Beyond the realm of the beautiful blue giant planet Neptune--the furthest major planet from the Sun--there exists a mysterious and strange region that we are only now beginning to explore. In this remote, dark, and frigid region, far from the light and warmth of the Sun, dwells a multitude of enchanting and mystifying objects. From this distant region, the Sun appears to be only a large star in the dark sky; not the enormous and luminous golden ball that greets our Earth each morning.
Once, not very long ago, schoolchildren were taught that Pluto was the furthest planet from the Sun--not Neptune. However, this beloved little world--that had a cartoon dog named in its honor--has recently been evicted from major planet status, and is now classified as a "dwarf planet". Why did this happen? Alas, poor Pluto!
The demoted little world that is Pluto was discovered the same year that Mickey Mouse's dog "Pluto" was created--and the pup was named for what was then considered to be a major planet. The American astronomer Clyde Tombaugh discovered Pluto in 1930, and the small, icy object, swathed in perpetual darkness, was appropriately named for the Roman god of the underworld. Pluto's largest moon, Charon, was discovered in 1978 by James Christy, and it is sometimes thought to be a chunk of Pluto itself, that may have broken off in a smash-up between Pluto and some other large object. When Pluto was still being classified as a planet, the Pluto/Charon system was considered to be a "double planet" by many astronomers. This is because Charon is roughly half the size of Pluto, making it larger in proportion to its "planet" than any other moon in our Solar System. The second runner-up is our own Earth/Moon system. Earth's Moon is about 27% the size of Earth. All of the other moons that circle the planets of our Solar System are considerably smaller than their host planet.
On August 24, 2006, the International Astronomical Union (IAU) struck Pluto from the pantheon of full-fledged planets, reclassifying it as a member of the newly created category of small worlds called "dwarf planets". When Pluto was first discovered in 1930, astronomers thought it was much bigger than it is, and also very much alone where it whirls in the frigid darkness beyond the realm of the giant planets. Now, astronomers know that Pluto is much smaller than originally supposed, and far from alone where it dwells in the outer limits of our Solar System.
The demotion of poor Pluto by the IAU came as the result of the discovery of a vast population of assorted, mostly icy objects that circled the Sun from an even greater distance than the very remote Pluto--particularly a world named "Eris", which appeared to be bigger than Pluto at the time of its discovery, but now is thought to be roughly the same size. In fact, Pluto and Eris are now considered to be "twin" worlds, whirling around our Sun in a remote region called the "Kuiper Belt".
Astronomers divide our Solar System into segments according to their distance from the Sun, as measured in Astronomical Units (AU). One AU is equal to Earth's mean distance from the Sun, which is 150 million kilometers, or 93 million miles. The innermost segment of the Solar System is our home, the area hosting the terrestrial (rocky) planets: Mercury, Venus, Earth and Mars, and it extends out roughly to 2 AU. The Asteroid Belt is next (2 to 4 AU), and it is situated beyond Mars, but nearer to the Sun than the largest planet in our Solar System, the gas-giant Jupiter. After the Asteroid Belt, the outer giant planets reign supreme in our Sun's family all the way out to the orbit of Neptune (Jupiter and Saturn are gas giants; Uranus and Neptune are smaller, but still gigantic, ice giants with thick gaseous atmospheres). The realm of the giant planets extends from 5 to 30 AU.
Next lurks the mysterious outer limits of our Solar System; its twilight zone veiled in bewitching, everlasting darkness, where Pluto, Charon, Eris and their enchanting icy-kind circle mysteriously in a vast kingdom far from the dazzling light and life-loving golden warmth of their parent Star. In this remote region, astronomers find it nearly impossible to spot dim, elusive worldlets. The brightness of an object plummets as the square of its distance. Hence, the same object observed at twice the distance from our planet will appear to be four times dimmer. This remote, dark, and frozen kingdom can be subdivided into the "trans-Neptunian" region of our Solar System, the "Kuiper Belt" (35-47 AU), and the "scattered disk" (35 to more than 100 AU). Beyond this may lurk the "Oort Cloud", a hypothetical shell of icy proto-comets loosely circling the Sun, extending halfway to the nearest star.
