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SIXTY YEARS ago this month, a young astronomer in Arizona began a systematic
search of the sky. His goal was to discover a new planet, something achieved
only twice before in human history. Within a year, Clyde Tombaugh had succeeded.
After looking in detail at the photographic images of two million stars,
he pinned down a ‘star’ that moved. It was the planet that we now call Pluto,
beyond the known bounds of the Solar System.

Over the years since its discovery, Pluto has provided more puzzles
and surprises than any other world in the Solar System. Immediately Tombaugh
discovered Pluto, he realised it was much fainter, and hence smaller, than
the planet he was expecting. Pluto’s orbit is more elongated than that of
any other planet, so that it actually crosses over the orbit of Neptune.
Pluto also turned out to be a ‘double planet’: it has a moon that is half
as big across as Pluto.

Some astronomers now doubt whether Pluto should be called a planet at
all: perhaps it is better described as a large asteroid. Tombaugh is convinced
that Pluto is a planet, if only because there are no signs of asteroids
orbiting the Sun at the distance of Pluto. The International Astronomical
Union agrees: so officially there are nine planets in the Solar System.
People have known about the first six planets, Mercury to Saturn, since
prehistoric times. They are all bright enough to be seen easily by the unaided
eye. In 1781, an amateur astronomer, William Herschel, stumbled across a
new planet beyond Saturn, the world we now call Uranus (named after the
father of Saturn).

Over the next four decades, astronomers found that Uranus was not moving
at a uniform pace around its orbit. They suspected that the gravity of an
eighth planet was affecting it. Two mathematicians, John Couch Adams in
England and Urbain Leverrier in France, independently calculated where this
planet should be. In 1846, Johann Galle, an astronomer at the Berlin Observatory,
found the new planet, Neptune.

Even allowing for the pull of Neptune, however, something was still
affecting Uranus. And as years went by, it seemed that Neptune was also
being pulled out of place. The hunt was on for a ninth planet. This time,
the calculations were made, independently, by two Americans, William Pickering
and Percival Lowell. Both predicted a planet several times as heavy as the
Earth lying beyond the orbit of Neptune.

Lowell had built an observatory at Flagstaff in Arizona to study the
‘canals’ on Mars, and, until his death in 1919, he used the telescopes here
to look for his predicted ‘Planet X’. In 1929, the observatory built a new
telescope to take photographic plates that could reveal Planet X. The director
hired Tombaugh to do the hard work of exposing the plates every night, and
inspecting them every day.

Tombaugh’s task was to compare plates taken a few nights apart, to look
for any ‘stars’ that moved. He started at Lowell’s predicted position for
Planet X. When the planet failed to show, he decided to ignore the prediction.
He would instead search all the way around the sky for objects as faint
as the telescope would show. By the end of January 1930, Tombaugh was looking
among the stars of Gemini. Suddenly, there was a point of light that moved
between two plates taken six nights apart. ‘When I saw the amount of motion
between the plates, I knew instantly it was beyond Neptune,’ Tombaugh recalls.

By this time, astronomers had given most of the names from classical
mythology to asteroids, but ‘Pluto’ was still available. The name of the
god of the Underworld seemed suitable for a planet exiled in the dark exterior
of the Solar System, and the first two letters were Lowell’s initials. Most
astronomers thought that this was entirely appropriate because Pluto did
seem to fit requirements to be Lowell’s Planet X. Tombaugh picked it up
only a few degrees away from the position that Lowell had suggested. Pluto’s
orbit was also very similar to the orbits of the planets predicted by both
Lowell and Pickering.

There was, however, one problem: Pluto was much fainter than Planet
X should have been. That was why astronomers had previously overlooked it
(including Tombaugh at the start of his search). How could Pluto be massive,
yet so dim? It might be a very dark body; or, conversely, it might have
a mirror-like surface that reflected a tiny image of the Sun, like a lamp
reflected in a ball bearing. The other answer could be that Pluto was not
Planet X. There was just no way to tell.

The answer came in 1978. Astronomers at the US Naval Observatory in
Washington DC had been taking regular photographs of Pluto for many years,
using a telescope not far from the Lowell Observatory in Arizona, to try
to measure its orbit more precisely. In June 1978, Jim Christy decided to
look at some photographs that had been rejected, because the image of Pluto
looked elongated, as if the telescope had accidentally moved during the
exposure. Christy noticed that the images of the stars on the same plates
were sharp and round: only Pluto’s image was elongated.

