Question

How can planet earth be considered a closed system?

Answer 1

Hint: Systems can be named open, closed, or isolated/confined. Open systems permit energy and mass to pass across the system limit. A closed system permits energy however not mass across its system limit. An isolated system permits neither mass nor energy to pass across the system limit.

Complete answer: The earth system all in all is a shut system. The limit of the earth system is the external edge of the atmosphere. Basically, no mass is traded between the Earth system and the remainder of the universe (aside from an incidental shooting star). But energy as sunlight based radiation passes from the Sun, through the climate to the surface. Earth then discharges radiation back out to space across the system limit. Thus, energy passes across Earth's system limit, yet not mass, making it a close system.
The interface between systems isn't in every case simple to distinguish. The interface between the hydrosphere and lithosphere at the shoreline is not difficult to perceive as a clear planar limit between solid and liquid. The interface between the climate and hydrosphere is less simple to observe as the hydrosphere includes both fluid water of the surface and water held noticeable all around.

Note: The Sea/Ocean is an illustration of an open system. The sea is a part of the hydrosphere and the sea surface represents the interface between the hydrosphere and the air that lies above. Sun oriented radiation goes through the atmosphere and is consumed by the sea.