All planets place a stress on the time space continuum for a distance of 3 light-years in all directions. Any starship that ends its turn within 3 light-years of a planet is in the planet's warp well (the green and yellow squares in the image below). If that starship is travelling faster than Warp-1, it fall into the planetary orbit automatically; the starship's waypoint will be unchanged. Starships with multiple waypoints set (see navigation) should be cautious of this.
Zooming into a single planet on the Starmap, the warp well becomes apparent as a grid around the planet.
Ships not affected by warp wells are non-moving ships and any ship with a warp speed of 1. Even ships equipped with Gravitonic Accelerators can move inside a warp well at warp 1. Ships under tow travel with their towing craft.
A ship that is hyperjumping is still affected by warp wells, unless it ends at exactly 3 light-years from the planet (the yellow squares in the image).