Did You Know?
Of the five recognized dwarf planets, only Ceres is visible through binoculars.
The dismissal of Pluto, and consequently Eris, split the world into two halves. One agreed with the International Astronomical Union (IAU), while the other contested their decision. As of now, the five accepted dwarf planets are Pluto, Eris, Ceres, Haumea, and Makemake.
Although we have five bodies that qualify as dwarf planets, we've rarely gotten a chance to conduct much research on Eris, Haumea, and Makemake. Reasons for this include distance, little or no available light for most of their revolution, and the fact that they are still quite new to us.
Definition of a Dwarf Planet
Let's take a quick look at how the IAU understands planets and dwarf planets. The definition is basically a set of three conditions that a body must meet to be called a planet. The body must -1. Be in the orbit of the Sun
This is the most obvious condition for any body to be considered a dwarf planet. It should revolve around the Sun, essentially being a part of our solar system.2. Have enough mass to achieve hydrostatic equilibrium
It means that the body should be spherical. When a planet is spherical, all of it moves at the exact same velocity. This isn't so simple, though. For example, Haumea isn't perfectly spherical, it is elliptical. It's a Jacobi-ellipsoid, to be precise. The planet is considered to have reached a relaxed state of equilibrium, ascertaining its status as a dwarf planet.3. Cleared its neighborhood
This is the real clincher. This is the reason why scientists are so divided over the definition. Essentially, for the body to be a true planet, it should have gotten rid of everything in its orbit already. Pluto still has not met this criterion, because its orbit lies in the Kuiper belt, and is therefore strewn with 'space stuff'. A significant opposition to this condition was presented when scientists like Mike Brown (who discovered Eris) and Alan Stern (another science heavyweight, with a lot of work involved around Pluto) added that even the main planets still haven't 'cleared their paths' yet. This includes Neptune, in whose path lies Pluto (implying that if Neptune was a planet, Pluto shouldn't even exist), or Jupiter with the Trojan asteroids. This includes Earth too, because of the thousands of near-Earth asteroids that keep buzzing around us. A lot of tweaks and twiddles were explained, most important of which is that a planet should be the one with the dominant gravity, compared to those with whom it shares its orbit.
A lot of other factors did contribute to the fuss, trying to add exceptions to every planet in the solar system, including the way the voting process was held, etc. But as of now, we still have only five dwarf planets.
The 5 Accepted Dwarf Planets
Moving on, we have five dwarf planets, for none of whom the term 'dwarf' decreases their wonder or importance to us.
Orbit Location: Kuiper belt
Orbit Radius: 39.48 astronomical units (AU)
Diameter at Equator: 2,306±20 km
Mass: 13.05 x 1021 kg
Surface Gravity: 0.58 m/s2
Atmosphere: Thin layer of nitrogen, carbon dioxide, and methane.
Pluto, the planet with the funky orbit, is pretty much like other bodies in the Kuiper belt - made of rock and ice. Pluto shares its space with five sidekicks (moons), Charon (the largest), Nix (changed from Nyx, which is the name of an asteroid), Hydra, and two more that were recently discovered in 2011 and 2012. Their proposed names have been favored as Vulcan and Cerberus. Pluto was discovered in 1930 by Clyde W. Tombaugh.
Unlike a few assumptions, the name of the planet is not derived from the cartoon dog. In fact, it has been said that the dog got its name in honor of the newly discovered planet, like the radioactive element plutonium. Pluto is named after the Guardian of the Underworld in Greek mythology, as suggested by an eleven-year-old girl, Venetia Burney.
In the year 2015, New Horizons, a spacecraft that was launched in 2005, should reach Pluto.
Orbit Location: Asteroid belt
Orbit Radius: 2.77 AU
Diameter at Equator: 974.6±3.2 km
Mass: 0.94 x 1021 kg
Surface Gravity: 0.27 m/s2
Named after the Roman goddess of harvest and motherly love, Ceres was discovered by Giuseppe Piazzi in 1801. The core is solid rock, covered by an ice mantle. Research has shown that there might be freshwater under the surface, more than what we have on Earth. The dwarf planet is 14 times smaller than Pluto, although it weighs out about one-third of the asteroid belt.
Ceres has been considered for space colonization in the future. It won't serve as a habitable place, but more of a transportation hub for a spacecraft between Earth and another object.
Orbit Location: The Scattered disc
Orbit Radius: 67.67 AU
Diameter at Equator: 2,326±12 km
Mass: 16.7 x 1021 kg
Surface Gravity: approx. 0.8 m/s2
Atmosphere: Similar to Pluto
Ceres is the ninth-largest body in our solar system, and the largest dwarf planet. Its highly eccentric orbit allows it to come closer to the Sun, with its orbit cutting Pluto's and coming close to Neptune's.
Eris was discovered in 2005 by Mike Brown, Chad Trujillo, and David Rabinowitz. For the shortest period of time, Eris was called the tenth planet, with Pluto being the ninth. It was actually the discovery or Eris and its classification as a proper planet that sparked the need for a new definition for the planets.
At its time of discovery, Eris was informally called 'Xena', after the TV warrior princess of the same name. Similarly, Xena's moon was called 'Gabrielle', after Xena's sidekick in the TV show. The planet was later formally named Eris, after the Greek goddess of chaos and discord.
Eris is the second-most reflective body in our solar system, after Saturn's moon, Enceladus.
Orbit Location: Kuiper belt
Orbit Radius: 43.13 AU
Diameter at Equator: 1,300 km (Gross estimation)
Mass: 4.01 ± 0.04 x 1021 kg
Surface Gravity: 0.44 m/s2
Another Trans-Neptunian Object, Haumea is just one-third of Pluto's mass. It is the third-brightest body in the Kuiper belt, after Pluto and Makemake. It's bright enough to be seen by a good telescope in the night sky.
The dwarf planet is named after the Hawaiian goddess of fertility and childbirth, in 2008 by the IAU. Its moons were named Hi'iaka and Namaka, after the goddess's daughters.
It is always difficult to ascertain the shape of a body far away by sitting right here on Earth. The process is very complicated and requires the body to pass along a specific place in its orbit for favorable measurements. As of now, no strong measurements have been done for Haumea, therefore its shape cannot be proved, only theorized. Using its mass, density, rotational period, and light curve, astronomers have deduced that Haumea is in the shape of an ellipsoid.
Orbit Location: Kuiper belt
Orbit Radius: 45.79 AU
Diameter at Equator: 1420±60 km
Surface Gravity: Unknown
Atmosphere: None/transient, similar to Pluto
Makemake was discovered in 2005 by Mike Brown and his team, with the unofficial name 'Easterbunny'. It is the second-brightest object in the Kuiper belt, after Pluto.
Makemake does not have a satellite (or none have been discovered yet), making it impossible to calculate its mass. It is about three-fourths the size of Pluto.
It takes an estimated 310 years for Makemake to complete one revolution. The dwarf planet is extremely cold, with a surface temperature of -240° C.
The Kuiper belt has not been fully explored yet, so chances are we'll come across more bodies that qualify as dwarf planets, like Orcus, Sedna, Quaoar, and 2007 OR10
(interestingly nicknamed Snow White, and the seventh potential dwarf planet discovered).
Mike Brown, one of the best astronomers of our time, agrees with our current 8-planet system. He makes a valid point that as long as you classify an object properly, you can call it whatever seems culturally appealing. Science and culture are deeply intertwined, but neither can stake a dominant claim over the other. The debate over planets and dwarf planets should be undertaken similarly; to understand the universe instead of blindly challenging it for no progressive purpose.