‘I realised what it meant immediately,’ he recalls, ‘but I didn’t really
believe it. I went away and got on with something else and then later thought:
‘I know it’s a moon!’ ‘ The elongated image consisted of two overlapping
images – of Pluto and of a fainter companion. Christy checked on plates
he had taken several years earlier, and found the same kind of image. He
and his colleague Bob Harrington discovered that they could fit all these
images with a moon that goes around Pluto roughly once a week, at a distance
of 20 000 kilometres.

Using Newton’s law of gravitation, Christy and Harrington could immediately
work out the combined mass of Pluto and its moon. It came out to be about
1/500 of the Earth’s mass. Lowell’s Planet X had to be several times heavier
than the Earth, to influence Uranus and Neptune. So Pluto was not ‘Planet
X’. The discovery of the new planet near to the predicted position of Planet
X, and the closely similar orbits, were just coincidence.

It fell to Christy, as the new satellite’s discoverer, to name it. He
settled on ‘Charon’. In Greek mythology, Charon was the ferryman who carried
souls to Hades across the river Styx. But Christy’s reasoning was more prosaic.
He wanted to name the satellite after his wife, Charlene, familiarly known
as Char (so the accepted pronunciation is ‘sharon’). As Charlene Christy
puts it, ‘Some husbands promise their wives the moon, but mine got it for
³¾±ð!’

Since those first photographs of Pluto and Charon blurred together,
other astronomers have obtained images that show the two bodies clearly
separated. These reveal that the two are not very different in size. Most
satellites are only a few per cent of the size of their parent planet, at
most, with the exception of our Moon, which is one-quarter the size of the
Earth. But the diameter of Charon is fully half that of Pluto. As a result,
the planet and its moon orbit around a mutual centre of gravity that is
between them. To many astronomers, the Pluto-Charon pair is best described
as a double planet.

Just as our Moon always keeps the same face turned towards the Earth,
so Charon keeps the same side turned towards Pluto. In each case, the gravity
of the planet tends to put a brake on the rotation of the satellite, so
it becomes ‘locked’ with the same orientation relative to the planet. Because
Charon is so large compared with Pluto, it has also managed to brake Pluto,
so that the same side of Pluto always faces its moon. If there were any
inhabitants on Pluto, people on one side of the planet would never see Charon
at all; those on the other side would see a large moon perpetually hanging
in the sky, six times the size of the Moon as seen from the Earth.

The orbit of Pluto and Charon is very tipped up compared to the plane
of their orbit around the Sun. From our vantage point on the Earth, we sometimes
see the orbit almost flat-on, so the two bodies seem to move in a circle
around one another, and sometimes edge-on, so that Pluto and Charon move
in front of one another as they perpetually orbit one another. Occultations,
or the period when they actually pass in front of one another, occur only
once every 124 years (half of the time it takes Pluto and Charon to orbit
the Sun) and last for only five years. Astronomers were exceedingly lucky
that one of these periods began just a few years after the discovery of
Charon.

In March 1985, astronomers began to see the two bodies clip each other’s
edge as they orbited one another. As the orbit became more accurately edge-on
to us, each body blocked off more of the other, until in 1987 and 1988 Charon
was disappearing completely behind Pluto on the far part of its orbit. On
the nearest part of each orbit, Charon passed squarely in front of Pluto,
hiding a portion of Pluto’s surface equal to its own size. Now we are past
the best. The two bodies are only partially hiding each other again. By
October 1990, the whole show will be over, until the 22nd century.

The actual view through a telescope shows nothing particularly spectacular
about these events. Because Pluto and Charon are so close, it is difficult
at the best of times to see them as two separate bodies. What astronomers
can do is to study changes in the light reflected from both Pluto and Charon.
This deceptively simple technique has provided more information on Pluto
than had been obtained in over five decades of previous studies.

Marc Buie, of the Space Telescope Science Institute in Baltimore, Maryland,
has been using a telescope on Hawaii to measure the combined light of the
system since the events began in 1985. As Pluto starts to hide Charon, or
the other way round, the total light from the system must decrease. By timing
accurately when the system starts to fade, and when it brightens again,
Buie has worked out the sizes of both bodies. More startling than the results
is the accuracy: the technique has a precision of a few kilometres in measuring
the sizes of bodies that are more than 4 billion kilometres away.

