Exploring the Ice Giants
The “ice giant” planets, Uranus and Neptune, have much in common. Their distance from earth makes them difficult to explore, but with a telescope or a sturdy pair of binoculars they can be seen orbiting across the ecliptic with the terrestrial and “gas giant” planets. For centuries, Uranus was thought to be a slow-moving comet until scientists in 1781 realized it was the seventh planet in the solar system. A century later, mathematical anomalies in the orbit of Uranus were observed, and astronomers agreed that an eighth planet was orbiting the sun, a theory that was confirmed when Neptune was identified in 1846.
Both planets are surrounded by icy rings and numerous small moons. Voyager 2 was launched from earth in 1977, arriving at Uranus 9 years later. The spacecraft took measurements of the planet’s magnetic field, studied the atmosphere, and discovered 10 new moons. NASA scientists finally had images of the featureless exterior of Uranus and its thin system of rings. Three years later, Voyager 2 flew past Neptune and its largest moon, Triton, before continuing on its journey into interstellar space. Neptune’s upper atmosphere was found to contain sparse clouds and volatile winds, including a storm system the size of the “great red spot” on Jupiter. In recent years, the ice giants are studied using ground and space-based observatories that can capture images billions of kilometers from earth.
Uranus and Neptune evolved in the deep cold of the outer solar system. They both have a gaseous atmosphere of hydrogen, helium, and the methane that causes the planets to appear blue-green
Uranus has the most unusual rotation of any planet in the solar system. This massive planet is tilted on its axis at a 98 degree angle, causing it to almost roll like a bowling ball around the sun. A year on Uranus is equivalent to 84 “earth” years, and the planet’s unusual position means that the sun is directed at the north and south poles rather than the equator.
Each of the planets in the solar system, including the earth, are protected by magnetic fields that originate from within an iron core and convect through a molten crust or layers of metallic hydrogen. Uranus and Neptune, the coldest and most distant planets, are composed of a small terrestrial core surrounded by methane and water-ice. As a result, their magnetospheres are not only unevenly distributed, they originate near the equator rather than streaming from the North to the South Pole.
Uranus appears to be featureless with only occasional clouds or “dark spot” storms being visible. Twenty-seven small moons are in orbit around Uranus, and 13 light rings were observed by Voyager 2 and have been photographed by ground based observatories on earth.
Neptune is the eighth planet in the solar system, orbiting approximately 4.3 billion kilometers from Earth at perihelion. A day on Neptune lasts 17 “earth” hours, and an orbit around the sun is completed every 164 years. The abundance of methane in Neptune’s upper atmosphere causes its dark blue color, and astronomers in the mid-1800’s named the planet after the Roman god of the sea.
Gases within the atmosphere of Neptune are the coldest of any planet, consisting of hydrogen, helium, and methane at approximately -200 C. However, its core is a superheated 5,000 C, hotter than that of Uranus. This variation causes cyclonic storms, cloud cover, and high winds measured at 2,100 kph.
Just beyond the orbit of Neptune is the Kuiper Belt, a region of icy comets and small planets believed to be remnants from the formation of the solar system 4.5 billion years ago
These “Kuiper Belt objects,” or KBO’s, include the dwarf moons Pluto, Haumea, and Makemake as well as numerous large comets. Neptune shares a gravitational “resonance” with large objects within the Kuiper Belt, while its gravity causes smaller comets to be jolted from their orbits and either collide with each other or spin off onto new – and potentially dangerous – pathways through the solar system.
Comets with orbital periods of 200 years or less are believed to have originated from within the Kuiper Belt. Halley’s comet, visible again from Earth in 2061, is one example, as well as comet 67P/Churyumov-Gerasimenko, which the Rosetta spacecraft caught up with in 2014.