Neptune’s Physical Properties



Neptune’s atmosphere is very similar to that of Uranus.

It’s about 80% hydrogen, about 19% helium, and the rest is ethane, methane, etc. The atmosphere absorbs red light and reflects blue light, giving Neptune an overall blue colour.

Neptune’s atmospheric composition is very similar to that of Uranus, but the atmospheric flows would be relatively active on Neptune.

This can be inferred from the fact that atmospheric tornadoes are visible on Neptune that are not visible on Uranus.

The atmospheric turbulence on Neptune is called the Great Dark Spot.


Neptune’s temperature is similar to Uranus.

With an average temperature of -214°C, it gives off more heat than it receives from the Sun.

This suggests that it has a heat source, much like Jupiter and Saturn, and can be seen as an effect of gravitational energy in the same way.

Internal structure

The size of Neptune is about 24,766 km, and its mass is about 1.021026 kg, and its density is about 1638 kg/m3, which is very similar to Uranus.

Therefore, it is assumed that the internal structure is also similar.

In other words, the liquid metal hydrogen that exists in the interior of Jupiter and Saturn cannot exist in Neptune, which has a relatively small internal pressure, and it is assumed that the ice of methane and ammonia in the atmosphere is ionised.

In addition, given the low pressure, similar density, and low temperature compared to Jupiter, it is thought that Neptune’s interior is similar to Uranus, with less hydrogen and helium, and rocks and ice.


Neptune’s axis of rotation is inclined at about 29.6° relative to its plane of rotation. It has a rotation period of about 16.08 hours, which is moderate compared to Uranus.


Neptune orbits at a distance of about 4.5 billion kilometres from the Sun. Neptune’s orbit is nearly circular, with a small eccentricity and a difference of less than 100 million kilometres between its farthest and closest points, which is very small compared to its orbital radius.

Neptune completes one revolution around the Sun in about 163.7 years at a speed of about 23.5 km/s.

Magnetic field

Neptune’s magnetic field was shrouded in mystery until Voyager 2 made close observations.

The observed strength of Neptune’s magnetic field is about 0.4 times that of Earth. And like Uranus, Neptune’s magnetic axis is heavily tilted relative to its rotation axis. Why this happens is still being studied.

Neptune’s rings

Neptune’s rings were discovered using the same method as Uranus’ rings. Although difficult to observe directly, the presence of Neptune’s rings was revealed by changes in the brightness of the starlight when Neptune caused an eclipse that obscured the background stars.

But it wasn’t until Voyager 2 that we saw it clearly. Like other planets with rings, Neptune is composed of several rings.

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