Buie, with David Tholen of the University of Hawaii, has found that
Pluto is 2284 kilometres across, with Charon almost exactly half Pluto’s
size, at 1192 kilometres in diameter. This makes Pluto only two-thirds the
diameter of the Earth’s Moon, and less than half that of the next smallest
planet, Mercury. An accurate measurement of Pluto’s diameter is not just
a way of fixing Pluto’s ranking in the general scheme of things. When combined
with the planet’s mass, it gives the density of Pluto, which in turn gives
us clues to the composition of the planet.

Although we know the diameters of Pluto and Charon individually, their
orbital characteristics tell us only the total mass of the system. In practice,
this is not too much of a handicap. If the two objects have the same density,
we can find this by dividing the total mass by the combined volume. Even
if they do not have the same density, the answer must be close to the actual
density of Pluto, because it contributes seven-eighths of the volume of
the system, and presumably the major proportion of the mass.

Buie and Tholen found that Pluto turns out to be close to twice as dense
as water. This came as a surprise, for two reasons. Spectra of Pluto had
shown that its surface is coated with methane, and solid methane has a low
density. If Pluto were made entirely of frozen methane it would have a density
little more than half that of water.

Astronomers have also found that, in general, the density of the solid
worlds in the Solar System decreases with distance from the Sun. The four
inner planets (including the Earth) are made of rock and iron, and are four
or five times as dense as water. The outer planets themselves are made largely
of gases, so are not a fair comparison. But the Voyager spacecraft have
measured the densities of their satellites, several of which are larger
than Pluto. The main satellites of Jupiter are between two and three times
as dense as water, while the smaller moons of Saturn have a density similar
to that of water. These worlds contain less rock than the inner planets,
with much of their bulk consisting of frozen water: some of Saturn’s moons
are virtually pure ice.

So astronomers had placed their bets on a Pluto composed mainly of either
water ice or methane ice: but it must be made largely of rock. William McKinnon,
of Washington University in Saint Louis, Missouri, and Steve Mueller of
the Southern Methodist University in Dallas, Texas, have investigated in
more detail. They had already built theoretical models of the satellites
of Jupiter and Saturn, tailoring the inner structure to fit the densities
and other measurements made by the Voyager probes. In the case of Pluto,
they find that three-quarters of its material must be rock. Most of the
rest is water ice, with a few per cent being methane. McKinnon and Mueller
say that the early Pluto probably suffered a meltdown, with about one-third
of the ice turning to water. As a result, the rock sank to the centre. Surrounding
this rocky core today is a mantle of water ice, with some methane trapped
in the surface layers.

The mutual occultations of Pluto and Charon also allow astronomers,
for the first time, to make a map of the surface features of both worlds.
Again, all they have to do is to measure accurately the total light from
the system. If Charon, for example, were the same brightness all over, without
any especially bright or dark regions, then the light from the system would
fade smoothly as it disappears behind Pluto. If it had a small bright spot,
then the light would drop rapidly when Pluto hid this spot. Similarly, we
can look for bright or dark regions on Pluto, by measuring changes in the
total brightness when Charon moves in front of it.

Simple though this technique is in principle, it requires a lot of computer
power to turn the observations into maps of Pluto and Charon. Buie’s first
approach was to build simple models of Pluto, with bright caps at the poles
and light or dark spots around the equator. These fitted the data fairly
well. Now he has used a sophisticated computer technique to turn the measurements
into a real map that shows how the planet would actually look if we could
see it through a sufficiently powerful telescope. The technique, called
the maximum entropy method, starts with a featureless globe, and adds the
minimum of details required to match the observations.

This first map of Pluto confirms that the planet has bright caps at
the poles, which are almost certainly composed of frozen methane. They are
three to four times as bright as the darker regions at the equator. The
equatorial region is slightly reddish in colour, and Buie suggests that
the surface here is coated with ‘dark organic residues, caused by the breaking
apart of methane and the formation of organic material over the age of the
Solar System’.

