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When it comes to planets, they are usually celestial bodies that revolve around the main sequence star such as the sun. Will white dwarfs and neutron stars have their own planetary systems? If so, are these planets still the previous ones?

In 2017, astronomers discovered a white dwarf star in Sloan survey data. Two years later, the researchers observed some special emission lines around it. Judging from the characteristics of the emission lines, there seems to be a solid signal. Excluding other causes such as companion stars and photoelectric instability, the researchers believe it may be a microplanet (SDSS J122831040b).

The concept of "microplanets" usually refers to a class of small celestial bodies called "stars". The so-called star son refers to the small pieces of solid matter formed by the gradual accumulation of gaseous matter, which generally appears in the protoplanetary disk when the star was born. After constant accretion and collision, these small stars gradually grow into big stars until they become a planet. So the star can be thought of as the embryo of the planet.

But for this microplanet, it is clearly not the kind of star that has just been born in the original planetary chart. After all, its host star is a weather-beaten white dwarf star.

In fact, many white dwarfs have their own planetary systems, so it is not uncommon for white dwarfs to accrete small bodies around them. But there is something different about this planet.

The first is size. As a microplanet, small it is really small. We know that the radius of the earth is about 6371 kilometers, and the radius of this microplanet is less than a fraction of it, perhaps only a few dozen kilometers, almost 1% of the radius of the earth. It can be said to be the smallest exoplanet ever discovered and confirmed.

Earth vs SDSS J1228 planet 1040b is a planet, strictly speaking, it is not a complete planet, but may only be the core of the planet.

Because it is observed that the orbital period of the microplanet is very short, it only takes about two hours to make a circle, so it should be very close to the white dwarf. Sure enough, it was later measured that its orbital radius was only 0.0034 astronomical units, less than 1% of the distance between Mercury and the sun.

Being so close to a dense star means that the planet is subject to strong tidal forces. Just like in wandering Earth, the earth's atmosphere is sucked away as it passes by Jupiter, except that the gravity of white dwarfs is stronger here.

The stills of "wandering Earth" under the action of powerful tides are not only the atmosphere, but also the earth's crust and mantle of the planet may be completely stripped off, leaving only a metal core. Even so, the kernel must have sufficient internal strength or density to avoid being destroyed.

In addition, white dwarfs have extremely high temperatures, usually reaching tens of thousands of degrees. As a result, the surface of the planet is also expected to have a temperature of thousands of degrees, which may look like a red-hot iron ball.

In terms of size alone, this microplanet may be the smallest exoplanet ever discovered, but it is not the smallest in terms of mass.

This is an exoplanet about 2300 light-years away in the constellation Virgo. Its mass has been measured more accurately, about 0.02 Earth masses, about the same as two moons. It is the least massive exoplanet known so far-PSR B1257Fe12b.

Seeing the beginning of "PSR" shows that its parent star is a pulsar, and this planet was discovered by pulsar timing. A pulsar is actually a highly magnetized rotating neutron star whose magnetic pole emits a strong beam of electromagnetic radiation. When the direction of the radiation beam sweeps across the earth, we will observe the corresponding pulse waves. Because the rotation speed of pulsars is almost constant, this pulsation has extremely regular characteristics. It is through the precise measurement of this regular pulse that astronomers determine whether there are planets around it.

The discovery of planets around pulsars came as a surprise to scientists. It makes sense for white dwarfs to have planets. After all, their death process is relatively mild. But neutron stars have experienced supernova explosions that are so violent that they release as much energy as our sun releases in its lifetime, so much so that the brightness of the explosion can illuminate its entire galaxy.

During the explosion, the star will throw most of the material out at a speed of close to the speed of 10 light, creating shock waves around it. So in theory, if there were planets around the star, it would have gone up in smoke. Even if it wasn't completely destroyed, I don't know where it went. But then people found that there were "survivors" here, which was really surprising.

So how on earth did these "survivors" avoid supernova explosions?

In fact, these so-called "survivors" may not have survived at all. No matter what kind of planet it is, it is a praying mantis in the face of a supernova explosion. Not to mention planets, that is, other stars as companions, should be afraid of three points in the face of such a companion.

Unless the planet is orbiting a binary star. As the saying goes, "the sky falls with a tall man against it." with another star to absorb the damage, the probability that the planet will survive is greatly increased.

