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Last time, we introduced an exoplanet Kepler-70 b, which was eaten and vomited by a star. Its surface temperature is even hotter than that of the sun. We have seen the hottest planet. How cold can the coldest planet be?

Most of the exoplanets we found previously have hot surfaces, especially hot Jupiter, which is very close to the star. But in 2006, an article published in the journal Nature said astronomers had discovered an ultra-cold exoplanet (OGLE-2005-BLG-390Lb) near a red dwarf.

In addition to its well-known longevity, red dwarfs are also notoriously low in temperature, usually at two or three thousand degrees Celsius. So the planets around it must get less heat, and if you add in the distance from the star, the temperature will not be high.

However, temperature is very special, and it is hard to imagine the difference between them in high temperatures (such as tens of thousands, hundreds of thousands or even hundreds of millions of degrees). But the low temperature is very straightforward, after all, absolute zero (0K) is there, at most a difference of one or two hundred degrees Celsius, no matter how low it is. The reason for this feeling may be that as carbon-based creatures, we already live near the lower temperature limit.

So we draw a habitable zone for each star, and the outer edge of the habitable zone is called the "snow line", which means that volatiles such as water and ammonia can only exist in solid form.

Of course, this is just a general concept, just like the equilibrium temperature of the planet last time, rather than the average temperature in reality. For exoplanets, conventional methods are difficult to observe when they are outside the snow line, but fortunately we have other methods, such as gravitational microlenses.

Gravitational microlens is an indirect method to detect exoplanets. The gravitational lens effect should be familiar to everyone, that is, when light passes by a massive celestial body, it is bent under the gravitational influence of that object, like a magnifying glass. The same is true of gravitational microlenses, except that the mass of the object that produces gravity in this case is relatively small (usually a constant star), and the lens effect it can produce is relatively weak, and it usually does not see an image like the "Einstein ring". It just causes a sudden change in the brightness of the background star, or a slight temporary dislocation. This method is often used to detect interstellar stray objects, such as the previously introduced stray planets.

Although the gravitational microlens can help us find some exoplanets beyond the snow line, Einstein's ring also has some drawbacks. For example, except for the star-to-planet mass ratio, parameters such as specific mass and distance cannot be determined directly and can only be estimated by other methods.

In 2005, astronomers suddenly detected a gravitational microlens event 20000 light-years away near the center of the Milky way. Later, confirmed by several telescopes, the researchers believed that the event was caused by a red dwarf star and a low-mass planet around it.

This is an exoplanet about 5.5 times the mass of Earth, 2 or 4 astronomical units from the parent star (about the distance from the asteroid to the sun). We know that the asteroid belt is already very cold, a full minus one or two hundred degrees Celsius. The planet's host star is also a red dwarf star 0.22 times the mass of the sun, which means that the planet receives only 0.1% of the radiation received by the earth. So astronomers speculate that its surface temperature may be less than 50K, or less than-220 ℃, making it one of the coldest exoplanets known.

If the planet were a gaseous planet, it would be an ice giant like Uranus and Neptune, but smaller. If it is a rocky planet, its surface is likely to be covered with thick ice. It's just that what makes up the ice may not be water, but ammonia, methane or even nitrogen. Such a planet might look similar to Europa and Titan, and if it had another molten core, there might still be liquid oceans under the thick ice. As a result, although the surface temperature of the planet is very low, it is not entirely impossible for life to evolve.

Europa's structure (concept map) in fact, the focus of the discovery of this planet is not on how low the temperature of the planet is, but on changing people's previous perception of exoplanets. Previously discovered exoplanets are mostly large gaseous giants, but this time gravitational microlens astronomers predict that planets with the mass of sub-Neptune may be more common around ordinary stars than large gaseous giants.

Sure enough, in the following decade, more and more exoplanets like Neptune and Super Earth were discovered. So far, these two have accounted for more than half of the exoplanets identified.

In 2016, with the help of gravitational microlenses, astronomers discovered a colder exoplanet (OGLE-2016-BLG-1195Lb).

This is an exoplanet called "hockey" with a surface temperature of only 33K (- 240℃). According to the latest data from 2023, its mass is about 10 times the mass of the earth, which is consistent with the distance from the parent star and the distance from our earth to the sun, that is, one astronomical unit. Since it's not too far away, why is the planet so cold?

Yes, because its host star is too cold.

The star is so small that it is only 1/10000 of the brightness of the sun that scientists are not sure whether it is a star or not. Because its mass is only 7.8% of that of the sun, it is likely to be just a brown dwarf that failed to ignite.

In addition, as I said last time, the reflectivity of the planet also directly affects the surface temperature of the planet. The planet has an extremely high reflectivity, about five times that of the moon. There wasn't much light, but it went out the other way around. Taken together, these factors make the surface of the planet extremely cold.

As I said at the beginning, after all, absolute zero is there, and extremely low temperatures are relatively common compared to extremely high temperatures. Such extremely cold places can actually be found in our solar system.

Such as Pluto, a dwarf planet in the Kuiper belt. Because it is 30 to 49 astronomical units from the sun, it takes several hours for sunlight to reach here, and it is conceivable that the amount of solar radiation it can receive is extremely limited. On the surface of Pluto, where the average temperature is only 44K (- 229 ℃), liquid nitrogen has to be frozen into lumps, and most of Pluto's surface is made up of frozen nitrogen ice.

In fact, if Pluto does not take into account the average temperature and laboratory environment, you may have no idea where the lowest temperature of the solar system is. It is in a place very close to us-the moon.

We know that due to the lack of atmosphere, the temperature difference between day and night of the moon is very large. Direct sunlight can have hundreds of degrees Celsius during the day and minus one or two hundred degrees Celsius at night. In this case, extreme low temperatures may exist in places that are not exposed to sunlight all the year round, such as polar craters.

At the bottom of some craters in the polar regions of the moon, it is unable to be exposed to the sun and has been in an extremely cold and dark environment. The probe previously measured an extreme temperature of 26K (- 247 ℃) in the Hermit crater in the North Pole, making it the coldest known place in the solar system.

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

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