Mars is small and made of rock. Is Mars an inner planet?

Yes, Mars is an inner planet, one of the four nearest the Sun.

Venus is small and made of rock. Is Venus an inner planet?

Yes, Venus is an inner planet too.

But Jupiter is big and made mostly of gas. Is Jupiter an inner planet?

No, Jupiter is an outer planet, one of the four farthest from the Sun.

The eight planets: their place from the Sun, their size, and what each is made of.

And Earth is small and made of rock. Is Earth an inner planet?

Yes, Earth is an inner planet, the third from the Sun.

So, when Uma said a spaceship could land on Jupiter and people could walk around on it, she was not right.

Jupiter is an outer planet, made mostly of gas, with no rocky surface anywhere to land on.