The Sky at Night Phần 4 ppt

41
P. Moore, The Sky at Night, DOI 10.1007/978-1-4419-6409-0_11,
© Springer Science+Business Media, LLC 2010
In the Sky at Night, I have covered several total eclipses, and the first of these way
back in 1961, was a pioneering effort. In 2006, the sky was clear, and in our television
programme all went well.
Chapter 11
Turkish Delight
Turkish eclipse (Credit: Pete Lawrence)
42 11 Turkish Delight
There are many glorious sights in Nature, but to me, as I have often said there is
nothing even remotely comparable with a total eclipse of the Sun. For the few
moments, when the brilliant solar disk is fully covered by the Moon, everything
seems to be put into a state of suspended animation; only the sky changes. I am not
surprised that ancient peoples were frightened and in backward countries this is still
true. An annular eclipse is fascinating, but no more, and it is hard to become really
enthusiastic about one that is partial.
I am lucky that I have seen seven totalities, and have presented Sky at Night
programmes for most of them; the first, way back in 1961, was a real pioneering
effort. But the eclipse of 23 March 2006 was one that I knew I would have to miss;
I simply wasn’t fit to travel to the track of totality, which ran from Brazil across to
Africa, cutting through Turkey and grazing Egypt before going on to end in
Mongolia. John Mason and his team from the South Downs Planetarium opted for
Egypt; the Sky at Night contingent, with Chris Lintott and including Pete Lawrence
and Bruce Kingsley, preferred the coast of Turkey. For once in a way, politics did
not interfere.
I was envious. From Selsey there was a small, partial eclipse – <20%, but as a
matter of principle I decided to photograph it. Joined by Alan Schultz and Tim
Wright, I watched it from my garden outside the observatory, talking to Chris enjoying
the pleasant Turkish heat. I had a mediocre view through scattered cloud
In Turkey, the sky was crystal-clear and conditions could not have been better.

Totality was due just before 2 o’clock in the afternoon Turkish time, and the expedi-
tion members began to make their final preparations.
No two totalities are alike. Sometimes, the sky becomes really dark; generally,
the light-level at mid-totality is about equal to full moonlight, but one never knows
quite what to expect. The shape of the corona is more predictable because it is
linked with the solar cycle; at sunspot maximum it is fairly symmetrical, while at
spot-minimum it is “spiky”, with streamers stretching out in all directions. In March
2006 we had just passed the low point of the cycle, so that the corona should have
been of the spot-minimum type, but Pete Lawrence’s H-alpha telescope showed
several prominences, so that the Sun was not entirely quiet even though no spot-
groups could be seen on the disk.
Quite apart from the corona, there are various phenomena to be observed. Before
totality there are shadow bands, wavy lines on the Earth’s surface seen when the disk
has been reduced to a narrow crescent. Then, there are Baily’s Beads, when the
sunlight streams through low-lying areas of the Moon’s limb; they were seen very
clearly at the last annular eclipse.
The Moon’s shadow rushes across the landscape at almost 200 mph, both before
and after totality. I remember seeing that, at the Cornish eclipse of 11 August 1999,
though from our site the eclipse itself was clouded out (a gentle rain fell throughout
totality), and we sheltered under umbrellas, muttering words such as “Tut, tut!” and
“Most annoying!”. And just as totality ends there is the wonderful “Diamond Ring”,
as the first segment of the photosphere pokes out from behind its temporary screen.
All these marvels were seen by the observers in Turkey, and they were suitably
impressed; “awesome” and “emotive” were two of the adjectives used by our
4311 Turkish Delight
commentators. The temperature dropped sharply by 5 or 6°, and just before the
corona shone out there was what was described as “a strange twilight”. The corona
itself was of typically minimum variety, with long streamers, and there were two
naked-eye prominences. This time the sky remained bright; Venus was prominent,
and Mercury could be glimpsed, but Chris reported that he could not see any of

the stars of Orion. Not that there was much time to look around; totality lasted
for a mere three and a half minutes – and as I know from experience, the time
seems to flash by.
One of the most interesting experiments was undertaken by Pete Lawrence.
During totality, he photographed the Moon, which was of course directly in front of
the Sun and so was illuminated solely by Earthlight. The pictures he obtained were
very good indeed; the outlines of the main maria were unmistakable, together with
some of the craters. Totality is the only time to see the completely New Moon.
Yes, Turkey was a great success, both as a television programme and, far more
importantly, to give dedicated observers a chance to carry out useful work. Nobody
who took part in the expedition is ever likely to forget it. Moreover, eclipse chasing
is addictive, and all the team members who went to Turkey began to make plans for
the next totality, on 1 August 2008, even though it did mean the slight inconve-
nience of travelling to North Greenland.

