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.

Courtesy of Erich Karkoschka,
U. Arizona





Larger false-color image.





By tracking atmospheric features
in Neptune's atmosphere, its day
is now estimated to last nearly
16 hours (more).

On May 20, 2011, Erich Karkoschka, a planetary scientist at the University of Arizona at Tucson's Lunar and Planetary Laboratory, published a paper revealing a more accurate determination of Neptune's rotation by tracking its atmospheric features over 20 years of observations. Except for Jupiter, accurate estimates of the rotational speed of the other gas giants in the Solar System Jupiter had yet to be determined. Karkoschka's estimated that a day on Neptune lasts precisely 15 hours, 57 minutes and 59 seconds. As neptune rotates faster than previously estimated at 16.1 hours using Voyager Mission radio data, the new estimate of its rotational period (at 16.0 hours) suggests that the planet's mass must be closer to the center than previously believed and so existing models of Neptune's interior may need to be revised (University of Arizona news release; Tammy Plotner, Universe Today, July 1, 2011; and Erich Karkoschka, 2011).

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.

As of July 15, 2013, Neptune is known to have at least 14 moons -- including the discovery of three new satellites announced in April 2003 and one in 2013. (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.

Luis Calçada, ESO



Larger and jumbo illustrations.



View of Neptune and the Sun
from the planet's largest's
largest moon, Triton (more).

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.

Results of numerical modelling released in March 2010 suggests that Neptune may have captured Triton from a planet with twice Earth's mass early in the Solar System's history, as it collided with that planet. In 2005, some astronomers proposed that Uranus and Neptune formed much closer to the Sun before migrating outwards and swapping places in the process (Maggie McKee, New Scientist, May 22, 2005), as a result enough material may have been left to form a planet with two Earth-masses that may have had Triton as a satellite. Triton, which is larger than Pluto, moves through its orbit in the opposite direction of Neptune's rotation, which suggests that the satellite did not form with Neptune but was later captured. This would explain Neptune's excess heat (irradiating 2.6 times more energy than its captured from the Sun) and the oddly eccentric and inclined orbit of Triton (Desch and Porter, 2010; and David Shiga, New Scientist, March 22, 2010).

NASA, JPL, Internet Archive



Larger and jumbo image montage.



Neptune may have captured it's largest
moon, Triton, from a planet with twice
Earth's mass, when it collided with
that planet (more).

On April 7, 2010, the European Southern Observatory (ESO) released infrared analysis of the atmosphere of Neptune's moon Triton, which indicates that the moon has entered its "summer" season in its southern hemisphere. A team of astronomers using the ESO's Very Large Telescope found carbon monoxide and made the first ground-based detection of methane in Triton's atmosphere. Their observations indicates that Triton's thin atmosphere varies seasonally, thickening significantly when warmed (ESO press release).

Scott S. Shepard,
DTM, CIW



Larger illustration.



The discovery of a high
inclination object among
the five known Trojans
around Neptune suggests
the presence of thousands
of such objects (more).

One of the recently discovered Trojans (2005 TN53) has an orbit that is more steeply tilted to the plane of the Solar System (ecliptic) than the other four. Because the methods used to observe these objects are not sensitive to objects so far out of tilt with the rest of the System, the detection of this Trojan suggests that there are many more like it, and that Neptune’s Trojans as a whole occupy thick clouds with complex, interlaced orbits. Subsequent numerical dynamical stability simulations indicate that as much as half of all Trojans around Neptune may have survived since the birth of the Solar System and the migration and interactions of its outer giant planets over four billion years ago. Color measurements of these pale red objects suggest that they share a common origin and evolutionary history different from that of classical Edgeworth-Kuiper Belt objects (David Powell, SPACE.com, January 30, 2007; Shepard and Trujillo, 2006; Trujillo el al, 2005; and orbital elements of 2005 TN53).

NASA






Another false-color image
of Halley's Comet






With a period of only 76 years,
Halley's is considered to be
short-period comet, which
may have originated as a
Centaur and even as a
Neptunian Trojan (more).

On July 17, 2010, two astronomers submitted a paper suggesting that Neptune's Trojan objects may be contributing to the reservoir of dynamically unstable Centaur objects (whose elliptical paths cross the orbits of the giant planets between Jupiter and Neptune), which can eventually be perturbed into the inner Solar System as short-period comets and even strike the Earth. Although there may be a million Centaurs more than a kilometer (0.6 miles) wide, only around 250 have been imaged by telescopes. As Centaurs have been modelled to last only three million years or so in their unstable orbits (before hitting a planet, the Sun, some other object, are ejected from the Solar System, or simply falls apart), they must be replenished continually over the eons (Eugenie Samuel Reich, New Scientist, July 30, 2010; and Horner and Lykawka, 2010). Some astronomers believe that most Centaurs come from the "scattered disk", part of the Edgeworth-Kuiper Belt of objects beyond Neptune. About one fourth of the risk of impactors hitting Earth comes from comets, rather than near-Earth asteroids.

Sheppard and Trujillo, 2010;
Subaru, NAOJ



More images



The sixth known Trojan object
discovered at Neptune's trailing
"L5" point, 2008 LC18 has
a diameter of around 60 miles
(or 100 kilometers) (more).

On August 12, 2010, astronomers announced their discovery of a sixth Neptune Trojan object (asteroid or dormant comet) with a 100-km (60-mile) diameter, which was apparently first detected in 2008 using the 8.2-meter Subaru telescope in Hawaii and designated 2008 LC18. While very difficult to detect due to their extreme dimness, the astronomers believe that as many as 150 objects of similar size as 2008 LC18 remain to be discovered at that same Trojan location, Neptune's trailing "L5" Lagrangian region, where 2008 LC18 is the first detected L5 or trailing Trojan. The astronomers believe that "[s]imilar populations and dynamics at both Neptune Lagrangian regions [leading L4 as well as trailing L5] indicate" that the Trojans were likely captured by Neptune during the early years of the Solar System when the planet was moving in a much different orbit that it is now, possibly during a relatively slow but smooth planetary migration outwards from our Sun, Sol, to its current location, before the giant planets settled into their current orbits when many planetesimals may have been stirred up onto unusual orbits before gravitational capture or ejection from the Solar System. They also estimate that there may be more large Trojan objects similar to 2008 LC18 in size at both of Neptune L4 and L5 regions than in the Main Asteroid Belt between Mars and Jupiter (Subaru press release; Sheppard and Trujillo, 2010; more images at astronomer Scott Sheppard's web site for 2008 LC18; Caitlin Stier, News Scientist, August 12, 2010; and Howard Falcon-Lang, BBC News, August 12, 2010).