The Sun’s Family
ATLAS OF THE UNIVERSE
T
he Solar System is the only part of the universe which
we can explore with spacecraft of the kind we can
build today. It is made up of one star (the Sun), nine
planets (of which the Earth comes third in order of dis-
tance), and various lesser bodies, such as the satellites,
asteroids, comets and meteoroids.
The Sun is a normal star (astronomers even relegate it
to the status of a dwarf), but it is the supreme controller of
the Solar System, and all the other members shine by
reflected sunlight. It is believed that the planets formed
by accretion from a cloud of material which surrounded
the youthful Sun; the age of the Earth is known to be
about 4.6 thousand million years, and the Solar System
itself must be rather older than this.
It is very noticeable that the Solar System is divided into
two parts. First there are four small, solid planets: Mercury,
Venus, Earth and Mars. Then comes a wide
gap, in which move thousands of midget worlds known vari-
ously as asteroids, planetoids and minor planets. Beyond we
come to the four giants: Jupiter, Saturn, Uranus and
Neptune, together with a maverick world, Pluto, which is
too small and lightweight to be classed as a bona-fide planet.
Pluto’s status does indeed seem questionable.
Numerous asteroid-sized bodies have been found moving
round the Sun close to and beyond the orbit of Pluto;
these make up what is called the Kuiper Belt (after G.P.
Kuiper, who suggested its existence). One Kuiper Belt
object, discovered in 2002 and named Quaoar, is
1250 kilometres (780 miles) in diameter – more than half
the size of Pluto – and there may well be others which are
even larger. It is entirely possible that Pluto is merely the
largest member of the Kuiper swarm.
Even more remarkable is Sedna, discovered in
November 2003. It may be larger than Quaoar (though
smaller than Pluto) and has a period of 12,300 years; at its
greatest distance from the Sun it is 990 astronomical units
away, and it always remains outside the Kuiper Belt.
There have even been suggestions that it was captured
from the planetary system of another star.
It seems that the four inner planets lost their original
light gases because of the heat of the Sun, so that they are
solid and rocky; the giants formed in a colder region, and
so could retain their light gases – mainly hydrogen.The Earth has one satellite: our familiar Moon, which
is much the closest natural body in the sky (excluding
occasional wandering asteroids). Of the other planets,
Mars has two satellites, Jupiter has over 60, Saturn over
30, Uranus 23 and Neptune 11. However, most of these
are very small and probably ex-asteroids; only four plane-
tary satellites (three in Jupiter’s system, one in Saturn’s)
are larger than our Moon.
Comets may be spectacular (as Comet Hale–Bopp
was, in 1997), but are of very low mass. The only sub-
stantial part of a comet is the nucleus, which has been
described as a ‘dirty ice-ball’. When a comet nears the
Sun the ices begin to evaporate, and the comet may pro-
duce a gaseous head, with a long tail. Bright comets have
very eccentric orbits, so that they come back to the inner
part of the Solar System only at intervals of many cen-
turies, and we cannot predict them. There are many short-
period comets which return regularly, but all these are
faint; each time a comet passes relatively close to the Sun
it loses a certain amount of material, and the short-period
comets have to a great extent wasted away.
As a comet moves along it leaves a ‘dusty trail’
behind it. When the Earth ploughs through one of these
trails it collects dusty particles, which burn away in the
upper air and produce the luminous streaks which we call
shooting-stars. Larger objects, which may survive the fall
to the ground, are termed meteorites; they come from the
asteroid belt, and are not associated either with comets or
with shooting-star meteors.
How far does the Solar System extend? This is not
an easy question to answer. It is possible that there is
another planet beyond Neptune and Pluto, and it is
thought that comets come from a cloud of icy objects
orbiting the Sun at a distance of around one to two light-
years, but we cannot be sure. The nearest star beyond the
Sun is just over four light-years away, so that if we give
the limit of the Solar System as being at a distance of two
light-years we are probably not very far wrong.MercuryVenusEarthMarsMercury
45.9 to 69.7 million km
Venus
107.4 to 109 million km
Earth
147 to 152 million kmJupiter
740.9 to 815.7 million kmSaturn
1347 to 1507 million kmMars
206.7 to 249.1 million kmThe Asteroid belt8-53 Atl of Univ Phil'05 3/6/05 12:55 pm Page 34