Neptune

Neptune has the fastest winds in the solar system, about 2,100 km/h

Neptune orbits at 30 AU (4.5 billion km) and takes 165 years to go round; it completed its first orbit since its discovery in 2011. It is slightly smaller than Uranus but denser and more massive, another ice giant of water, ammonia and methane under a hydrogen and helium atmosphere. Unlike Uranus, it radiates more than twice as much heat as it receives from the Sun, and that inner heat drives violent weather.

Neptune weather: -200 °C and supersonic winds driven by inner heat

Sunlight at Neptune is 900 times weaker than at Earth, yet its weather is the most violent of any planet, powered by heat escaping from inside.

Found with a pencil. Uranus kept drifting from its predicted path. In 1846 Urbain Le Verrier in Paris worked out where an unseen planet must be to explain it, and on September 23 Johann Galle in Berlin found Neptune within 1° of the predicted spot on his first night of searching. John Couch Adams in England had made a similar prediction. Galileo had seen Neptune in 1612 and 1613 but took it for a star.

Not as blue as the pictures. Voyager 2's famous images were contrast-boosted, making Neptune look much deeper blue than Uranus. A 2024 reprocessing of Voyager and Hubble data shows the two planets are actually a similar pale greenish blue, Neptune a little bluer.

Neptune needs binoculars and a finder chart; a telescope shows a tiny blue disk

At magnitude 7.7 to 7.9, Neptune is too faint for the naked eye but within reach of any binoculars once you know which "star" it is. A finder chart is essential; it moves only about 2° a year. In a telescope at 200x it shows a tiny blue-gray disk 2.3″ across, and a 6-inch or larger telescope can pick up Triton at magnitude 13.5.

1. Binoculars

   A faint star; identify it by comparing with a chart over two or three nights

2. 4-inch

   A small bluish disk at high power, clearly different from the stars

3. 6- to 8-inch

   Triton, close to the planet, under dark skies

4. 12-inch and larger

   Triton easily; Nereid (magnitude 19) only with cameras

Filters and tips: No filter is needed. Plan with a chart for the current year and use high magnification to confirm the disk.

Neptune oppositions to 2032, in Pisces

At opposition Neptune is up all night. Oppositions come about two days later each year. In the late 2020s Neptune sits in Pisces, near the border with Cetus.

Computed by this site with Astronomy Engine; dates are UTC. Magnitudes from Mallama and Hilton (2018).

Neptune compared with Earth

How Neptune measures up against Earth. Sunlight is the strength of sunshine above any atmosphere; on Earth that is 1,361 watts per square meter.

Neptune's Adams ring holds clumped arcs, unique in the solar system

Neptune's five rings are faint and dark. Its outer Adams ring is not uniform: it holds bright clumps called arcs, probably held in place by the gravity of the small moon Galatea. Distances are from the planet's center.

Triton holds 99.5% of the mass of Neptune's moons

Neptune has 16 known moons in the site's dataset. Triton, 2,707 km across, holds almost all the mass of the moon system. It orbits backwards, against Neptune's spin, which means it was probably a Kuiper Belt object captured by Neptune. Voyager 2 saw dark plumes erupting from it. Tides are slowly pulling Triton inward; in a few billion years it will be torn apart into a ring.

1 spacecraft have visited Neptune or its moons

From the site's spacecraft dataset. Arrival is the date of orbit insertion, landing or closest flyby.

Reaching Neptune

Only Voyager 2 has visited, in August 1989. At 4.5 billion km away, a mission takes 12 years or more, and no Neptune orbiter is funded. Neptune as a destination, on SolarSystemOS

Neptune physical and orbital data

Sources: NASA Neptune fact sheet; Voyager 2 results; Irwin and others, 2024 (true colors of Uranus and Neptune). Values from the site's planets dataset (planets_q426), which lists a source for every row.