Shulou(Shulou.com)12/24 Report--

We know that the earth revolves around the sun for a year. Different planets have different times to circle the sun, so each planet has its own "year". A planet like Mercury, which is closer to the sun and has a smaller orbit, has less than 88 days in a year. (of course, the "sky" here is said according to a day on earth. On the other hand, for planets like Neptune, which are relatively far from the sun, the "year" on it is equivalent to 165 Earth years. If some asteroids with high orbital eccentricity are taken into account, their "year" will be even longer. For example, Sedna, an extraterrestrial object like Neptune, has an orbit period of 11,400 years, while the asteroid 2015 TG387 has an orbit period of more than 30,000 years!

However, the solar system is still too small after all. 36 light-years away, there is a planet (COCONUTS-2 b) that takes 1.1 million years to orbit its host star!

What is the concept of 1.1 million years? If the earth were this planet, it would still be dominated by Homo erectus at this time last year, and Homo sapiens hadn't even appeared yet. At this time "next year", after another 1 million years of evolution, we do not know how far human beings will evolve at that time.

1 million years is an extremely long time for most species on earth. It means that we can no longer feel the change of seasons, when you and I are like those short-lived insects, "summer insects cannot talk about ice".

As mentioned before, most exoplanets are discovered by the transit method and the apparent velocity method. In fact, these two methods do not see the planets, they are the observation of the stars to which the planets belong. That is, first identify the target star, and then look for traces of the existence of planets on it. Even in direct imaging, in most cases, you have to identify the star, then use a coronometer to block out the starlight, and then look around to see if there are any planets.

Apparent velocity method

Lingxing method

Direct imaging, but this planet was directly detected by a wide-field infrared survey satellite. This means that except for the weak light, the planet is not much different from the surrounding stars, so people did not know that it was a planet at the time.

Although in terms of energy alone, the energy output of this planet is more than 1 million times weaker than that of stars like the sun, it can be directly captured by infrared, indicating that it is still somewhat different from other exoplanets. For example, first of all, it is relatively close to us, only 36 light-years away.

However, Proxima b is only 4 light-years old, and it can only be found by the apparent velocity method. Is it reasonable that these 36 light-years can be seen directly?

This brings us to the second point, that is, it is big enough, otherwise there would not be so much light radiating out, right? It is estimated that this is a gaseous giant planet about the size of Jupiter but with a mass of 6.4 times Jupiter. So at the time astronomers thought it might be an isolated interstellar object, such as a sub-brown dwarf.

In 2021, however, researchers at the University of Hawaii found that the wild sub-brown dwarf seemed to have a gravitational relationship with another star (COCONUTS-2 A).

It is an M-type dwarf star (red dwarf) with only the mass and radius of the sun. It is between 1.5 and 800 million years old, so it is relatively young. There is a gravitational relationship between the two celestial bodies, indicating that they are either a planetary system of stars + planets, or a binary system composed of one large and one small. Whether it is "mother and son" or "partner", in short, these two celestial bodies have their own "families". It's just that the two family members live apart for many years, separated by 6,000 or 7,000 astronomical units (a full 0.1light-year), so that it takes more than 1 million years for them to circle each other.

The distance, coupled with the fact that the stars are not very bright, makes the "sun" actually seen above the planet not even as bright as some stars. This means that if you come to this planet, you will find that there is no difference between day and night at all, because the "sunshine" here is so weak. So in fact, what you see will always be a dark starry sky, and the so-called "day" is just a red "star" in the sky.

It is so far from the star that the temperature on its surface must be very low. After all, you can't receive much stellar radiation at all. In fact, however, the surface temperature of the planet is as high as 434K, or 160 ℃. By contrast, Jupiter, which is only five astronomical units from the sun, has a surface temperature of just over 100 degrees below zero. Why is that?

First of all, the mass of the planet must be a key reason. After all, despite the size of Jupiter, the planet weighs six times as much as the latter, enough to show how stressful the planet's heart is. Under high pressure, the interior can naturally generate more heat, so it is not surprising that the surface temperature is high.

