Nobody knows whether life exists outside Earth yet. Giordano Bruno’s theory about the multiplicity of habitable worlds and about the unavoidable emergence of numerous cradles of life in the Universe has been engaging people’s mind for the past 4 centuries. What is astonishing is, that for these 4 centuries, Bruno’s ideas have never contradicted the contemporaneous scientific knowledge. The only thing that needs to be done is to update the concepts for Universe, life, sentience, etc. with which Bruno worked.


During Giordano Bruno’s lifetime, widely spread was the belief, that in the centre of the Universe was the Earth and that it is unique in the Universe. And today (as of August 2018) the number of confirmed exoplanets (a term we use for extrasolar planets that revolve around other stars) is 3798 in 2841 stellar systems with planets, of which 633 systems have more than one planet. Based on statistical estimates, the total number of exoplanets in the Milky way must be over 100 billion. Of these, 5 to 20 billion probably are Earth-like planets.

But is it possible for life to develop on one of these alien worlds? Here, we will talk mostly about life similar to that we know exists on Earth, because this is our only point of reference and it is very difficult to imagine (scientifically) other life forms. What are the most basic conditions, that need to be met for life to emerge on an extrasolar planet?

First, just like the Earth, we suppose that the planet has to be rocky, and thus not very large (as we know, the larger planets in the Solar system are gaseous spheres with no solid surface). Up to date (August 2018) the astronomers have discovered over 950 exoplanets, similar to the Earth, which we call terrestrial type planets (mass around 1 Earth mass and radius between 0.8 and 1.25 Earth radii), and another about 1100 so-called “superearths”. The superearths have masses greater than the Earth’s mass, but smaller that the masses of Uranus and Neptune (i.e. less than 15 Earth masses). They have larger sizes that the terrestrial planets, but their radii do not exceed 2 Earth radii. The term “superearth” concerns only its mass and size, but is not in any way connected to the conditions on its surface of its potential habitability. Some scientists think, that superearths whose radii are around 2 earth radii are the most favourable for life to develop. Their higher gravity ensures thicker and denser atmospheres, higher level of erosion and as a consequence – more leveled topography. The end result would be an “archipelago planet” with shallow oceans with numerous island chains of moderate heights that are ideal for reach biodiversity.

The second condition is that there should be conditions for the presence of liquid water on the planet’s surface. Around each normal star (including the Sun, luckily for us), there is a zone with distances at which this condition is met. For hotter stars this zone will be further away from the star, for cooler stars it will be closer. It is called habitability zone (fig. 1), or the “Goldilocks Zone” (after Goldilocks from the tale “Goldilocks and the Three Bears”, in which a little girl chooses from sets of three items, ignoring the ones that are too extreme – large or small, hot or cold, etc. – and settling on the one in the middle, which is "just right").

If the planet is closer to the star than the beginning of the habitability zone, it is going to be too hot on its surface for liquid water to be present, if it is further away, after the end of the habitability zone, it is going to be too cold.

Fig. 1 Habitability zone.

The third condition concerns time. There should be enough time for life to emerge on the planet. The first, most simple, organisms on Earth emerged about a billion years after its formation, and for the more complex forms (plants and animals) another two billion years were needed. The more massive and bright stars, however, end their life much faster than the Sun – in several million years (the most massive stars) to several hundred million years. Due to their short life, such stars are not suitable for the development of alien life of terrestrial type. Much better candidates are the less massive stars (of about a solar mass down to one tenth of a solar mass) which live for tens of billions of years.