Astrophysicists at the University of Leiden (Netherlands) Michel Kama and Alessandro Patruno proved that planets suitable for life can exist around neutron stars. So, in the presence of certain conditions, the super-earths PSR B1257 + 12 d and PSR B1257 + 12 c, which received the names Fobetor and Poltergeist, find themselves in the habitable zone of the PSR B1257 + 12 star, called Lich. The study on this topic was published by the authors in one of the specialized publications.
At the moment, scientists know about three thousand neutron stars, but only two of them reliably have planetary systems, and some may have such systems. It should be noted that the first exoplanets were discovered precisely near a neutron star. It happened in 1991. The discovery was made by the Polish-American radio astronomer A. Wolschan, who discovered two exoplanets near PSR B1257 + 12 - Fobetor and Poltergeist. Each of them is about four times heavier than our planet. A year later, this discovery was confirmed by the Canadian astronomer Dale Frail.
After some time, another exoplanet, PSR B1257 + 12 b, was discovered there, which turned out to be 50 times lighter than Earth. It is located very close to a neutron star, so the conditions on it are not suitable even for the most extreme life. As for the Poltergeist, this exoplanet is 4.3 times heavier than the Earth, on its surface the temperature reaches 51-652 Kelvin. The planet revolves around the pulsar at a distance of 0.36 astronomical units with a period of 66 days. The second exoplanet, Phobetor, is farther from the pulsar and slightly heavier than the Poltergeist.
The star PSR B1257 + 12 itself is located in the constellation Virgo, at a distance of 2.3 thousand light years from our planet. It is about 1.4 times heavier than the Sun, but about 125 trillion times smaller than it (the radius of the pulsar is only 10 kilometers). Astronomers estimate the age of PSR B1257 + 12 at about one billion years, that is, the pulsar is four times younger than the Sun. The star rotates with a period of 0.06 seconds, high-power X-rays emanate from it into the surrounding space. It was previously thought that life on these two exoplanets was impossible, but Patruno and Kama were able to prove that this was not the case.
The formation of neutron stars occurs as a result of a supernova explosion, after which there is often enough matter in orbit to form a protoplanetary disk. In addition to the PSR B1257 + 12 pulsar, exoplanets were also discovered around PSR J1719-1438. The carbon-rich satellite PSR J1719-1438 b may well have previously been a white dwarf. Scientists also admit that an asteroid belt may exist near PSR J1937 + 21. In addition, scientists interpret some astronomical phenomena, in particular the GRB 101225A gamma-ray burst, as a collision of a neutron star and an asteroid or comet.
Researchers have traditionally identified three types of planets that may be near neutron stars. The first type includes typical planets, which are a byproduct of star formation and which formed even before the supernova explosion and the appearance of the neutron star itself. The second type includes planets that are formed from the matter that was left after a supernova explosion near a neutron star. Planets of the third type are planets that were formed from the matter of a destroyed satellite of a neutron star (for example, PSR J1719-1438 b). This type is typical for satellites of millisecond stars, in particular, for PSR B1257 + 12 and PSR J1719-1438.
Scientists speculate that planets around neutron stars are the exception rather than the rule. High-energy gamma and X-rays, as well as the so-called pulsar wind, can destroy any object over a period from a million to a billion years. At the same time, a relatively small celestial body, which is far enough from the star, has a chance to maintain a stable orbit for a long time. For this reason, despite the relatively small number of pulsars with planets, due to the large number of neutron stars themselves (about a billion) within the Milky Way, the number of planetary systems around them reaches 10 million.
Planetary systems near pulsars do not have to be similar to those found near main sequence stars. So, for example, the habitability of a planet is usually defined by terms such as the equilibrium surface temperature, the given radiant energy received from the host star. This energy is calculated at a first approximation as blackbody radiation reaching its maximum in the optical, infrared or ultraviolet ranges. In this case, typical habitable zones are identified at a distance ranging from a few shares to astronomical units.
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The habitable zone, which is much smaller in size than near the stars of the main sequence, is calculated for white dwarfs (the Sun will turn into an object of this kind in 8 billion years). When in 3 billion years the star cools down to a temperature of about 10 thousand kelvin, the location of the habitable zone will be at a distance of 0.005-0.02 astronomical units. When it comes to neutron stars, the brightest blackbody radiation corresponds to X-rays, when many high-energy ionizing particles are observed. At the same time, ultraviolet, optical and infrared radiation is practically absent.
The authors of the study used special software that analyzes photographs of the PSR B1257 + 12 system, which were obtained on May 3, 2007 using the Chandra X-ray space telescope. In addition, they used observational data from May 22, 2005 to compare their findings with those of other scientists. According to preliminary estimates, the surface temperature of the pulsar reaches 1.1 million kelvin, and near it, at a distance of a fraction of-astronomical units, a dust disk may exist.
