Neptune, the Third Most Massive Planet

‍Neptune is the eighth planet from the Sun and the third-largest. It is also the last of the planets discovered by astronomers in ancient times. It is the only planet that has a blue color and its name was given after another god of water, Neptune. For almost two centuries after its discovery, there was no other way than to use telescopes to study this planet. In addition, until 1962 when Voyager 2 passed by it on its way out of the solar system for good, very little was known about Neptune. Since then we have learned much more about its structure and characteristics thanks to various probes and satellites launched by NASA and European Space Agency (ESA).

Interesting Facts about Neptune

- The Great Dark Spot: The most notable feature discovered on Neptune was one of its storms, the Great Dark Spot. This storm was detected in Hubble Space Telescope images in 1994 and was the most prominent feature seen on the planet.

- Gravity: Neptune’s gravity is enough to pull you down into the atmosphere, even if you are in orbit around it. Using a slingshot maneuver, the New Horizons spacecraft was able to take a look at Neptune with very little fuel. In fact, when New Horizons was squeezed by Neptune in 2007, it was going so fast that it briefly entered orbit around the planet.

- The Equatorial Bulge: Neptune’s equatorial region is 40% higher than the rest of the planet. This is due to the fact that its composition is different near the equator than it is near the poles. The equatorial region is composed of ice, while the polar regions are made of gases. This causes the equator to bulge out more than the poles.

- Atmosphere: Neptune’s atmosphere is made mostly of hydrogen and helium, like Uranus. It also has a thin layer of clouds made of methane and hydrogen cyanide. The clouds are so thin that they are transparent enough for Hubble to see Neptune’s photosphere.

Its Orbit and Rotational Period

Neptune has an orbit that is the most eccentric of all the planets of the Solar System. Its distance from the Sun varies between 4,500 and 10,500 million km. It takes 164 years to make a complete orbit around the Sun (almost 30 years longer than Pluto!). Neptune's orbit around the Sun is very unusual. Most of the planets in the Solar System are in orbits that are almost circular. Neptune's orbit, on the other hand, is very eccentric. That means that its path around the Sun is very squashed and not very circular. Its distance from the Sun varies between 4,500 and 10,500 million km. It takes 164 years to make a complete orbit around the Sun. Neptune takes so long to complete one orbit because it is the most distant planet from the Sun.

Its Atmosphere

Neptune’s atmosphere consists almost entirely of hydrogen and helium, with traces of methane and other hydrocarbons. The abundance of methane is the main reason for the blue color of Neptune. The methane molecules absorb red and yellow wavelengths from sunlight while reflecting back blue wavelengths. So Neptune appears blue. The other two blue planets, Uranus and Neptune, are cooler than the gas giants. Also, their blue color comes not from the Sun, but from the internal absorption of red and yellow wavelengths by methane. The density at the surface of Neptune is 10-7 times lower than on Earth. The escape velocity at Neptune's equator is 12.5 km/s. It means that the molecules of the atmosphere are not held on the surface of the planet, but they escape into space. For this reason, Neptune has an increasing and thick atmosphere. The thinning of the atmosphere at an altitude of 30-40 km is due to the action of strong ultraviolet and X-ray radiation of the Sun. The ultraviolet photons decompose molecular hydrogen into hydrogen atoms, which create a strong atmospheric outflow.

Appearance and Magnitude

Neptune is 2.8 times larger than Earth and has a mass 17 times greater. Its radius is the smallest among the eight planets. It has a very high equatorial tilt and the planet's axis is almost parallel to the plane of the Solar System. Its rotational period (one day) is 16 hours longer than its orbital period (one year). Neptune is the third most distant planet from the sun. The equatorial radius of Neptune is 30,775 km and its mass is 17 times greater than that of Earth. The blue color of Neptune is due to the absorption of red and yellow wavelengths of sunlight by methane in the atmosphere of the planet. The temperature of the upper atmospheric layers of Neptune is about -235 degree Celsius.

It's Interior and Composition

Neptune is mainly composed of ice mixed with gases, including hydrogen, helium and methane, which means that it has a similar structure to Uranus. The core of Neptune is ice and gas that are under high pressure, which corresponds to a temperature of 12,000 degrees Celsius. The composition of Neptune is mostly water, ammonia and methane. The core of Neptune is similar to that of Uranus. In the upper layers of Neptune, water and ammonia are present. The abundance of methane is the main reason for the blue color of Neptune.

Continents or Moons?

Neptune has 16 moons. The largest among them is called Triton. It is the only large moon in the Solar System with retrograde orbit (i.e. orbits in the opposite direction to the planet's rotation). The distance between Neptune and Triton varies from 2,800 to 7,600 km, which means that the satellite completes its orbit in 6 days. Triton was discovered in 1846 and was originally considered the first discovered planet in the Solar System. The largest Neptunian moon is named Rhea. Rhea's equatorial radius is about twice that of the Earth's.