The Yggdrasil Star System

Artist Hoai Son Nguyen at https://sonnguyen.artstation.com/

The Sun - the egg of a primordial Novaspawn, the inhabitants of the system call Jormungandr. When it neared it's hatching time, in order to stave off the nova that would result, the Allfather, the primary diety of the system, grew the tree around the egg preventing the hatching as long as the tree remains. 

The Tree - Sustained by both the Novaspawn egg, and planar energies drawn from the planets of the system. As long as it remains, the sun survives, and the egg remains unhatched. 

Tree Branches - Branches reach out from sun towards each of the planets. When a branch reaches the planet, they enter a portal at the point they would meet the atmosphere. Each planet specifies planes of existence they are aligned with. 

The Realms of the World Tree

The first thing any explorer would note about this system, other than the giant plant dominating it, is that the orbits of the planets do not share the same flat plane like most solar systems do. Instead, the planets in this system orbit the sun like the electrons orbit an atom. 

Muspelheim - Closest Planet to sun. Vertical orbit, Z axis. A burnt, volcanically active rock, similar to mercury in that it's too close to the sun. Is aligned with the plane of fire. Has sparse, but poisonous, radioactive looking clouds. Not only is the surface covered in industry, but much of the near orbit is covered by a massive space station stretching across the planet, just far enough outside the atmosphere for it to maintain a stable orbit, but covering almost a quarter of the planet.

Midgard - 2nd planet from sun. Earth-like planet. Closer to earth than any of the others. Aligned evenly with all inner cosmology planes.

Asgard - 1 of 2 planets that share being 3rd from the sun. Horizontal orbit, x/y axis. One of the habitable planets in the system. This one is distinct because of the leylines running across it's surface, so powerful they can not only be seen visually, but seen from space. Otherwise earth-like with many biomes. Aligned with more with the astral, shadow, and ethereal plane than any of the elemental planes. 

Vanaheim - 2 of 2 planets that share being 3rd from the sun. Horizontal orbit, x/y axis. This world would be Asgard's twin. Opposite of it in the same orbit, always with the sun between them constantly. Same leyline effects. It has slightly different planar connections though, being more connected with the Fey realms than the astral plane.

Jotunheim - 4th planet from sun. Imagine earth, but in winter 100% of the time. A frozen planet, but in the habitable zone. Rocky and Watery, but less heavily forested biomes. It has military structures so large they can be seen from space. Aligned mostly with earth and water planes. 

Niflheim - 5th planet from the sun. A Jupiter-esque gas giant. One one side it had a single massive eternal storm, but opposite of it on the other side it has a hole, like a giant whirlpool in the atmosphere that might just be a tiny black-hole-esque singularity. This planet is also a "Comet catcher" and as such, it's core has been struck so hard and often that it has left a trail of chunks of ice in it's orbit similar to an asteroid belt, but icy rather than rocky. The planet is heavily aligned with the plane of water, and has a frozen, icy core. Visually, I'd like it to feel like a gloomy place. 

Elivagar - The Ice Belt sharing Niflheim's orbit, constantly colliding with that gas giant, and simultaneously being restored by it. The gravity of the other planets pull some of the icy rocks out of the belt towards each planet, where they eventually break up in the atmospheres supplying most of the water in the solar system. 

Alfheim - 6th planet from the sun. Vertical orbit, Z axis. It's been magically terraformed to juxtapose just enough of the sun's corona around it, just outside the atmosphere, to allow the planet to develop as if it were in the habitable zone. It is a colorful place with large amounts of crystalline deposits throughout the planet, causing the light it steals to bounce from reflection to reflection, changing with each one. It is most heavily aligned with the fey realms, but also carries a strong alignment to the plane of air. 

Svartalfheim - 7th planet from the sun. This planet is the ultimate expression of terraforming. It is wholly artificial, but constructed out of natural elements (it's not a huge machine). It is however perfectly symmetrical in all ways, with perfectly measured land masses, 4 moons that are perfectly synchronized, and even the movements of it's tectonic plates are scheduled and never deviate from this perfection. It is hollow however, and all civilization on the planet is actually deep inside it. This planet is heavily aligned with the plane of earth and air. 

Helheim - Furthest planet from sun. This planet's orbit is the horizontal plane, the X and Y axis. An Ice Giant like Neptune or Uranus. Largely unnoticeable except for one thing: It is strongly aligned with both the plane of water, the plane of shadow, and the plane of fire. 

Another time I will tell you about the people who call this star system home, including their playable stats.