[30] Colour collage of Triton,
captured by Voyager 2 in 1989.
[31] Dwarf planet Pluto, as
constructed from multiple Hubble
Space Telescope photographs taken
from 2002 to 2003 (NASA).3031oblige the gentleman from Paris.” A young astronomer, Heinrich D’Arrest,
overheard the conversation and asked to be allowed to join in.
Galle and D’Arrest lost no time and on the first clear night went to the
dome to check stars near the position Leverrier had given. Galle used the
telescope while D’Arrest checked the map. A new map had recently been
prepared and had not been widely distributed. D’Arrest discovered a star not
on the map in only minutes. Both astronomers looked carefully and fancied
they could detect a small disc – which no star can show – and they followed
the body until it set. The next night it had moved by exactly the amount to
show it was a planet. A triumph of ingenuity.
Remember that the hunt at Berlin was undertaken purely on the basis
of Leverrier’s work: the observer had never heard of Adams. By this time
Sir George Airy had been roused to activity and instructed one of his
astronomers, James Challis, to look for the planet. Challis had no up-to-
date maps, and adopted a laborious method of checking, but did indeed
find an object with a small disc. He did not call in other astronomers to
confirm his findings. When he did return the sky was overcast, and Galle
and D’Arrest’s announcement was already made.
When the discovery of the new planet, Neptune, was announced by Sir
John Herschel, the French accused the British of trying to steal the glory of
the discovery. This could have led to an unpleasant incident but fortunately
it did not. When Adams and Leverrier met they struck up a friendship which
lasted the rest of their lives, even though neither spoke the other’s language.
In size and mass Neptune and Uranus are near twins. Uranus is slightly
larger and slightly less massive. Neptune's magnitude, between 7 and 8,
means it is too dim to be seen with the naked eye, but binoculars show it
easily, and a telescope shows a small disc which is not green like Uranus,
but bluish.
In other ways, too, Uranus and Neptune differ. Neptune does not share
the remarkable axial tilt of Uranus so the seasons there are more
conventional. Neptune also has a source of internal heat so there is definitelya rocky core at a fairly high temperature. It too qualifies as an ice giant
though it does have a hydrogen atmosphere with a considerable percentage
of helium.
One satellite, Triton, discovered soon after Neptune itself, in 1846, is
unique in being the only major satellite to have retrograde motion – i.e.
to move round its primary in the direction opposite to that in which the
primary rotates. There seems almost no doubt that Triton was originally an
independent body ensnared by gravity and unable to break free.The Kuiper Belt
We are not yet at the end of the planetary system. Out beyond Neptune
we come to another swarm of asteroidal-type worlds, known as theAstronomer Book