Second, in addition to the relatively large mass, the way planets are formed may also be a reason. Given that the planet is so far from the star, researchers believe it may not have formed in the protoplanetary disk around the star. Because if it is formed in the circumstellar disk around the star, then throw it so far away, but can not completely shake off, this strength is really difficult to grasp, to put it bluntly, the probability is too small. So astronomers believe that the planet may have experienced a thermal start.

What is "hot start"? This brings us to the question of the formation of planets.

According to the traditional theory of planet formation in the protoplanetary disk around stars, planets formed in the protoplanetary disk are generally formed slowly by accretion. Dust first forms small particles by electrostatic adsorption, and then under the action of gravity, these small particles slowly gather into larger chunks of matter until they form a planetary core several times the mass of the Earth. The core then gathers more gas together in the form of an accretion disk, which is called "core accretion". The process of core accretion is so long that it usually takes tens of millions of years to form a giant planet.

But for hot start, the whole process is very fast. In this way, a large amount of gas and dust will collapse directly together due to the local instability of the circumstellar disk, eventually forming a huge celestial body. Because it does not experience the slow accretion process of an accretion disk, the gas does not lose much entropy and the resulting planet can have more heat, so this method is called the "hot start" of the planet.

There are two main sources of energy for thermal start-up: one is the heat released by the decay of radioactive elements in the rock, and the other is the gravitational potential energy released by the impact of matter with the surface of the planet during collapse. The planets formed in this way are often giant gaseous giants with larger sizes and higher temperatures. By contrast, the method of core accretion is called "cold start".

Although planets formed by hot start are usually gaseous giants, this rule is not absolute. In recent years, for example, astronomers have realized that Pluto, the dwarf galaxy of our solar system, may have been formed by thermal activation.

Astronomers now believe that there should be a global liquid ocean beneath Pluto's thick ice. Since Pluto is already located in the Kuiper Belt and is very far from the sun, it was previously thought that Pluto was a very cold ice pimple at the beginning of its formation. Then as the internal heat is released, Pluto changes from a "hockey puck" to a "water polo". Ice turns into water, which means that the volume of the planet shrinks. Then, with the passage of time, the planet gradually cooled, so the previous "water polo" was frozen into a "hockey puck" again. The water turns to ice, which in turn causes the planet to expand again.

But the problem is: on Pluto, we only find evidence of expansion of "water to ice", but never find evidence of contraction of "ice to water". So the researchers tried to use the "hot start" model to explain these characteristics of Pluto, and found that the model was in good agreement with the observed results. This suggests that Pluto may have quickly formed a hot "water balloon" by hot start, and then as it cooled, the surface of the planet gradually began to freeze, leaving only unfrozen oceans inside. These provide some evidence for Pluto's thermal start-up, whether from the fault structure of Pluto's crust, the thickness of the surface ice and the duration of the internal ocean.

However, whether it is cold start or hot start, the actual situation is often more complicated than expected. Cold and hot are relative, some "cold start" may not be so cold, some "hot start" may not be so hot. In addition, the differences between the two ways of planetary formation can be easily identified in the early stages of planetary formation, but with the passage of time, after thousands of years of evolution, the differences between them will become smaller and smaller until disappear.

Reference:

[1] https://en.wikipedia.org/wiki/(308933)_2006_SQ372

[2] https://en.wikipedia.org/wiki/COCONUTS-2b

[3] https://exoplanet.eu/catalog/coconuts_2_b--7920/

[4] https://www.hawaii.edu/news/2021/07/27/massive-coconuts-exoplanet-discovery-uh-grad-student/

[5] https://iopscience.iop.org/article/10.3847/2041-8213/ac1123

[6] https://beyondearthlyskies.blogspot.com/2014/08/giant-planet-formation-cold-start-vs.html

[7] https://www.nature.com/articles/s41561-020-0595-0

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

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