For three-quarters of a century, astronomers thought that Pluto was alone where it dwelled in the remote shadowy outskirts of our Solar System beyond Neptune. However, all this changed in 1992, with the discovery of the first Kuiper Belt Object (KBO), and later with the discovery of a multitude of larger objects in this region, especially Eris.
The IAU was forced to come up with a new definition of "planet". The new definition stated that a "planet" is an object that orbits the Sun without being some other object's moon, is large enough to be rendered spherical by its own gravity (but not so massive that it is capable of nuclear fusion, like a star), and has "cleared its neighborhood" of most other orbiting objects.
Because it was finally realized that Pluto is not a solitary object where it whirls, but instead actually shares orbital space with a multitude of other worldlets in the Kuiper Belt--the region of icy objects beyond Neptune--it was ousted from major-planethood by the IAU.
Pluto possesses an elliptical orbit that is not in the same plane as the eight major planets of our Solar System (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune). On average, this remote dwarf planet twirls around our Star at a distance of 5.87 billion kilometers (3.65 billion miles). It requires 248 Earth-years to complete a single orbit.
Because Pluto is so remote from the light and heat of our Sun, it is one of the most frigid worlds in the Solar System, with surface temperatures of roughly minus 375 degrees Fahrenheit (minus 225 degrees Celsius).
Pluto has four other, much smaller known moons, in addition to Charon. Astronomers currently think that Pluto is about 70 percent rock and 30 percent ice--predominantly nitrogen ice. The dwarf planet also sports a very thin atmosphere of nitrogen, methane and carbon monoxide, which reaches out roughly 3,000 kilometers (1,860 miles) into space. When the dwarf planet twirls closest to our Sun, it is surrounded by an ethereal atmosphere--but when it travels to the more distant, colder region of its orbit, this atmosphere freezes and then falls to its surface as "snow", giving the worldet a sparkling pinkish frosting of mainly nitrogen ice.
Astronomers will learn more about this beloved little world beginning in July 2015, when the New Horizons probe reaches this distant region of our Solar System. This will be the first time a spacecraft has come to explore this very remote, mysterious object, where it dwells with others of its kind in the outer limits.
California Institute of Technology (Caltech) astronomer Dr. Michael Brown led the team that discovered Eris in 2005. Eris is thought to be one of the brightest little worlds in our Solar System, with a diameter of about 1,445 miles--equivalent to the diameter of Pluto which is thought to have a diameter of between 1,429 and 1,491 miles across. Because Eris is smaller than originally thought, its surface is considerably brighter than first thought. This is because the quantity of light astronomers detected emanating from it originated from a tinier than anticipated surface area.
The new study suggesting that Eris is smaller than first estimated was authored by Dr. Bruno Sicardy and colleagues. Dr. Sicardy is a planetary scientist at Pierre and Marie Curie University and Observatory in Paris, France. When Eris fortuitously passed in front of a background star more recently, it provided astronomers with the clearest view of it since it was first spotted by Dr. Brown and his team. The new analysis was released in October 2011 to the British journal Nature. The authors think that Eris's brightness is the result of a millimeter-thick layer of methane-and-nitrogen frost that covers its surface. This frost, they suggest, was probably once an atmosphere about 10,000 times thinner than Pluto's. This very thin atmosphere froze onto the surface, as a result of the extraordinarily cold temperatures Eris experienced as it traveled further and further away from our Sun on its 557-year orbit. Despite its brightness, the scientists think that Eris's surface should have been darkened by a heavy shower of micrometeorites, as well as numerous strikes from cosmic rays.
Even though Eris's size has probably been roughly ascertained, it is still unknown whether it is actually smaller or larger than Pluto. This is because Pluto's size has not been precisely determined.
