Shulou(Shulou.com)11/24 Report--

Thanks to CTOnews.com netizens for the clue delivery of the holy Buddha! We know that it has been 13.8 billion years since the birth of the universe. During these 13.8 billion years, countless planets have been formed in the universe. Some of these planets have just been born in the protoplanetary chart, some are in the prime of life like Earth, while others have long since vanished with the death of their host stars. So which is the oldest planet that has been found among the living planets?

These stars visible to the naked eye in the night sky are usually massive stars at least several times the mass of the sun. We know that the larger the mass of the star, the shorter the lifespan, so for these massive "short-lived ghosts", even if there are planets around them, they will not be too old.

So red dwarf stars with small masses can be called "as long as the sky", and their lifespan can usually reach tens of billions, hundreds of billions or even trillions of years. Is it possible for the oldest planets to appear around red dwarfs?

As mentioned before, the primary stars in the universe should be supermassive stars without metal elements, and it is clear that red dwarfs should not have appeared so early. This has also been confirmed in the observation that almost all the red dwarfs observed are rich in metals, indicating that the materials that make up them are not without the participation of the previous generation of stars.

In addition, the primary stars do not contain metal elements, which is very detrimental to the formation of planets. According to the traditional theory of planet formation, a planet needs to have a metal core before it can form a planet large enough by rapidly increasing its mass by accretion. So not to mention metal-free stars, even metal-poor stars, the planets around them are very rare.

So where would the ancient planets be?

In fact, the conjecture just now is generally correct: for planets to be old enough, stars cannot be young. So where to find older stars, globular clusters can be found.

Known as a "stellar nursing home", the stars in the globular cluster are usually tens of billions of-year-old people. Some star clusters themselves are even older than their host galaxies, just like some old people in life, who often witness the rise and fall of their towns.

But whether they are massive stars or second-generation stars with relatively small masses, they will eventually come to an end over time. Once the host star experiences a supernova explosion, the planets nearby want to survive. It's not impossible.

In 1993, astronomers found an obvious anomaly in the rotation rate of a pulsar in a binary system in the globular cluster M4 in the constellation Scorpio.

After calculation, the researchers believe that the pulsar should have more than one companion star, and there may be a second companion star at a greater distance. Given that the mass of the second companion star is very small, perhaps only about 1% of the mass of the sun, this means that it may be a brown dwarf or a planet.

The pulsar was originally named PSR B1620-26A, and its companion name is the same, except that the suffix is changed from A to B (PSR B1620-26B), so the suffix of the second companion star is named c (PSR B1620-26c). Later, it was found that the companion star with the suffix B had a mass of only 0.34 times the mass of the sun, so it was not a neutron star at all, but a white dwarf star, so it was renamed WD J1623-266. The little guy with the suffix c is classified as one of their planets, PSR B1620-26 (AB) b.

As can be seen from this name, the planet revolves not around pulsars (A) or white dwarfs (B), but around both of them (AB). Yes, this is a ring double star system!

In any case, since almost all the stars in the cluster were born at the same time, and the planets are usually formed with the main star, the age of the planet may be as old as the cluster.

Globular cluster M4 is expected to be around 12.7 billion years old, so if the planet is as expected, it should also be around 12.7 billion years old, born at dawn 1 billion years after the Big Bang.

As the oldest planet ever discovered, it is called Methuselah (Methuselah) by the oldest people in the Hebrew Bible.

Note: this and the "Methuselah" which is "older than the universe" are two different things. That Methuselah is a star (HD 140283) with an estimated age of about 14.4 billion years. However, because of the great uncertainty of the numerical value, it does not conflict with the age of the universe.

Going back to the planet Methuselah, its origin is still uncertain, and studies suggest that it probably formed not in the place where it was discovered today, but with a star, such as the predecessor of this white dwarf, captured by a pulsar.

As mentioned in the previous pulsar planet issue, a pulsar planet is usually either newly formed around a pulsar, or happens to be captured by a pulsar, or may even evolve from a companion star.

Astronomers' guesses about the planet were more complicated, perhaps at the time:

A planet forms along with a star on the periphery of the core of the cluster. As time goes by, slowly they begin to migrate to the core. Later, they met a pair of neutron star binaries, and then the alien star joined the binary system with its own "child" (that is, the planet) as a third party, and then in the unstable three-body motion, the "original" of the neutron star was kicked out, so the original neutron star binary became a neutron star, a main sequence star, and a planet.

Then, over time, the main sequence star gradually expanded into a red giant, and its outer material began to be stripped off by the neutron star. A characteristic of a neutron star is that the larger its mass, the smaller its size. So as matter falls into the neutron star, the neutron star becomes smaller and smaller and rotates faster and faster until it takes only a few milliseconds to rotate around, that is, a millisecond pulsar. The red giant next to it eventually became a white dwarf after a later explosion. As for the planet brought by the main sequence star, it is now orbiting the neutron star and the white dwarf star.

But there is a problem with this explanation, that is, globular cluster M4 is a very low-metal cluster. As I just said, according to the traditional theory of planet formation, it is difficult for planets to form directly in such a low-metal environment. So someone gave another guess:

Another binary system orbiting each other, the difference is that the two stars are so "close" (so close) that in the later stages of evolution, one star expands enough to wrap the other in. Then, with the loss of material, an astrolabe was slowly formed on the periphery of the two. Then, under the action of the stellar wind, the astrolabe even extended to more than 100 astronomical units.

At this time, a star passes by, and under the gravitational disturbance of the star, the matter in the astrolabe gradually begins to gather, and a planet is born. This formation process does not require a metal core as a prerequisite, so planets can theoretically form in globular clusters that lack metal elements.

Either way, the age of the planet will not be too young. at present, it has revolved around the binary star for tens of billions of years, so will it continue to rotate like this in the future until it is old?

Because the system is still slowly moving towards the core of the cluster, as the star density continues to increase, hundreds of millions of years later, when the system approaches a star again, because low-mass objects are more likely to be thrown out, so the future fate of the planet may break away from the globular cluster and eventually become a wandering planet in interstellar space.

references

[1] https://en.wikipedia.org/wiki/PSR_B1620%E2%88%9226_b

[2] https://en.wikipedia.org/wiki/PSR_B1620%E2%88%9226

[3] https://en.wikipedia.org/wiki/HD_140283

[4] http://exoplanet.eu/catalog/psr_b1620-26_(ab)_b/

[5] https://www.nature.com/articles/365817a0

[6] https://iopscience.iop.org/article/10.1086/307771/

[7] https://academic.oup.com/mnras/article/355/4/1244/992851

[8] https://www.science.org/doi/10.1126/science.1086326

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

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