Until recently, it was believed that in the first hundreds of millions of years the Universe was "empty and formless": there were no stars in the dark space, let alone their planets. Gradually, the picture is changing, and a new study has shown that the first planets began to appear already in the first few hundred million years after the Big Bang. This seriously pushes back the date of possible life.
The traditional view of the evolution of the Universe in recent years—especially through observations by the James Webb Space Telescope—has shown multiple cracks, with radiation being absorbed by the not-yet-ionized interstellar gas, to be incompatible with reality. Even 300 million years after the beginning of the Universe's history, galaxies are already fully observable.
But the question remains: when did classical planetary systems begin to form, that is, not just stars that make up galaxies, but those near which life is possible? The very first stars almost had heavy elements, since they were still produced in the depths of supernovae (after all, there were simply no supernovae before the first stars). Without heavy elements, the formation of planets and protoplanetary disks is unlikely. At least, there was definitely no talk of rocky planets: without heavy elements, it is impossible to create a planet consisting of heavy elements like our Earth.
The authors of the new study, available on the Cornell University preprint server, set out to model exactly how planets formed in the early universe. Specifically, they modeled the rate of evolution of pair-unstable supernovae and their impact on the environment.
Pair-unstable supernovae are particularly massive stars — 130 or more times "heavier" than the Sun — that explode as supernovae by an unusual mechanism. When a star is so powerful, the total energy of thermonuclear reactions in its interior reaches enormous values, creating powerful gamma radiation. It is so strong that it creates pairs of electrons and positrons.
This process is triggered when a very high-energy photon is in specific conditions: for example, in the field of a massive charged particle or an atomic nucleus. In this case, "out of nothing" (but actually from the energy of the photon) a "particle-antiparticle" pair arises, where the role of the particle is played by an electron, and the role of the antiparticle is played by a positron (the antiparticle of the electron).
The process of pair formation can be explosive, and while it is going on, the pressure exerted by gamma radiation from the core on the outer layers of the star decreases sharply. At the same time, the pressure of the outer layers of the star on the inner ones does not decrease. Therefore, the pressure balance between the inner and outer layers is disturbed. The star partially collapses, which seriously increases the temperature and pressure inside its core.
In this case, thermonuclear reactions can occur, which are normally energetically impossible. For example, in a normal star, nuclear fusion reactions stop at carbon (or oxygen or neon, depending on the force), because further fusion of atomic nuclei absorbs more energy than it releases. But with pair instability, there is so much energy in the core of a star that there is a massive production of such a heavy element as iron.
An important difference between such a supernova explosion and a normal one is that, due to the very high energies, all the material of the star is ejected into the surrounding space. No neutron star or black hole is formed: the entire supernova collapses without a trace.
Ordinary stars of our era cannot explode like this. First, they do not have enough mass (today such massive stars simply do not form). Second, for this to happen, the star must be almost completely devoid of elements heavier than helium. The modern Universe simply does not have the raw material for stars that is so poor in elements heavier than helium. On the other hand, 13,5 billion years ago, there were almost no heavier elements, so the very first generation of stars could often explode in this way.
The authors of the new work calculated the evolution of such stars and its impact on the interstellar medium close to it in the early Universe. It turned out that after its explosion, the content of heavy elements near the exploded supernova can be large - sometimes even more than in the substance of the Sun. At the same time, the explosion generates serious instability in the surrounding gas. The gas is “clumped” by the blast wave so much that protostellar clouds with a mass of up to one solar mass arise in it.
Importantly, these clouds contain not only gas, but also dust of heavy elements, from which a planetesimal can already form - a body formed from cosmic dust, which then serves as a "brick" for building planets. can reach five Earth masses. This is not very much by the standards of modern planetary systems, but still. enough to form a rocky planet of the Earth's mass.
Astronomers' calculations have shown that such systems would have a central star with a mass of up to 0,7 solar masses. In the range of orbits of 0,46-1,66 astronomical units (one such unit is equal to the distance from Earth to the Sun), there should be enough water to form a planet capable of having oceans.
From all this, scientists have concluded that the first inhabited planets could have formed as early as the first 200 million years of the universe's history. They could have arisen even before the appearance of the oldest galaxies. Moreover, such planets can be discovered in the coming years by studying the oldest known stars in our Galaxy.