Neptune
Found with a pencil before anyone pointed a telescope at it. Where it is right now, how big and how heavy, what goes round it, and what is still unexplained.
Neptune on its axis
Turning here at one rotation every 12 seconds, which is 4,833 times real speed. What is not sped up: which way it turns, how far the axis leans, and how its day compares with every other planet's. The deepest blue of the eight, and that colour is methane absorbing red light. The dark spot is schematic in position: Neptune’s come and go over a few years, and the one Voyager 2 photographed in 1989 had gone by 1994. The bright smudges are the methane clouds that trail them — the highest clouds on the planet.
Wed, August 26, 2026
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Distances from the sun are in AU — one AU is the Earth’s average distance, 149,597,870 km — and the angle is where the body sits around its orbit, measured from the March equinox direction.
Neptune by the numbers
The mass is worked out from Neptune’s gravitational parameter and the gravity from that and its radius; the year comes from Kepler’s third law and the width is the same figure the simulator draws with. None of them is typed in beside the picture, so none of them can disagree with it.
The moons of Neptune
Neptune has 16 confirmed moons, of which the 12 below are large enough to be worth drawing. The rest are mostly a few kilometres of captured rubble on distant, tilted orbits. Every distance and period here is real, and they are not independent: each pair has to satisfy Kepler’s third law against Neptune’s mass, which is how this table is checked rather than trusted.
| Moon | Distance | Orbit | Width | |
|---|---|---|---|---|
| Larissa | 73,548 km | 13.3 h | 194 km | Battered and irregular, inside the ring system |
| Proteus | 117,647 km | 1.12 d | 420 km | About as large as a body can get and stay this lumpy |
| Triton | 354,759 km | 5.88 d ↺ | 2,707 km | Orbits backwards, so it was captured — and is spiralling in |
| Nereid | 5,513,818 km | 360.1 d | 340 km | The most eccentric orbit of any large moon: 1.4 to 9.6 million km |
| Naiad | 48,227 km | 7.1 h | 66.0 km | Closest in, and tilted 5 degrees to the rest - still settling after Triton arrived |
| Thalassa | 50,075 km | 7.5 h | 82.0 km | Dances an avoidance pattern with Naiad every time they pass |
| Despina | 52,526 km | 8.0 h | 150 km | Shepherds the inner edge of the Le Verrier ring |
| Galatea | 61,953 km | 10.3 h | 176 km | Holds the Adams ring arcs together - the ring is clumpy, and this is why |
| Hippocamp | 105,283 km | 22.8 h | 34.8 km | Found in 2013 in old Hubble frames: chipped off Proteus by a comet strike |
| Halimede | 16,611,000 km | 1,879.7 d ↺ | 62.0 km | Probably a chip off Nereid - same colour, crossing orbits |
| Sao | 22,228,000 km | 2,914.1 d | 44.0 km | Prograde, unlike most captures, and takes eight years to go round |
| Laomedeia | 23,567,000 km | 3,167.9 d | 42.0 km | Found in 2002, and named after a Nereid of Greek myth like the rest |
↺ marks a moon going round backwards. This system is the picture at the top of this page — press Play and watch these periods run.
The moons of Neptune, one at a time
The table above has every figure — how far out, how long a lap takes, how wide. What follows is what those figures cannot tell you: what each one is actually like, who found it and when, and what is still argued about it.
Proteus
Just about the largest a body can be and still not be round.
- It sits right at the size where gravity starts winning against material strength, and it has not quite lost.
Triton
It orbits backwards, which means Neptune did not make it — Neptune caught it.
- It is a captured Kuiper Belt object, and the only large moon in the solar system on a retrograde orbit.
- Geysers of nitrogen erupt several kilometres up and are blown downwind into dark streaks. Voyager 2 caught them in 1989.
- At 38 kelvin it is the coldest surface ever measured anywhere in the solar system.
- Its "cantaloupe terrain" — dimpled, and almost uncratered — means the surface is being renewed.
Still unsettled
What drives the geysers is still argued: a solid-state greenhouse warming nitrogen ice from below is the leading idea, and it has never been observed directly.
Because it orbits backwards, tides are dragging it in. In about three and a half billion years it will pass Neptune’s Roche limit and become a ring.
Nereid
The most eccentric orbit of any moon in the solar system.