Buie advises caution in interpreting the brighter and darker regions
around the equator. ‘We’ve traditionally been fooled by low-resolution maps
of the planets: in the case of Mars, we were completely wrong.’ Views from
the Earth had led astronomers to interpret Mars’s dark markings as oceans,
and then as areas of growing vegetation. Spacecraft eventually showed that
they were regions of desert swept clear of bright dust by seasonal winds.
Buie thinks that the markings on Pluto are related to the amount of methane
on the surface. Conditions on Pluto are such that a small increase in temperature
can produce a large increase in the amount of methane evaporating from the
surface.

Two different worlds

Charon, Buie says, is quite different from Pluto. It is far more uniform,
with difference in brightness amounting to only 10 or 20 per cent, similar
to the contrasts on the Earth’s Moon. Charon probably does not have polar
caps, nor a band around the equator. The main features seem to be bands
at latitudes that correspond to the temperate zones on Earth; a dark band
in one hemisphere and a bright band in the other.

Rick Binzel, at the Massachusetts Institute of Technology, is also mapping
Pluto and Charon. He has been using telescopes at the McDonald Observatory,
deep in Texas. Binzel says he is not as far along as Buie, and their results
so far are a little different, but consistent. The important thing, he stresses,
is to map Pluto with independent observations and with different computational
techniques, and see if the answers come out the same. ‘If they do, then
we really have found out something about the surface of the planet.’

Eventually, this technique should produce a map of Pluto revealing as
much detail as the naked eye can see on our Moon, but both Buie and Binzel
see this as being many months away yet. Buie also hopes to check whether
the brighter patches do contain more methane. He has been monitoring the
infrared brightness of Pluto-Charon at wavelengths that correspond to spectral
lines of methane. The resulting map should pin down just where the methane
lies on Pluto.

Infrared observations have already pointed up a difference between Pluto
and Charon. Because the two bodies are so close, it is almost impossible
to separate the infrared spectra of Pluto from that of Charon. Instead,
Buie and his colleagues observed the combined infrared spectrum and then
the spectrum of Pluto alone, when it was hiding Charon. By subtraction,
Buie obtained the spectrum of Charon.

As expected, Pluto’s infrared spectrum showed strong lines of methane.
But Charon shows no sign of methane at all. Its spectrum has instead a strong
line at 2 micrometres, which is characteristic of water ice. It is similar
to the spectra of the icy moons of Jupiter, Saturn and Uranus. Buie says
Charon and Pluto may have started off with similar surfaces, containing
methane. Because Charon has a lower gravity, however, its original complement
of methane has escaped into space, exposing its mantle of water ice.

This observation ties in neatly with the idea that the bright patches
seen on Pluto, but not Charon, are made of methane ice. Fresh methane ice
should be brighter than the dull water ice of the mantle, just as a fresh
snowfall on the Earth is whiter than ice or snow that has been exposed for
some time. So we might expect Pluto to reflect sunlight better than Charon,
and an analysis of the amount of light reflected by each body separately
shows that Pluto reflects about half the light falling on it, while Charon
reflects only one-third.

If Pluto has fresh falls of ‘methane snow’, then it should have an atmosphere
of methane, however tenuous. Telescopes could not show such an atmosphere
directly, and the infrared spectra cannot distinguish methane ice from methane
gas. But astronomers recently had another stroke of luck. Last June, Pluto
passed in front of the distant star. If Pluto had no atmosphere, then the
star’s light should be cut off abruptly as the planet moved in front of
it. Any atmosphere around Pluto would cause the star to become dimmer before
it disappeared. Astronomers prepared months in advance for the event. It
was visible only from the south Pacific ocean, New Zealand and Australia.
Observatories in New Zealand and Australia, some staffed by amateur astronomers,
installed electronic equipment to monitor precisely the star’s changing
brightness.

The American astronomers who had been studying Pluto did not intend
to miss out. Bob Millis, of the Lowell Observatory, set up a telescope in
Australia. Jim Elliot, of the Massachusetts Institute of Technology, booked
the Kuiper Airborne Observatory, a flying observatory, run by NASA. It is
a converted Lockheed transport plane that carries a telescope almost a metre
in diameter. In 1977, Elliot had discovered the rings of Uranus from this
plane high over the Indian Ocean. Now he was flying over the Pacific, to
look out for an atmosphere around Pluto.

Elliot’s group saw the star begin to fade as its light passed 1500 kilometres
from Pluto’s centre, indicating the beginnings of an atmosphere. The light
dimmed gradually at first, then more quickly. It finally faded out without
any sharp cutoff to indicate that it had passed behind the solid edge of
the planet.

