The first habitable worlds formed even before the creation of the first galaxies.

The traditional view of the evolution of the Universe has faced numerous cracks in recent years — particularly due to observations from the James Webb Space Telescope. The concept that the Universe experienced the Dark Ages, a period lasting up to half a billion years after the Big Bang during which there were no stars and radiation was absorbed by yet-to-be-ionized interstellar gas, has proven incompatible with reality. Even 300 million years after the beginning of the Universe's history, galaxies are already clearly observable.

However, the question remains: when did classic planetary systems begin to form, meaning not just stars that make up galaxies, but those near which life could potentially exist? The very first stars had almost no heavy elements because they had not yet been produced in the cores of supernovae (as the first stars could not have formed supernovae). Without heavy elements, the formation of planets and protoplanetary disks is highly unlikely. At the very least, rocky planets were definitely not on the table: without heavy elements, it's impossible to create a planet made of heavy elements, like our Earth.

The authors of a new study, which can be explored on the Cornell University preprint server, aimed to model how planet formation occurred in the earliest Universe. Specifically, they simulated the evolution rate of pair-instability supernovae and their impact on the surrounding environment.

Pair-instability supernovae refer to particularly massive stars — 130 times more massive than the Sun and above — that explode as supernovae through an unusual mechanism. When a star is this massive, the total energy of nuclear reactions in its core reaches enormous levels, generating powerful gamma radiation. This radiation is so intense that it creates pairs of electrons and positrons.

This process is triggered when a high-energy photon is in specific conditions: for example, in the field of a massive charged particle or an atomic nucleus. In this case, a pair of "particle-antiparticle" emerges "from nothing" (but actually from the energy of the photon), where the electron serves as the particle and the positron (the electron's antiparticle) is the antiparticle.

The process of pair production can be avalanche-like, and while it is ongoing, the pressure exerted by gamma radiation from the core on the outer layers of the star sharply decreases. Meanwhile, the pressure of the outer layers on the inner layers remains unchanged. This means that the pressure balance between the inner and outer layers is disrupted. The star partially collapses, significantly increasing the temperature and pressure within its core.

During this, nuclear reactions that are normally energetically impossible can occur. For instance, in a typical star, fusion reactions stop at carbon (or at most oxygen or neon), because beyond this, the fusion of atomic nuclei consumes more energy than it releases. However, under pair instability conditions, the energy in the star's core is so high that there is mass production of a heavy element like iron.

A key difference between such a supernova explosion and a regular one is that due to the very high energies, all the material of the star is expelled into the surrounding space. No neutron star or black hole forms: the entire supernova disintegrates completely.

Regular stars of our era cannot explode in this manner. Firstly, they lack sufficient mass (today, such massive stars simply do not form). Secondly, for this to occur, the star must have almost no elements heavier than helium. In the modern Universe, there is simply no material for stars that are so deficient in elements heavier than helium. However, 13.5 billion years ago, there were almost no heavy elements, which means the very first generation of stars could often explode in precisely this way.

The authors of this new work calculated the evolution of such stars and its influence on the nearby interstellar medium of the early Universe. It turned out that after their explosion, the concentration of heavy elements near the exploded supernova can be quite high — sometimes even greater than that found in the material of the Sun. Additionally, the explosion generates significant instability in the surrounding gas. The gas "clumps" due to the shock wave so much that protostellar clouds with masses up to one solar mass form within it.

Importantly, these clouds contain not only gas but also dust of heavy elements, from which planetesimals can already form — bodies that develop from cosmic dust and serve as "bricks" for building planets. The total mass of such planetesimals in the calculated early systems of the Universe can reach five times the mass of Earth. While this is not a large amount by the standards of modern planetary systems, it is still sufficient for the formation of a rocky planet with Earth-like mass.

Astronomers' calculations showed that such systems will have a central star with a mass of up to 0.7 solar masses. Within the orbital range of 0.46 to 1.66 astronomical units (one such unit equals the distance from the Earth to the Sun), there should be enough water to form a planet capable of hosting oceans.

From all this, scientists concluded that the first habitable planets could have formed within the first 200 million years of the Universe's history. They may have arisen even before the most ancient galaxies appeared. Moreover, such planets could be discovered in the coming years through the study of the oldest known stars in our Galaxy.