Vulcan is a former rogue planet that, with human intervention, was moved closer to Sol in 7021 and became the new first planet as well as the most massive and densest planet in the system, passing Earth.
Vulcan was once a rogue frozen R-Class object hurtling through the universe until its path was determined to crash into Europa. As Europa was the homeworld of the at the time still reclusive Kya, the Sol System planetary government decided to alter its course so that it would become the new first planet of the Sol System and named it Vulcan in honor of the hypothetical planet of the same name.
Vulcan is thought to have formed around a particularly violent star that would eventually throw the planet and its moon out of its parent star system. Tracebacks have determined that Vulcan originated from RS 8409-1353-8-11843423-10, a white dwarf 11.843 light years from Sol.
It's hypothesized that Vulcan was once in a distant orbit due to the lack of evidence of a previous atmosphere. The collapse of it's original parent star into a white dwarf caused Vulcan to drift away in the direction of Sol around the time Sol was actually forming.
Vulcan was first detected by the Extrasolar Body Tracking System, or EBTraS. on January 26, 7011. The EBTraS AI initially determined that Vulcan's trajectory would cause it to pass through Jupiter's orbit. Upon further extrapolation, EBTraS determined Vulcan would crash into Europa. The EBTraS AI immediately alerted Jovian authorities who in turn contacted the Sol planetary governments.
The planetary government hired Terragen to help alter Vulcan's trajectory at that time. They started by altering the orbits of a few hundred asteroids so that they swung by the moving rogue planet, then out to Jupiter and then back, using the gravitational attraction to move Vulcan in the desired direction. This slowed its approach and altered its vector so that it would enter into a close orbit of Sol. As the planet approached Sol, the increasing heat from the sun melted long frozen gases on Vulcan and its closely orbiting moon Hephaestus, creating atmospheres on both worlds and turning both into hot Ae-Class worlds.
The planet and its moon immediately became a popular destination for tourists as well as an ideal location for human scientists to study the sun from up close.
Vulcan is too close to the sun to terraform, so most of the inhabitants live near the poles of the planet while most of the remote scientific equipment is located all around the world near the equator facing Sol. The planet is home to over 17.58 million human and alien scientists, who study Sol as well as the planet itself.