45
P. Moore, The Sky at Night, DOI 10.1007/978-1-4419-6409-0_12,
© Springer Science+Business Media, LLC 2010
In the spring and summer of 2006, Saturn was well-placed, with the ring system
still widely open. For this programme, I was joined by John Zarnecki, Carl Murray,
and photographers Pete Lawrence, Damian Peach and Dave Tyler. Indications of
liquid areas on Titan had been found, but true revelations about the “Lake District”
came later – see The Lakes of Titan.
What is the most beautiful object in the sky? Many people will favour a spiral
galaxy, such as the Whirlpool; others will opt for a lunar crater, or a great comet
with a glowing head and long, gently-curved tail – but my vote goes unhesitatingly
Chapter 12
Ringed World
Saturn from Cassini (Credit: NASA)
46 12 Ringed World

to Saturn, the planet with the rings. There are plenty of spirals, many lunar craters
and the occasional spectacular comet, but there is only one Saturn.
It is a giant world, almost 75,000 miles across; it has a gaseous surface, and
though there is a hot, presumably silicate core, the overall density of the globe is
less than that of water. It is often said that if you could drop Saturn into an ocean,
it would float – but finding an ocean of sufficient size would be quite a problem!
Saturn’s mean distance from the Sun is 886 million miles, but even from this range
it still outshines most of the stars; when it is at its best, only Sirius and Canopus
outrank it. Its slow movement and its dull, yellowish glare led to its being named
after the God of Time – Jupiter’s father, and his predecessor as ruler of Olympus.
Saturn takes 29½ years to compete one journey round the Sun, but its day is only
10¼ days long, shorter than for any other planet apart from Jupiter; the quick spin
means that the equator bulges out, and any small telescope will be good enough to
show that the disk is markedly flattened. The upper atmosphere, rich in hydrogen
together with some helium, is very cold, at a temperature of about −180°C. Eight
satellites can be seen with a good, modern amateur-owned telescope, but of these
only one (Titan) is bigger than our Moon. There are over 30 much smaller satellites,
some of which have retrograde motion and are almost certainly captured asteroids.
In some ways, Saturn is not unlike Jupiter. It also has cloud belts and brighter
zones, but the surface is less obviously varied than Jupiter’s, and neither are there
any vivid colours, so that the disk seems comparatively bland, and there has never
been anything to rival the Jovian Great Red Spot. However, there are strong winds
and storms, and between latitudes 35 and 36 we find a turbulent region nicknamed
“Storm Alley”. It has to be admitted that for really detailed views we have to rely
on space probes, notably the Voyagers of a quarter of a century ago and now the
Cassini mission, which was launched in 1997 and was still sending back invaluable
data over 10 years later (of course, it carried the Huygens lander which made a
gentle touch-down on Titan). But we must not forget the Hubble Space Telescope,
which has monitored Saturn and has sent back remarkable pictures of aurorae there.
Like ours, the aurorae are caused by particles of the solar wind which are trapped

