Can a planet be covered in forest or swamps?

The Star Wars universe includes planets entirely covered by forest (Endor) or swamps (Dagobah). But could such planets really exist?

An alien planet could not be covered in forest of trees that are biologically similar to the ones that we know from Earth. It would require a climate in which no areas are so cold or elevated that they are permanently covered in snow and ice, or so dry and warm that desert takes over. Such a climate could only exist by means of forceful ocean currents carrying energy from the equator to the poles throughout the year (atmospheric winds are not enough). So the planet could not be 100% forest, as major oceans would be needed for the required climate.

A planet covered in swamps makes even more demands on biology, climate and geography. A swamp is a wetland with large-growth vegetation – on Earth either trees or bushes. Very few species of Earth’s plants like having their roots in water, so it requires a targeted evolution of species. Moreover, precipitation must be at a sufficiently high rate throughout the year to maintain such vast shallow areas, and the geography would need to be so flat that no islands of drier land occur. The existence of a swamped planet is therefore highly unlikely, but not impossible.

Star Wars scenes on the Endor moon were shot in a Californian forest.

In StarWars:EpisodeV, Luke Skywalker meets his mentor Yoda on Dagobah – a planet covered in swamps. It’s a highly unlikely planetary ecology.