Philips Atlas of the Universe

Pluto

ATLAS OF THE UNIVERSE

E

ven after the discovery of Neptune, there were still

tiny irregularities in the movements of the outer giants

which led Percival Lowell (of Martian canal fame) to

make fresh calculations in the hope of tracking down yet

another planet. In 1930, 16 years after Lowell’s death,

Clyde Tombaugh used a specially obtained telescope at

the Lowell Observatory to identify a new planetary object

only a few degrees from the predicted place. After some

discussion it was named Pluto – a suitable name, since

Pluto was the god of the Underworld, and the planet

named after him is a gloomy place even though sunlight

there would still be 1500 times brighter than full moon-

light on Earth.

Pluto has a curiously eccentric orbit, and when closest

to the Sun it moves well within the orbit of Neptune,

though since Pluto’s path is inclined by as much as 17

degrees there is no fear of collision. The last perihelion

passage fell in 1989, and not until 1999 was Pluto’s dis-

tance from the Sun again greater than that of Neptune. The

revolution period is almost 248 years; the axial rotation

period is 6 days 9 hours, and the axis of rotation is in-

clined by 122 degrees to the perpendicular, so that the

calendar there is very complicated indeed.

The main puzzle about Pluto is its small size and mass.

The diameter is a mere 2324 kilometres (1444 miles),

which is less than that of the Moon or several other plan-

etary satellites, including Triton. The mass is no more than

0.002 that of the Earth, and obviously Pluto can have no

measurable effect upon the motions of giants such as

Uranus and Neptune. Either Lowell’s reasonably accurate

prediction was sheer luck (which is hard to believe), or

else the real planet for which he was hunting remains to

be discovered.

The density is over twice that of water, so that there

must be a fairly high percentage of rock in its globe; there

could be a silicate core surrounded by a thick mantle of

ice, but we have no definite information, because no

spacecraft has been anywhere near Pluto. One thing we do

know is that there is a thin but extensive atmosphere.

When Pluto passes in front of a star, and hides or occults

it, the star fades appreciably well before it is covered up,

so that for a brief period its light is coming to us by way

of Pluto’s atmosphere. The atmosphere may be methane,

nitrogen or a mixture. When Pluto moves out to the far

part of its orbit the temperature will become so low that

the atmosphere may freeze out on the surface, so for part

of the Plutonian ‘year’ there is no gaseous surround at all.

The next aphelion is due in 2114, but the atmosphere will

probably condense out long before that.

In 1977 it was found that Pluto is not a solitary trav-

eller in space. It is associated with a secondary body,

which has been named Charon in honour of the somewhat

sinister boatman who used to ferry departed souls

across the River Styx on their way to the Underworld.

Photographs taken with the Hubble Space Telescope show

the two bodies separately, even though they are less than

20,000 kilometres (12,500 miles) apart. Charon has a

diameter of 1270 kilometres (790 miles), more than half

that of Pluto itself; the mass is one-twelfth that of Pluto,

and when the two are shining together Charon contributes

only 20 per cent of the total light. Its orbital period is

6.3 days, which is the same as the axial rotation period

of Pluto, so that the two are ‘locked’ and an observer on

Pluto would see Charon hanging motionless in the sky.

By a fortunate chance there were mutual eclipse and

occultation phenomena during the late 1980s – a state of

affairs which will not recur for 120 years. When Charon

passed behind Pluto it was completely hidden, and Pluto’s

spectrum could be examined alone; when Charon passed

in front of Pluto the two spectra were seen together, and

that of Pluto could be subtracted. Pluto appears to have a

surface coated with methane ice, perhaps with some ice of

 Discovery plate of Pluto,

taken by Clyde Tombaugh in

1930. Pluto is indicated by the
      arrows; it looks exactly like a
      star, and was identified only
      because of its motion from
      night to night. The very over-
      exposed star image is that of
      Delta Geminorum, magnitude
      3.5; the magnitude of Pluto
      was below 14.

▲ Size of Pluto. Pluto

is shown here compared

with the Moon, Triton

and Charon. It is clear the

Pluto–Charon pair cannot

be regarded as a planet-and-

satellite system; Charon has

more than half the diameter

of Pluto.

▲ Clyde Tombaugh,

discoverer of Pluto; this

photograph was taken in

1980 at the 50th anniversary

of the discovery.

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