by Saturn’s strong magnetic field and plunged down into the upper atmosphere,
making it glow. Saturnian aurorae are brightest in high latitudes north and south
because the rotational axis and the magnetic axis virtually coincide.
White spots sometimes appear on the disk. The brightest of modern times have
been those of 1933 (discovered by W.T. Hay, better remembered by most people as
Will Hay, the stage and screen comedian) and 1990, but there was a reasonably
noticeable white spot in 1996. When a new white spot appears, as no doubt will
happen before too long, there is a good chance that it will be first seen by an amateur.
When I began the Sky at Night series, in 1957, there was no telescope anywhere
which could be used to match these – but we have entered the Electronic Era, and
it has to be said that photography now looks decidedly old-fashioned.
Cassini, now happily moving round the planet, is the first Saturn orbiter; Pioneer
11 and the Voyagers were fly-by missions because the Pioneer encounter was really
an afterthought on the part of NASA and the Voyagers were on their way to the
outer Solar System. Cassini began its main work immediately after arriving, and
4712 Ringed World
one picture was particularly spectacular; viewed from the space-craft, the Sun
passed directly behind Saturn, so that Cassini lay in the planet’s shadow and the
rings were brilliantly back-lit. This lasted for twelve hours, and the NASA planners
wasted no time. An entirely new ring was discovered, engulfing the midget satel-
lites Janus and Epimetheus, and this was something of a surprise. It had been sug-
gested that meteoritic impacts on both these satellites might send a certain amount
of fine material into orbit, but nobody had really expected a complete ring, albeit a
very tenuous one. (En passant, I could have been the discoverer of Janus. In 1966,
I was using the 10-in. refractor at Armagh Observatory, in Northern Ireland, and
made three observations of the then-unknown Janus, but as I did not recognise it as
being new I can claim absolutely no credit!) Early images from Cassini also showed
the diffuse, extensive E-ring, with one of the familiar satellites, Enceladus, sweep-
ing through its outer part. Cassini also took a picture of our Earth, the first time our
world had been imaged from a range of over 900 million miles. It appears as a tiny,

featureless dot.
The F ring, outside the main system, is both “clumpy” and variable. The particles
are kept in their orbits by two small shepherd satellites, Prometheus and Pandora.
Cassini showed that Prometheus, a mere 63 miles in diameter, is interacting with
the ring and actually pulling particles off it: Pandora is rather smaller, but no doubt
acts in the same way.
Enceladus, discovered by William Herschel as long ago as 1787, proved to be an
amazing world even though it is so small (300 miles in diameter). There is an exces-
sively thin atmosphere; the surface is icy, and reflects almost 100% of the sunlight
falling upon it, so that the albedo is higher than for any other body in the Solar
System. The main surprise was the discovery of geysers in the South Polar Region,
sending water-ice particles high above the surface, so that there must be a heat-
source below – just about the last thing that anyone had expected. Earlier, it had been
found that Enceladus causes disturbances in Saturn’s magnetic field as it moves
along in its orbit, and this indicated the presence of a conducting medium (water?)
below the ice-sheet. Very probably, there really is an underground sea of ordinary
water, though it would be premature to speculate about Enceladan life-forms.
Hyperion and Iapetus, the two outer satellites of the “original eight”, have also
perplexed us. Hyperion is cratered, and does not have captured or synchronous rota-
tion; it takes 21½ days to complete one orbit, but is “tumbling along”, and at the
moment it spins round in a mere two days. This is not the main puzzle. Hyperion is
not regular in shape; it measures 255 × 162 × 137 miles, but because its density is 1½
times that of water it ought to have become a sphere. Why hasn’t it? And if it is the
broken-off half of a larger body, where’s the other half? There is no sign of it
Iapetus is larger, almost 900 miles across, and has an orbital period of 79 days;
this is the same as the axial rotation period. The distance from Saturn is 2,200,000
miles. Parts of its surface are bright and icy, while other parts are as black as a black-
board. Cassini results indicated that the blackness is due to a surface deposit rather
than material welling up from below. (Why Cassini? Because the Italian observer
G.D. Cassini paid close attention to Saturn during the seventeenth century; he

discovered Iapetus, Rhea, Dione and Tethys, plus the main division in the ring
48 12 Ringed World
system, now named after him. Rather confusingly, perhaps, the main dark
area on Iapetus has been christened Cassini Regio.) Iapetus is curiously-shaped;
928 × 930 × 891 miles, making it seem slightly squashed. The most peculiar feature
is the equatorial ridge running over 800 miles through the middle of the Cassini
Region; it follows the equator almost perfectly, but does not extend on to the bright
areas. It is around 12 miles wide, and in places rises to 12 miles above the ground,
making it much higher than Everest and comparable with anything on Mars. All
sorts of theories have been put forward to account for it. One involves a collision
between two smaller bodies which merged, while according to another idea both the
ridge and the dark patches were created when Iapetus grazed the outer edge of the
ring system long ago. Or could Iapetus itself have had a ring? We have to admit that
we simply do not know.
Titan is the giant of Saturn’s family, and is unique among planetary satellites
inasmuch as it has a thick atmosphere, denser than ours. The Huygens lander, carried
most of its way by Cassini, made a controlled touch-down upon “spongy” ground,
and sent back excellent images; obviously, it could not transmit for long, but it
exceeded all expectations. Since then Cassini has made regular passes, and radar
has shown beyond reasonable doubt that there are extensive seas, not of water but
of a mixture of ethane and methane.
Saturn and its satellites have already given us plenty of surprises, and no doubt
more are in store. Which intrigues you most? The new ring, the chemical seas of
Titan, the polar aurorae, the towering equatorial ridge of Iapetus or the spongy
tumbling Hyperion It is not easy to decide, but all in all I would choose the fountains
of Enceladus. When William Herschel first glimpsed the satellite over 200 years
ago, he surely could not have expected that the tiny speck seen in his home-made
telescope would prove to be an active world, with geysers hurling ice-crystals high
into space.
49

