The three outer planets in the solar system— Uranus, Neptune, and Pluto—were unknown to astronomers before the late eighteenth century. The planets were discovered following improvements to optical instruments and the subsequent discovery of gravitational effects on the orbit of Uranus.
Uranus was discovered by the German-born British astronomer William Herschel in 1781, using a 6.4-inch (16-centimeter) reflecting telescope. Its distance from the sun is almost exactly where it was predicted by Herschel according to the Titius-Bode law of the positioning of planetary orbits. There is evidence, however, that the planet had been seen by other astronomers before its date of discovery, but its true nature was not recognized. Herschel believed this discovery to be a new comet, and it was not until two months later that the find was considered to be a planet.
A small telescope reveals Uranus to be a faint blue-green disk. More powerful instruments show that the planet is accompanied by 15 satellites, the brightest of which is Oberon, with a magnitude of 14. This satellite, and the satellite Titania, were also discovered by Herschel. The orbital plane of five of the satellites reveals that Uranus’ equator is tilted at nearly 98° to the plane of its orbit, which means that the planet lies virtually on its side as it travels around the sun. The axial rotation period (the length of a day on Uranus) is thought to be 17 hours and 8 minutes.
The speed of rotation of Uranus causes the planet to be flattened at its poles, so that the polar radius is 900 miles (1,500 kilometers) less than the 16,200-mile (26,000-kilometer) equatorial radius. Because of the axial tilt, the poles usually point alternately toward the sun. This means that the polar regions alternately remain in light and dark for durations of up to 42 years. Seen from Earth, the direction of rotation of Uranus changes, at times turning clockwise and at others rotating from top to bottom.
Uranus receives little of the sun’s heat because it is so far away, and probably has a surface temperature as low as —357° F. ( — 216° C). In addition, the planet does not seem to have any internal source of heat. Such conditions restrict the type of atmosphere Uranus can have: most substances subjected to such low temperatures turn to a solid state. The bluegreen color of the disk is due to the absorption of red light by large quantities of methane in the planet’s atmosphere. Molecular hydrogen and helium have also been found in abundance and, although not observed at cloud-top level, ammonia is expected to be present as well. The internal composition of Uranus remains virtually unknown. One model proposes that beneath the atmosphere lies a solid interior, consisting of a rocky core with a radius of about 5,000 miles (8,000 kilometers) covered by a crust of ice.
Astronomers observed an occultation of the star SAO 158687 by Uranus in 1977. To their surprise, the star’s light was obscured five times before the main body of Uranus covered it completely, and the “winking” was repeated on the other side of the planet. This observation indicated the existence of a system of 11 sharply defined rings around Uranus, the innermost one lying 28,000 miles (45,000 kilometers) from the planet’s center and extending out to about 33,000 miles (53,000 kilometers). The rings are far narrower than those of Saturn, most being only about 6 miles (10 kilometers) in width, and only one, the outermost, Ring E, reaches 60 miles (100 kilometers) wide. This ring varies in width from 12 to 60 miles (20 to 100 kilometers) and this variation is reflected in its changing distance from Uranus— a fluctuation of about 500 miles (800 kilometers). It lies at about 1.95 planetary radii from Uranus. The rings are composed of charcoal-dark icy particles and are almost circular. Their rigid shape is maintained by small satellites.
Soon after the discovery of Uranus, astronomers noticed that the planet strayed from the orbit predicted for it if only the sun and the known planets were exerting their gravitational effects. It was suggested that the discrepancy was the result of the gravitational force of an undiscovered planet beyond Uranus’ orbit. John Couch Adams, using the Titius-Bode law, attempted to determine the position of this new planet. However, it became apparent that the law did not apply to the body, although it did help to narrow the range of the planet’s possible orbits. Adams’ findings were met with skepticism in Britain and further research was consequently hindered. As a result, work on the same problem by Urbain Le Verrier in France caught up, and with the help of Johann Galle at the Berlin Observatory, he found the planet in 1846 within one degree of the predicted position. As with Uranus, it was later found that the planet—called Neptune-had been observed before that date but was unrecognized as such. It was calculated that Neptune appeared very close to Jupiter in the sky in January 1613. The most likely person to have observed this event would have been Galileo Galilei. A search through his notes produced drawings that indicated that Galileo had seen Neptune and had observed its movement over the period of a month, but had thought it to be a star and not a planet.
Both Neptune and Uranus are grouped with Jupiter and Saturn as being the giant planets of the solar system. In several ways, Neptune is the twin of Uranus: Neptune’s radius is about 5 per cent less than that of Uranus, and
its mass is about 16 per cent greater. With a maximum magnitude of 7.7, Neptune is never visible to the naked eye. Viewed with a telescope, however, the planet appears as a bluegreen disk similar to Uranus, and it is also flattened at the poles as a result of its rotation.
Unlike its neighbor, however, Neptune is a world of surprising turbulence. Winds sweep the planet at the rate of 400 miles (640 kilometers) per hour. And a storm system the size of Earth churns counterclockwise around the planet at the rate of 700 miles (1,120 kilometers) per hour. Scientists call this violent hurricane in Neptune’s southern hemisphere the Great Dark Spot.
