. This forms part of one of the large-class science missions that the European Space Agency ESA is launching between 2035 and 2050. "We’re reaching the limit of what we can achieve with ground-based telescopes, because all the molecules we’re looking for also appear in the Earth’s atmosphere, and the temperature of the Earth is similar to that of the exoplanets that interest us," he says. "If we want to escape the tremendous background noise created by the Earth, we have to head into space. It may well be the only way to detect traces of life in the exoplanet atmospheres."
Unfortunately, however, there is no way of installing telescopes in space that are as large as those in the Atacama Desert. Quanz and his colleagues have therefore proposed a bold project, known as the Large Interferometer for Exoplanets (LIFE). The idea is to position four additional small telescopes at the second Lagrange Point, which is where the James Webb Space Telescope took the spectacular images that recently wowed the world. "By combining measurement signals from multiple small telescopes, we can achieve a resolution similar to that of a single, larger telescope!" says Quanz. "This will enable us for the first time to directly image and chemically characterise dozens of Earth-like planets."
Before this can happen, scientists will need to resolve a whole series of technical challenges: the telescopes need to fly in a very precise formation that changes each time a new planetary system is targeted; the measurement signals from the individual satellites have to be synchronised with tremendous precision; and the telescopes must be equipped with extremely sensitive sensors designed to capture the little light emanating from the planet. Equally critical is the question of how the satellites will be powered, since repositioning them requires substantial amounts of fuel.
All this is technically feasible, says Quanz, though it will require a major effort not only by scientists, but also at a research-policy level. "Ultimately, it’s a matter of priorities," he says. "For the first time, we have the chance to offer an empirical answer to the question of whether alien life exists. Finding that answer would fundamentally transform our view of the world – it’s not an opportunity we should miss."