Planetary motion as perpetual motion?

[GiantSheeps]

Could a planet orbiting around a sun be considered an example of perpetual motion? I know that the planet wouldn't be doing any work, since it goes back to the same spot every year, but does an object have to be performing for it to be considered perpetual motion? The two might have nothing to do with one another, but I admittedly don't have very much experience with physics and all that.

 

[russ_watters]

Your instincts are correct: though Newton's first Law might fit the dictionary definition for "perpetual" motion, scientists don't use that word to describe it. They only use the word to describe motions that violate the laws of physics (thermodynamics).

 

[dean barry]

Essentially, its stable orbit is a rare balance of gravity opposed by just the correct velocity of the planet, as space is a vacuum, no drag occurs to slow the planets endless orbiting.

 

[Chronos]

Planetary orbits eventually decay. They ultimately either escape the host star, or are consumed by it. This process can take many billions of years. So an orbit can be robust, but, not perpetual.