The modern planets are those which were not known due to their faintness until the invention of the telescope. They include Uranus, Neptune and Pluto.
In 1781, the English astronomer William Herschel discovered Uranus during a search for comets in the constellation of Gemini. Since Herschel's discovery was just luck, Uranus is sometimes called the serendipity planet, i.e. the word serendipity means to discover by accident.
In a telescope, the planet appears as a green object whose rotaton is retrograde like Venus and whose spin axis is tilted more than 90 degrees. Why the planet rotates backwards and on its side is a mystery. One pausable theory contends that the planet might have collided with another large object in the early days of our Solar System. This collision event apparently disrupted the planet's rotation and obliquity.
The internal structure of Uranus is believed to include a rocky core, a large shell of liquid water, a smaller shell of liquid molecular hydrogen and an atmosphere of hydrogen, helium and methane (CH4) gases. It is the methane gas in the atmosphere which gives the planet its green color.
There is a system of 11 rings that encircle the planet. These rings are narrow and faint and are likely made of debris coated with a black tar-like substance. It should be noted that the rings can not be easily observed from earth as can Saturn's.
Twenty-seven moons orbit Uranus. The largest, Titania is one-half the size of our moon. All of the moons of Uranus are icy/rocky objects and most have irregular shapes.
The Voyager 2 is the only spacecraft to have visited Uranus. This fly-by mission in 1986 passed within 50,000 km of the planet.
Soon after Uranus' discovery, astronomers began to observe irregularities in its motion as the planet moved along the ecliptic. To explain this behavior, two astronomers, Adams and LeVerrier, using Newton's Law of Gravitation, independently predicted that an unknown planet beyond Uranus was gravitationally disturbing Uranus' motion. A search was started in 1846 and the new planet, later to be named Neptune for the mythological god of the seas, was found.
In a telescope, Neptune is blue object. Like Uranus, its interior includes a rocky core, liquid shells of water and molecular hydrogen, and a H, He and methane atmosphere. A higher concentration of methane is probably the reason why Neptune is blue rather than green like Uranus.
There are at least 13 icy/rocky moons that orbit Neptune. The largest moon Triton has a nitrogen atmosphere. However, this atmosphere is negligible in comparison to the dense nitrogen atmosphere of Saturn's moon Titan.
Neptune has a ring system with four narrow, faint sections that are not directly observable from earth.
When Voyager 2 flew by Neptune in 1989, the spacecraft found a dark colored weather storm (Great Dark Spot) in the planet's atmosphere. In 1994 Hubble Space Telescope failed to re-observe the storm. Evidently, Neptune's earth-size weather disturbance had dissipated unlike Jupiter's quasi-permanent Great Red Spot.
One should note that Uranus and Neptune are similar in mass and size as is the earth and Venus.
Pluto which looks like a faint star even in a large telescope was discovered by the American astronomer Clyde Tombaugh in 1930. This smallest planet, an object smaller than our moon, is made of water, methane and ammonia ices with a tenuous atmosphere of N2, CH4 and CO (carbon monoxide). The surface of this cold world (-389 F) has dark patches. Pluto spins on its side (high obliquity), rotates in a retrograde direction and has one just moon, Charon which is one half its size.
Pluto travels in the most elliptical orbit of any planet. Due to its high eccentricity, Pluto is closer to the Sun than Nepture near the time of perihelion passage. The most recent perihelion for Pluto was 1989 and 1980 and 1999, Neptune was the furthest planet from the Sun. It should be noted that since Pluto's long, "skinny" orbit is tilted with respect to Neptune's, the orbits do not intersect and the planets will never collide.
Because Pluto is neither a terrestrial nor a Jovian planet, there is a strong suspicion that it is not one of the original planets of our Solar System. One theory claims that Pluto and Charon might be pieces of a moon of Neptune which was gravitationally torn away from Neptune when a large object (planetismal) passed near Neptune. Another theory maintains that Pluto is a trans-Neptunian object which originally belong to the Kuiper Belt, a distant ring of icy debris located in the outer regions of the Solar System.
No spacecraft has yet visited Pluto but a mission called New Horizons is now being planned. If the spacecraft meets its 2006 scheduled launch it will reach the smallest planet in 2015.