An incredible amount of water, one of the simplest of molecules, resides
in the solar system. As the Sun emerged from gas and dust, tiny bits of ice and
rock debris began to gather in a frigid, flattened disk of planetesimals sur-
rounding the infant star. The temperatures in some parts of the disk might
have been warm enough for liquid water to exist on the first solid bodies in
the solar system. In addition, water vapor in the primordial atmospheres of the
inner terrestrial planets might have eroded away by planetesimal bombard-
ment and blown beyond Mars by the strong solar wind of the infant Sun.
Once planted in the far reaches of the solar system, the water coalesced to cre-
ate icy bodies that streak by the Sun as comets.
The solar system is quite large, consisting of nine known planets and
their moons (Fig. 3), although controversy remains whether Pluto is actually
a planet or some other type of body.The image of the original solar disk can
be traced by observing the motions of the planets. All of them revolve
around the Sun in the same direction it rotates, and all but one, Pluto, orbit
within 3 degrees of the ecliptic. Because Pluto’s orbit inclines 17 degrees to
the ecliptic, it might be a captured planet or possibly a moon of Uranus
knocked out of orbit by collision with another body or a comet from the
Kuiper belt.
Figure 3The structure of
the solar system.
(Photo courtesy NASA)PLANET EARTH