Saturn
noun
The ground star is Saturn. That's why it's called "sat"urn. Because the first people to find it sat on the ground there. It's a star made completely of sit-on-able ground.
どせい
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
土星は大きいです。
Saturn is big.
どれが土星か、分かりますか。
Do you know which one is Saturn?
土星にはどうやって行けばいいですか？
How can I get to Saturn?