According to Elliot, those observations show that Pluto’s atmosphere
consists of two different regions. The upper part is a layer of transparent
gas some 300 kilometres thick, which caused the first gradual fading of
the star’s light. Below is a layer of haze that is at least 46 kilometres
thick. The haze is not opaque enough to obscure the features that Buie and
Binzel are mapping on the surface of Pluto, but it can hide a star near
the edge of the planet because we are then looking a long distance through
the haze.

The pressure at the bottom of Pluto’s atmosphere is just a few millionths
of that on Earth, but the extent of the outer part of the atmosphere is
huge when compared with the size of Pluto. Although Pluto is only one-fifth
the diameter of the Earth, its atmosphere stretches about twice as far upwards.
Elliot says the atmosphere must contain some methane, but this is not necessarily
the main component, which could be nitrogen or argon. The observations show
that the atmosphere could consist purely of methane at 68 K, or mainly of
nitrogen at 107 K.

At the Lowell Observatory, Millis has analysed the results from all
the ground-based observatories in Australia and New Zealand. He is confident
that the atmosphere of Pluto consists mainly of methane. ‘Methane is the
only substance that shows up in the infrared spectra,’ Millis points out.
The lower temperature needed for a pure methane atmosphere also fits better
with other observations of Pluto, such as those made by the Infrared Astronomical
Satellite in 1983.

What is puzzling the researchers now is what happens at the bottom of
the hazy layer. The star disappeared when its light was passing 1142 kilometres
from the centre of Pluto, just where Buie and his colleagues put the surface
of Pluto, from their observations of Charon. Millis and Elliot think it
is possible that Charon too was disappearing behind Pluto’s haze, not behind
its solid surface. In other words, Pluto could be even smaller than the
diameter of 2284 kilometres that is now generally accepted.

Millis cites some new measurements that may support this conclusion.
His team at the Lowell Observatory has been measuring as precisely as possible
the positions of Pluto and Charon not just relative to each other, but to
the background stars. The centre of mass of the double planet must move
in a smooth path through space, and by measuring the wobble of Pluto and
Charon individually to either side of this path, Millis hopes to work out
the proportion of the total mass in each. His first results show that Pluto
contains less of the total than people expected. This means either that
Pluto has a lower density (with the corollary of a very high density for
Charon) or that it is smaller than the accepted value.

The 1980s and 1990s are a particularly good time for studying Pluto’s
atmosphere. Pluto follows a very elongated orbit about the Sun, and in September
this year it reaches its closest point. Since 1979, Pluto has been closer
to the Sun than Neptune, and will stay that way until 1999. Alan Stern of
the University of Colorado and Larry Trafton of the University of Texas
have calculated that the extent and density of Pluto’s atmosphere depend
strongly on its distance from the Sun. At Pluto’s far point, the atmosphere
condenses onto the surface, as methane snow. Each time it comes back to
the Sun, some of the methane evaporates again, and the atmosphere builds
up.

Pluto’s strange orbit has puzzled astronomers since the 1930s. If it
can cross the orbit of Neptune, then why has it not collided with the larger
planet, or at least come so close that Neptune’s gravity would fling it
out of the Solar System? The simple answer is that Pluto leads a charmed
life. The time it takes to orbit the Sun is exactly 1.5 times the period
of Neptune. This ensures that whenever Pluto is relatively close to the
Sun, Neptune is at a position in its orbit that is a safe distance away.
Pluto follows a very tilted orbit, which takes it well below the plane of
Neptune’s orbit when Pluto is closest to the Sun. This geometry is assured
by the repeating gravitational pull of Neptune prevents Pluto’s orbit swinging
round in such a way that the point where the two orbits cross could coincide
with Pluto’s closest point to the Sun.

For these reasons, the textbooks say that Pluto is safe enough. But
Jack Wisdom and Gerald Jay Sussman, of the Massachusetts Institute of Technology,
have recently taken a closer look. They have made a detailed calculation
of Pluto’s motion over hundreds of millions of years, including the gravitational
pulls of Jupiter, Saturn, Uranus and Neptune. Most important, they have
also taken account of the newly developed theory of chaos.