There are many other enticingly mysterious frozen "oddballs" in this faraway, dark and cold region. For example, Haumea, discovered by Dr. Brown's team in late 2004, is easily one of the most bizarre objects in our Solar System. Haumea is roughly about 1,931 km (1,200 miles) across--rendering it nearly as wide as Pluto. However, Haumea is merely one-third as massive as Pluto because it is not round. Instead, Haumea is shaped like an enormous football. This weird denizen of the Kuiper Belt, that restlessly circles our Star in an orbit that is only slightly more distant than Pluto, is one of the fastest spinning bodies in our Solar System, completing one full rotation in less than four hours. Astronomers discovered that about three-quarters of Haumea's bright, frigid surface is covered with crystalline water ice--much like the familiar ice that we find in our home freezers. However, an energy source is necessary to maintain this homey type of highly organized ice, and some astronomers have suggested that this requisite energy may result from radioactive elements hidden deep within Haumea, enhanced by tidal heating generated by the gravitational attraction that this worldlet and its two moons (Hi'iaka and Namaka) exert on one another.
Makemake is another weird little world twirling around in the outer fringes of our Solar System. Discovered in 2005 by Dr. Brown's team, Makemake's precise size remains undetermined, but the best current estimate makes it about 75% as big as Pluto. This renders it the third-largest currently known dwarf planet in the Kuiper Belt after the twin-sized worlds, Pluto and Eris. Makemake completes its long orbital journey around our Star every 310 years, and is only slightly further out than Pluto, at an average distance of 6.85 billion kilometers (4.26 billion miles), and displays a reddish hue in the visible light spectrum. Astronomers think that its surface is coated with a glistening frost of frozen methane. This remote and mysterious world sports no known moons.
Hundreds, perhaps thousand, of remote and as-yet-undiscovered dwarf planets circle our Sun in the frigid outer limits of our Solar System.
Billions of years ago cryovolcanoes disgorged mushy water-ice over 50 percent of the surface of the faraway dwarf planet affectionately dubbed "Snow White"--or less poetically 2007 OR10. In cases of cryovolcanism, or icy volcanism, mushy ice composed of such volatiles as water, methane or ammonia is substituted for the fiery lava that we are familiar with on our own planet. Cryovolcanism also occurs on other bodies in the outer Solar System, such as Enceladus, a small icy moon orbiting the planet Saturn.
Snow White, which was discovered in 2007 as part of the doctoral thesis of Dr. Brown's then-graduate student, Dr. Meg Schwamb, circles our Star in the remote Kuiper Belt, and is about half the size of Pluto. This makes Snow White the fifth largest known dwarf planet in our Solar System. At the time of its discovery, Dr. Brown thought that this icy little world was a large chunk of Haumea that had broken off and gone its separate way. Dr. Brown nicknamed the weird little world "Snow White" in honor of what was then thought to be its white color.
However, follow-up studies of the object showed that Snow White is rose-red. In fact, it is one of the reddest bodies in our Solar System. Some astronomers now suggest that this red-tinged dwarf planet may also be coated with a film of methane-frost, the lingering relic of an ancient atmosphere that is gradually being puffed into interplanetary space.
Snow White is not the only red object in the remote Kuiper Belt. For example, Quaoar, a small dwarf planet twirling around in the Kuiper Belt, is also rosy-red. Quaoar was discovered in February 2007 by Dr. Brown's team.
For a long time astronomers erroneously thought that Snow White, though relatively large, was unremarkable--just another one out of several hundred potential dwarf planets that romp around with hundreds of thousands of kindred worlds in the Kuiper Belt.
Dr. Brown noted to the press in October 2011 that "With all of the dwarf planets that are this big, there's something interesting about them--they always tell us something. This one frustrated us for years because we didn't know what it was telling us."
In 2010, Drs Brown, Adam Burgasser and Wesley Fraser used a new instrument with the 6.5-meter Magellan Baade Telescope in Chile, to get a better view of Snow White. Snow White was quite red, as was expected by this time, but the big surprise was that it was covered in water ice. Dr. Brown continued to explain that "Water ice is not red." Ice is very common in the cold outer reaches of our Solar System, but it is almost always white.
According to Dr. Brown, "That combination--red and water--says to me, 'methane'. We're basically looking at the last gasp of Snow White. For four and a half billion years, Snow White has been sitting out there, slowly losing its atmosphere, and now there's just a little bit left." Over time, exposure to the radiation from space converts methane--which is composed of one carbon atom bonded to four hydrogen atoms--into long hydrocarbon chains, which look red. Like a light whisper of snow freezing on the streets on a wintery twilight, the irradiated methane may cling to Snow White's frigid surface, making it look rosy-red.