1.13 added Datapacks and since nobody ever used the API (you should all feel bad) its possible to do most of the stuff the API can via datapacks & jsons.

And example is here:

• nether: this registers the nether as a planet you can jump to, and adds a recipe to craft the coordinates.
• AR space

the jsom must be located at data/<mod name>/planets/<dimension name>.json. This mean for the dimension foo:bar the planet json would be at data/foo/planets/bar.json or minecraft:the_nether and data/minecraft/planets/the_nether.json

Key words (mandatory):

• texture: the texture path for the planet image
• name: the name of the Dimension, this will the player see as the planet name
• upgrades: the upgrades needed to get to this planet Key words (optional):
• breathable_atmosphere: if the player can breath there, if not this will enable futurepacks no air system, if a mod dimension with similar properties is choosen (moon from Galacticcraft, Space from Advanced Rocketry) then both sstems will be active and the player will probaply die (because the AIR systems of the mods are not compatible with each other).
• oxygen_gravity_velocity: only important if breathable_atmosphere is false, determines how fast AIR will sink to the ground (negative values are possible)
• oxygen_spread_velocity: only important if breathable_atmosphere is false, determines how fast the AIR will spread to surounding blocks.

Example:

{
	"texture": "futurepack:textures/gui/planet_envia.png",
	"name": "Envia",
	"upgrades": [
		"lifesupport"
	],
	"breathable_atmosphere": false,
	"oxygen_gravity_velocity": 0.4,
	"oxygen_spread_velocity": 0.2
}