In chaos theory, a small effect can give rise to strikingly large consequences:
the classic example is the flutter of a butterfly’s wings in California
causing a storm in New York. Wisdom and Sussman ran the calculations twice,
with Plutos that started off in very slightly different places. They found
that the motions of the two Plutos were entirely different after only 20
million years, less than 1 per cent of the age of the Solar System.

Although the calculation proves that Pluto’s orbit is ‘chaotic’ in a
strictly mathematical sense, it does not necessarily mean that Pluto must
whizz off into a completely new orbit, either in the future or in the past.
Other mathematicians have recently shown that the motions of Mercury and
Mars (and probably Venus and the Earth, too) are also chaotic in the mathematical
sense, and we are pretty sure that they have been following much the same
paths since the birth of the Solar System. But Pluto is a much smaller body,
under the gravitational sway of much larger planets, rather like the asteroids
in the belt between Mars and Jupiter. Wisdom has specialised in working
out the paths of asteroids. He has found that he can sometimes follow an
asteroid’s motion for 100 000 orbits with nothing happening, and then there
is a sudden change into a completely different orbit.

‘We have no ability to predict the future of Pluto,’ Wisdom concludes,
‘and we cannot deduce where it came from.’ Until the discovery of Charon,
many astronomers thought that Pluto was an escaped moon of Neptune. But
the idea that both Pluto and Charon could have escaped together from Neptune
is so unlikely that this idea has been abandoned. Pluto could either have
formed in much the same orbit as it now occupies, or in a different orbit
that was altered by the gravity of the massive outer planets. We can never
tell, from its orbit alone, which is correct.

The most promising way of learning more about the origin of Pluto is
to compare its composition with the make-up of other bodies in the outer
part of the Solar System. Astronomers hope that the Voyager spacecraft will
provide some clues when it flies past Neptune this August. Neptune’s larger
moon, Triton, is very similar in size to Pluto and at the moment is as roughly
as far from the Sun.

¿ìè¶ÌÊÓƵs in the US and Europe are also planning spacecraft that will
follow up the success of the Giotto mission to Halley’s Comet, and investigate
the solid nuclei of other comets. These objects have come in towards the
Sun from regions even farther out than Pluto.

For Pluto itself, however, the immediate prospects are bleak. After
Pluto and Charon finish their game of hide-and-seek in 1990, Buie says ‘the
leaps and bounds in discoveries will go away; new results will come very
much harder’. About that time, astronomers should be using the Hubble Space
Telescope, an instrument above the Earth’s blurring atmosphere. Even this
instrument will hardly show Pluto as a disc, but it will allow astronomers
to measure the brightnesses and spectra of Pluto and Charon independently.

What about a spaceprobe to Pluto? ‘That’s a dream right now,’ says Buie.
The path of Voyager 2 will not take it on to Pluto. Even another Voyager
would not be much help. Voyager 2 was sped on its way by the gravitational
help of Jupiter, Saturn and Uranus. As a result, it reached Neptune, beyond
the current distance of Pluto, in only 12 years. Now the planets are no
longer arranged in a suitable way. Buie says a Pluto probe would take 60
to 70 years to arrive at its goal.

But that applies only to a probe with rocket engines that boost it up
from Earth, and then let it coast. Buie is envisaging instead a craft that
generates thrust continuously throughout the flight. ‘If it carried an engine
that gave it a thrust equivalent to only one-hundredth of the Earth’s gravity
it would reach Pluto in only two months.’

Space scientists in Europe are already working on a suitable type of
rocket engine, to send a probe to rendezvous with a comet’s nucleus. These
ion drives replace chemical reactions by an electric field that discharges
ionised gases. The actual thrust is much less than that produced by a chemical
rocket, but it can carry on for months or years.

In a meeting in a month’s time, Buie will start discussing these ideas
with other colleagues who are studying Pluto. They hope that their enthusiasm
may fire the planners of the next generation of interplanetary spacecraft.
One of the reasons for sending spacecraft to the planets is to understand
their development and the formation of their Solar System, so casting new
light both on the history of our Earth, and on the processes that may create
planets around other stars.

The recent observations of Pluto indicate that it may be a new type
of world. It is clearly different from the rocky planets near the Sun, and
from the giant planets of gas. Pluto now seems to differ, too, from the
icy moons of the outer planets. It may be a unique object in our Solar System,
telling us how a planet is formed in the most distant reaches of a planetary
system, far from the warmth of a central star. For that reason alone, Pluto
is worth further exploration.