When you move up to the next planet, you get rewarded with a player that you already have on your roster at level 1 but on the new planet. However, since it's not possible to have more than 1 of the same player on your team, the 2 players merge into a more powerful player from the new planet. Although it may seem that his level is lower, at one point you will notice that his value will be significantly higher than it was at the same level on the previous planet.
Not all of your players will be moved up to the new planet with you. To continue improving your players, you will need to sign their version from the new planet. You can sign players from the new planet when you are on that planet.
Here is the list of additional levels received when you upgrade your players to different planets: