Now that the International Astronomical Union has redefined a planet specifically as a body which is massive enough to be rounded by its own gravity (but not massive enough to cause thermonuclear fusion) and which has also cleared its neighbouring region of planetesimals, there will probably never be a tenth planet. For that matter, there is no longer a ninth planet: the twin planets Pluto and Charon have been reclassified as a dwarf planet and its moon. As such, they join the many rocky masses of the Kuiper and asteroid belts which may fit the new criteria, including relatively nearby Ceres and such distant icy bodies as Eris and Makemake. While there remain gravitational orbit anomalies which have yet to be explained, most believe that they are too small to have been created by anything with more gravitational mass than Pluto or Eris … yet there are possibilities.
Certainly other massive solar system objects do exist. In 2005, Michael Brown, Chad Trujillo, and David Rabowitz discovered Makemake, a new dwarf planet whose orbit averages a quarter further from the sun than Pluto’s, with an inclination twice as large. It takes over 309 years to complete its orbit. Unlike Pluto and Eris, it has no satellites. Like all Kuiper belt objects (KBOs), it is named for a creator deity, in this case that of the Rapanui, the native people of Easter Island, because the discovery was made shortly after Easter. At 750 kilometres in radius, it is the third largest of the KBOs, after Eris and Pluto.
Makemake is actually the second trans-Neptunian dwarf planet to have been discovered by Michael Brown and his team of astronomers. The first was Eris, which had been spotted as early as 2003 but which was not identified as a planet until 2005. It is slightly larger than Pluto and 27% more massive. Its orbit is so sharply elliptical that it starts to look like a comet’s orbit, with its aphelion three times as far away as its perihelion. Its average distance from the sun is 67 astronomical units (AUs), twice as far away as Pluto’s, and its orbital period is 557 years. Because of the co-existing controversy over just what constitutes a planet and the differing conventions for naming planets and dwarf planets, Eris was not formally named for over a year after its identification. One of the original nicknames it had been given was Xena, a name teasingly preserved in the name finally given to Eris’ moon, Dysmonia (“lawlessness”), the demon daughter of Eris: since Lucy Lawless had played the character of Xena. Following the convention for KBO dwarf planets, Eris is named for the Greek goddess of primal chaos.
The third Knuiper belt object to have beeh discovered by Michael Brown and his team is Haumea, named for the Hawai’ian goddess of childbirth and fertility. Unusually for trans-Neptunian objects, Haumea is distinctly ovoid, being twice as long along its longest diameter as along its shortest. Because of this unusual shape, its size has not yet been accurately determined, although it is believed that it may be the fourth largest of the trans-Neptunian objects, after Eris, Pluto, and Makemake. It also has a very high albedo, believed to be due to crystalline water ice, and its day is only four hours, faster than any other known solar system object larger in diameter than 100 km. Haumea’s orbit has a lower eccentricity than most objects in the area, only 1.89, although its inclination from the ecliptic is over 28 degrees. It has two moons, Hi’iaka and Namaka. Even as these two children of Haumea sprang from different parts of her body in Hawai’ian mythology, the two moons are believed to have broken away from the body of Haumea in some ancient collision. Although the rotational dynamics of the three bodies suggest that such a collision must have happened over a billion years ago, near the birth of the solar system, the coating of water-ice paradoxically suggests that the dwarf planet is very young, less than 100 million years old. The Haumea collisional family lies an average of 43.3 AUs from the sun, and takes 285 years to complete a single orbit.
Although as yet these three are the only new trans-Neptunian dwarf planets, they are not the only named trans-Neptunian objects. Also discovered in 2004 by Michael Brown’s team, Sedna is among the most distant solar system objects currently known. Its closest approach to the earth is still three times further than the orbit of Neptune, while its furthest reaches take it 975 AUs away from the sun. It takes 10,500 years to complete a single orbit. It is an unusually deep red in colour, nearly as red as Mars, although at its standard temperature of only 33 Celsius degrees above absolute zero this colour cannot be caused by iron oxidation. Not much else is yet known about Sedna, although it seems to be about a third smaller than Pluto and a moon is inferred from the slowness of its rotation. Not surprisingly, it is named for the Inuit goddess of the deep, dark, cold places under the sea.
