For sure, this is one of the most exciting question you can ask. It was already raised in the ancient time by Metrodorus von Chios (4th century BC). Immanuel Kant, the famous philosopher, supposed that all stars are suns and might be surrounded by worlds similar to our own.
The following frequently asked questions are answered by Dr. John Lee Grenfell (DLR, Institute of Planetary Research, Berlin), including the latest developments in research.
What is the probability that there is extraterrestrial life elsewhere in the universe, at least in the form of simple single-celled organisms?
Of course, I really hope that there is extraterrestrial life. But it is not possible to give a probability because of the complexity of this question. What might be helpful is to consider the processes and environments which enable life to develop.
Which factors determine whether life exist on Earth?
Life in its simplest forms has existed on Earth for about 3.9 billion years. When compared to Earth’s age of 4.6 billion years, we can see that life on Earth developed relatively quickly. We don’t know exactly how, where and when life started to evolve. There are three factors which seem to be important: liquid water, a source of energy and sufficient elements like carbon, hydrogen, nitrogen, oxygen, phosphor and sulfur (C, H, N, O, P, S).
The amount of water on a planet is dependent on the bombardment of impactors, e. g. comets, in its early development phase, which is closely related to the dynamics of the system, i. e. the orbital parameters. Whether there is liquid water on the surface of a planet, is correlated with the climate, which in turn is coupled to the distance between star and planet and the atmospheric properties. All of these are the focus of current research in extrasolar planets. Several exoplanets have been found which orbit in the habitable zone of their stars. The habitable zone is a range of distances from a star where liquid water could exist. Models suggest that the proportion of water on Earth is relatively low compared to other terrestrial planets of comparable distance to their star. It is interesting to mention that impacts on the early Earth delivered not only water, but other important organic molecules, such as amino acids which are essential for life.
Source of Energy
The incoming solar flux is the most important energy source on the surface of modern Earth. Through photosynthesis, almost all life on Earth relies, directly or indirectly, on sunlight. Nevertheless, microorganisms exist which take their energy from chemical bonds.
In the early phase of development, the planetary abundance of elements as C, H, N, O, P, and S is fixed. The frequency depends on the composition of the gas and dust disc in which the planet forms.
Besides water, energy and the key elements C, H, N, O, P, S, what influences the develoment of life?
The Sun: compared to other G-type stars, our Sun has a very stable brightness.
The Moon: our Moon is relatively large. It may have fixed the rotation axis of Earth over a long period, thereby stabilizing Earth’s climate. But this is very much disputed at the moment.
Gas planets: Giant planets like Jupiter might have saved the Earth from impacts by catching asteroids due to gravitational force. But there is also significant evidence for the contrary: Jupiter may enhance the bombardment rate by scattering asteroids in to the inner solar system.
Earth’s magnetic field: the magnetic field shields our atmosphere against high energy particles generated in the Sun or elsewhere in the Galaxy. These particles have the potential to damage DNA and hence are generally bad for life.
Plate tectonics: neither Venus nor Mars show plate tectonics which seems to stabilize the climate of Earth and to distribute nutritions.
What is the current status of exoplanetary research and the search for life?
Exoplanetary research is just beginning to learn about life supporting factors such as the importance of the Sun and the giant planets in the outer solar system. Data considering other influences are still missing.
There are a lot of discussions about the possibility of life in the solar system, on Venus, Mars, Titan (a moon of Saturn) and some of Jupiter’s moons. To advance, we need measurements of biosignatures, signs of life, on many exoplanets. Up to date, there is no data showing clear evidence. However, the big telescopes which will be constructed in the coming decades will be able to measure the tiny signals of biosignatures.
Is there life on the exoplanets of other galaxies?
The focus of the current research is on planets in our Galaxy. We assume that in regions lacking the CHNOPS elements or with a high density of supernovae, life could not have developed. But without data, there is only speculation.
What is your estimation about extraterrestrial life at a similar developed stage as on Earth?
There are periods in the Earth’s history where life evolved faster than at other times, e. g. the period of the Cambrian Explosion which started half a billon years ago. In my opinion this evolution was trigged by several factors. Examples are the climate change or the change in cosmic or UV radiation. It may be supposed that large changes are bad for the evolution of life and cause extinction events, whereas small changes might be drivers for the evolution of life.
We don’t know much about such changes on exoplanets and how they might influence the development of life, and so we can’t make a serious statement.
The SETI program (Search for Extraterrestrial Intelligence) is looking for highly developed, intelligent life, but hasn’t succeeded so far. The Drake equation is the starting point for trying to estimate the number of advanced civilizations in the Galaxy. However, several of these input parameters can’t be determined in a scientific way, such as the probabililty that a civilization destroys itself.