One of the biggest areas of confusion and debate around the planets of our solar system is whether or not Pluto is a planet.
Read below for the definitive and up-to-date answer in 2018.
1. Is Pluto a planet?
So let’s get straight to it – no, Pluto is not a planet.
It used to be considered the ninth planet of our Solar System but was declassified as a planet when a standard global definition was agreed in 2006 by the International Astronomical Union (IAU).
The Solar System therefore has just eight planets and Pluto is not one of them.
The reason that Pluto is not a planet is essentially that it is too small. When it was first discovered in 1930 nobody knew how big it was, but later in the 20th century it was discovered that it was tiny in comparison to the other planets of the Solar System and that there were many other objects that were larger or of similar size in the same region. It was therefore downgraded to a Dwarf Planet.
> To read more on this, see how many planets are there?
1.1 Wait, I thought I heard that Pluto became a planet again?
No, it didn’t. Pluto has not been a planet since 2006 but debate about this continues and it’s possible it could change in the future.
In early 2017, a NASA team put forward a proposal for a new definition of a planet that, if agreed upon, would mean reclassifying Pluto as a planet. The academic paper on this can be seen here.
If this definition were accepted, over 100 more objects would be classified as planets in our Solar System.
As it stands though, the 2006 definition remains. The Solar System has eight planets and Pluto is not one of them.
2. Frequently asked question about Pluto
2.1. Does Pluto have moons?
Pluto has five moons. These are:
Charon is the largest and actually so big that it was previously considered to classify Pluto and Charon together as a double dwarf planet (but this was eventually rejected).
2.2. How big is Pluto?
Pluto is just one-sixth the size of Earth and smaller than our moon.
Some key measures of size are:
- Radius: 739 miles (1,189.9 km)
- Diameter: 1,473 miles (2,370 km)
- Surface area: 3.5% of Earth
- Mass: 2% of Earth
2.3. Where is Pluto? How far is it from the Earth and Sun?
Pluto lies beyond Neptune at the edge of our Solar System in an area known as the Kuiper Belt, which has around 100,000 currently known objects.
Depending on where the planets are, Pluto is between 2.66 billion miles and 4.67 billion miles (4.28 billion km to 7.5 billion km) from Earth. The average is 3.7 billion miles (5.9 billion km).
Pluto’s average distance from the Sun is 3.67 billion miles (5.9 billion km). The closest Pluto gets to the Sun is 2.76 billion miles (4.44 billion km). The farthest Pluto gets from the Sun is 4.58 billion miles (7.38 billion km).
2.4. What missions has NASA sent to Pluto?
NASA’s New Horizons spacecraft was launched in 2006 with the primary objective of flying by Pluto, which it successfully did in 2015.
Pluto’s distance from Earth that it makes it difficult for us to study. Before New Horizons, the main NASA examination of Pluto had been through the Hubble telescope.
It took New Horizons 3462 days to reach its closest point to Pluto on 14 July 2015. The mission was able to map the surface composition of both Pluto and Charon, analyze Pluto’s atmosphere, and provide images of Pluto that were vastly superior to anything previously seen.
The last transmission of data from New Horizons of Pluto was received on October 25 2016. The spacecraft has then carried on to its secondary objective of exploring other objects in the Kuiper Belt.
2.5. What’s the temperature of Pluto?
Pluto is very cold in comparison to Earth as it is much further from the Sun. The average surface temperature on Pluto is 229 Celsius (or -380 Fahrenheit).
This ranges from a minimum of -233 degrees Celsius (-387 degrees Fahrenheit) up to -223 degrees Celsius (-369 degrees Fahrenheit).
2.6. What’s on Pluto?
Pluto is about two-thirds rock and one-third ice. This was learned only when New Horizons supplied pictures of Pluto back to NASA on Earth.
Images from New Horizons have indicated that Pluto may have “ice volcanoes” that spew out a cold mixture of ice, nitrogen, and methane.
There are also giant mountains of ice and plains that are thought to be comprised of nitrogen.
Pluto does have an atmosphere which is made up of mostly nitrogen, plus smaller amounts of methane and carbon monoxide.
2.7. What does the name ‘Pluto’ mean?
Pluto is the ancient Greek god of the underworld.
The name was proposed by 11-year British schoolgirl Venetia Burney in 1930. She suggested it to her Grandfather who had worked at Oxford University in England who then passed it on an astronomy professor he knew in the USA. This was then passed to the Lowell Observatory in Arizona, USA who voted to officially adopt the name.
Pluto’s moon, Charon, is named after the mythological boatman who ferried souls across the river Styx to Pluto for judgment.
3. New Horizons Pluto Photos
NASA’s New Horizons spacecraft provided us with some amazing photos of Pluto in 2015.
There was a buzz across the world when the famous image of Pluto’s “heart” was shared (this feature of Pluto is now called Tombaugh Regio).
The famous image of Pluto shared by NASA’s New Horizons in July 2015.
See here for the full gallery from NASA of New Horizons pictures of Pluto.
4. Five quick facts about Pluto
- Sometimes Pluto is inside the orbit of Neptune therefore closer to Earth and the Sun than the eight planet
- Pluto is the only object in our Solar System beyond Neptune that is known to have an atmosphere
- Pluto orbits the Sun every 248 years
- A single day on Pluto is equal to 6.4 Earth days
- Pluto’s real name is 134340 Pluto. This occurred when Pluto was declassified as a planet and was given a name in line with other non-planet objects
5. Take the Pluto quiz
6. Sources and further information about Pluto
Check out these resources to learn more about Pluto:
- Wikipedia: Pluto
- NASA New Horizons
- A great article from The Guardian on the Pluto “planet” debate and some of the interpersonal rivalries behind it
- NASA documentary of the New Horizons mission to Pluto:
Find more Skies & Scopes articles on astronomy and the planets here.