CHAPTER 24 | URANUS, NEPTUNE, AND THE DWARF PLANETS 539
Original papers related to Adam’s calculations were lost for
decades, but when they were found in 1998, they painted a dif-
ferent picture. Adams did the calculations correctly, but he com-
puted only the orbit for the new planet. He didn’t actually
calculate its position in the sky along the orbit; that was left to
the English astronomers conducting the search. Once the new
planet was discovered, the English astronomers, out of national
pride, pressed Adam’s case further than they should have.
For 150 years, astronomers took sides or gave both astrono-
mers equal credit. In fact, it seems clear that Leverrier deserves
credit for making an accurate, useful prediction of a position on
the sky and then pressing it aggressively on an astronomer who
had the right skills to make the search. You should still give both
astronomers credit for solving a diffi cult problem that was one of
the great challenges of the age. A modern analysis shows that
both Leverrier and Adams made unwarranted assumptions about
the undiscovered planet’s distance from the sun. By good fortune
their assumptions made no diff erence in the 19th century, and
the planet was close to the positions they predicted. Do you
think they deserve less credit for that reason? Th ey tried, and the
other astronomers of the world didn’t.
Leverrier and Adams could have been beaten to the discov-
ery had Galileo paid a little less attention to Jupiter and a little
more attention to what he saw in the background. Modern stud-
ies of Galileo’s notebooks show that he saw Neptune on December
24, 1612, and again on January 28, 1613, but he plotted it as a
star in the background of drawings of Jupiter. It is interesting to
speculate about the response of the Inquisition had Galileo pro-
posed that a planet existed beyond Saturn. Unfortunately for
history, but perhaps fortunately for Galileo, he did not recognize
Neptune as a planet, and its discovery had to wait another
234 years.
Historians of science have recounted the discovery of
Neptune as a triumph for Newtonian physics: Th e three laws of
motion and the law of gravity had proved suffi cient to predict the
position and orbit of an unseen planet. Th us, the discovery
of Neptune was fundamentally diff erent from the discovery of
Uranus. Uranus was discovered “accidentally” in the course of
Herschel’s attempt to systematically observe the entire sky,
whereas the existence of Neptune was predicted by Leverrier and
Adams using basic laws.The Atmosphere and Interior
of Neptune
Little was known about Neptune before the Voyager 2 spacecraft
swept past it in 1989. Seen from Earth, Neptune is a tiny, blue-
green dot never more than 2.3 arc seconds in diameter. Before
Voyager 2, astronomers knew Neptune is almost four times the
diameter of Earth, or about 4 percent smaller in diameter than
Uranus (Celestial Profi le 10), with a mass about 17 times
that of Earth, 20 percent more than Uranus. Spectra revealedNeptune
Uranus and Neptune are often discussed together. Th ey are
about the same size and density and are very similar planets in
some ways. Th ey do diff er, however, in certain respects. Unlike
Uranus, Neptune has a signifi cant amount of heat fl owing out
from its interior. Also, Neptune has especially complicated ring
and satellite systems. Even the discovery of Neptune diff ered
from the discovery of Uranus.
The Discovery of Neptune
Th e discovery of Neptune triggered one of the greatest controver-
sies in the history of science. For over a century people told the
story and took sides, but only in the last few years has the real
story become known. You can follow the story and decide which
side you favor.
In 1843, the young English astronomer John Couch Adams
(1819–1892) completed his degree in astronomy and immedi-
ately began the analysis of one of the great problems of 19th-
century astronomy. William Herschel had discovered Uranus in
1781, but earlier astronomers had seen the planet as early as
1690 and had mistakenly plotted it on charts as a star. When
19th-century astronomers tried to combine all those data, they
didn’t quite fi t together. No planet obeying Newton’s laws of
motion and controlled only by the gravity of the sun and the
other known planets could follow such an orbit.
Some astronomers suggested that the gravitational attraction
of an undiscovered planet was causing the discrepancies. Adams
began with the observed variations from the predicted positions
of Uranus, never more than 2 arc minutes, and by October 1845
he had, through a laborious and diffi cult calculation, computed
the orbit of the undiscovered planet. He sent his prediction to
the Astronomer Royal, Sir George Airy, who passed it on to an
observer who began a painstaking search of the area star by star.
Meanwhile, the French astronomer Urbain Jean Leverrier
(1811–1877) made the same calculations and sent his predicted
position of the planet to Johann Galle at the Berlin Observatory.
Galle received Leverrier’s prediction on the afternoon of September
23, 1846, and, after searching for 30 minutes that evening, found
Neptune. It was only 1° from the position predicted by Leverrier.
Th e discovery of a new planet caused a sensation, but
English astronomers didn’t like a Frenchman getting all the
credit. After all, they said, the planet was found only 2° from the
position predicted by the orbit computed by their own astrono-
mer, Adams. When the English announced Adams’s work, the
French suspected that he had plagiarized the calculations, and
the controversy was bitter, as controversies between England and
France often are. For over a century, historians of science have
repeated the story of the young English astronomer who missed
his chance because the astronomers in charge of the search were
careless and slow.
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