Galileo Galilei first spotted the planet Neptune with his primitive "spyglass" on December 28, 1612. He observed it again on January 27, 1613. Unfortunately, on both occasions, Galileo thought that the giant, remote planet was a fixed star, appearing near the planet Jupiter in the dark night sky. Because of this mistake, Galileo is not credited with the discovery of Neptune.
The beautiful, banded, blue ice-giant planet, Neptune, is the furthest major planet from the Sun. It is also orbited by a very weird large moon that may not have been born a moon at all. The moon, Triton, is about 1,680 miles in diameter, and sports features that eerily resemble those found on the dwarf planet Pluto. Pluto is a denizen of the Kuiper Belt. The Kuiper Belt is a reservoir of comets and other icy bodies--some large, some small--that circle around our Sun beyond the orbit of Neptune, at a distance of about 30 to 55 Astronomical Units (AU) from our Star. One AU is equal to the average distance of Earth from the Sun--approximately 93,000,000 miles.
Triton and Pluto share roughly the same bulk composition and density, as well as similar atmospheres. In addition, both remote bodies move in unusual orbits. Pluto has a highly eccentric orbit, and is sometimes closer to the Sun than Neptune! Furthermore, Pluto orbits in the opposite direction around our Sun than do the eight major planets of our Solar System. Triton revolves around Neptune in a direction counter to that of its planet--and its retrograde orbit indicates that it is a captured object. Because of the unusual nature of both Triton's and Pluto's orbits, as well as the similarities of their bulk properties and atmospheres, it has long been thought that there is some sort of historical connection between them. Indeed, it was once thought that Pluto was an escaped moon of Neptune, but this is now considered unlikely. It is much more likely that long ago Triton, like Pluto, circled the Sun independently, but was unluckily captured by its adoptive planet--whereas Pluto was left to wander freely.
Neptune, the eighth major planet from the Sun, and its neighboring sister-planet, Uranus--the seventh planet from the Sun--are both classified as ice-giants because their large cores are icy, and they never managed to acquire the immense gaseous envelopes of the two true gas-giants, Jupiter and Saturn. The gas giants are possibly composed entirely of gas and liquid, although they may have small solid cores. In contrast, the ice-giants have large solid cores and thinner atmospheres. The two gas-giants, being mostly atmosphere, are very lightweight for their size. Saturn is the lightest planet in our Solar System, despite its immense diameter. In fact, Saturn is light enough to float like a huge raft in water, provided there was an ocean big enough for it to bob around in.
The spacecraft Voyager 2 flew past Uranus in 1986, and Neptune in 1989. Voyager 2 sent back images of Neptune to Earth that revealed a strikingly beautiful deep blue planet, that sported stripes and bands, and spot-like storms akin to hurricanes. Neptune's bands and spots are different shades of blue--and these lovely shades of blue are caused by atmospheric methane, not oxygen. Some of Neptune's frothy storms are white, and look like whirling marshmallows.
Triton is the largest of Neptune's 13 moons. It is an unusual world, twirling around its planet in the wrong direction. Many astronomers think that some time in the remote past, Triton was nudged out of its home in the Kuiper Belt, and during its wanderings in the darkness of interplanetary space, at last swept close enough to Neptune to feel the irresistible lure of that planet's gravity. As Neptune drew Triton into its gravitational embrace, that luckless wanderer from the Kuiper Belt underwent a sea-change from a comet-like denizen of our Solar System's outer limits, to a moon of one of the major planets. So, there Triton whirls around in its new home, circling its planet Neptune, but circling it backwards. And like all moons, it is now a dependent of its parent planet. As a matter of fact, the moon was given the name of Triton as an allusion to the demigod Triton's dependence on the sea-god Neptune in Greek mythology.
Like Earth's own large Moon, Triton is locked in synchronous rotation with its planet--one side always faces Neptune. However, because of Neptune's odd orbital inclination, both of the moon's polar regions take turns facing the Sun. Spacecraft images of Triton reveal mounds and round pits formed from icy lava flows (cryovolanism), as well as smooth volcanic plains. The surface of the moon is only sparsely cratered, indicating that its surface is new--that is, it is constantly being resurfaced, probably by the "lava" flow from icy volcanoes. Triton is very bright--its fresh, sparkling, new ice-coating is believed to cover a heart of metal and rock. Triton's high density suggests that it contains more rock in its interior than the icy moons of Saturn and Uranus.
Triton also possesses a thin atmosphere composed mainly of nitrogen, and a smaller quantity of methane. This atmosphere probably is the result of Triton's cryovolcanism, which is enhanced by seasonal heating from the Sun. Although little is currently known of Pluto's atmosphere, it is thought to be primarily composed of nitrogen with some carbon monoxide and methane added to the mix--and it is extremely tenuous. Pluto's very thin atmosphere may exist as a gas only when Pluto is nearest to the Sun (perihelion). For most of Pluto's very long year, the atmospheric gases are frozen in the form of ice on its extremely frigid surface. One year on Triton is almost 248 Earth-years long--or 90,471 Earth-days!
Triton is one of the coldest bodies in our Solar System. In fact, it is so cold that most of its nitrogen atmosphere is condensed as frost, giving its surface a very bright, mirror-like surface, that reflects about 70% of the sunlight that reaches it.
Astronomers suspected for a very long time that Triton was not born a moon of Neptune, but was instead a luckless refugee from elsewhere that had been kidnapped by its planet. It was not until 2006, however, that a convincing theory explaining how Triton was ensnared by its adoptive parent was proposed. This theory suggests that Triton once had a companion as it orbited the Sun. According to this scenario, Neptune's strong gravitational embrace tugged Triton away from its sister world. This research was reported in the May 11, 2006 issue of the journal Nature.
"We've found a likely solution to the long-standing problem of how Triton arrived in its peculiar orbit. In addition, this mechanism introduces a new pathway for the capture of satellites by planets that may be relevant to other objects in the Solar System," explained Dr. Craig Agnor, a researcher from the University of California, Santa Cruz, in the May 10, 2006 issue of Time Magazine.
The model indicates that Triton originated as part of a binary system, much like Pluto and its large moon Charon. "It's not so much that Charon orbits Pluto, but rather both move around their mutual center of mass, which lies between two objects," Agnor added.
Gravity can pull binary systems apart when the sister objects travel too close to a massive body--such as the planet Neptune. The orbital motions of the two sister objects results in one member traveling slower than the other. This can disrupt the system and permanently alter the orbital companion. This mechanism is termed an exchange reaction, and it could have shot Triton into a number of different orbits around Neptune, Agnor continued.
In 2006, NASA dispatched the New Horizons spacecraft to visit the outer limits of our Solar System--the Kuiper Belt where the dwarf planet Pluto dwells, along with trillions of icy comets, and a multitude of other larger icy bodies--and where it is thought that the adopted moon Triton was born. The spacecraft will reach this mysterious and unexplored region in July 2015, when it flies by the icy dwarf planet and its moons--including the large moon Charon. New Horizons will shed light on the weird worlds and bizarre objects dwelling in the outskirts of our Solar System.
As for Triton--it's a doomed world. It circles around its parent planet in the wrong direction, and as it does so it moves ever closer and closer inward. Eventually, Triton will crash into Neptune!
I am a writer and astronomer whose articles have been published since 1981 in various journals, magazines, and newspapers. Although I have written on a variety of topics, I particularly love writing about astronomy because it gives me the opportunity to communicate to others the many wonders of my field. My first book, "Wisps, Ashes, and Smoke," will be published soon.