Neptune

The eight planet from the Sun at just over 30 times the Earth-Sun distance, Neptune is the farthest of the four gas giants. Slightly smaller but denser than Uranus, it has a diameter of over 49,500 km (30,800 miles) which is four times Earth's and its mass is more than 17 times Earth's. Like the other giant planets, Neptune also has a fast rotation that gives it a "day" lasting just over 16 hours on Earth, but its distant orbit takes almost 164 Earth years to complete.

NASA


Larger image.


Long light-colored
cirrus-type clouds
can be found floating
high in Neptune's
atmosphere (More at
Astronomy Picture of
the Day).

Like Uranus, Neptune appears to be composed of mostly "ices" and rock, with about 15 percent hydrogen and a little helium. According to astronomer Richard Crowe, the planet may have a rock-ice core of about 1.2 times Earth's diameter -- 30 to 40 percent of Neptune's diameter -- and around 10 times Earth's mass -- 60 to 70 percent of Neptune's total mass ("Ask Astro", Astronomy, October 2004). Like all the gas giants, however, its atmosphere is made of about three-fourths hydrogen and and one-fourth helium with a small amount of methane which produces wispy clouds. In fact, and Neptune's bluish color is due to the same light scattering and red wavelength absorption by methane also found on Uranus.

NASA -- larger image

The Great Dark Spot.

Neptune has a great storm in the southern hemisphere called the "Great Dark Spot" that is about half the size of Jupiter's Great Red Spot and so is roughly the same diameter as the Earth, and at least one other smaller storm spot has been detected as well. Like the other gas giants, there are rapid winds confined to bands of latitude, including one band that is moving the Great Dark Spot westward at over 1,100 km (or 700 miles) per hour. Indeed, Neptune has the fastest planetary winds in the Solar System, reaching as fast as 2,000 km (over 1,200 miles) per hour.

Courtesy Jet Propulsion Laboratory. Copyright (c) California Institute of Technology, Pasadena, CA. All rights reserved.

The energy for such high-speed winds and ever-changing clouds is not coming from the Sun. Indeed, Neptune radiates 2.6 times as much energy as it absorbs from the Sun. Like Jupiter and Saturn, Neptune appears to have an internal heat source and so it radiates more than twice as much energy as it receives from the Sun. Planetary scientists believe that deep inside Neptune, pressure builds (and heat) until much of its formerly gaseous hydrogen turns into liquid metallic hydrogen, again like Jupiter and Saturn. Under such conditions, the methane found in Neptune's atmosphere also decomposes, as the bonds holding methane's four hydrogen atoms dissolve and the carbon atoms may bind to one another in the extreme pressure to form diamonds (according to a new hypothesis by a team at the University of California at Berkeley and experiments conducted by Robin Benedetti). Hence, a rain of diamonds may be falling toward Neptune's core, which release heat through friction with its heavy atmosphere (Curtis Rist, Discover, September 2000).

Observations with the Hubble Space Telescope from 1996 to 2002, however, confirm that Neptune's cloud bands have been getting wider and brighter in response to seasonal variations in sunlight, like the seasonal changes seen on Earth. This finding is consistent with observations made by G. Wesley Lockwood at the Lowell Observatory, which indicated that Neptune has been gradually getting brighter since 1980. The planet's brightness in near-infrared wavelengths is much more sensitive to high altitude clouds than to visible light, and so the recent trend of increasing cloud activity was also seen in near-infrared light as well in Keck Telescope observations made from July 2000 to June 2001, by a team of astronomers including Heidi B. Hammel. As Neptune takes almost 165 years to orbit the Sun, in a single season on the planet can last more than 40 years. Therefore, if what astronomers have been observing is truly seasonal change, the planet should continue brightening for another 20 years. (More discussion on Neptune cloud brightening at STScI.

Lawrence A. Sromovsky, Patrick M. Fry,
Sanjay S. Limaye, Kevin H. Baines,
U. of Wisconsin-Madison, JPL,
STScI, NASA

Larger image.


Responding to sunlight received,
seasonal changes since 1996
have lead to increased clouds
encircling Neptune's southern
hemisphere, which is seen on
Earth as a gradual brightening
(more at STScI).

Like Earth, Neptune spins on an axis that is tilted at an angle of 29 degrees toward the Sun, only a little more than Earth's 23.5-degree inclination. Remarkably, Neptune appears to exhibit seasonal change although the Sun is 900 times dimmer than it is from Earth. Hence, there is much less Solar energy available to drive the reversal of seasons that each hemisphere receives at a given time as it progresses through the season. (More information is available from Professor Larry Sromovsky on the Atmospheric Dynamics of Neptune.

Neptune's magnetic field is highly tilted and off center (47° degrees from its axis of rotation) like that of Uranus. And, like Uranus, it is believed that a giant spherical shell of electrically conductive substances such as water in its middle layers may be in motion to generate that electro-magnetic energy.

NASA -- larger and other images, and fact sheet


Lastly, The planet also has a set of rings like the other gas giants that are probably made of dust particles smashed off of Neptune's moons by micro-meteorites. (See an animation of the rings' orbits, known "shepherd moons", and other satellites around the Jupiter, with a table of basic orbital and physical characteristics.)

More on the Neptunian ring system is available from NASA's Planetary Rings Node.

NASA -- larger and jumbo images.


Neptune's largest moon, Triton.

Neptune is known to have at least 13 moons -- including the discovery of three new satellites announced in April 2003. (See an animation of the orbits of these satellites around Neptune, with a table of basic orbital and physical characteristics, or a NASA fact sheet.) Thirteen are relatively small between 35 and 320 km (22 to 200 miles) in diameter. However, one named Triton is a magnitude bigger at about 2,700 km (1,700 miles) across, which is roughly three-fourths the size of Earth's Moon and larger than Pluto.

Oddly enough, four of Neptune's moons are orbiting the planet within its Roche limit. Inside that limit, Neptune's gravitation pull is so strong that, in theory, no Solar nebular material could have agglomerated into those satellites, and so the satellites must have been captured or dragged within the Roche limit by tidal forces on the planet's surface (Rist, 2000). According to Carolyn Porco of the University of Arizona's Lunar and Planetary Laboratory, these satellites may eventually disintegrate to create a ring system as striking as those of Saturn.

NASA -- larger and jumbo images

Simulated view of Neptune on Triton's surface.

Triton's orbit is retrograde and inclined which suggests that the moon is actually a captured Edgeworth-Kuiper Belt ice body similar to Pluto and Charon. It has a thin atmosphere of molecular nitrogen and methane with a high haze layer, and there are what appears to be methane and nitrogen ices covering its relatively young, uncratered surface. There are plumes of darkish smoke rising from the surface like geysers which suggests that Triton is being heated internally by tidal interactions with Neptune.