50 Fun Facts About Neptune

Where does Neptune rank in distance from the Sun?

Since Pluto's 2006 demotion it has been the outermost planet, about 30 times farther from the Sun than Earth.

Neptune is unique among the planets in how it was found. How?

Wobbles in Uranus's orbit told Adams and Le Verrier where to look.

Who first observed Neptune through a telescope, on 23 September 1846?

He found it within a degree of Le Verrier's predicted position on the very night the letter arrived.

Which two mathematicians independently predicted Neptune's position from Uranus's orbit?

A Franco-British row over credit followed; the consensus now is that both deserve it.

Whose 1821 tables of Uranus's orbit first revealed that something unknown was tugging on the planet?

His hypothesis of an unseen perturbing planet set Adams and Le Verrier to work after his death.

Which famous astronomer plotted Neptune as a 'fixed star' in 1612 and 1613 without realising what it was?

It happened to be near Jupiter and had just turned retrograde, so it barely moved.

Neptune is named after the Roman god of what?

Le Verrier proposed the name, then tried to have the planet named after himself instead.

What does Neptune's astronomical symbol represent?

It is the three-pronged spear of the sea god.

Which name did Galle himself propose for the new planet?

Challis in England suggested Oceanus; Le Verrier briefly campaigned to name it after himself.

How long does Neptune take to orbit the Sun?

It only completed its first full orbit since discovery in July 2011.

In what year did Neptune complete its first full orbit since its discovery?

It returned to the spot north-east of Iota Aquarii where Galle had first seen it.

Neptune's mass is about how many times Earth's?

That is only about a nineteenth of Jupiter, yet it is the densest of the giant planets.

Which planet has the fastest winds in the Solar System?

They reach almost 600 m/s, over 2,000 km/h, faster than the speed of sound.

Roughly how fast are Neptune's strongest winds?

Most of them blow in the opposite direction to the planet's rotation.

What was the huge storm photographed on Neptune in 1989 called?

It resembled Jupiter's Great Red Spot but had vanished by the time Hubble looked in 1994.

What had happened to the giant 1989 storm when Hubble looked for it in November 1994?

A new dark storm had appeared in the northern hemisphere instead.

What nickname was given to a fast-moving white cloud group on Neptune seen before the 1989 encounter?

It zipped along faster than the Great Dark Spot; the Small Dark Spot was another 1989 storm.

What gives Neptune its blue colour?

Voyager's images exaggerated the contrast, making it look far deeper blue than Uranus when the two are actually similar.

Neptune and Uranus are classed together as what type of planet?

They are smaller than Jupiter and Saturn and much richer in water, ammonia and methane.

Which is the only spacecraft to have visited Neptune?

It made its closest approach on 25 August 1989, then dived past Triton because it was the last planet on its route.

How long did radio signals take to travel from the spacecraft to Earth during the 1989 Neptune encounter?

Over four hours each way, so the flyby had to run on pre-planned commands.

The 1989 flyby's accurate measurement of Neptune's mass disproved which long-standing hypothesis?

The mass came out 0.5% lower than calculated, which explained the leftover orbital wobbles.

What is Neptune's largest moon?

It holds more than 99.5% of all the mass in orbit around the planet.

Neptune's largest moon is unique among the big moons of the Solar System for what?

That backwards orbit shows it was captured, probably a former dwarf planet from the Kuiper belt.

How soon after Neptune's discovery was its largest moon found by William Lassell?

None of the other moons were found by telescope until the 20th century.

What is the eventual fate of Neptune's largest moon as it slowly spirals inward?

In roughly 28 billion years it will cross the Roche limit and be shredded.

What did the 1989 flyby see erupting from the surface of Neptune's largest moon?

That makes it one of the few worlds with active eruptions ever observed.

Neptune's largest moon, measured in 1989 as the Solar System's coldest known object, was roughly what temperature?

Its bright frozen-nitrogen surface reflects most of the feeble sunlight away.

How many known moons does Neptune have, according to the article?

Voyager 2 alone found six of them in 1989.

Which moon of Neptune has a highly eccentric orbit, its farthest point seven times its nearest?

Its eccentricity is 0.75; it was the second Neptunian moon discovered.

Neptune's outer ring, Adams, is famous for containing bright arcs named after what?

Courage, Liberté, Egalité and Fraternité; the moon Galatea keeps them from spreading out.

Which small moon's gravity is thought to keep Neptune's ring arcs from spreading into a full ring?

It orbits just inside the Adams ring.

Neptune's magnetic field is tilted from its rotation axis by how much?

Like Uranus's, it is also offset from the planet's centre by more than half a radius.

Neptune's gravity dominates which region of small icy worlds beyond its orbit?

It stretches from about 30 to 55 AU and includes Pluto.

Between 1979 and 1999, Neptune was temporarily the ninth planet from the Sun. Why?

Pluto's stretched orbit crosses inside Neptune's for about 20 years of each circuit.

Can Neptune be seen with the naked eye?

At magnitude 7.8 it can be outshone by Jupiter's moons and several asteroids.

Neptune's name translates as what in Chinese, Japanese, Korean and Vietnamese?

In modern Greek the planet is simply Poseidon; in Maori it is Tangaroa.

In 2009 the Academy of the Hebrew Language chose which official Hebrew name for Neptune?

Rahab is a sea monster mentioned in the Psalms, though most Hebrew speakers still say Neptun.

Neptune's thermosphere is anomalously hot, at roughly what temperature?

The planet is far too distant for sunlight to explain it, and the cause remains obscure.

Roughly how long is Neptune's rotation at the equator versus the poles?

The magnetic field, tracking the interior, turns once every 16.1 hours.

Neptune has the smallest angular diameter of any planet as seen from Earth: roughly how big?

That is why serious study had to wait for Hubble and adaptive optics.

How long does Neptune spend, on average, crossing each constellation of the zodiac?

Its 164-year orbit divided by twelve; it is currently heading for perihelion in 2042.

Which Neptunian moon is about as large as an irregular body can be before gravity rounds it?

It is the second-most-massive moon of Neptune yet only 0.25% the mass of Triton.

Le Verrier predicted Neptune's position as roughly 'five degrees east of' which star?

Galle found the planet within a degree of that spot, and 12 degrees from Adams's estimate.

Astronomers use Neptune's name as a shorthand for what?

Just as they talk about 'Jupiters', they call Neptune-mass worlds around other stars 'Neptunes'.

What did the 1989 flyby images of Neptune become the basis for as they arrived?

'Neptune All Night' aired the pictures as they came in.

Where does Neptune rank among the Solar System's planets by diameter?

Neptune is slightly smaller than Uranus but more massive and dense, making it the densest of the giant planets.

Roughly how long does each season last on Neptune?

Its axial tilt of 28.32° is similar to Earth's, so it has Earth-like seasons, but each stretches across a quarter of its 165-year orbit.

Which Chinese probe concept, IHP-2, was planned to fly by Neptune in January 2038?

The 2020s concept called for two Voyager-like probes to explore opposite ends of the heliosphere.

Neptune's magnetic field is offset from the planet's centre by at least what fraction of its radius?

Combined with its 47° tilt, the off-centre field resembles that of Uranus, which showed the oddity was not caused by Uranus's sideways spin.