♆ Neptune

How to follow Neptune live

The panel above recomputes Neptune every second in your browser: how far it is from the Sun and from Earth (in AU and in kilometers), its apparent magnitude, the illuminated fraction, the phase angle and the elongation from the Sun. The magnitude tells you at a glance whether Neptune is visible to the naked eye tonight or needs a telescope.

The top-down map of the Solar System then shows exactly where Neptune sits among the other planets right now. Run time with the day slider, zoom and pan, click a second planet to compare distances, or press "Next event" to jump to the next opposition or closest approach to Earth, the best moment to observe it.

Quick facts

About Neptune

Neptune has been the most distant planet from the Sun since 2006, when Pluto was reclassified as a dwarf planet. It orbits about 30 astronomical units out, so far that sunlight takes more than four hours to arrive and the Sun appears in Neptune's sky as nothing more than a very bright star, with no perceptible warmth.

It is an ice giant and close cousin of Uranus in size and makeup: beneath an atmosphere of hydrogen, helium and methane lies a thick mantle of water, ammonia and methane in a supercritical state. Methane absorbs red light and returns a deep, intense blue, far richer than Uranus's pale tone. Despite the extreme distance, Neptune radiates internal heat, which keeps its atmosphere active and turbulent.

Physical characteristics

With an equatorial diameter of about 49,500 km, Neptune is slightly narrower than Uranus but denser and more massive: 17 times Earth's mass against Uranus's 14.5. It is the smallest of the four giants by diameter but the most compact. It has no solid surface: anyone diving in would pass through gas layers of growing density until reaching the fluid ice mantle under crushing pressure and temperatures of thousands of degrees Celsius.

Neptune's atmosphere holds the fastest winds in the Solar System, with documented gusts above 2,100 km/h, faster than the speed of sound on Earth. Those winds are partly powered by the planet's internal heat, which radiates more energy than Neptune receives from the Sun. When Voyager 2 flew past in 1989, it photographed the Great Dark Spot, an Earth-sized storm in the southern hemisphere. Hubble images in the following years showed it had vanished; new dark spots were detected in 2018 and 2021, confirming a restless atmosphere that spins up and dissolves storms within a few years.

Orbit and slow drift across the sky

Neptune takes about 165 Earth years to complete a single orbit around the Sun. Discovered on 23 September 1846, it only completed its first orbit on astronomical record on 12 July 2011, returning to the patch of sky where it had been found. That is one of the most unsettling time perspectives in planetary astronomy: a full human lifetime does not cover this planet's year.

That slow pace means Neptune barely shifts among the stars from one year to the next: it creeps about two degrees per year and stays in the same constellation for years at a time. In 2025 and 2026 it sits in Pisces. Even at its annual opposition (its closest point to Earth), its brightness reaches only magnitude 7.8, below the naked-eye limit of roughly magnitude 6.

How to observe Neptune

Neptune is never visible to the naked eye. An instrument is required: good binoculars mounted on a tripod under a truly dark sky will show a faint, non-twinkling bluish point, and a small telescope is the most reliable way to find and confirm it.

• Use an up-to-date star chart or an astronomy app to learn which region of which constellation holds it, since it moves so slowly.
• Look for a bluish point that does not twinkle like the surrounding stars; at higher magnification (150x or more) it shows a tiny grey-blue disc with no visible surface detail through amateur instruments.
• The annual opposition is the best time: Neptune reaches its closest point to Earth and peak brightness (around magnitude 7.7 to 7.9).
• Triton, the largest moon, is within reach of telescopes from about 250 mm aperture on good nights.

Triton and the other moons

Neptune has 16 confirmed moons. Triton, by far the largest, is 2,707 km across and holds more than 99.5% of the total mass in orbit around the planet. The most striking fact about Triton is that it orbits in a retrograde direction, opposite to Neptune's rotation and to the vast majority of moons in the Solar System. That is strong evidence that Triton was captured from the Kuiper Belt at some distant point in the past rather than forming alongside Neptune.

Triton's surface temperature is about -235 °C, making it one of the coldest bodies measured in the Solar System. Voyager 2 photographed geysers shooting nitrogen gas up to 8 km high into Triton's thin atmospheric envelope. Over time, Neptune's tidal forces will slow Triton's orbit until it crosses the Roche limit and breaks apart into fragments, possibly forming a ring system similar to Saturn's, within about 3.6 billion years. Nereid, the second best-known moon, has the most eccentric orbit of any large moon in the Solar System, ranging from 1.4 to 9.6 million km from the planet.

The mathematical discovery: Le Verrier and Adams

Neptune was the first planet found by mathematics before the telescope, one of the most impressive milestones of nineteenth-century science. After the discovery of Uranus in 1781, astronomers noticed that the perturbations in its orbit could not be fully explained by the known gravitational influences alone: something else was tugging on Uranus.

Two mathematicians reached the same conclusion independently: Frenchman Urbain Le Verrier and Englishman John Couch Adams. Le Verrier sent his calculations to astronomer Johann Galle at the Berlin Observatory, who on 23 September 1846 pointed a telescope at the predicted position and found Neptune less than one degree from where the math said it would be. Adams had made similar calculations months earlier, but the Greenwich Observatory did not act in time. The priority dispute between France and England generated controversy for decades.

Exploration: Voyager 2 and the James Webb Space Telescope

Only one spacecraft has visited Neptune: NASA's Voyager 2, on 25 August 1989, more than 12 years after its launch. The probe passed about 4,950 km above the cloud tops, flew past Triton and revealed the Great Dark Spot, the nitrogen geysers and six new moons. Since then, Neptune has been studied only remotely.

The James Webb Space Telescope (JWST) photographed Neptune in September 2022 with a clarity that surprised astronomers: the rings, hard to catch even with Hubble, appeared with remarkable sharpness, alongside storm features and atmospheric structure. In 2023, JWST identified carbon monoxide ice signatures in Neptune's atmosphere for the first time. No orbiter mission to Neptune has been approved yet, but scientific proposals have been submitted to NASA and ESA, targeting launch windows between 2030 and 2040.

Neptune facts and records

• It was the first planet found by mathematics before the telescope, in 1846, when Johann Galle spotted it almost exactly where Le Verrier had calculated.
• It has the fastest winds in the Solar System: gusts above 2,100 km/h, faster than the speed of sound on Earth.
• It has faint, uneven rings with denser dust arcs that were given names such as Liberty, Equality and Fraternity.
• Triton is one of the very few moons in the Solar System with confirmed geological activity: nitrogen geysers filmed by Voyager 2 in 1989.
• Neptune completed its first recorded orbit in 2011, 165 years after its discovery.
• The only spacecraft to visit it, Voyager 2, is still active and speeding away from the Solar System, with no plans to redirect it back.

Other planets

• ☿ Mercury
• ♀ Venus
• ♂ Mars
• ♃ Jupiter
• ♄ Saturn
• ♅ Uranus