P. Moore, The Sky at Night, DOI 10.1007/978-1-4419-6409-0_13,
© Springer Science+Business Media, LLC 2010
Of all the problems faced by a modern astronomer, that of “dark matter” is one of
the most baffling. In the Sky at Night we return to it periodically, and for this programme
I was joined by Professors Gerry Gilmore and Bob Nichol.
Look up into the sky, and you will see bodies of all kinds – planets, stars, galaxies.
The Universe seems to be a crowded place. Yet, we now know that most of it is
invisible. We can make out less than 10% of it. The rest cannot be seen at all.
The man who first realised this, during the second half of the twentieth century,
was Fritz Zwicky, who was Swiss by blood, born in Bulgaria, and spent most of his
Chapter 13
Matter We Cannot See
Fritz Zwicky (Caltech)
50 13 Matter We Cannot See
working life in America. It is fair to say that he was one of the most eccentric
astronomers of his (or any other) time, but of his brilliance there was no doubt at
all. He examined the cluster of galaxies in the constellation Coma, measured their
motions, and realised that they were moving around so quickly that they should
fly apart. Yet they didn’t. Something was “glueing” them together; the cluster must
contain a vast amount of invisible material.
Next, the stars in rotating galaxies were not moving as they ought theoretically
to do, because they did not obey Kepler’s laws. In the Solar System the centre of
motion is the Sun, which also contains over 99% of the mass. Kepler’s Laws state
that planets closest to the Sun must move the fastest, and the furthest must be the
slowest, which is exactly what we find; Mercury is the quickest (which is why it
was named after the scurrying Messenger of the Gods) and remote Neptune is the
most leisurely. Now consider a spiral galaxy, such as M.31 in Andromeda or, for
that matter, our own Milky Way Galaxy. The stars are rotating round the nucleus of
the system, and Kepler’s Laws should apply. The Sun is about 25,000 light-years
from the galactic centre, and takes 225 million years to complete one orbit, a period

often referred to as the “cosmic year”. Stars further out should move more slowly,
but this is not true. The rotation is more like that of a solid, spinning cartwheel.
How can this be so? Again Zwicky had the answer. In a galaxy, the mass is not
concentrated at the centre, but is spread through the entire system. This explains
why the stars behave in the way that they do, but what precisely is the unseen material –
Zwicky’s “missing mass”? He did not know, and neither do we, well over 50 years
later. All kinds of suggestions have been made. Among these are vast numbers of
low-mass stars, too dim to be detected; material locked up in Black Holes and
therefore cut off from the rest of the universe; ordinary matter, but so tenuous that
it evades us; neutrinos, with a certain amount of “rest mass” – all these were inves-
tigated, and found to be wholly inadequate. More popular today are “WIMPs” – Weakly
Interacting Massive Particles, which are not the same as the matter we know, and
are beyond the reach of our equipment. This may sound plausible, but it is really
fudge, and an admission that we simply do not know. The one certain fact is that
unless all our measurements are wrong, dark matter definitely exists.
Back to Zwicky. In our Galaxy we have occasional stellar explosions which
are far more violent than ordinary “new stars”, or novae, which are not really new at
all; what happens is that the white dwarf component of a binary system suffers an
outburst which makes it flare temporarily up to many times its normal bril-
liancy before subsiding back to its former state. The more cataclysmic outbursts
are different; for them, Zwicky coined the term “supernovae”. They are of several
different kinds, but a Type 1a supernova involves the total destruction of a white
dwarf, which literally blows itself to pieces. All supernovae of this kind reach
about the same luminosity, which means that we can find their distances – and
since a 1a can become as powerful as all the stars in a galaxy combined, it can be
seen across vast stretches of the universe.
During the past 1,000 years only three supernovae have been seen in our Galaxy,
the stars of 1006, 1054, 1572 and 1604 (the most celebrated of these was that of
1054, which was not a 1a; we see its remnant now as the Crab Nebula). Zwicky
5113 Matter We Cannot See

