The Solar System Mod currently adds 8 planets: Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto (dwarf planet that everyone wanted me to add). Mercury is a place with no living creatures. Lava flows from under cracks. The same with Venus, because they are both incredibly hot. Mercury is tan, I changed the texture as of v0.5, and Venus is orange(see pictures).

Mars is cooler than Earth, so there is no lava. Mars is reddish, no living creatures.

Jupiter is like Mars, but more orange-reddish.

Saturn is mostly plain and is golden.

Uranus is sky blue and ice is beneath the surface. Snow covered. Ice Lakes.

Neptune is blue, with ice beneath the surface. Snow covered. Ice Lakes.

Pluto is dark brown ish green ish and gray ish. Snow covered. Ice Lakes.

All planets are made out of their own material. Ex. Mars is made of Mars Rock

You can get to to the planets by activating custom frames of portals. All portals can be lighted with fire, and they are in the shape of a normal Nether portal.

Mercury: Light a GOLD BLOCK Portal.

Venus: Light a NETHERRACK Portal.

Mars: Light a NETHERBRICK Portal.

Jupiter: Light a BRICK Portal.

Saturn: Light a GLOWSTONE Portal.

Uranus: Light a IRON BLOCK Portal.

Neptune: Light a SNOW BLOCK Portal.

Pluto: Light a Lapis Lazuli Block Portal.

Aliens now spawn naturally! Fixed as of v0.7.1. Notice, you won't find them commonly, they spawn on Easy+, and at night.

Aliens are quite strong and deal a decent amount of damage. But then again, they drop Alien Gems, which can be crafted into Alien Swords. They have 1000 uses and are as strong as iron. Aliens hold Alien Swords as of v0.7.1!

As of v0.8, I've added the Alien tool set! With Alien Pickaxes, Shovels, Axes, and Hoes. These are all like iron tools.

As of v0.9.2, I've added ALIEN HOUSES! Yes they naturally generate only on the planet Mars. They are mostly made of a green block called Alien Blocks. You can't craft them. The only way to get them is travel to Mars and mine out the houses. Check out the pictures in the download.

As of v0.9.5, you can now craft Alien TNT using this recipe:

AGA
GAG
AGA

A = Alien Block

G = Gunpowder

Alien TNT blows up with a little more damage then regular. Must use redstone to activate.

As of v0.9.7, there are now Deadly Aliens! They are really deadly, they do 4 hearts of damage every hit. Watch your back! Luckily they don't spawn as commonly as aliens. When you kill them, they drop the ultimate Planetary Gem! These can be crafted into Sword of the Planet, the awesomest thing in the blocky world of Minecraft. It'll kill everything, monsters, creepers, and all that stuff in one hit! Craft using the default sword recipe, with Planetary Gems.

As of v0.9.8, there are now Meteorite Ore! They are quite rare and only appear in a maximum of 6 in one vein. Smelt them into Meteorite Ingots to craft into Meteorite tools! No sword yet. The tools are about the same as diamond (alien tools are now same as iron due to the easiness of getting alien gems).

The Space Suit first appeared in v0.9.4. It is currently useless right now, except for a decent defense. NuclearBanana is working on making it so it hurts when you go on planets without the suit.