8️⃣ Neptune

Neptune is the eighth and farthest known planet from the Sun in the Solar System. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times the mass of Earth and slightly larger than Neptune.[d] Neptune orbits the Sun once every 164.8 years at an average distance of 30.1 AU (4.5 billion km). It is named after the Roman god of the sea and has the astronomical symbol♆, a stylised version of the god Neptune’s trident.

Neptune Neptune symbol.svg
Neptune Full.jpg

Neptune’s Great Dark Spot and its companion bright smudge; on the west limb the fast moving bright feature called Scooter and the little dark spot are visible.
Discovered by
Discovery date 23 September 1846
Pronunciation /ˈnɛptn/ (About this sound listen)
Adjectives Neptunian
Orbital characteristics[6][a]
Epoch J2000
Aphelion 30.33 AU(4.54 billion km)
Perihelion 29.81 AU(4.46 billion km)
30.110387 AU(4.50 billion km)
Eccentricity 0.009456
367.49 days[4]
5.43 km/s[4]
Inclination 1.767975° to ecliptic
6.43° to Sun‘s equator
0.72° to invariable plane[5]
Known satellites 14
Physical characteristics
Mean radius
24,622±19 km[7][b]
Equatorial radius
24,764±15 km[7][b]
3.883 Earths
Polar radius
24,341±30 km[7][b]
3.829 Earths
Flattening 0.0171±0.0013
7.6183×109 km2[8][b]
14.98 Earths
Volume 6.254×1013 km3[4][b]
57.74 Earths
Mass 1.0243×1026 kg[4]
17.147 Earths
5.15×10−5 Suns
Mean density
1.638 g/cm3[4][c]
11.15 m/s2[4][b]
1.14 g
0.23[9] (estimate)
23.5 km/s[4][b]
0.6713 day[4]
16 h 6 min 36 s
Equatorial rotation velocity
2.68 km/s (9,650 km/h)
28.32° (to orbit)[4]
North pole right ascension
19h 57m 20s[7]
North pole declination
Albedo 0.290 (bond)
0.41 (geom.)[4]
Surface temp. min mean max
1 bar level 72 K (−201 °C)[4]
0.1 bar (10 kPa) 55 K (−218 °C)[4]
8.02 to 7.78[4][10]
19.7±0.6 km
Composition by volume

Neptune is not visible to the unaided eye and is the only planet in the Solar System found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to gravitational perturbation by an unknown planet. Neptune was subsequently observed with a telescope on 23 September 1846[1] by Johann Galle within a degree of the position predicted by Urbain Le Verrier. Its largest moon, Triton, was discovered shortly thereafter, though none of the planet’s remaining known 13 moons were located telescopically until the 20th century. The planet’s distance from Earth gives it a very small apparent size, making it challenging to study with Earth-based telescopes. Neptune was visited by Voyager 2, when it flew by the planet on 25 August 1989.[11] The advent of the Hubble Space Telescope and large ground-based telescopes with adaptive optics has recently allowed for additional detailed observations from afar.


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