The Solar System

544 PART 4^ |^ THE SOLAR SYSTEM

Th e observatory director turned to other projects, and

Tombaugh, working alone, expanded his search to cover the

entire ecliptic. For almost a year, Tombaugh exposed plates by

night and blinked plates by day. Th en, on February 18, 1930,

nearly a year after he had left Kansas, a quarter of the way

through a pair of plates, he found a 15th magnitude image that

moved (■ Figure 24-17). He later remembered, “ ‘Oh,’ I thought,

‘I had better look at my watch; this could be a historic moment.

It was within about 2 minutes of 4 pm [MST].’ ” Th e discovery

was announced on March 13, the 149th anniversary of the dis-

covery of Uranus and the 75th anniversary of the birth of Percival

Lowell. Th e object was named Pluto after the god of the under-

world and, also in a way, after Lowell, because the fi rst two letters

in Pluto are the initials of Percival Lowell.

Th e discovery of Pluto seemed a triumph of discovery by

prediction, but Tombaugh sensed something was wrong from the

fi rst moment he saw the image. It was moving in the right direc-

tion by the right amount, but it was 2.5 magnitudes too faint.

Clearly, Pluto was not the 7-Earth-mass planet that Lowell had

predicted. Th e faint image implied that Pluto was a small world

with a mass too low to noticeably alter the motion of Neptune.

Later analysis has shown that the variations in the

motion of Neptune, which Lowell used to predict the loca-

tion of Pluto, were random uncertainties of observation and

could not have led to a trustworthy prediction. The discovery

of the new planet only 6° from Lowell’s predicted position

was apparently an accident.

Pluto as a World

Pluto is very diffi cult to observe from Earth. Only a bit larger

than 0.1 arc second in diameter, it is only 65 percent the diameter

of Earth’s moon and shows little surface detail even when observed

with the Hubble Space Telescope, although as you will learn,

The Dwarf Planets

In 1930, a world was found orbiting beyond Neptune. Although
a solid object smaller than Earth’s moon rather than a low-density
Jovian planet, the public welcomed it as the ninth planet, and it
was named Pluto. At the end of the 20th century, with much
improved telescopes, astronomers found more such small worlds,
and it became clear that Pluto was just one of a large family of
similar objects.
In 2006, the International Astronomical Union voted to
move Pluto out of the family of planets and make it part of a
larger family of small worlds labeled “dwarf planets.” To under-
stand this controversial subject, you can start with Pluto and its
discovery.

The Discovery of Pluto

Percival Lowell (1855–1916) was fascinated with the idea that an
intelligent race built the canals he thought he could see on Mars
(see Chapter 22). Lowell founded Lowell Observatory in Flagstaff ,
Arizona, primarily for the study of Mars. Later, some say moti-
vated to improve the reputation of his observatory, he began to
search for a planet beyond Neptune.
Lowell used the same method that Adams and Leverrier had
used to predict the position of Neptune. Working from what
were understood to be irregularities in the motion of Neptune,
Lowell predicted the location of an undiscovered planet beyond
Neptune. He concluded it would contain about 7 Earth masses
and would look like a 13th-magnitude object in eastern Taurus.
Lowell searched for the planet photographically until his death in
1916.
In the late 1920s, 22-year-old amateur astronomer Clyde
Tombaugh began using a homemade 9-in. telescope to sketch
Jupiter and Mars from his family’s wheat farm in western
Kansas. He sent his drawings to Lowell Observatory, and the
observatory director Vesto Slipher (see Chapter 16) hired him
without an interview. Th e young Tombaugh bought a one-way
train ticket for Flagstaff not knowing what his new job would
be like.
Slipher set Tombaugh to work photographing the sky along
the ecliptic around the predicted position of the planet. Th e
search technique was a classic method in astronomy. Tombaugh
obtained pairs of 14  17-inch glass plates exposed two or three
days apart. To search a pair of plates, he mounted them in a blink
comparator, a machine that allowed him to look through a
microscope at a small spot on one plate and then, at the fl ip of a
lever, see the same spot on the other plate. As he blinked back
and forth, the star images did not move, but a planet would have
moved along its orbit during the two or three days that elapsed
before the second plate was exposed. So Tombaugh searched the
giant plates, star image by star image, looking for an image that
moved. A single pair of plates could contain 400,000 star images.
He searched pair after pair and found nothing.

24-3

■ Figure 24-17

Pluto is small and far away, so its image is indistinguishable from that of a

star on most photographs. Clyde Tombaugh discovered the planet in 1930

by looking for an object that moved relative to the stars on a pair of photo-

graphs taken a few days apart. (Lowell Observatory photographs)

Visual-wavelength imagesVisual-wavelength images

a b

Pluto was here

on the first plate.