What are the dimensions of the solar system?

Answer 1

The size of the Solar System depends partly on what are considered to be its outermost reaches. There are three main candidates:

• the aphelion of the orbit of the farthest planet,
• the farthest observable object which orbits the sun,
  
• or the edge of the heliosphere.

The furthest distance from the sun reached by the outermost planet, Neptune, is approx 4.5 billion kilometres and this would give the Solar System a diameter of 9.1 billion km. (This definition depends on Eris remaining a dwarf planet and not attaining planetary status.)

The most distance observable object in the Solar System is Sedna. It is 143.7 billion km from the Sun, thus giving the Solar System a diameter of 287.5 billion km or approx 1922 AU.

The heliosphere is the region within which the solar wind pushes against the interstellar medium, or where the Sun's gravity is stronger than that of other stars. The edge of the heliosphere, called the heliopause is reached at a distance of 90 AU giving the heliosphere a diameter of 180 AU.

Incidentally, although at its aphelion, Sedna is outside the heliosphere, its orbit is very eccentric and brings it to 76 AU at its nearest approach to the sun - well within the heliosphere.