And that’s inside and outside our solar system. “But it’s important to define what exactly is meant by life,” says junior professor Lena Noack, a geophysicist and astrobiologist at Freie Universität. She does not necessarily expect aliens with two arms, legs, and eyes. “But I do assume there is life out there in different forms and levels of complexity.”

Single-celled organisms were the only life on earth for two billion years. “This life form could be very widespread in the universe,” Noack says. “The likelihood of multicellular organisms like plants or animals is much lower.” What conditions does a planet need to offer for life to emerge there? How important is liquid water?

How exactly did the planets in our solar system form? How would we recognize life in the first place if it is not at all similar to life on earth? Questions like these were the subject of discussion for more than 200 scholars, scientists, and researchers from all over Europe at the annual conference of the European Astrobiology Network Association (EANA 2018), which was held on the Freie Universität earth sciences campus in Lankwitz in late September.

Several thousand planets outside our solar system – known as “exoplanets” – have already been discovered. Some have a toxic atmosphere, while others are much larger, denser, and heavier or significantly smaller than the earth. Many of them revolve around very dim stars known as “M-dwarf” stars, which would make photosynthesis, and thus plant growth, difficult to impossible.

Noack is interested in how “Earth-like” other planets actually are. “It’s not about looks, so whether there are continents and oceans, but instead astrophysical and geophysical properties that could favor the emergence of life: the optimum distance from the star, the intensity of the star’s radiation, internal activity, stable surface temperatures, and possibly plate tectonics.” When it comes to the habitability of a celestial object, these are important factors.

Plate tectonics, combined with volcanic eruptions and earthquakes, might seem at first glance to be bad for life on the surface. But in the long term, the consequences are very positive indeed, and even essential to long-term habitability, at least on Earth. After all, these aspects stimulate the global carbon cycle. “As oceanic plates subside, parts of the ocean floor where there are deposits of carbonates, crustaceans, and other organic waste matter are pushed back into the earth’s mantle. Over time, the material melts there, and carbon dioxide is returned to the atmosphere through volcanic activity.” At the same time, the sinking crust cools the mantle, which keeps the magnetic field produced in the earth’s iron core stable in the long term.

Noack would also like to better understand how the early earth developed and create a theoretical physical model of it. “This will allow us to define parameters that help us look at the thousands of exoplanets that have already been discovered and pinpoint five, say, where Earth-like development is most likely.”

So far, the only way to sense exoplanets has been indirectly, using the transit method: Astronomers observe how bright a star is and then, if its light is dimmed periodically, they can conclude that there is a planet revolving around it. Radius, mass, and the distance between it and the star, which can be used to roughly estimate the surface temperature – to date, that’s about all the information there is on exoplanets. No human will ever be able to set foot on one of them, since even the closest one, Proxima Centauri b (discovered in 2016) is more than four light-years away from Earth.

Experiments on the International Space Station (ISS) also planned

Not long from now, there will be space telescopes that can be used to look at exoplanets directly from space. “They will collect light quanta from the planet’s atmosphere on which we can perform spectral analysis to determine the chemical composition of the atmosphere. The spectral analysis will then also allow us to identify possible biosignatures,” says Andreas Elsässer, who holds a doctorate in biophysics and, like Noack, was involved in organizing the EANA 2018 event.

While Noack focuses primarily on the geophysics of planets, Elsässer is interested in tiny particles in their atmospheres. He hopes to identify biomarkers that can be used to search for extraterrestrial life within our solar system and beyond. “In the past, we only paid attention to very simple molecules, like oxygen, ozone, or water. But oxygen can also arise from non-biological processes, so it isn’t a clear sign of life.”

With this in mind, Elsässer is thinking about organic compounds like methane and amino acid decomposition products, or long-chain hydrocarbons. “These hydrocarbons are not normally gaseous. But radiation causes them to break. They get smaller and become volatile,” he explains. “And that’s when we might also see them in a planet’s atmosphere.”

To find out just what he actually needs to look for, Elsässer is simulating possible planetary conditions inside planetary simulation chambers where the pressure, temperature, and gas composition can be varied. He is also exposing organic molecules to radiation and analyzing their decomposition products. “As far as radiation is concerned, we have limited options. Simulating the entire solar spectrum is extremely difficult.”

All that should change soon. In cooperation with researchers from the European Space Agency (ESA) and the U.S. National Aeronautics and Space Administration (NASA), Elsässer is working on an experimentation platform that is to be attached to the outer hull of the International Space Station (ISS). There is no protective terrestrial atmosphere there. Starting in about the year 2020, it will then be possible to use spectrometers in the ultraviolet, visible light and infrared ranges to observe what happens to biomolecules in the test chambers under real-world conditions in space.

Will we discover extraterrestrial life before this century is over? Yes, Noack says with conviction. “I’m sure we will find interesting indications. But it’s hard to find clear proof of life on an exoplanet. I assume there will always be scientists who dispute extraterrestrial life unless and until there is communication with another life form.”