Uranus and Neptune | Ice giants of the Solar system

Far beyond the orbit of Saturn lie ice giants. Planets in the coldest regions of the Solar System.  Let’s explore these “ice giants”.

Uranus and Neptune (Left to Right)


The first planet to be discovered by humans with a telescope, 14 and half times massive than the Earth and having radius 4 times that of Earth here comes the Uranus.

Uranus; as seen by Voyager 2
Orbiting at 3 billion km from Sun, about 19 times farther than Earth is from Sun it takes 84 earth years to complete one orbit around the Sun. And one Uranian day is equal to 17 hours, 14 minutes and 24seconds.

Orbit of Uranus 
Most strange fact about Uranus is its axis of rotation, tilted at 97.77˚ it literally rolls around the Sun. This makes seasons weird on Uranus, the planet’s the North Pole experiences 21 years of nighttime in winter, 21 years of daytime in summer and 42 years of day and night in spring and fall.

Tilt of Uranus
The reason for its axial tilt is still not clear but scientists have suggested that during the formation of the Solar System, there would have been few collisions with large asteroids or smaller protoplanets. These collisions could have knocked Uranus’s axial tilt.

Uranus compared with Earth
Uranus has 27 moons largest of which is Titania.

Uranus holds the title of the coldest planet of Solar System with temperatures as low as -224 ˚C.

Voyager 2 near Uranus, (Artists Imagination)

The only probe that ever encountered Uranus was NASA’s Voyager 2. It approached Uranus back in 24, January 1986 and investigated Uranus’s moons, atmosphere and magnetosphere before going to Neptune.

Structure of Uranus

So it is time to put you on Uranus, we have dropped the phrase “put you on the surface” because there is no surface of these giants. The atmosphere of Uranus has no definite boundary as it lacks a solid surface. It is primarily made of hydrogen and helium. But these gases are majorly present in the outer layers of the atmosphere. The outermost layer of atmosphere, the thermosphere extends from its rings to 4000km above from the proposed surface. (Surface here refers to a revolving oblate spheroid set at the point at which atmospheric pressure equals 1 bar.) Opposite to what expected thermosphere has a temperature of about 800-850K source of heat required for maintaining these temperatures is unknown. So falling through thermosphere you will feel the warmth of it. As you cross thermosphere you will encounter stratosphere here temperature gradually drops from 850K to 53K. Between 300km to 100km from the surface, there is a layer of hydrocarbons at a pressure of 0.1mbar to 10bar. These hydrocarbons are methane, acetylene, and ethane. Ethane and acetylene condense at these temperatures causing the bland appearance of Uranus. So as you pass through these layers you will witness freezing temperatures (don’t forget to bring your blankets) and a gradual increase in pressure. This hazy layer will soon block your vision as sunlight can’t penetrate below this point. Your falling speed will gradually decrease so no splash. Just above surface of Uranus is troposphere at a temperature ranging from 320K to 53K. Here pressure and densities are very high increasing to 100 bar. At this crushing pressure, your vehicle would be crushed and you will be frozen. So Uranus is also no go.


Farthest planet from the Sun in the solar system at a distance of 4.5 billion km or 30 times farther than Earth is from Sun here comes sister planet of Uranus, Neptune. It takes around 165 years to complete one orbit around the Sun. One day last about 16 hours.

Orbit of Neptune
It is the only planet discovered by mathematical observation rather than by empirical observation.
It has 14 moons largest of which is Triton. Triton is the only large moon in the Solar system which orbits in the direction opposite to that of planet’s rotation. It indicates that it was once probably a dwarf planet in Kuiper belt and captured by the gravity of Neptune.

The orbit of Triton compared to Motion of Neptune
Neptune is 4 times bigger than Earth with the radius of 24,764km. Its mass is 1.024×1026kg, 17 times the mass of Earth.

Size of Neptune compared to Earth
It was visited by Voyager 2 on 25 August 1989. It takes 246 minutes (4hrs, 6 minutes) for light to travel from Earth to Neptune thus Voyager 2 was preloaded with instructions for its maneuver around Neptune.

Voyager 2 near Neptune, (Artist’s Imagination)
Next planned mission for Neptune is still in the planning phase.

Structure of Neptune 
So let’s put you on, Neptune. First, you would fall through its atmosphere’s upper layer called stratosphere where temperature decreases with altitude decreasing so things are about to get chilly. After falling for few kilometers temperature would drop to about -200˚C. Around 48 km deep you would enter ammonia and hydrogen sulfide clouds with the pressure of 5 bars. Here wind speeds are up to 2400 kmph so are likely to be thrown out of your planned trajectory. At this depth, no light from Sun is able to reach to you. So keep a flashlight handy. After about 150km water clouds would appear around you will see the flashes of lightning. After these clouds pressure increases to 50 bar and temperatures of 26 ˚C. Falling through these clouds temperatures increase to 1000 ˚C. After falling for 6400km you will reach mantle of Neptune at the pressure of 10,000bar and temperatures increasing 4000 ˚C. As most of the human body is made of organic matter which is Carbon mostly, at this pressure and temperature you would be transformed into the purest form of Carbon, Diamonds. So your journey ends as you are a glittering diamond now. See you then.

Mystery in the creation of Ice giants

These planets are so mysterious that their mere existence is questionable.  The existence of such large planets this far from their parent star is doubtful.  In the early solar system when the formation of planets was happening, terrestrial planets formed by the accumulation of planetesimals (solid objects thought to exist in the protoplanetary disc) and gas giants are thought to be formed by accretion of gaseous envelopes from solar nebula over course of million years.

Protoplanetary Disc (Artist’s Imagination)
But Uranus and Neptune could not form by this method, at least not where they are today. Collision accumulation process in the outer edges of solar nebula is very slow due to large orbits and low surface densities of solids. In longer orbits of 20-30 AU (Astronomical Units) escape velocity from the Solar system is 8kms-1 comparable to orbital velocities. And due to the presence of Gas giants as Jupiter and Saturn in their vicinity, their probability of getting ejected into hyperbolic orbits or colliding with Gas giants was very high.
Best guess about their formation is that they were formed between Jupiter and Saturn at the same time as the gas giants were formed, and they were gravitationally scattered outward to their current orbits.
(Source: NASA Astrophysics Data System, Wikipedia, Space.com)

Categories: Solar System


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