For possible life on Phobetor and Poltergeist, the main danger and at the same time, the main source of heat can be X-rays, which can provoke a significant heating of the atmosphere of the planets. Gamma and hard X-rays penetrate the atmosphere much deeper than soft X-rays and ultraviolet radiation. However, in the event that the gas envelopes are wide, hazardous radiation cannot reach the planet's surface.
According to the assumptions of Kama and Patruno, the planets that revolve around isolated pulsars should evolve like celestial bodies revolving around the main sequence stars, which emit strong X-rays early in their evolution. On our planet, X-rays are quickly blocked by the thermosphere, in which gas is ionized when it interacts with ultraviolet and X-rays. This layer has a fairly high temperature, which is hundreds - thousands of Kelvin. At the same time, this layer is ineffective as a heat source because it is rarefied.
According to the generally accepted thesis, the habitable zone is the area around a star in which an Earth-like planet (that is, a planet that has an atmosphere of carbon dioxide, nitrogen, and water) can have a sufficient amount of liquid water on its surface. Very often a necessary but insufficient condition for the habitability of the planet, scientists believe that the indicator of its equilibrium temperature does not fall below 270 Kelvin. Kama and Patruno calculated the habitable zone around the pulsar PSR B1257 + 12 using estimates of the radiation reaching Phobetor and Poltergeist, hypothesizing that the equilibrium temperature of the two super-Earths is 175-275 Kelvin.
This is quite possible, since the atmosphere of large planets has a higher temperature gradient than on Earth, the atmosphere of which is quite homogeneous. Based on this, the researchers concluded that if X-rays are the main source of energy for the planets, then all three planets of the PSR B1257 + 12 system are unsuitable for life, because it is too cold there. But if we take into account the gamma radiation that occurs due to the pulsar wind in the atmosphere of the planets, then the boundaries of the habitable zone are shifted by a distance of 2-5 astronomical units.
Between these two possible scenarios, there is a space of parameters in which Fobetor and Poltergeist fall into the habitable zone. In addition, the authors of the study proved that the most ancient planet known to man - PSR B1620-26 - even in the most optimistic case, cannot be habitable. Regarding the pulsar PSR J1719-1438, scientists currently have too little data on X-ray radiation, so no definite conclusions can be drawn. According to scientists, the X-ray luminosity of most of the isolated pulsars with the outflow of matter into a companion onto a neutron star (the so-called Bondi-Hoyle accretion) is much higher than that of PSR B1257 + 12, which is atypical in this sense.
In other words, for Earth-like planets, the habitable zone around a neutron star exists for a relatively short time. And for super-lands with a dense atmosphere, the habitable zone lasts much longer. Scientists have calculated that if our planet was 1-10 astronomical units from PSR B1257 + 12, and if its atmosphere accounted for about one percent of the mass of the entire planet, then the Earth would lose its gas shell in about 10 million years. Under the same conditions, super-Earths with thick atmospheres would have lost their gas envelope in about a trillion years.
As the researchers note, the biggest danger to the atmosphere is not X-rays, but pulsar winds. They act at a certain time - there is a kind of death line that determines the moment when the neutron star stops producing wind. In young pulsars, this happens in about a million years, and in millisecond stars, billions of years. However, according to scientists, this eliminates the source of the planet's energy, as a result of which its temperature drops sharply, and any possibility of determining the habitable zone is excluded. However, in this case, Bondi-Hoyle accretion remains, which can generate enough X-ray radiation, thus heating the planet. In addition, the temperature can be maintained by tidal heating.
In the event that the axis of rotation of the neutron star and the magnetic axis strongly diverge, the pulsar wind may not reach the planet's surface at all. In the equatorial plane, in which the planets are often located, there is no pulsar wind, there is only X-ray radiation. Scientists for such a case calculated that the atmosphere of Phobetor and Poltergeist over 850 million years has lost approximately 0.0005 Earth masses, which is approximately 0.0001 of its own mass. This is very small, especially if the atmosphere PSR B1257 + 12 d and PSR B1257 + 12 c account for, according to the generally accepted assumption, about one percent of the mass of the planets.
This study does not provide an opportunity to draw unambiguous conclusions that super-Earths near PSR B1257 + 12 are within the habitable zone. At the moment, its determination is impossible for pulsars, including the neutron star PSR B1257 + 12. At the same time, the study showed that if Phobetor and Poltergeist have a powerful and dense atmosphere, then theoretically these planets can be suitable for life.