The planets around neutron stars are made up mostly of carbon, which turns into diamond under pressure.
Scientists from Columbia University (USA) have proposed an explanation for the mysterious and previously unexplained mechanism of planet formation in neutron star systems. Based on their model, all previously discovered planets in such systems are mainly composed of diamonds. A preprint of the relevant article is available on the Cornell University website.
The era of exoplanet discovery a quarter of a century ago began with pulsar planets - bodies orbiting pulsars (neutron stars with a magnetic field tilted relative to its axis of rotation). For a long time astronomers thought that the appearance of bodies like our Earth around pulsars was very strange. The fact is that neutron stars appear after supernova explosions. Such a powerful event should destroy all the planets previously available to the star or throw them over a huge distance, so that earthly astronomers would simply not notice them. How is it that whole planetary systems of neutron stars have already been discovered?
Researchers at Columbia University tried to answer this question using a completely unexpected scenario. They modeled the long-term interactions between a neutron star and a white dwarf. Stars like the Sun at the end of their life become white dwarfs. They lack mass to explode like a supernova and form a neutron star. Today, it is believed that most of the stars in the Universe should exist in binary, triple or even larger systems in terms of the number of stars. Thus, in nature there is a significant probability of accidental formation of a neutron star - white dwarf pair. They were originally a pair consisting of a sun-like star and a more massive blue-white star.
Modeling has shown that in about one percent of cases, the gravity of the neutron star will gradually destroy the white dwarf with powerful tidal forces. Taking into account the abundance of neutron stars and white dwarfs, even one percent is enough for the pulsar planets to be quite numerous in our Galaxy.
A neutron star is very dense - with a mass comparable to the Sun, it has a diameter not 1.4 million kilometers, but only 20-25 kilometers, and therefore, the gravity of such a body is extremely strong. Since the edge of the white dwarf closest to it will be subjected to greater gravitational effects than its distant "edge", in some cases the neutron companion will destroy the dwarf, literally tearing it apart.
In this case, a disk is formed around the neutron star from the matter of the white dwarf destroyed by it. Since the latter is a kind of "corpse" of a normal star, all the fuel for thermonuclear reactions in it has long since burned out. Therefore, there is no hydrogen and light elements. The dwarf is dominated by carbon and oxygen, the "waste" of past nuclear reactions in the interior of the star. In the disk from its substance, as shown by modeling, the formation of rather large planets is possible. Due to the absence of light elements, they will not be gas giants. But such bodies are not similar to our Earth either. There is no water, little iron and silicates. But there will be carbon under the thin planetary crust. Due to the enormous pressure of the outer layers, it will take the form of diamond or lonsdaleite there.
Since there will be almost no other elements in the composition of such planets, the total weight of diamonds in their composition is estimated by the authors of the work to be quite high - up to 100 octillion carats (one with 29 zeros). The atmosphere of such a "diamond planet", covered with graphite crust, most likely will not be too thick. It will consist of carbon monoxide (CO) and oxygen, "knocked out" of carbon monoxide molecules by ionizing radiation from the vicinity of the neutron star.
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It should be emphasized that ionizing radiation there will be extremely strong. A significant part of cosmic rays reaching the Earth's surface came to us precisely from the vicinity of distant neutron stars, whose magnetic fields can play the role of a particle accelerator - and much more powerful than the Large Hadron Collider. The radiation on the planet near the neutron pulsar star will be such that not only people, but also the electronics they have, would not withstand local conditions even for a short time.