"Unfortunately for sky-scryers, Oblu is not very photogenic from orbit. This world could best be described as a temperate Venus, a planet that, like Earth's evil twin, is cloaked in a thick, crushing sea of carbon dioxide and suffocated beneath a runaway greenhouse effect. Unlike Venus, however, Oblu is not heated nearly so intensely by its dimmer stars, and although the pressure at the surface would still crush a human like a walnut under a triphammer (thank you, Sir Arthur Conan Doyle!), the temperature is well below Venus' Hadean temperatures, with an average of about 40˚C. Still too hot for humans or most other multicellular Earth life, of course, but below boiling--and that's what counts.
To really do justice to it, I'd need to do a separate post entirely devoted to Oblu, so I'll leave off with that extremely brief summary, pausing only to leave an exercise for the reader. For insight into one of Oblu's chief oddities, I recommend looking up the phase diagram of carbon dioxide, the chief component of Oblu's atmosphere, and then...thinking about it, and what I said above, for a bit."
A bit coy, I 'spose, but the post was dragging on and I didn't really have time to write a full exploration of Oblu. The "oddity" to which I was referring was that, at the temperature and pressure described, carbon dioxide ceases to behave as a gas, transitioning into a supercritical fluid. Supercritical fluids are a phase of matter with properties similar to both gases and liquids, expanding to fill containers like a gas but able to dissolve other materials like a liquid. The result of this, on Oblu, is that the entire surface is covered by an atmosphere/ocean of supercritical carbon dioxide, with oxygen, water, nitrogen, and a small amount of salt dissolved into the fluid and the life of Oblu living within this alien sea of not-quite-gas and not-quite-liquid.
Now, in my Xenoastronomy post, I included a picture of both Gavanna and Oblu. As noted in the quote above, Oblu is not particularly interesting when viewed from on high, but perhaps once one sinks beneath the all-obscuring clouds into the carbon dioxide fluid at its surface, its beauties will resolve themselves a bit more clearly:
Hm. Or, um, maybe not. There are a few shapes roughly visible: some sort of rugose, coralline growth off to the lower right, with a little thingumbob attached to it that might be a growth of the coral-thing, or might be some other organism entirely. In the background is a...thing, multi-stalked with some sort of bulbous shape perched atop, and a few indistinct patches of light further off. Other than that, all is unclear.
The reason for this is simple. Oblu's clouds are far more effective at shielding light from the surface than even those of Venus, leaving the planet's surface in a permanent starless pseudo-night. Complicating matters further is the carbon dioxide fluid covering the surface of the planet; although it itself is perfectly transparent, Oblu is a very wet world--and liquid water is only slightly soluble in supercritical carbon dioxide, and not all that much denser. The result is an emulsion of minute water droplets, drifting through the lower atmosphere/ocean in a dense fog that quickly scatters the few photons of visible light that manage to find their way down to the surface.
Despite this, Oblu is a rich, living world, full of its own plant and animal-equivalents, and many of the latter are extremely active creatures, bounding and leaping through their inky world with a boldness and surety that only sighted animals have any right to possess. Their senses are obviously up to a task that our poor eyesight, confined to a tiny fragment of the full electromagnetic spectrum, falls far short of. Let's look at this same scene, then, but through the eyes (or equivalent thereof) of one of the native inhabitants of Oblu:
So, what exactly are we looking at here? The native animals of Oblu ("Animal" is used here in the sense of "mobile heterotroph," by the way, not in the sense of "member of the phylum Animalia") are incapable of seeing in visible light and all other wavelengths, except for radio waves and low-energy microwaves. They are, however, exceptionally sensitive to pressure waves (read: sound), and have organs similar to the lateral lines of Earth's fish, letting them detect slight disturbances in the fluid around them. They share another similarity with fish in that they are also capable of detecting electrical potential in the slightly-conductive (thanks to the low but nonzero concentration of salts dissolved in the fluid) carbon dioxide atmosphere/ocean. The combination of these senses gives us the picture above: Color here represents different wavelengths of radio and microwave light, with red being long wavelengths (radio, low energy) and blue short wavelengths (microwave, high energy). The saturation of the colors indicates the charge density; a faded out, near-gray object has very little net charge/static charge separation, while a vividly-colored, fully-saturated object is either very strongly charged or has an overall neutral charge, but with many isolated patches of strongly positive and negative charge scattered across its surface. Finally, white highlights indicate pressure waves, and turn up wherever something is moving or making noise, "illuminating" its surroundings with sound.
...At this point, I started to write out a rather involved paragraph examining the various features of the landscape, but things got far too complicated far too quickly; the picture above is, after all, an alien world seen through alien eyes, and there's precious little in the way of a common reference frame for we Earth-dwellers. Rather than go into an insanely detailed description of what every little shade of color, gradation of vividity, and flash of white mean, then, I'll just give a general description of the scene. If y'all have any questions about specifics ("Why is most of the mountain in the background red, but a small portion both rainbow-hued and highlighted?" "What's with the wonky clouds?" "Why on Earth did you go to the trouble of drawing a completely incomprehensible image when everyone would have gladly forgiven you for waving the wand of Artistic Convenience and rendering Oblu's surface well-lit and clear for the purpose of the illustration?"), then heigh-ho for the comments section, and I'll answer 'em as best I can without boring those who may not be quite as interested. Sound good? Excellent.
Okay, background. The plant-equivalents on Oblu (distantly related to our fungi, and referred to collectively as "gume") are not photosynthetic but electrosynthetic; that is, they rely on electrical discharges for their energy. As the entire planet is covered with what is effectively one gigantic thunderhead, this is a more effective strategy than it would be on Earth, and most gume have evolved organic lightning rods rather than leaves, which they rear up as near to the top of the atmosphere/ocean of carbon dioxide as possible. Some of these gume are tree-like, growing up as a single spike, but others grow outward and upward in a vast, flimsy, spongy mass--a strategy which takes longer to pay off but yields far greater dividends, as it allows them reach greater heights than the "trees," building themselves up into bulging sponge-mountains. One of these organic mountains is visible in the background, with the red spikes atop it being its lightning rods, reaching up for the skies (the spikes on the middle peak have just been hit, and are blue in this picture because the sudden jolt of electricity and the rapid pulse of current have caused them to emit higher-energy light. Note that the "lightning" strike doesn't look particularly lightning-y, as the electricity is flowing through a conductive medium, and is thus not confined to single, jagged paths like lightning in our own insulating atmosphere).
The plain beneath the mountains is the de facto surface of Oblu, a "sea" of microorganisms with a gel-like consistency. Similar mats of of microbes covers most of the planet's surface, and are detritivores, absorbing any dead organisms that fall to their surface--or living organisms, for that matter, if they stay still long enough. Unlike the similar layer of bacterial slime at the floor of Earth's oceans, these bacterial seas function as meta-organisms, funneling nutrients to different parts of the sea as needed and engaging in vast, slow chemical warfare with neighboring bacterial mats. Detritus is not the only source of nutrients for these huge collectives, as some are capable of harnessing geothermal energy and actively tap immense amounts of energy from what would be, in other circumstances, active volcanos. None of the inhabitants of Oblu or Gavanna have quite managed to figure out whether these bacterial mats are capable of thought; they occasionally develop intricately connected webs of conductive "nerves," theoretically capable of immensely sophisticated thought, but as these neural nets tend to dissolve back into the ooze within a few Oblu days (a month or so of Earth time) they don't seem to be used for any permanent thinking.
Finally, the animals of Oblu, referred to as ghembals (I named 'em some years ago, when I had only recently learned the word "gimbal." I regret nothing). With the exception of some very specialized fliers, all ghembals are spherically symmetrical, with no defined up, down, left, or right. They are often covered with long, retractable limbs, each bearing a flattened pad-foot at its tip with a mouth in the center of each pad, and with conductive fibers running the length of the limb, used to detect radio and microwaves when fully extended. They move not by walking, but by rolling; a ghembal pushes itself forward by bending in the direction of motion, starting to fall, and then extending limbs from its "front" to catch itself while retracting its "hind" limbs. The extent to which the limbs are retracted or extended varies; rapidly-rolling species (like the large critter seen at central-left) tend to have very long limbs, while slower-moving, smaller ghembals (like the little spherical beastie to the lower-right) may make do with much stubbier appendages. Ghembals have managed very well with this mode of locomotion, adapting it to "running," cernuating (The word "cernuation" is courtesy of a fellow who goes by Sigmund Nastrazzurro, who has been worldbuilding since before I was born. I highly recommend investigating his blog, which is a never-failing fountain of speculative biology delights), and even flight (a form of modified cernuation, with the flying ghembal rapidly inflating and deflating hydrogen bladders at the ends of its limbs and swinging from them like a gibbon swinging from one branch to another, with air resistance holding the inflated bladders in place while the ghembal swings beneath. It obviously doesn't work too well in strong winds, so ghembals that fly using this method tend to stick to the porous interior of gume tangles, where wind isn't an issue).
That's Oblu life in a nutshell, then. Only one other thing really needs noting; just as on Gavanna, there are Elu on Oblu. The large ghembal in the center-left of the picture is an Elu, as it happens; the vivid green claw-teeth on its limbs are intentionally garish, meant to broadcast its presence and to signal that it's harmless and safe to approach. Despite their presence, I've never written any short stories focused specifically around an Elu from Oblu. It's hard enough for me to write the completely alien mental environment of Gayenni Elu; adding on a completely alien sensory and physical environment has, hitherto, been a bit too much of a challenge for me. Ah, well. Someday.