a very large round object in space that moves around the sun or another star

行星:

Mercury is the smallest of all the planets.

水星是行星中最小的一颗。

Is there life on other planets?

其他行星上有生命吗？

the future of Planet Earth

地球的未来

sb is (living) on another planet, what planet is sb on?, [spoken] used humorously to say that someone's ideas are not at all practical or sensible

某人异想天开〔幽默用法，表示某人的想法毫不实际或不明智〕:

He thinks motherhood is glamorous – what planet is he on?

他觉得当母亲令人向往 — 他是外星人吗？

the planet, the world – used especially when talking about the environment

地球〔尤用于谈论环境〕:

a safer future for the planet

地球更安全的未来

the planetary system

行星系