- Its distance from Neptune varies by a factor of seven over one orbit — it is almost certainly a survivor of whatever disruption Triton’s capture caused.
When Earth and Neptune are next closest
Both planets are moving, so the gap between them swings enormously — and the closest approaches are not all equal, because the orbits are ellipses rather than circles. These are the next four, solved from the orbits rather than looked up.
Click a date to take the simulator there. These are minimum-distance moments, which fall near — but not exactly on — opposition.
Neptune: things worth knowing
- It was predicted before it was seen. Irregularities in Uranus's orbit were used to calculate where an unseen planet had to be, and it was found in 1846 within a degree of the predicted spot.
- The fastest winds in the solar system blow here — up to about 2,100 km/h — on the planet that receives the least sunlight of the eight.
- Neptune radiates 2.6 times as much energy as it gets from the Sun. Where that heat comes from, on a planet the same size and age as Uranus, is unknown.
- Triton orbits backwards, which means it did not form there: it was captured, almost certainly from the Kuiper belt. It is also geologically active, with nitrogen geysers, and it is spiralling inward.
- One Neptune year is 164.8 Earth years. It completed its first full orbit since discovery in 2011.
- The Great Dark Spot photographed by Voyager 2 in 1989 had vanished by the time Hubble looked in 1994. Others have appeared and gone since.
What we still don’t know about Neptune
Every one of these is genuinely unsettled — not simplified for the page, not waiting on a textbook update.
Why is Neptune so much warmer inside than Uranus?
Same size, same composition, same era — and one has an internal heat engine while the other does not. Every explanation offered (a different impact history, layered convection, helium raining out of the interior) is a hypothesis without a decisive measurement, and only a mission to one or both of them would change that.
What powers those winds?
Neptune receives about 0.1% of the sunlight Earth does, and generates the strongest winds anywhere. Internal heat is the presumed engine, but the mechanism that turns it into 2,000 km/h zonal jets has not been demonstrated.
How was Triton captured without wrecking the system?
Capturing a body that large normally requires shedding an enormous amount of energy. The favoured route is that Triton arrived as half of a binary and its partner was flung away, leaving Triton bound. Whatever happened, it would have destroyed Neptune's original moons — which is consistent with the odd, sparse system that is there now.
Recently learned about Neptune
Findings reviewed August 2026. Space science moves; a date on a finding is part of the finding.
The rest of the system
Inward: Uranus. Outward: Pluto. Or step out: all the planets, a picture and a paragraph each, and the solar system simulator, where every orbit runs at once.
Make a link to a particular view
The date, the zoom, the span, the speed, the layers and any flight path are all in the address bar, so copying the URL shares exactly what is on screen. Set it up above, then take the link — it is the quickest way to hand a class one specific thing to look at.
The other simulator
This one is about the whole system. If the question is where the sun and the moon are from where you are standing — what time the sun comes up, why tonight's moon is the shape it is — that is the Sun, Earth & Moon movement simulator, which has a page for every city and a slider over a day, a week or a month. Between the two is the three bodies moving together — Earth going round the sun, the moon going round the Earth, on one screen and openly not to scale, keeping only the real ratio between the two periods.
And for why any of it stays up: the orbital velocity simulator takes one planet and lets you set its distance and its sideways speed by hand, so you can watch the balance that holds every orbit here — and break it, into an ellipse, an escape, or a fall into the sun.
Also: the classroom guide · sunrise & sunset by city · moon phase & moonrise · lunar eclipses · how the positions are worked out
Neptune: questions without settled answers
Why is Neptune so much warmer inside than Uranus? Same size, same composition, same era — and one has an internal heat engine while the other does not. Every explanation offered (a different impact history, layered convection, helium raining out of the interior) is a hypothesis without a decisive measurement, and only a mission to one or both of them would change that.
What powers those winds? Neptune receives about 0.1% of the sunlight Earth does, and generates the strongest winds anywhere. Internal heat is the presumed engine, but the mechanism that turns it into 2,000 km/h zonal jets has not been demonstrated.
How was Triton captured without wrecking the system? Capturing a body that large normally requires shedding an enormous amount of energy. The favoured route is that Triton arrived as half of a binary and its partner was flung away, leaving Triton bound. Whatever happened, it would have destroyed Neptune's original moons — which is consistent with the odd, sparse system that is there now.