The scale and structure of the universe

In order to obtain some perspective on the solar system, it is useful to contemplate the scale of the universe as we perceive it at present, as “on the galactic scale of things, the solar system is a rather small place” [1]. The mean distance between the Sun and the Earth is 149.6×106 km or one astronomical unit (AU). The diameter of the Sun is 1.392×106 km, close to 0.01 AU. Mercury is close to the Sun at 0.4 AU, but many of the extrasolar planets are within 0.05 AU of their parent star. Our planetary system extends out to the orbit of Neptune at about 30 AU. Beyond that, from about 35 to over 1000 AU, lies the Edgeworth–Kuiper Cloud of icy comets. Further out is the inner Oort Cloud extending from 5000 to over 50,000 AU. The classical Oort Cloud of comets marks the outer bound of the solar system extending from 50,000 to about 105 AU. All these distances are very small on a galactic scale.

Titius–Bode Rule

The distances of the planets from the Sun can be expressed as a series 0.4, 0.7, 1.0, 1.6, 2.8, 5.2 etc., that is close to their separation in AU. This sequence of numbers can be arrived at by adding a constant 0.4 to the doubling sequence of 0, 0.3, 0.6, 1.2, 2.4, 4.8 etc. There are many other more sophisticated formulations.