Neptune (Poseidon), god of the sea, first object discovered by a search based on perturbations (of Uranus). From observed discrepencies in the orbit of Uranus, and with an assumed mass and distance given by the Titius-Bode rule (38.8 AU) John Adams (1845) and independently Urbain Leverrier (1846) predicted where such a perturbing body would be. As luck would have it, the incorrect distance led to a correct position and the two predictions were within a degree of one another. But Adams submitted his observation to the Astronomer Royal, Sir George Biddell Airy, who gave Adams a simple problem as a test. Unfortunately, Adams did not respond immediately. Later, Leverrier gave Airy his similar predictions and unbelievingly Airy asked him the same question. When Leverrier promptly answered the question, Airy decided the planet was worth a search. Justice prevails! Airy urged James Challis, Director of the Cambridge Observatory to drop everything and look for the planet. Meanwhile, Leverrier had written to the Berlin Observatory astronomer Johann Gall who eagerly searched for the planet and on 23 September 1846, with a student assistant Heinrich Louis d'Arrest, spotted the eighth planet. It turns out that Cambridge had observed but not recognized the planet. Galle was a bit fortunate in that he had recently prepared charts of the region and the eighth magnitude planet was easily located.

This picture (*) of Neptune was taken by Voyager 2. The picture shows the Great Dark Spot and its companion bright smudge; on the west limb the fast moving bright feature called Scooter and the little dark spot are visible. North of these, a bright cloud band similar to the south polar streak may be seen.

Since Voyager's visit the great dark spot seems to have disappeared when Hubble looked at Neptune in 1994. So the weather is variable.

Neptune's rotational axis is tilted 30 degrees to the plane of its orbit around the Sun. Its seasons last an incredible 40 years: during the southern summer the south pole is in constant sunlight for about 40 years, and in northern summer the north pole is in constant sunlight for about 40 years.

Similar to the case at Uranus, Neptune's magnetic field is tilted 47 degrees away from the planet's spin axis and offset from the planet's center by about half its radius.

Neptune's atmosphere is made up of hydrogen, helium, and methane, the last of these giving the planet its blue color (since methane absorbs red light). Winds here are the fastet on any body in the solar system, blowing at speeds over 2,400 km (1,500 miles) per hour.

Neptune has been visited by Voyager 2 (Aug 1989) which revealed a great dark spot similar to and about half the size of Jupiter's Great Red Spot.

Neptune has a large moon, Triton, about the same size as our moon, which is in retrograde orbit and a smaller moon, Nereid, which has the most eccentric orbit (e=0.75) of any moon in the solar system. Very strange. There are six other known moons and a set of four faint rings discovered by Voyager 2.

Neptune, like Jupiter and Saturn (but not Uranus), generates more heat than can be accounted for by insolation.

Pluto

After the mathematical discovery of Neptune, you can bet that more planets were sought after. Notable amongst the seekers of truth buried in noise were Galliot (1909), Lowell (1915) and Pickering (1928). In 1930 Tombaugh found Pluto. It was within 6° of Lowell’s prediction and had Lowell’s orbital elements. Unfortunately, recent observations indicate that the Neptune orbital "residuals" which led to the discovery of Pluto were spurious and the accord with Lowell’s Pluto prediction with fact was a fluke!

Walker and Hardie found a brightness variation with a period of 6.3867 days indicating that Pluto has a long rotation period with respect to the outer giants. In fact, the lonely sentinel has a companion, Charon (found by I.W. Christie). This satellite orbits the planet with the above period. The fact that Pluto has a moon is especially handy as we can now calculate the mass from

This value is ten times more accurate than the guessed value prior to 1978. So if <r >~1.5r _water we find it has a diameter of ~2700km. Kuiper says it should be 6000km.

The orbit of Pluto has the highest i (17° ) and the largest e (1/4) of the planets.

Pluto takes 248 years to orbit the Sun, coming as close as 30 AU and as far as 50 AU. The sun would appear to be a brilliant point source (5/400 of a degree across). Pluto's most recent close approach to the Sun was in 1989. Between 1979 and 1999 Pluto has actually been closer to the Sun than Neptune has been, although the i is such that Pluto and Neptune can (?) never collide. Most of what we know about Pluto we have learned since the late 1970s from ground-based observations, the Infrared Astronomical Satellite (IRAS), and the Hubble Space Telescope.

The smallest, coldest and most distant planet from the Sun, Pluto is sometimes not even considered a planet! We know that it is more like an asteroid or a comet, while others call it a double planet system because its moon, Charon, is about the half the diameter of Pluto. It is also sometimes called the largest of a group of objects that orbit in a region called the Kuiper belt--a disk-like zone of comets beyond the orbit of Pluto.

But never fear, astronomers are very resistant to change, and Pluto is in no danger of having its planetary status removed. This is so ordered by the IAU.

Pluto appears to have a bright layer of frozen methane, nitrogen and carbon monoxide on its surface. While it is close to the Sun, these ices thaw, rise, and temporarily form a thin atmosphere, with a pressure one-millionth that of Earth's atmosphere. As Pluto moves toward its farthest point from the Sun, this atmosphere freezes and falls back onto the surface of the planet.

Pluto is about two-thirds the diameter of Earth's Moon, and may have a rocky core surrounded by a mantle of water ice. Its mass, due its lower density, is about one-sixth that of the Moon.

Pluto's moon, Charon, is about half the size of Pluto itself. The two move through space always showing the same face to each other. In contrast to Pluto's methane surface, Charon's surface is mostly water ice, and the moon has no atmosphere. Charon may have been born through a head-on collision between Pluto and another large ice body, in much the same way as the Earth-Moon system is believed to have formed. According to computer models, some of the debris from this giant impact on Pluto went into orbit around Pluto and coalesced to form Charon.

It may be the last survivor of a lost population of ice dwarfs that inhabited the primeval solar system. It may even be an escaped satellite of Neptune.

In 1995 the first object in the so-called Kuiper Belt was discovered. Since then about 60 more objects, made of rock and ice and a few hundred km in size, have been found in the solar system's cold outer reaches between Neptune and the Oort cloud.