believed that he would be able to detect supernovae in external galaxies, and as usual
he was right. Much later, it was found that these explosions could tell us something
more. We know that the universe is expanding, and has been doing so ever since
the Big Bang 13.7 thousand million years ago, and we know the speeds at which the
galaxies are racing away from us, but the 1a supernovae show that they are further
away than they ought to be if the rate of expansion is constant. The recessional
velocities of far-away systems ought to slacken off, because of the effects of gravity.
Instead, the velocities are increasing. We live in an “accelerating universe”.
Albert Einstein once introduced a force which he called the cosmological constant –
the opposite of gravitation. He subsequently rejected it, but it now seems that it
really does exist, and is known as dark energy. There is a tug-of-war between gravi-
tation and dark energy, and gravitation seems to be losing. What will eventually
happen we do not know. And if we are puzzled by dark matter, we are even more
so by dark energy. Its nature is completely unknown, and we cannot even make
reasonably plausible speculations.
For the moment we have to admit defeat – but only for the moment. New tech-
niques, new instruments, new theories come along with almost bewildering rapidity,
and moreover there may well appear a new Newton or a new Einstein to make a
fundamental breakthrough. If so, I hope he will be willing to come to a Sky at Night
studio, and talk to whoever has succeeded me as presenter of the programme!

53
P. Moore, The Sky at Night, DOI 10.1007/978-1-4419-6409-0_14,
© Springer Science+Business Media, LLC 2010
The universe is a violent place, calm though it may often look. Supernovae are
powerful enough, but gamma-ray bursters are even more so. I was joined by Nial
Tanvir and Julian Osborn; we also heard from Helen Fraser, and Chris Lintott went
down to see Tom Boles, working hard in his observatory during one of his nightly
supernova hunts.
Chapter 14

Gamma-Ray Bursters
Gamma-ray burster grb080916 NASA Swift (Stefan Immler)
54 14 Gamma-Ray Bursters
The role of an amateur astronomer has changed. When I started out, in 1930 (!),
an amateur was concerned mainly with the Solar System, and usually with one
particular object, in my case the Moon. There were some amateurs who were variable
star enthusiasts, but not many and amateur photography was rudimentary. Today all
this has changed, and though the Solar System is not neglected, amateurs have
extended their scope. Photography has given way to electronics; the modest 15-in.
reflector in my observatory can be used to produce images far better than any profes-
sional could have managed a couple of decades ago. Sadly, I have to admit that these
developments came just too late for me; all I can do now is to watch – and admire.
Searching for supernovae in outer galaxies is one favourite pastime. When a
supernova appears, observations are needed quickly, and this sort of investigation
does have to be left to the professionals, but a supernova cannot be predicted, and
this is where amateur help is invaluable. Two amateur hunters – Tom Boles in
England and the Reverend Robert Evans in Australia – have each made over a 100
discoveries. But powerful though they are, supernovae are outmatched by gamma-
ray bursters, which are unbelievably violent.
Gamma-rays are ultra-short and energetic. Fortunately for us, our atmosphere
shields us, and gamma-ray astronomy could not really begin until space research
became practicable, but during the late 1960s it was found that US satellites, sent
up to search for evidence of Soviet nuclear tests, were picking up bursts of gamma-rays.
The Russian nuclear tests proved to be as unreal as Iraq’s weapons of mass destruc-
tion, but the gamma-ray bursts were genuine, and astronomers were intrigued. On
5 April 1991 NASA launched the Compton Gamma-Ray Observatory to study them
(it was named after the American Nobel Prize winner Arthur Holly Compton, a
pioneer in gamma-ray research). The CGRO orbited until 4 June 2000, and carried
out a full survey of the sky, discovering almost three hundred sources. Many of
these could be identified; for example, the Crab Nebula is a gamma-ray emitter. But

