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

Can you imagine a fluffy planet like cotton candy? One might say,"Well, gas planets are mostly gas, right? "

As we said earlier in the Jupiter Landing issue, gaseous planets are more about the difficulty of defining where the planet's surface is than about the fact that the planet is mostly gaseous. For Jupiter, a few thousand kilometers below the clouds, it's already a mixture of gas and liquid, with liquid metallic hydrogen and a solid core below. So the average density of a typical gaseous planet, though not as dense as a rocky planet like Earth, is not as cotton-like as one might expect.

Jupiter's internal structures, such as Jupiter, have an average density of 1.326 g/cm³, and even Saturn, the lowest density in the Solar System, has an average density of 0.687 g/cm³. Yes, Saturn could theoretically float if you threw it into water. But the planet we're talking about today, with an average density of only 0.27 g/cm³, does look like a giant cotton candy.

In August 2022, an article published in Astronomical Journal reported that astronomers using TESS telescopes and ground-assisted observations found a very special wood-like planet near a star 590 light-years away.

The planet is designated TOI-3757 b. As can be seen from the number b, it is the first planet we have found around the star TOI-3757.

TOI-3757 is an orange dwarf star (K-type main-sequence star) slightly smaller than the Sun, which is already small enough to approach the size of a red dwarf star (M-type main-sequence star). And this planet is also the least dense exoplanet we've ever found near a red dwarf.

It is generally believed that large gaseous planets are difficult to form near stars such as red dwarfs. Because red dwarfs are small and have low surface temperatures, they have very long lifetimes. The red dwarfs in the universe today are like rebellious teenagers, very active and prone to flares. Therefore, it would be fine if we stayed away from it, but if we wandered in front of it every day, we would inevitably be affected. Over time, the atmosphere above would have been blown almost, so we could not form a particularly huge planet.

But this TOI-3757 b has the same size as Jupiter, but only a quarter of its mass.

Its parent star, an orange dwarf, has a mass and radius equal to at least 0.6 suns. With this planet, it is less than 0.04 astronomical units (AU) away from the star, about 1/10 of Mercury's distance from the sun, almost a face-to-face flight. So why does it have so much atmosphere at such close range?

First, the researchers wondered: Could the planet be so close to the star that it received too much heat and was "baked" to swell?

There is some truth to this statement, and it is also the reason why hot Jupiters are generally less dense. Because some of the star's heat goes deep into the planet's atmosphere and causes it to expand through various mechanisms. But the researchers observed that the planet was not hot enough to expand to this extent, so sunlight was not the main cause of the planet's expansion.

Then the researchers wondered: since this star was originally observed by transits, if it is lying on its axis like uranus, is it possible that we are thinking of its planetary rings as the planet itself?

Uranus For a planet so close to its star, it is likely to be tidally locked. So if it has planetary rings, the planetary rings must be in the same plane as the orbit. Even if there is a certain angle, this angle is not too large. So from our perspective, even if the planet has planetary rings, it's a thin line, with very limited obscuring of the star's brightness. So this explanation is unlikely.

Elemental analysis later revealed that the planet was very low in metals (that is, elements other than hydrogen and helium) compared to other gaseous planets near red dwarfs. The low metal content means that the protoplanetary disk from which it was formed dissipates easily, and even if there is still material left, the accretion process for a low-metal planet is very slow. To put it bluntly, the planet ate slowly at the beginning of its development, and the surrounding food was blown away by the stellar wind before it could eat much. In the end, the planet weighed only 1/4 of Jupiter. Low mass means weak gravity, and weak gravity means loose internal matter, which leads to today's low density results.

In addition to this low-metal explanation for planetary accretion, the researchers offer another plausible explanation: although the temperature of the planet's surface indicates that it is not receiving enough heat from the star. But if the heat comes from within rather than from outside, it could also affect the size of the planet. But the problem was, for this planet, it was already relatively old. Logically speaking, the core should have cooled down. Where did the heat come from?

At this point, researchers found that the planet's orbit eccentricity is relatively high. Such elliptical orbits heat the planet's interior by tidal action, which may be one reason for the expansion. However, it is not clear how big this impact will be. More detailed studies of the planet's internal structure and more accurate estimates of orbital eccentricity are needed to know.

TOI-3757 b isn't the least dense planet yet, as astronomers discovered two extremely fluffy planets in 2012 near Kepler-51, about 3,000 light-years away. Kepler-51b has a radius 0.6 times that of Jupiter but a mass equivalent to only two Earths. The other Kepler-51c is even more exaggerated, with a mass of only 4 Earth masses, but a radius of 0.8 times Jupiter and a density of less than 0.1 g/cm³. Perhaps this is the real "gaseous planet."

Kepler-51c Mass and Radius In addition, the planets around Kepler-51 (and the stars themselves) are very young. If the 4.5 billion-year-old Solar System is middle-aged, Kepler-51 is still a 500 million-year-old child. Plus, these planets that are puffier than cotton candy are dissipating their gas so rapidly that over the next billion years their puffy bodies will slowly become slimmer and eventually Neptune-like.

Kepler-51 Finally, for these hot Jupiters very close to the star, there is no life, especially today's TOI-3757 b, a low-metal planet. After all, whether it is carbon-based or silicon-based, it is useless to say that there are not enough raw materials.

This article comes from Weixin Official Accounts: Linvo Says Universe (ID: linvo001), Author: Linvo

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