Kepler's Law of Planetary Motion

• Johannes Kepler, a German astronomer who lived in the early 17th century, proposed several theories regarding the orbits of planets around the sun. Kepler's First Law of Planetary Motion states that the "orbits of the planets are ellipses, with the Sun at one focus of the ellipse." While there is nothing in the laws of physics preventing a planet, or any object for that matter, from orbiting the sun in a perfectly circular orbit, gravitational pull from other planets orbiting the sun play a role in the orbit of every planet.
  
  

Physics

• In an elliptical orbit there are two focal points, known as "foci." Both foci are within the elliptical oval, and the distance from both of them combined to the edge of the ellipse is always the same. In our solar system, the sun forms one of the foci of the planet's elliptical orbit, while generally nothing is located in the other focal position. Because the sun is not at the center of the ellipse, the distance of a planet to the sun is always changing.
  
  

Eccentricity

• The orbits of most planets, if they were to be shown on paper, would not be obviously an elliptical shape. The extent to which a planet's orbit deviates from a perfect circle is known as its eccentricity. Most planets in our solar system have very slight levels of eccentricity in their orbits. Pluto, however, is different. Of all the planets it has the greatest eccentricity in its orbit, circling the sun in a distinct oval shape when plotted on paper.
  
  

Hyperbolic Orbit

• In contrast to elliptical orbit, an object in hyperbolic orbit does not make a complete circuit of a planet. An asteroid may approach the sun, for example, with enough speed for its trajectory to be merely curved by the sun's gravitational pull but not captured into a complete orbit. These types of orbit can be roughly illustrated as a letter C or U, with the sun in the middle and the meteorite following the shape of the letter.
  
  

Spiral Orbit

• If an asteroid were to approach the sun without enough energy to create a hyperbolic orbit, it may end up in a spiral orbit, otherwise known as an impact orbit. In this case, its lack of energy or speed when approaching the sun does not give it enough power to "sling-shot" away from the sun, causing it instead to curve into the sun in a spiral fashion.