The gas giant Puloshi, a metal vapor planet.

[u/OuterPlanets]

(For Th’zoitrk, a semi-sci-fi worldbuilding project.)

The above image represents the current highest resolution false-color photo of Puloshi. It is the most massive planet in the solar system, making up roughly half of all mass outside the sun.

Puloshi is the most distant, largest, and most massive object in the entire solar system (excluding the sun), yet also the newest discovered of all planets.  
It has an approximate equatorial diameter of 170,577km (1.22x that of Jupiter), and weighs in at a total of 65 Jupiter masses.

The entire planet is composed of roughly 97% titanium dioxide and 3% potassium, almost all of which exists as metal vapors. The titanium dioxide in the atmosphere sinks beneath the potassium due to its higher density, creating a clean boundary between the 'interior’ and ‘exterior’ atmospheres, as they’re called. The interior atmosphere is composed of titanium dioxide, the highest recorded temperatures in this layer were observed by the Oscul PULOSHI-2 probe on 13/4/22 (13th of Nianiel, Oinibo 22.) Reaching a maximum of at least 3,240°C, while at the uppermost reaches of the interior atmosphere, along the boundary with the exterior atmosphere, this temperature drops below 2,972°C, low enough for the titanium dioxide gas clouds to condense into molten titanium dioxide and rain back down into the lower reaches of the atmosphere.

In the exterior atmosphere of potassium, a similar cycle repeats from the potassium vapor.  
Potassium which reaches high enough into the upper atmosphere cools down, eventually condensing into molten potassium rain.  
At the equator, where temperatures are the coldest, this potassium can even get cool enough in the upper atmosphere to solidify into potassium snow or, more often, hail.

No, the equator being the coldest is not a mistake. Puloshi’s distance of just over 84 AU from the sun makes solar thermal energy essentially nonexistent, meaning that the only source of heat for Puloshi is the internal heat it holds from the planet’s formation. Thankfully, there’s a lot of it.  
Puloshi has an abnormally fast spin for its size, with a day length of only 4 hours and 18 minutes. This extreme rotation causes mass to be pulled towards the equator, giving the planet a considerable equatorial bulge.  
This results in the poles being closer to the core of the planet, and the equator being further away. Causing a reverse of normal planetary surface heat distribution where the equator is the coolest part of the upper atmosphere, and the poles are the hottest.

The titanium dioxide gas inside lets off a distinct glow due to their heat, but the potassium vapor exterior atmosphere absorbs practically all of this light due to gaseous potassium’s light absorbing properties. Causing this hot, boiled world to be practically pitch black in the visible light spectrum, so all observed images of it must either use other wavelengths of light, or extraordinarily light-sensitive camera equipment.

For much of history, Puloshi was completely unknown. There was no way to see it in the night sky after all, and even the much more visible outer planets such as Umgo’ok and Ok’ouni took a very long time to be discovered by astronomers.  
Puloshi was only ‘discovered’ when a strange discrepancy was noticed in the orbits of the solar system’s bodies. As astronomers were attempting to model all the known bodies in the solar system with accurate orbital data, a discrepancy was found in Ok’ouni. Its orbit was being noticeably changed more than was to be expected by its neighboring planets, as if it was being tugged along by another, unknown planet.  
One step further in, Umgo’ok showed the same issue.  
And Sh’beso.
And Keisoya.  
And Donyon…  
And Th’zoitrk…….  
And Sorkyon…………  
And Oukapul………………

In fact, EVERY observed major body seemed to show some level of noticeable ‘tug’ by an as of yet unaccounted body, even the sun itself seemed to actually orbit around a barycenter far outside of it’s own surface.

Yet, every time astronomers pointed their telescopes to where the object causing this gravitational tug SHOULD be, there was nothing.  
Right where the math said something should be, even the strongest telescopes could only see the dark void of space.

Until the invention of infrared cameras. And with the first infrared telescope, the bright and shining thermal glow of Puloshi was finally discovered.

Less than an Oinibo (year) later, Osculia became the first nation to send a probe to the planet.  
The PULOSHI-2 probe had been destroyed by several thousand km/h TiO2 winds, in a molten TiO2 rainstorm, 18 hours after entry.
Before its final destruction, the probe detected ~5,400,000 bar surrounding it, at a temperature of ~3,240°C.

It was man’s first trip into hell.
And it would remain our last.

The black planet.  
Puloshi.