But the pulsar planet we are going to talk about today is different. In a 2013 article published in the monthly Proceedings of the Royal Astronomical Society, researchers observed the pulsar for two years and concluded that the planets around it were newly formed in the original astrolabe. Note: the protoplanetary disk here is not the protoplanetary disk when the pulsar predecessor star first formed, but the pulsar itself after the supernova explosion.

The researchers speculate that, like most pulsars, this pulsar was originally in a binary system. There was originally a massive companion star next to it. As the matter of the companion star was slowly absorbed by the pulsar, an accretion disk gradually formed around the pulsar.

Then the companion star came to the end of its life, followed by another supernova explosion. But the blast did not have a destructive effect on the previous pulsar. Instead, it provided a lot of new food for the pulsar. With the continuous evolution of the accretion disk, the protoplanetary disk was born. Then, under some kind of disturbance, the stars began to form here, and the pulsar's own planet was born.

This is one of the explanations for the origin of pulsar planets. In addition to this, there are two other possibilities for the origin of pulsar planets.

The first is that the pulsar captures a passing celestial body by its own gravity, making it its planet. This is just like some planets becoming their moons by capturing small celestial bodies.

Another situation is a little surprising: the predecessor of the planet may be the companion star himself. The companion star first survived the explosion of its companion's supernova, then expanded into a red giant, and finally became a white dwarf. Under the strong gravity of the pulsar companions, all the hydrogen and helium in the outer layer of the white dwarf star are sucked away, leaving only a lone carbon crystal core, which becomes a planet of the pulsar. The exoplanet PSR J1719 − 1438 b is such an example. If we are interested, we will talk about it later.

What kind of environment will these pulsar-dependent planets look like? Is there any possibility for life to survive on it? Or, for those dense stars, does the concept of habitable zone still exist?

First of all, it is known that the habitable zone of white dwarfs with a surface temperature below 10000 degrees is about 0.005 to 0.02 astronomical units, which is about 2 to 8 months away. After all, the size of the white dwarf is too small, and the livable zone is certainly very close in terms of temperature.

However, the surface temperature of these planets mainly comes from the radiation of white dwarfs in the near infrared, visible and ultraviolet bands, but neutron stars (especially pulsars), including their accretion processes, produce a large number of high-energy X-rays and even gamma rays. There is almost no radiation with relatively long wavelengths, so different mechanisms need to be considered, and the situation is more complicated.

In a 2017 paper published in the journal Astronomy and Astrophysics, researchers analyzed radiation from neutron stars and planetary atmospheres.

The planet's atmosphere generates a lot of heat when exposed to X-rays, causing the planet's atmosphere to evaporate. For rocky planets like Earth, the atmosphere is relatively thin and can easily evaporate, so the habitable time is very short. And for the dense atmosphere of super-Earth, especially gaseous planets, its habitable time is theoretically much longer. That is, for pulsars, it may not have a fixed habitable zone, and livability has more to do with the characteristics of specific planets.

Around today's pulsar, researchers have so far discovered three planets. In addition to the 0.02 times the mass just mentioned, there are two super Earths of 3.4 and 4.3 times the mass of the earth. For these two super-Earths, as long as their atmospheric share is high enough, even the most penetrating gamma rays may not be able to reach the surface of the planet.

If you take into account the magnetic field, the habitable life of the planet is even longer, even billions of years. You know, it only took about 1 billion years from the birth of Earth to the emergence of life, which means that it is not impossible to give birth to life on pulsar planets. It's just that if the atmosphere is too thick, the pressure below will be so high that there may be large amounts of liquid or superfluid matter above the solid surface. So if there is life there, it may also be similar to the deep-sea species on Earth.

Reference link:

Http://exoplanet.eu/catalog/psr_1257_12_b/

Https://exoplanets.nasa.gov/discovery/exoplanet-catalog/

Https://en.wikipedia.org/wiki/List_of_smallest_exoplanets

Https://en.wikipedia.org/wiki/SDSS_J1228%2B1040_b

Https://en.wikipedia.org/wiki/PSR_B1257%2B12_A

Https://arxiv.org/abs/1904.02163

Https://arxiv.org/abs/2101.08033

Https://physicsworld.com/a/astronomers-find-smallest-exoplanet/

Https://academic.oup.com/mnras/article/433/1/162/1029800

Https://www.aanda.org/articles/aa/full_html/2017/12/aa31102-17/aa31102-17.html

This article comes from the official account of Wechat: Linvo says ID:linvo001, author: Linvo

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