Daring Do and the Cave of Skulls

Things were first dark but Daring Do found a flashlight. Never leaves home without it. She looked around... and saw a skeleton. Looking at a wall... and it said... "Pluto". Pluto, Pluto, Pluto. Maybe that meant something. And that skeleton must have been someone who went in the cave. But how'd that happen?

Soon, Daring couldn't see the entrance anymore. She wanted to go back... but she couldn't... this was a quest. This was important.
She still ran back. But the Exit was gone. A way in... but no way out.

She then noticed another mural. The solar system. Mercury... Earth... Jupiter... Pluto. She rubbed her hoof over Pluto. It went through. Yikes.
And then a voice began to talk. No one seemed to hear it... Being no way out. "That is the portal. You have found it. If you find a way to get through, I will let you in." it said.