The Sun Station was first added in the Alpha Demo, it is not present in the regular Alpha. It consists of a large platform connected to a smaller platform underneath with a large spiral.
A Nomai text on the bottom platform contains a secret message for players to read, asking the first player to tell the developers what the Nomai are searching for.
The only way of reaching the Sun Station is by landing on the large platform which is quite challenging due to the close proximity of the Sun and its strong gravity.