There are two ways to look at the question ”how many planets are there”? One is to limit ourselves to the number of planets in our solar system and the other is to expand our thoughts to cover the known universe. So, let’s have a quick look at both ways of thinking.
How Many Planets Are There In Our Solar System
There are currently eight bodies in orbit around our Sun that are considered to be planets. To many adults that is both confusing and annoying. We spent hours memorizing the nine planets and their order from the Sun only to have Pluto reclassified as a dwarf planet in 2006. That leaves us with Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
So, why was Pluto demoted, so to speak? It all started with some improvements in the technology used for observing the skies. With those improvements came the discovery of Eris in 2005. Mike Brown, Chad Trujillo, and David Rabinowitz were using images taken at the Palomar Observatory when they noticed a large trans-Neptunian object(TNO). Eris is remarkable because it is the largest of the dwarf planets, larger than Pluto, and sparked the debate about whether it should be classified a tenth planet or if Pluto’s planetary status should be reevaluated. The International Astronomical Union(IAU) met in 2006 to settle the debate. In order to do so, the IAU needed to redefine what constitutes a planet. Here are the current requirements for planethood in our Solar System.
1. A body has to orbit the Sun.
2. A body must have enough gravity to pull itself into a sphere.
3. A body must have cleared its orbit of other objects. This is where Pluto failed to meet the test. It only accounts for a fraction of the mass within its orbital area, whereas Earth accounts for the vast majority of material within its orbital area.
Meet The Five Dwarf Planets
Ceres was the first dwarf planet ever discovered. It was initially classified as an asteroid. It is composed of rock and various forms of ice. It is also the smallest of the dwarf planets. The Dawn spacecraft is scheduled to arrive in orbit around Ceres in 2015.
Haumea not much larger than Ceres. It barely has enough gravity to be classified as a dwarf planet. In fact, it has only pulled itself into an ellipsoid(egg shape). Despite only having about one-third of the size of Pluto, it has captured two moons of its own. (136108) Haumea I Hi-iaka and (136108) Haumea II Namaka are both extremely small rocky bodies that are only known because of perturbations in the light emitted by their primary.
Makemake, unlike the other dwarf planets, is a Kuiper Belt object(KBO). The others are considered to be TNOs. It is nearly the size of Pluto. Like Eris and Haumea, its discovery is contributed to Mike Brown. Its distance from Earth means that scientists can only estimate its composition and temperature. Based on best estimates, it is extremely cold and is most likely covered in ices of the gases methane, ethane, and nitrogen.
Eris helped turn the entire planetary classification system on its ear. It is more massive than Pluto, making it the largest dwarf planet at this time. Eris has 27 percent more mass than Pluto. It also has the distinction of being the ninth most massive body orbiting our sun. Dysnomia is the only known moon of Eris.
Pluto is now the tenth largest body orbiting the Sun. Relatively little is known about Pluto despite its having been discovered nearly 100 years ago. It has five known moons: Charon, Nix, Hydra, S/2011 (134340),also known as P4, and S/2012 (134340) 1 also known as P5. The New Horizons spacecraft will fly within 10,000 km of Pluto in 2015, marking the closest approach to the dwarf planet.
Remember the mnemonic that we all used to remember the order of the planets? Here is a new one that accounts for the demotion of Pluto: My Very Excellent Mother Just Served Us Noodles.
How Many Planets Are There In The Universe
Not long ago, no one had observed a single planet outside of our solar system. Now, there are several hundred known exoplanets, with more being discovered each month. Quite a few of them are thought to be within the habitable zone of their stars. With billions of stars in our galaxy alone, there is the potential for several billions of planets to exist. Some scientists estimate that there may be as many 100 billion planets in the Milky Way. For years many scientists and science fiction buffs have speculated that there could be life elsewhere. The existence of these planets exponentially increases the likelihood of life elsewhere in the universe.
There are too many references to celestial bodies in this post to place a link to each one, but you can find plenty of information about each by going to www.nasa.gov.