What is the next planet to go to after the jungle planet on astro knights island?

You can do the three planets (jungle, ice, and fire) in any order. The jungle is the easiest (if you remember to pick up the Laser Lance), followed by the ice planet, and then the very difficult dragon to complete the fire planet.

Once you have all three knights with you, you can go to the Asteroid Belt and find the secret castle of the Binary Bard (it's near X-11 Y-82).