In all likelihood this is a product of the stellar forge. OK, obviously everything is a product of the stellar forge, but I'll explain what I mean.
Elite: Dangerous, to my empirical knowledge, appears to simulate stellar evolution and solar system evolution. Stellar evolution is how stars change in nature as they age; the most significant version of stellar evolution is what happens when the star begins to die and eventually dies, which is why we have giant stars and stellar remnants. Empirical evidence of this would be the existence of multiple red giant stars in a single system: if you're lucky to find one you'll notice they're very similar in age and mass because otherwise, they wouldn't be giant stars (one would still be main-sequence or have become a stellar remnant). There are many other tidbits of evidence but that isn't actually too related to the topic at hand.
Solar system evolution is actually much more complicated and still full of open questions. It's how orbital mechanics modify the solar system over the eras. Orbits aren't static and change over millions and billions of years due to orbital resonance, perturbations, and even disturbances from passing stars. Even our own solar system may have had a fifth gas giant that was ejected a long time ago. The game, to my empirical knowledge, simulates some aspects of this too. Planets of young stars appear to be more likely to have chaotic orbits. That is because not enough time has passed for the orbits to settle in a way such that some planets get ejected from the solar system. Since our lifespan is simply a snapshot of time compared to the age of the universe, stars, and solar systems, seemingly rare things can persist like this. Trojan moons and even trojan planets can form. Many explorers may have encountered a hot jupiter (a gas giant exceptionally close to its parent star). Scientists believe hot jupiters are a product of solar system evolution, where gas giants slowly migrate closer and closer to their parent star.
A specific aspect I've noticed the game simulate is captured planets. Perhaps after the game "generates" the solar system, or maybe even while the game is generating the solar system (we don't know how the game does this, only the end result, hence its empirical nature), the game may generate a captured planet. You can tell it's likely a capture planet because in orrery view, the planet is not in the plane of the solar system. Planets that formed with their parent star form from a protoplanetary disk, so they will all be in roughly the same plane when they finally "grow up" and become full fledged planets. Captured planets, originally being rogue planets, can come from any direction in a 360 degree sphere and get nudged into an orbit. Thus, they have no reason to be in the plane of the solar system. Another way you can tell is if the planet has an eccentric orbit. Most planets that form with the solar system have close to circular orbits (orbital eccentricity ~ 0) because they formed from a protoplanetary disk that is near circular. Small adjustments arise from post-formation orbital nudges and such (the other solar system evolution things I mentioned). Captured planets wander into the solar system with a wide range of speeds. As a result, they often have eccentric orbits. In your screenshot the planet has orbital eccentricity of 0.2061, so it is oval shaped. Some planets can have eccentricities nearly 1!Additionally, captured planets tend to, but are not required to be in very large orbits. Such planets appear to override the natural limits placed by the game on how far "natural" planets are allowed to generate. Sometimes they spawn fairly close, even as one of the inner planets. However, such orbits are not stable, and we are less likely to find them. Distant orbits are more likely because nothing can disturb them that far out. Lastly, since captured planets may "override" whatever forces generated "normal" planets, they are sometimes the only planet in the system other than stars. It appears that the game sometimes chooses to generate no planets in a solar system, but a captured planet spawns separately. As a result, if you find a solar system with a single lone planet, it is likely to be a captured planet, since if the game chose to generate "normal" planets, then in all likelihood the game would have generated more than one.
Finally, captured planets can be anything. They can be gas giants, high metal content worlds, water worlds, and yes, even earth-like worlds. It is unknown if the planet type is chosen before or after the orbit is decided by the game, since, once again, much of this is empirical. Boring planets are uncommon, but not very rare. However, finding terraformable water worlds and earth like worlds around O class stars is likely only possible if they are captured planets, because the goldilocks zone for these planets is farther out what the game can generate for "normal" planets. Great find CMDR.
