(ORDO NEWS) — Scientists now and then discover traces of cosmic cannibalism, when burned-out stars literally swallow their own planets and asteroids. Recently, researchers stumbled upon a particularly interesting case.
It is difficult for a planet to survive the death of its star. For example, luminaries with a mass of ten or more suns end their lives in a supernova explosion. The shock wave of this explosion is capable of sweeping away any planetary system.
But even less massive stars often take their children with them. Having turned into a white dwarf (below we will describe in detail what it is), the star often breaks apart and absorbs its own planets.
Ashes of swallowed worlds remain in the atmosphere of a cooling star. These, so to speak, leftovers give astronomers a rare chance to find out what distant exoplanets are made of.
Agony of the stars
Why do the stars shine? Because in their bowels there are thermonuclear reactions that release energy. Chief among these processes is the conversion of hydrogen into helium. In flippant astronomical jargon, this is called the burning of hydrogen.
Of course, thermonuclear reactions have nothing to do with real combustion: combustion is a chemical process, it does not affect the nuclei of atoms. But “burning hydrogen” sounds much shorter than “fusion reaction in which hydrogen is consumed”, so we will also say that.
A star consists of several layers, which differ from each other in pressure, temperature and density of matter. The central layer of a star is the core. Here, the pressure is highest, because all the overlying layers of the luminary press on the core with their weight.
This pressure compresses matter to a solid density: for example, the core of the Sun is about 20 times denser than steel. The temperature in the core is also very high (for the Sun it is 15 million degrees). The overlying layers of the star are colder than the core and not as dense.
Almost all its life, a star is provided with energy by the combustion of hydrogen in its center. But sooner or later, the hydrogen in the core of the star ends.
Then a thin layer of hydrogen adjacent to the core from the outside begins to burn. When it burns out, the next layer lights up, and so on. A powerful stream of radiation literally inflates the star, and it becomes a red giant.
A dying star goes through several more metamorphoses associated with the burning of helium, and sometimes even heavier elements. We will not describe this agony in detail. It is important that a significant part of the star’s mass is dissipated in space.
Stars more massive than ten suns scatter it in a violent supernova explosion. More modest luminaries part with their substance not so spectacularly. But the result is the same: in the place of the former star there remains a cloud of rarefied matter and a dense hot core, in which no thermonuclear reactions take place.
Gravity tends to compress this core. The pressure of the substance tries to prevent this, but it does not always succeed. Stars with a mass of ten or more suns leave behind too massive remnants (1.5 solar masses or more).
Such an object is doomed to shrink into a neutron star, or even into a black hole. But if the remainder is not so massive, the compression stops before this metamorphosis. True, the “star corpse” still turns out to be very dense: comparable in mass to the Sun, it resembles the Earth rather in size. This is the white dwarf.
The name “white” is a convention. Sirius B and 40 Eridani B, the first to be discovered, are white, they gave the name to all their brethren. In general, the real color depends on age.
A typical white dwarf has a surface temperature of about 50,000 degrees shortly after its birth (that is, the death of its parent star) and appears blue. But gradually it cools down, because thermonuclear reactions, we recall, have already ended.
In a billion years, the temperature of the white dwarf drops by a factor of five, and now it really looks white.
After another few billion years of cooling, it goes through stages from yellow to red, and after ten billion years (a time comparable to the age of the Universe) it ceases to emit light at all. But all the while, it’s been called a white dwarf, not yellow, red, or brown.
Doom of worlds
What happens to the planets during all these dramatic metamorphoses? Alas, usually nothing good.
Even with the transformation of the luminary into a red giant, the planets closest to it will not be in trouble. For example, the Sun at this moment will expand so much that it will literally swallow Mercury, Venus and the Earth. The only consolation is that this catastrophe is still about 5 billion years away.
Worlds more distant from their sun have a chance to survive, but they are not safe either. For example, the stream of incandescent plasma that flows out of a star losing mass is by no means harmless. It is not as destructive as the shock wave of a supernova, but can easily deprive an Earth-like planet of the atmosphere.
However, the main danger for the planets that have escaped the mouth of the red giant comes from… from neighboring planets. More precisely, from their gravity.
Planets are most strongly attracted, of course, to the star. But their attraction to each other cannot be neglected either. These mutual influences can easily bring the body out of a stable orbit and drop it on the local sun (or, conversely, throw it into interstellar space).
Isaac Newton, the discoverer of the law of universal gravitation, was well aware of this problem. He explained the stability of the solar system no more no less than the direct intervention of the Creator.
In defense of the genius, let’s say that in his time the solar system almost coincided with the known universe, so that its manual adjustment was a matter quite befitting a deity.
Scholars today dispense with theological arguments. They believe that planetary systems go through a turbulent youth when planets collide, swap places, and so on.
The solar system (and any other) system has gone through several unstable configurations, not lingering in any of them precisely because they were unstable. So its current power-law motion is, in a way, the result of natural selection.
However, we recall that in the process of turning into a white dwarf, the Sun will lose a significant part of its mass.
With such difficulty, the balance of gravitational forces found will be irrevocably disturbed. So, the system will again begin bacchanalia. And in this leapfrog, some planets can come dangerously close to a white dwarf.