the brief, super-energetic flashes, detected by BATSE (the Burst sand Transient
Source Experiment) were quite different, and nobody could make out what they
were. For a time it was thought that they might be relatively local, but on 23 January
1999 Compton found that one violent burst left an “afterglow” which could be
examined spectroscopically, and was obviously a long way away; its distance was
given as 4.5 thousand million light-years.
Now we have the Swift satellite, which was launched on 20 November 2004 and
was put into an almost circular orbit at an altitude of 370 miles (600 km). It was an
immediate success, and its BAT (Burst Alert Telescope) is on average detecting one
burst per day. When a burst is found, Swift can slew round and use its X-ray and
ultra-violet telescopes to follow the sequence of events. So far, the remotest burst
observed lies at a distance of 12.3 thousand million light-years.
Gamma-ray bursts (GRBs) are of two main types: short (less than 2 s in duration)
and long (several seconds). Their origins are different. A long burst is believed to
be due to the collapse of a hypergiant star, at least 40 times as massive as the Sun,
to form a black hole. When the star runs out of fuel and energy production comes
to an abrupt stop, its matter swirls downwards towards the core, and the infall
results in a pair of jets emerging from the rotational poles of the doomed star; the
5514 Gamma-Ray Bursters
shock waves break into space, and their immense energy is released in the form of
gamma-rays. A short burst is more probably due to a collision between two neutron
stars; which hit each other and fuse to form a black hole. The whole process takes
only a second or two, and there is no afterglow. It is also possible that some flashes
are due to flares from magnetars, which are stars with unusually strong magnetic
fields, but we know little about magnetars, because only five had been discovered
by the end of 2006.
Could we be in danger from a gamma-ray burst? If it occurred within a few light-
years from us, the answer is “yes”. Even a supernova would make things very
uncomfortable from a range of, say, 200 light-years, and comparing a supernova
with a GRB is like comparing a match with a searchlight. However, our Galaxy

does not seem to be of the type prone to GRBs, and our particular region is reas-
suringly quiescent. We can survey them from a respectful distance, and see how
they behave – the biggest bangs since the original Big Bang almost fourteen thousand
million years ago.

57
P. Moore, The Sky at Night, DOI 10.1007/978-1-4419-6409-0_15,
© Springer Science+Business Media, LLC 2010
Jupiter was prominent in the night sky during the summer of 2006, so this seemed
to be a good time to devote a programme to the giant planets and their past
wanderings. I was joined in my study by Drs Richard Nelson, David Rothery and
John Rogers, while in my observatory Chris Lintott and a group of observers
including Pete Lawrence, Damian Peach, Bruce Kingsley and Ian Sharp turned the
15-in. reflector toward Jupiter, now showing two Red Spots instead of only one.
Our Solar System is in some respects an orderly place. There are eight planets,
in two well-defined groups. The four inner planets, Mercury to Mars, are rocky and
comparatively small; beyond them comes the main belt of asteroids, of which only
one Ceres is over 500 miles in diameter, and the only one, Vesta is ever visible with
the naked eye. Next come the four giants, Jupiter to Neptune, and then the Kuiper
Belt objects, of which the best-known, though not the largest, is Pluto. But things
have not always been as straightforward as this.
We have a good idea of the age of the Solar System, because we are confident
that the Earth is about four and a half thousand million years old. The planets were
formed in a rotating disk of material round the youthful Sun, which was not then as
Chapter 15
Wandering Giants
Jupiter, photographed by Damian Peach
58 15 Wandering Giants
luminous as it is now. Instabilities in this disk led to the gradual formation of
“cores” containing both ice and water. These cores built up to bodies around 15 times

