by Ludwig Scheibe (TU Berlin), December 2024
One of the most exciting questions in the field of exoplanets is undoubtedly: is there life as we know it on any of these alien worlds? It is a question that people have been asking since ancient times. Now, in the age of exoplanet research, we have the chance to get to the bottom of it – even if a definitive answer will probably be a long time coming, if there can be one at all.
The first step in the search for new life must be to know where we can look at all. What are the most promising planets on which life could also have developed? Life as we know it needs four things: an energy source, the possibility for complex molecular chains, a form of nutrients and a solvent, in our case liquid water. This last point is the key to the so-called habitable zone.
For a planet to have liquid water on its surface, it must not be so cold that all the water would freeze, and not so warm that all the water would evaporate. The classic habitable zone is therefore the area around a star in which a planet receives just enough energy from it to allow liquid water on its surface. As the temperature of a planet is also strongly influenced by its atmosphere, an atmosphere like that of the Earth is taken as a basis here.
In our Solar System, this zone extends from just outside the orbit of Venus to just inside the orbit of Mars, with the Earth right in the middle. But these distances are strongly dependent on how hot the star is and how brightly it shines. A cooler star than the sun, which therefore also shines less brightly, has a much more compact and closer habitable zone than a hotter one. The drastic difference in the distances can be clearly seen in the comparison image between the TRAPPIST system and the inner planets of the Solar System:
Artist’s impression of the TRAPPIST-1 planetary system and, in comparison, the inner planets of the solar system. The planet sizes are to scale, but not the distances. The habitable zone is shown in green. Note that it is much closer to the star in the case of the faint TRAPPIST star than in the solar system. Credit: NASA/JPL-Caltech
We have already found rocky planets in the habitable zone of their star. One example would be the TRAPPIST system pictured above. But be careful: just because a planet can theoretically have liquid water does not mean that it actually has water. And even if it does, the existence of liquid water is by no means a guarantee that life also exists.
Many of the planets that we have found so far in the habitable zone orbit small, faint M stars in very close orbits. It is therefore likely that they are tidally locked, i.e. have a permanent day and night side, which would significantly influence the atmosphere and climate conditions on them. In addition, M stars, especially at the beginning of their development, emit disproportionately high levels of UV radiation, which is not conducive to the development of life. A planet with the mass and size of Earth and orbiting a sun-like star in the habitable zone has not yet been found, as of 2024.
And even if we do find such a planet – as the PLATO mission intends to do – it may look very different there than it does here, and life may not necessarily have developed. It is therefore essential to study such promising candidates more closely and, for example, to examine its atmosphere for biosignatures – signs of life. An atmosphere is important not only because of the gases that life needs, but also to mitigate harmful UV radiation.
Infographic on the role of the atmosphere in supporting life. Credit: SPP 1992 / Patricia Klein
On the other hand, the classical habitable zone is not the only place in a star system where we might find liquid water. It is conceivable that strong tidal interactions could generate heat beneath the surface of a gas giant’s moon, creating deep subsurface oceans. Such oceans are suspected under the surface of Jupiter’s moons Europa and Ganymede, for example.
Thus, finding a planet in the habitable zone is just one of many clues to discovering life outside our solar system. And even if we haven’t yet found a second Earth with a thriving biosphere, we’ve only scratched the surface of what’s out there, and many exciting discoveries are still to come. And who knows, maybe one day we will be able to answer the age-old question: is there life beyond Earth?