What is, and what is not, a planet?
Pluto just didn't fit the criteria for a planet, so it was placed in a new category with others of its kind, dwarf planets. So what is a planet, and what is Pluto?
What is a Planet?
In 2006, the International Astronomical Union decided that there were too many questions surrounding what could be called a planet, and so refined the definition of a planet.
According to the new definition, a planet must:
- Orbit a star.
- Be big enough that its own gravity causes it to be shaped as a sphere.
- Be small enough that it isn’t a star itself.
- Have cleared the area of its orbit of smaller objects.
The dwarf planets of our solar system are exciting proof of how much we are learning about our solar system. With the discovery of many new objects in our solar system, astronomers refined the definition of a dwarf planet in 2006.
According to the IAU, a dwarf planet must:
- Orbit a star.
- Have enough mass to be nearly spherical.
- Not have cleared the area around its orbit of smaller objects.
- Not be a moon.
The reclassification of Pluto to the new category dwarf planet stirred up a great deal of controversy. How the classification of Pluto has evolved is an interesting story in science.
From the time it was discovered in 1930 until the early 2000s, Pluto was considered the ninth planet. When astronomers first located Pluto, the telescopes were not as good, so Pluto and its moon, Charon, were seen as one much larger object (Figure below). With better telescopes, astronomers realized that Pluto was much smaller than they had thought.
Pluto and its moon, Charon, are actually two objects.
Better technology also allowed astronomers to discover many smaller objects like Pluto that orbit the Sun. One of them, Eris, discovered in 2005, is even larger than Pluto.
Even when it was considered a planet, Pluto was an oddball. Unlike the other outer planets in the solar system, which are all gas giants, it is small, icy, and rocky. With a diameter of about 2,400 km, it is only about one-fifth the mass of Earth’s Moon. Pluto’s orbit is tilted relative to the other planets and is shaped like a long, narrow ellipse. Pluto’s orbit sometimes even passes inside Neptune’s orbit.
In 1992, Pluto’s orbit was recognized to be part of the Kuiper belt. With more than 200 million Kuiper belt objects, Pluto has failed the test of clearing other bodies out its orbit.
From what you’ve read above, do you think Pluto should be called a planet? Why are people hesitant to take away Pluto’s planetary status? Is Pluto a dwarf planet?
Pluto has three moons of its own. The largest, Charon, is big enough that the Pluto-Charon system is sometimes considered to be a double dwarf planet (Figure above). Two smaller moons, Nix and Hydra, were discovered in 2005. But having moons is not enough to make an object a planet.
Pluto and the other dwarf planets, besides Ceres, are found orbiting out beyond Neptune.
Ceres is by far the closest dwarf planet to the Sun; it resides between Mars and Jupiter. Ceres is the largest object in the asteroid belt (Figure below). Before 2006, Ceres was considered the largest of the asteroids, with only about 1.3% of the mass of the Earth’s Moon. But unlike the asteroids, Ceres has enough mass that its gravity causes it to be shaped like a sphere. Like Pluto, Ceres is rocky.
Is Ceres a planet? How does it match the criteria above? Ceres orbits the Sun, is round, and is not a moon. As part of the asteroid belt, its orbit is full of other smaller bodies, so Ceres fails the fourth criterion for being a planet.
This composite image compares the size of the dwarf planet Ceres to Earth and the Moon.
Makemake is the third largest and second brightest dwarf planet we have discovered so far (Figure below). With a diameter estimated to be between 1,300 and 1,900 km, it is about three-quarters the size of Pluto. Makemake orbits the Sun in 310 years at a distance between 38.5 to 53 AU. It is thought to be made of methane, ethane, and nitrogen ices.
Largest Known Trans-Neptunian Objects. Makemake is named after the deity that created humanity in the mythology of the people of Easter Island.
Eris is the largest known dwarf planet in the solar system — it has about 27% more mass than Pluto (Figure above). The object was not discovered until 2003 because it is about three times farther from the Sun than Pluto, and almost 100 times farther from the Sun than Earth is. For a short time Eris was considered the “tenth planet” in the solar system, but its discovery helped to prompt astronomers to better define planets and dwarf planets in 2006. Eris also has a small moon, Dysnomia, that orbits it once about every 16 days.
Astronomers know there may be other dwarf planets in the outer reaches of the solar system. Haumea was made a dwarf planet in 2008, so the total number of dwarf planets is now five. Quaoar, Varuna, and Orcus may be added to the list of dwarf planets in the future. We still have a lot to discover and explore.
- There are currently five dwarf planets in our solar system: Pluto, Eris, Haumea, Makemake, and Ceres.
- Most dwarf planets are similar to planets, except that they haven't cleared their space of debris.
- Pluto was thought to be larger than it is because its large moon made the dwarf planet look bigger than it is.
- Why isn't Pluto still a planet?
- Why did people think that Pluto was a planet in the decades after its discovery?
- If Pluto is a dwarf planet, are any other of the eight planets in our solar system at risk of losing their planetary status?
- Why do some people still insist that Pluto is a planet?
Use these resources to answer the questions that follow.
- What is found in the outer solar system?
- What characteristics must a planet have?
- Which characteristic did Pluto fail? Why?
- What are the characteristics of Eris and what is it?
- What is Pluto made of? What is its surface?
- Why is Pluto so cold?
- What is Charon?