2829Jupiter, Uranus and Neptune also have rings, but they are dark and difficult
to detect. The bright icy rings of Saturn are in a class of their own.
All the planets we have discussed are easy naked-eye objects, and some,
particularly Venus and Jupiter, are strikingly brilliant. Beyond Saturn there
are two more giants but they are much fainter and further away.
Uranus
First comes Uranus, discovered by William Herschel in 1781. Herschel, by
profession an organist, was not looking for a planet and did not recognize
it for what it was. In his paper to the Royal Society he gave an “account of a
comet”. Only when its orbit was calculated did it become clear that the object
was a true planet and a giant one. It takes 84 years to go round the Sun and
is just visible without optical aid provided you know where to look.
Its diameter is approximately 30,000 miles, less than half that of Saturn,
and ordinary telescopes show little other than its pale green disc. It is a giant
but not a gas giant similar to Jupiter and Saturn. Most of its material is in the
form of ices, in a hydrogen-rich atmosphere.
Whether there is a solid nucleus is uncertain. Jupiter and Saturn do have
solid cores – both emit far more radiation than they could if they just received
radiation from the Sun alone – and the same is true of Neptune. But Uranus
appears to have very little by way of internal heat, which means its make-up
is different.
In another way Uranus is peculiar. Most planets spin with their rotation
axes at a reasonable angle to the orbital plane. In the case of the Earth, our
axis is tilted by 23.5 degrees, while with Jupiter it is only a few degrees and
the planet moves in an upright pose. With Uranus the tilt is 98 degrees, more
than a right angle so the rotation is technically retrograde or backwards.
The rotation period is just over 16 hours. The orbital period is 84 years and
we see Uranus through all kinds of angles. Sometimes we are facing a pole
while at others the equator is presented. The reason for the tilt is unclear.
It used to be thought it was hit by a massive body and tipped on its side,
but this does not seem very plausible bearing in mind we are dealing with
a giant of considerable size and mass. Today it is generally believed that
interactions with other planets are responsible, but we still do not know why
it is inclined at such an angle.
Like the other giants Uranus has some satellites. Only five are in the range
of telescopes used by amateurs and all are less than 1000 miles in diameter.
The most interesting is the closest-in, Miranda. It has a very varied surface –
mountains, craters and ridges: a geologist’s paradise.
Neptune
When Uranus was discovered, immediate efforts were made to work out
its orbit and the way in which it might be expected to move. Orbital period
was easily fixed as being 84 years but as time went by, it became painfully
obvious that something was wrong, and that Uranus was not behaving
exactly as the mathematicians had forecast. Slowly but surely it wandered
away from its calculated path until the errors became too great to be
ignored. What caused them?
One suggestion was that Uranus was being pulled out of position by
another large planet at a still greater distance from the Sun. In the mid-19th
century several astronomers began to look at the problem. One was the
Cambridge graduate John Couch Adams. By following the position of Uranus
he could calculate where the perturbing planet was. In 1846 he came out
with what he thought was an accurate position and sent his calculation to the
Astronomer Royal, Sir George Airy at the Royal Observatory, Greenwich. Airy
was not impressed and no immediate search was made. Meanwhile there
were developments in France. A Paris mathematician, Urbain Leverrier,
also became interested in the problem and began working on the same
lines as Adams. Note there was no contact between the two and they had
never heard of each other. By 1846 Leverrier too had his proposed position
and contacted the Paris observatory. Again, nothing was immediately
done. Patience, however, was never Leverrier's strong point – one of his
compatriots said he was not the most detestable Frenchman, he was the
most detested.
Before long Leverrier became tired of waiting and contacted Johann
Galle, who worked at the Berlin observatory. Galle was enthusiastic and
it so happened there was a telescope suitable for planet hunting. Galle
went to the observatory director, Encke, and requested to begin the search.
Encke was suspicious, but ultimately decided to cooperate, saying: “Let usThe planets
[28] Proteus, one of six new moons discovered at Neptune by Voyager 2 (NASA).[29] Neptune and Triton imaged by PL using a 14-inch telescope.