294 The Poetry of Physics and The Physics of Poetry
planetary and lunar orbits. The planets and the moons all orbit in the
same direction as the Sun rotates, from west to east with the exception of
Venus and Uranus. The planets near the Sun (Mercury, Venus, Earth and
Mars) are smaller and denser than the outer planets and are composed of
silicates (rocks) and metals. The outer planets of Jupiter, Saturn, Uranus
and Neptune are composed primarily of hydrogen and helium. Another
distinction between the inner and outer planets is the number of moons.
The outer planets have 22 moons altogether while the inner planets have
only three moons. The planets and satellites posses 99% of the angular
momentum of the Solar System but only 0.1 of 1% of its mass.
Pluto was once considered to be a planet despite its small size when it
was first discovered in 1932 but that all changed in 2006 when the term
dwarf planet was minted by the International Astronomical Union (IAU)
to deal with the controversy of the status of Pluto. As other objects,
namely Haumea and Makemake, were discovered orbiting the Sun
further out than Pluto but with masses less than that of Pluto the status of
Pluto was called into question. With the discovery of Eris, which is
larger than Pluto and further out, the controversy of Pluto’s status as a
planet came to a head. The controversy was resolved by a vote taken at
the IAU annual meeting where it was decided that Pluto would be
demoted to the status of a dwarf planet and that it would be joined in this
category by Haumea, Makemake and Eris. It was also decided that Ceres
in the Asteroid Belt would be promoted from an asteroid to a dwarf
planet bringing the number of dwarf planets to five for the moment.
Perhaps the most fascinating feature of the Solar System is the
spacing of the planetary orbits from the Sun, which follows a pattern
known as Bode’s law. The radii of the planetary orbits are given by the
formula R = Ro (0.4 + (0.3) 2n), where n has the values –∞, 1, 2, 3, 4 etc.
and Ro is equal to the radius of the Earth’s orbit, 150 million km. The
values n = –∞, 1, 2, 3, 4, 5, 6, 7 correspond to Mercury, Venus, the
Earth, Mars, the Asteroid Belt, Jupiter, Saturn and Uranus respectively.
The law does not hold for Neptune, but this planet has a somewhat
confused situation because the dwarf planet Pluto is apparently an
escaped moon of Neptune. The moons of Jupiter and the moons of
Saturn also obey a form of Bode’s law.
An explanation of the regularities of the Solar System including
Bode’s law will require an understanding of how the Solar System
formed, which at the moment is not fully understood. It is believed that
the planets and their satellites condensed out of the gas cloud that