Is Pluto a planet?
IT has a surface temperature of minus 225 degrees Celsius and was once the ninth and most distant planet from the Sun.
But Pluto's status as a planet is one of the most controversial issues in the world of astronomy.
Is Pluto a planet?
No, Pluto is not currently classed as a planet. It is a minor, or dwarf, planet.
Pluto used to be the ninth planet in our solar system until the International Astronomical Union created a new system for classifying space objects in 2006.
It set out three characteristics that define a planet:
- The object must have an established orbit around the sun
- The object must have a strong enough field of gravity to make it "round-ish"
- The object must have a gravitational pull strong enough to "clear the neighbourhood" around its orbit.
Pluto failed on the third point.
Unlike Earth and the other eight large planets, Pluto has not cleared its surrounding wider area of any other large objects.
However, many astronomers were not happy about the decision to reclassify Pluto and the debate about its planetary status has rumbled on.
In 2017 a new idea was put forward, where a planet could be any round object in space smaller than a star.
This would reclassify many moons as planets, including Earth’s moon, and all the dwarf planets.
When was Pluto discovered?
Pluto was discovered at around 4pm on February 18, 1930.
Astronomer Clyde Tombaugh had spent every night for months looking at photographic plates of the night sky at Lowell Observatory in Arizona.
At that time, technology wasn’t advanced enough to see how big Pluto was, or to notice the other bodies beyond Neptune.
Pluto – the key facts
Here's what you need to know, according to Nasa...
- Pluto measures roughly 1,400 miles across, around half the width of the USA
- It orbits the Sun around 3.6billion miles away on average – 40 times farther than Earth
- It takes 248 Earth years for Pluto to orbit the Sun – with each day lasting 153 hours
- Pluto has a thin atmosphere of nitrogen, methane and carbon monoxide
- Unlike the eight "proper" planets in our Solar System, Pluto is officially classified as a dwarf planet
- Pluto has five moons, the biggest of which is called Charon
- Nasa's New Horizons is the only spacecraft to visit Pluto, passing by in July 2015
By 1992 more of this was becoming clear, and in 2005 Mike Brown identified Eris, a frozen body nearby which was then thought to be larger than Pluto.
Later, if Pluto had remained classed as a planet, then so might similar sized objects such as Eris, Ceres (the largest asteroid) and Charon (a moon of Pluto).
This could have increased the number of planets in our solar system to 12.
Where is Pluto?
Pluto is in the same solar system as the Earth, but much further out, beyond the farthest planet from the sun, Neptune.
It is around 3.7 billion miles from the sun, and one of many minor planets beyond Neptune’s orbit, called trans-Neptunian objects.
Pluto is also in the Kuiper belt, a collection of hundreds of thousands of icy minor planets and asteroids orbiting the sun out beyond Neptune.
Astronomers began to notice these far-out frozen objects in 1992, and they include Sedna Namaka, Hydra, Nyx, Styx, Kerberos, and Makemake, as well as Pluto.
How far is Pluto from Earth?
Pluto is between 4.67 billion miles and 2.66 billion miles from Earth.
Its distance from Earth changes because both planets move around the sun in elliptically-shaped orbits.
Pluto is so far away it took great technological advances such as the Hubble telescope before we could see it properly.
NASA's New Horizons mission provided valuable high-resolution pictures, when it flew past Pluto in 2015.
How big is Pluto?
Pluto is about one-sixth the width of Earth, at just 1,420 miles in diameter.
It is far smaller even than Earth’s moon, and 30 times smaller than Mercury, the smallest planet in our solar system.
New Horizons identified Pluto as the largest object in the Kuiper belt and, crucially, larger than Eris.
Charon is the largest of Pluto’s five moons, and is half the size of Pluto, so they are sometimes together called the double planet.
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New Horizons showed Pluto not only to be larger than previously thought, but to have a thicker atmosphere and to be more geologically dynamic.
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Pluto’s surface has broad plains and valleys, several kilometre-high mountain ranges, even volcanos.
The range of chemicals and colours found on Pluto differentiates it from others in the Kuiper belt, and some regions of Pluto appear new whereas others are very old.