Rip and swallow
What does “dangerously close” mean? This means that the planet can be torn apart by tidal forces.
Tidal forces are basically a very simple thing. The hemisphere of the planet facing the star is somewhat closer to it than the opposite, which means it is attracted more strongly. It is this effect from the Sun and Moon that causes the tides in the Earth’s oceans.
But the planet itself is also slightly stretched, deformed. If you get too close to the star, this deformation will exceed the tensile strength, and the planet will literally be torn to pieces. The dust cloud into which it will turn will gradually settle on the star.
How close do you have to get to a white dwarf to fall prey to it? Approximately one solar radius (the own radius of a white dwarf, we recall, is comparable to the earth’s).
In many systems, such cataclysms have already occurred. Clouds of dust and debris, sometimes quite large , are observed around some white dwarfs . True, such eloquent traces of catastrophes are rarely observed (most likely only because they are difficult to notice with our instruments). But there is other evidence as well.
White dwarfs have thin atmospheres made almost entirely of hydrogen and helium, the pitiful remnants of the parent star’s past reserves. But in the atmosphere of about every fourth object, heavier elements are also observed, up to iron. Such white dwarfs are called polluted.
These heavy elements can hardly belong to the cooling star itself. The powerful gravity of a superdense body would not allow them to frivolously soar in the atmosphere. It is believed that these are the remnants of planets recently torn apart and swallowed by a white dwarf.
“Recently” means “within hundreds of millions of years.” During this time, the “evidence” does not have time to completely go into the depths of the white dwarf.
What is interesting about these hot gases that were once planets? They give us a rare chance to understand what exoplanets are made of.
Usually all we have is the radius of the planet or its mass. One has to almost guess the composition, relying on the fact that there are no rocky planets the size of Jupiter or gas planets with the mass of the Earth.
Of course, the laws of physics give us good reason to think so. But verdicts like “gas giant” or “planet of solid minerals” can hardly be called an exhaustive description.
It is good when the mass and radius of the exoplanet are known simultaneously. Then you can calculate its average density without relying on assumptions. However, this is so rare that the European Space Agency specifically launched the CHEOPS telescope into orbit to remedy the situation.
And in any case, the average density does not say much about its composition. Even the mass fractions of the rocky core, hydrosphere, and atmosphere can vary widely. And there is no need to talk about such trifles as replacing sodium with magnesium or oxygen with nitrogen.
There is another possibility to determine the chemical composition of the exoplanet. Most of the known planets periodically pass between their sun and our telescopes (due to this, exoplanets are mostly discovered).
Then the rays of the star shine through the atmosphere of the planet through and through, and traces of its gases appear in their spectrum.
But the composition of the atmosphere says little about the composition of the surface. In addition, modern telescopes can only do this trick with giant planets or at least super-Earths. Meanwhile, of course, we are most interested in earth-like worlds.
This is where polluted white dwarfs give astronomers a chance. Indeed, it is precisely those atoms that made up the destroyed planets that fall into the atmosphere of the killer star.
Having found out the relative content of chemical elements, one can estimate what molecules and even what minerals the dead worlds consisted of.
Recently, scientists from the University of California at Los Angeles announced an interesting discovery. They examined the polluted white dwarf G238-44, located about 86 light years from Earth. Astronomers have processed data from several instruments, including the famed Hubble.
Thanks to this, they measured the content of nitrogen, oxygen, magnesium, silicon and iron in the G238-44 atmosphere. This led them to an interesting conclusion.
The high content of iron suggests that metal asteroids or planets with iron cores like the Earth fell on the white dwarf.
On the other hand, the abundance of nitrogen indicates icy bodies such as comets or Pluto. Moreover, we are talking about asteroids rather than planets: according to researchers, the total mass absorbed by G238-44 is less than the lunar one.
It is unthinkable to combine two such different compositions in one celestial body. All theories of planet formation claim that rocky asteroids are formed near the star, and blocks of ice – on the far fringes of the system.
True, these models are based on the structure of the solar system and in this sense can be biased. But in the end you can’t fool physics. Small icy objects like Pluto cannot form close to a star: they would simply evaporate under its rays.
Thus, G238-44 became the first known white dwarf, on which objects of both the first (“iron”) and the second (“ice”) types fell.
What’s interesting about this? First, it turns out that the chaos caused by the transformation of a star into a white dwarf affects even the outer edges of the system, where icy bodies live.
And secondly, evidence has been obtained that not only the Sun has an outer belt of icy bodies. Some other facts also speak of this, for example, observations of exocomets .
Note that some researchers attribute to comets an important role in supplying the newborn Earth with water and organic matter. It is possible that for the emergence of life, not only refractory bodies like the Earth are needed, but also such icy guests.
And if we are already firmly convinced of the existence of numerous rocky exoplanets, then the prevalence of “ice belts” is still in question.
Well, at least the G238-44 system seemed to have both ingredients. This inspires some optimism, unless we accidentally stumbled upon a very atypical system (which is unlikely).
Although, of course, one can only speculate if life ever flourished under the light of G238-44. And if it bloomed, it hardly survived the transformation of its sun into a white dwarf.
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