A planetary shield is a system that projects a single force field that encompasses a planet or projects multiple interlocking fields that provide significant planetary coverage. The shield exists to protect the planet from attack by space ships and other phenomena, although the degree of protection varies.
Planetary Shields in Star Wars
A planetary shield in Star Wars is a roughly-spherical shape that encompasses an entire planet, protecting it from orbital bombardment. Several references to planetary shielding technology and its effects are made throughout the Star Wars film-canon.
Star Wars IV: A New Hopedestruction of the planet Alderaan shows the 'breaking point' of a core-world's planetary shield. The Death Star I fires its planet destroying superlaser at Alderaan, a beam with a calculated energy of 1E38 J. This energy beam strikes the planetary shield, but the planet does not begin to sustain damage for another tenth of a second. This may mean that, before giving out, the shield was able to dissipate 1E37 J of energy. However, Star Wars turbolasers seem to have a visible "tracer" component, as well as an invisible portion. Since a second explosion is visible during the destruction, it is likely that the invisible portion struck later. This will make the actual amount of energy absorbed lower. It certainly doesn't seem likely that the Empire designed their ultimate weapon with only 900% overkill against existing civilian targets.
Star Wars V: The Empire Strikes Back
In Star Wars V, the Rebel Alliance is able to utilize a "theatre shield" to protect their base on Hoth from orbital bombardment. The shield was strong enough to resist fire from several Imperial Star Destroyers and a Super Star Destroyer for long enough to make a surface assault that could slip under the shield necessary, leading to the Battle of Hoth.
Star Wars VI: Return of the Jedi
This establishes the range of the shield-net itself. A planetary shield generator is employed on the sanctuary moon of Endor, prior to the Battle of Endor. The shield not only extends to the upper atmosphere of the moon, but all the way out into space to entirely encompass the Death Star II as well. Aboard Home One, during the briefing given to the pilots prior to the attack on the Death Star II, a holographic projection of the shield indicates that it does indeed reach out in a conical structure to surround the orbiting Death Star. The full extent of the shield is evident from the fact that the shield needs to be dropped in order for a shuttle to land on the moon, not just to approach the Death Star.
Further, in the novelization, it is shown that when a ship crashes into this planetary shield the collision destroys the ship.
Planetary Shields in Star Trek
Planetary shields are occasionally mentioned in Star Trek but generally are not powerful enough to protect an entire planet from a major attack.
- The planet Elba II had a planetary shield that blocked transporter function and could withstand phaser fire from at least one Constitution-class starship, but it's not clear whether the shield could have withstood a determined attack. Commander Scott, commanding in the absence of the captain and first officer, was concerned about damaging the Elba II asylum, which was a sealed habitat in a toxic environment. Consequently, his efforts at penetrating the shield were restrained by the need to avoid rupturing the asylum. Nonetheless, the Enterprise did try to penetrate the shield using the starship's main phaser banks on full power, attacking a point on the far side of the planet from the asylum where the shield would be weakest, and the attempt failed. Phaser attacks against surface targets have been sustained for substantially longer, though.
- The Federation penal colony on the planet Tantalus V was protected by a force field that blocked transporter function, but no attempt was made to defeat the field with starship weaponry.
- The Federation science outpost on Calder II had a deflector shield to protect it and the archeological ruins being studied there, but there was no indication that this shield protected the entire planet. Phaser fire from a single mercenary starship was capable of defeating the shield (an effort made easier with trickery known to a Federation starship captain). 
- Chakotay told Annorax that if Captain Janeway had given other ships temporal shielding technology, they would have shared it with their home worlds, which would be able to protect their planets from the Krenim time ship's temporal weapon. This exchange suggests that such civilizations may have had the ability to shield entire planets. However it should be noted that temporal shields are essentially the normal shields and deflector on Voyager modified by Seven of Nine and Lt. Tuvok to the frequency and inverse pulse of the Krenim's chroniton torpedoes. Once the frequency modification was in place, they provided protection against the otherwise conventional warheads onboard the torpedoes. Consequently, a defensive fleet of starships with appropriate shielding may well have been sufficient to keep the Krenim at bay.
- In the 2150's, the Klingon outpost and colony of Qu'vat had at a defensive shield that could withstand a prolonged bombardment from a D-5 battlecruiser's main disruptors. There was, however, no indication that the shield protected the entire planet, and an NX-class starship was also able to withstand fire from the cruiser's weapons (albeit for a much shorter period of time).
- The planet Aldea had a planetary shield that could block transport and cloak the planet, but it was never tested against weaponry. Furthermore, the planetary cloaking shield was an advanced alien technology that the Federation has not successfully reproduced.
- In the Delta Quadrant, the Federation starship USS Voyager encountered the Kraylor, whose homeworld had a planetary shield grid capable of resisting energy weapons and preventing ships from physically reaching the surface.
- TOS "Whom Gods Destroy"
- TOS "Who Mourns for Adonais?"
- TOS "Dagger of the Mind"
- TNG "Gambit, Part I", "Gambit, Part II"
- VOY "Year of Hell, part II"
- ENT "Divergence"
- TNG "When the Bough Breaks"
- VOY "Nightingale"