the mass of the Earth, a process which took at least a million years. Their gravita-
tional pulls were now great enough to collect more material from the solar nebula,
and the cores or “protoplanets” making up our Jupiter and Saturn were particularly
massive. The growth must have been quick by comical standards; even so, the cores
of our Uranus and Neptune did not become gravitationally powerful until much of
the nebular material had been dispersed. These two are composed largely of ices,
and each may have no more than two Earth masses of gas.
There was also the “left-over” material, with a total mass which was far from
negligible; at least 35 times that of the Earth, and this had a very marked influence
upon the sequence of events. Initially, Neptune may have been closer-in than Uranus.
Interactions between the ice-giants and the scattered material meant that there may
have been what is usually termed planetary migration, which obviously took a long
time; Uranus and Neptune were driven outward, possibly exchanging places, though
Jupiter and Saturn were less affected because they had built up to bodies of much
greater mass. By now the migrations have stopped, and the Solar System has settled
into its current stable form, but in its early history the situation was chaotic. At one
time, Jupiter and Saturn may have been in a 2:1 resonance, Jupiter making two orbits
round the Sun for every one of Saturn’s.
Uranus has an unusual axial tilt – 98°, more than a right angle to the orbital
plane. It has been generally assumed that this was due to the impact of a large body
which literally “tipped Uranus over”. This does not sound very plausible. Moreover,
the Uranian satellites share the same inclination, and an impact could hardly have
taken them along. It is much more likely that interactions with the other giants and
with the debris caused a slow, steady tip-over.
In the inner part of the forming Solar System, another important factor had to be
taken into account. The Sun went through a period of great activity, sending out a strong
“wind” – known as a T Tauri wind, because it has been identified in other stars, of which
the faint variable T Tauri was the first. Most of the light gas (hydrogen, with some
helium) was blown away, leaving only the rocky materials. This is why the Earth is
comparatively deficient in hydrogen, which is the most plentiful element in the universe

as a whole (atoms of hydrogen far outnumber the atoms of all the other elements put
together). There was a period too, when debris bombarded all the newly formed planets;
we still see the effects in as much as solid planets and satellites are crater-scarred,
though in some cases (notably Earth) most of the craters have been eroded away, while
in others (Venus, and Jupiter’s satellite Io) the craters have been removed because
volcanic activity has provided total re-surfacing long after the Great Bombardment
ended, 3.9 thousand million years ago.
No large planet could be formed in the zone now occupied by the main-belt
asteroids, because of the disruptive pull of Jupiter. In the Kuiper Belt, the bodies
are widely spaced; so far as we know there is no dense swarm. Eris is the largest
known KBO at present, but even so it is no more than about 1,500 miles in diameter.
There may be another major planet way out beyond the Kuiper Belt; this now seems
5915 Wandering Giants
unlikely, but is not impossible. If it exists, is bound to be so faint that its discovery
will be largely a matter of luck.
There are “irregular” objects, often with highly eccentric and inclined orbits,
such as Sedna, which has an orbital period of over 10,000 years, and whose path it
takes from rather beyond the Kuiper Belt out to the region of the much more distant
Oort Cloud, far beyond the reach of even the Hubble Space Telescope. Some satellites
of the giant planets have retrograde motion, and are certainly captured bodies rather
than bona-fide satellites. The most important of these is Neptune’s attendant Triton,
which is larger than Eris or Pluto. Neptune could not have captured it from the
Kuiper Belt had it been single; it was probably one member of a pair – in fact a
binary KBO. When approaching Neptune the two were wrenched apart; one compo-
nent was “kicked away”, and the other put into a path round the ice-giant which was
initially eccentric, but gradually became more circular. Binary KBO’s are very
common, and indeed Pluto and its companion Charon make up such a binary; two
more members of the group, Nix and Hydra, have recently been found, though both
are very small.
At the moment the Solar System is going through a “calm” period. Migration is

to all intents and purposes suspended, and there is little chance of dramatic change
in the near future. But this state of affairs cannot continue indefinitely, because the
Sun itself will eventually change. In perhaps a thousand million years it will have
become so luminous that our Earth will be too hot for mankind to survive; subse-
quently it will swell out to change into a red, giant star. However, it will have lost
an appreciable amount of mass, and its gravitational pull will have weakened, so
that the orbits of the planets will spiral outward, and the Earth’s distance from the
Sun may increase enough to save it from being incinerated. After this may come
the planetary nebula stage, and when the nebulosity has dispersed, the Sun will be
reduced to a tiny, super-dense globe – a white dwarf, still orbited by the ghosts of
its remaining planets. This may sound depressing – but we will not be there to see.
Humanity may have departed to find a better home.
At least the crisis is not imminent. We have plenty of time to work out what we
can do!