Two of Neptune’s satellites, Triton and Nereid, had been detected with the use of a telescope. Six more moons were found when the Voyager 2 probe flew by Neptune in 1989.
Neptune lies at an average distance from the sun of 2,798,800,000 miles (4,504,300,000 kilometers), with the result that it receives very little heat and its temperature is extremely low—about —353° F. ( — 214° C). Unlike Uranus, however, Neptune does seem to have an internal heat source. The planet’s small size would imply that any heat source would also be small, but Neptune has been observed to give out heat at a rate of 0.03 microwatts per ton of mass. This emission has the effect of heating the planet to about the same temperature as Uranus, even though it is much farther from the sun. Being so far from the sun, Neptune takes nearly 165 earth-years to complete a single orbit. As a result, it will not return to the point at which it was discovered until 2011.
Neptune’s atmosphere contains hydrogen, helium, methane, and acetylene. Ammonia may also exist in the lower levels of the atmosphere. A cycle of updrafts and downdrafts pushing methane gas creates cloud formations. Gaseous methane drifts high into Neptune’s atmosphere where it freezes into ice particles. The clouds are then dragged down to warmer regions where they are broken up. This process produces cloud cover over almost half of Neptune, including the Great Dark Spot. The clouds help the planet retain sunlight, which is absorbed by the upper atmosphere. The absorption of sunlight by methane gives Neptune its blue color.
Auroras spread over a wide area have also been sighted in Neptune’s atmosphere. Radiation belts similar to those encircling Earth surround Neptune. Charged particles in these belts appear to plunge into Triton’s atmosphere and generate auroras at its equatorial plane. Images taken by Voyager2 have revealed that, like the other giant planets, Neptune is encircled by a system of complete rings.
Radio signals have disclosed that Neptune completes one rotation every 16 hours and 7 minutes. The Great Dark Spot, however, takes about 18 hours to complete a rotation. Its strong winds sweep westward against Neptune’s rotation. Neptune possesses a magnetic field that is tilted about 50° from the rotation axis. Uranus’s magnetic field is similarly tilted. The internal structure of Neptune is also thought to be similar to that of Uranus, with a rocky core covered by a crust of ice.
In 1930, during a painstaking search among photographic plates of the region of the sky near the constellation Gemini, Pluto was discovered by the American astronomer Clyde Tombaugh. Mathematical predictions of the position of a new planet had been made based on the observed deviations in the orbits of Uranus and Neptune, but it now appears that the closeness of these predictions to the true position of Pluto was a coincidence.
Pluto’s orbit is the strangest of all the planets in the solar system. It is the most elliptical and comes inside Neptune’s orbit for 8 per cent of the 249 earth-years that Pluto takes to complete one revolution. Because of this, Neptune will be the outermost planet in the solar system until March 1999. Also, Pluto’s orbit is highly inclined, at an angle of more than 17° to the ecliptic.
Pluto appears in the largest telescopes as only a tiny, rather fuzzy disk. At its greatest distance from the earth its size is equivalent to that of a small coin seen from a distance of about 6 miles (10 kilometers). Recent estimates of Pluto’s diameter suggest that it is about 1,430 miles (2,300 kilometers). Pluto is therefore the smallest planet in the solar system. Its small size has made all observations of the planet liable to considerable inaccuracy.
In 1978, Pluto was discovered to have a satellite, Charon. It is about one-third the size of the planet, having a diameter of about 500 miles (800 kilometers). This makes it the largest satellite, relative to its parent planet, in the solar system. Charon’s orbital period is 6.39 earth-days, and the satellite orbits about 11,000 miles (18,000 kilometers) from Pluto. Observations of Charon’s orbit about its parent planet have revealed that Pluto has a mass of only about 0.002 times that of the earth, which makes it the lightest planet in the solar system. Pluto has a mean relative density of 2.03. With such a low mass, it is unlikely that much of an atmosphere can exist around the planet. Its estimated maximum temperature—369° F. (-223° O— also makes the presence of a gaseous atmosphere unlikely. Methane frost was, however, detected on the planet’s surface in 1976, indicating a somewhat tenuous atmosphere containing methane. For this atmosphere to be retained, heavier elements (such as neon) must also be present because the planet has such a low density. The icy crust is thought to comprise a layer a few tens of miles thick, below which may lie a solid core consisting of rock and ice.
Planets beyond Pluto
The latest estimates of Pluto’s mass, based on observations of the orbital motion of Charon, are much lower than would account for the deviations of Uranus and Neptune from the orbital paths that they should follow. This raises the question of the existence of another planet, orbiting beyond Pluto, which may be exerting sufficient gravitational force to cause the observed discrepancies. The two methods by which planets outside the orbit of Saturn have been detected were constant observation and mathematical analysis. Neither method will prove practical in the search for a trans-Plutonian planet This is because a search of magnitudes low enough involves expenditure in precious observatory time, and too many variables exist to enable mathematical analysis to narrow the field down sufficiently. The belief now is that no planet similar to Pluto seems to exist out to a distance of 60 astronomical units from the sun.