The last of the four objects currently redefined as a dwarf planet is Ceres, the largest known asteroid in the asteroid belt between Jupiter and Mars.
The Knuiper belt is named for Gerard Kuiper, an astronomer who had proposed in 1951 that the region around Pluto’s orbit should be fairly clear of planetesimals, as per the definition of planet. Almost from the very beginning, there was a problem with this proposal just as soon as it became clear that Pluto was much smaller than had originally been thought, and also regularly crosses Neptune’s orbit in an orbit that is at one and the same time stable, yet unpredictable. In 1992, the first of the KBOs was discovered within the space that should have been swept clear, quickly followed by hundreds of others. This field of discovery is a very intimate one: David Jewitt, who led the team which discovered 1992 QB1, the first of the trans-Neptunian objects to be discovered after Pluto and Charon, was also the graduate advisor to Chadwick Trujillo.
So many planetoid-sized objects have now been discovered, in fact, that a new system of distribution naming was needed. Those objects which travel in orbital resonance with Neptune, meaning that they exert a regular, periodic gravitational influence upon each other which keeps their respective orbits stable, are now called resonant objects. Orbital resonance is measured as a ratio of orbital length. For example, objects at a 1:2 resonance would orbit the sun exactly twice for every single orbit of Neptune’s. Those objects at 1:2 resonance are called twotinos, and those at 2:3 resonance are called plutinos (after Pluto, the first known of this type). Objects clustered at Neptune’s Lagrangian points are called the Neptune Trojans.
Next come cubewanos, or classical Kuiper belt objects, which occupy a discrete belt outside Neptune’s orbit. This differentiates them from plutinos, which tend to have much higher orbital eccentricities and often cross Neptune’s orbit. Cubewanos derive their unusual name from 1992 QB(1): since all later objects found in that region were nicknamed QB1-o’s.
At the far reaches of our solar system can be found the objects of the scattered disk, whose aphelia can be further than 1000 AUs. These are marked by high eccentrities, and are thus capable of being gravitationally influenced by Neptune despite their great distance. Eris lies in the region which overlaps the Kuiper belt and the scattered disk. In contrast, detached objects are those planetoids which never approach close enough to be gravitationally influenced by Neptune, and which cannot have resulted from gravitational scattering. Sedna is generally held to belong to this group.
Furthest away of all would be the objects of the hypothetical Oort cloud, the region from which comets are believed to originate. At a distance of 50,000 AU, the Oort cloud would be located a quarter of the way to the next nearest star, Alpha Centauri. As yet, there are no confirmed direct observations of anything within this region.
Finally, the relatively nearby objects which have been found in orbits overlapping those of the gas giants are called centaurs. Because these solar system objects have some or most of their orbits within the areas dominated by the gas giants, their origins are particularly diverse, their orbits are unstable, and they continually flirt with annihilation. Some of these orbits are so highly eccentric, their perihelions actually take them inside the orbit of earth! Thus far the largest of these objects to have been discovered is 10199 Chanklo, only 250 km in diameter, which was found in 1997. Because several of these objects show comet-like traits, such as displaying a coma when near perihelion, they are classified both as asteroids and comets.
With all these objects contributing to outer planet gravitational twitches, could another solar system object which meets true planetary criteria still exist? The answer, surprisingly, is yes. Sedna’s orbital pattern absolutely requires another significant object to have been involved at some point, whether a passing star or brown dwarf in the distant past, or a planet-sized object exerting continual and regular influence. The size of such a planet would depend directly on its distance from the sun, ranging from an earth-like mass at a thousand AU distance to another possible Jupiter at 5000 AU distance.
The two Voyager spacecraft are on their way out there right now. Who knows what they might find?
