"Bright moonlight before bed, suspected frost on the ground". We know that although the moon is called moonlight, this light is not emitted by the moon itself, but reflected sunlight. The same goes for planets. Although the moon looks bright, that's largely because it's so close to us, not because it reflects light. The moon's albedo is actually very low, only about 10 percent.

The least reflective of the solar system's eight planets is Mercury, which, like the moon, lacks an atmosphere, with an albedo of less than 9 percent. Other planets are not too reflective if they have an atmosphere at all. Like Earth, its albedo is about the same as those of the gaseous planets, around 30%. Jupiter is a little bigger, 50 percent. But Venus has the highest albedo. Thanks to its thick atmosphere and unique sulfuric acid clouds, Venus has an albedo of 76 percent! So it can be said that Venus is the brightest object in the sky after the sun and the moon.

In order for a planet to be "the prettiest", in addition to its appearance (high albedo), it must also be close enough to its star. Venus, for example, not only blows away all its competitors in albedo, but it's also in a very hot relationship with the sun, just 0.72 astronomical units away from the Sun (3/4 of the distance from Earth), second only to Mercury. So the brightest planet outside our solar system, it must also be very close to its host star.

In 2019, astronomers discovered a rare planet called LTT 9779 b (TOI-193 b) next to a star 264 light-years away. According to the transit method, the planet is very bright, with an albedo of 80 percent, higher than Venus. And sure enough, it's very close to its host star, only 1/42 of the distance from Venus to the Sun (0.017 astronomical units). So close to the light source and so reflective, you can imagine how bright it must be.

The planet is a gaseous planet with 29 Earth masses and 4.6 Earth radii. Given its size and density, it is classified as a Neptunes object. This object is rare not because it has a high albedo or because it is a Neptane-like object (a third of all confirmed exoplanets are Neptane-like objects). It's rare because it's too close to its host star for a Neptune object to be here at all!

Normally, planets that fly close to their stars are either huge gas giants (such as "hot Jupiters") or rocky planets about the size of Earth. Because if you're not a flesh shield like the former, you'll be eaten and stripped by the stars in a very short amount of time (say, 100 million years), leaving you with a small solid core.

This is especially true when it comes to young stars. For example, the planet's host star (LTT 9779), which is about 80 percent the size of our sun, is also a G-sequence star. But compared to the sun's stately 4.6 billion year old "middle-aged uncle", the star is still a "young guy" less than 2 billion years old. When faced with a young star with very strong radiation, it would be almost impossible for any planet the size of Neptune to lock in its outer atmosphere by its own gravity. Its hydrogen and helium should have been stripped away, leaving it with a bare rocky core.

Look directly at the graph of planetary radius and orbital period, its ordinate is the planetary radius (unit: Earth radius), and its abscissa is the orbital period (unit: day). It can be seen that very close to the star (the orbital period is very short), there are basically planets one or two times the radius of the Earth; At slightly greater distances, large gas giants can be stable; And the Neptune-like objects in the middle, they're mostly farther away. Neptune-like objects are rarely found in the triangle, so this region is also known as the "Neptune desert."

But the planet in question (the pentagram in the picture) is one of the few examples of a "Neptune desert." Because it is so close to its star, it has a very small orbit, going around the star in 0.8 days, which means that a "year" above it lasts only 19 hours.

This close to the star, the planet's surface temperature must not be cool. Yes, its equilibrium temperature is nearly 2000K, which is close to the surface temperature of a red dwarf, so it is also called Ultra-hot Neptune. So the question is: how can a tiny, gaseous planet, dominated by hydrogen and helium, hold on to its atmosphere at such extreme temperatures?

Some scientists have speculated that the planet may have been a Jupjup-sized giant before it was stripped of its material by its star, leaving it with a body the size of Neptune. But it's hard for a giant planet to lose that much mass in a short period of time with stellar winds and hot baking (light evaporation) alone. So the planet may also be experiencing other ways of outflowing material, such as a Roche Lobe Overflow (RLO).

In a paper published in October 2023 in the journal Monthly Royal Astronomical Transactions, researchers looked at X-rays from the planet's host star using the XMM-Newton space telescope. They found that the star was actually much softer than we expected. Not only does it have an unusually slow rotation, but the X-rays it emits are not nearly as strong as expected, only 15 times as strong as its peers. Well, I thought he was a spirit boy, but I did not expect to be a weak scholar. Weak stellar radiation may be one reason the planet is able to maintain an atmosphere.

Now the question is: as a hot Neptune, what explains its 80 percent superhigh albedo? The gas planets in our solar system have, at best, 50 percent the albedo of Jupiter. With such high reflectivity, there must be something special about this planet, and its atmosphere may be hiding some secrets.

Fortunately, the planet is not too far away (only 264 light-years), and with the help of space telescopes with infrared capabilities, we can see what's in its atmosphere through the transmission spectrum.

Silicates are basically things like stone, sand, and glass, and rocky planets like Earth are basically made of silicates. Depending on the composition, the boiling point of silicates is generally more than two thousand degrees (or even more than one thousand degrees for glass). Given the planet's equilibrium temperature of nearly 2,000 degrees, it really could be vaporized if it had any sand on it. But that's not all. In addition to these silicates, scientists have found that the clouds also contain the metal titanium. In other words, the surface of the planet is covered with a layer of "titanium sand cloud", no wonder the reflection ability is so strong, together with the whole planet is a big mirror.

1. Although it is close to its star, its host star is very weak in X-rays and its stellar wind is not strong;

2. The metal content of the planet's atmosphere is very high, which makes its entire atmosphere very heavy and difficult to blow away;

3. The high albedo caused by the metal cloud blocks most of the star's radiation, which also prevents the planet from overbaking.

These reasons seem plausible so far, but the mystery of this super-hot Neptune is only tentatively solved. It may be observed in more detail by the JWST in the future, in the hope that more evidence will help solve the mystery.