According to the most widely accepted definition of a planet, there are eight planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto, Eris, Haumea, Makemake, and Ceres are dwarf planets. But, there are a host of other bodies in the solar system.
Here is a look at what a planet is, why Pluto doesn’t qualify, and how many planets may exist in the galaxy and universe.
- There are eight planets in the solar system, excluding dwarf planets like Pluto and large moons.
- Astronomers predict the presence of a ninth planet.
- The Milky Way galaxy contains an estimated 100 billion planets.
- The universe contains an uncountable number of planets.
What Is a Planet?
Astronomers are not in agreement on the definition of a planet. But, the International Astronomical Union (IAU) defines a planet according to the following criteria:
- A planet orbits its star. In other words, it is not a moon.
- It has enough mass to be round and have hydrodynamic stability.
- It has cleared its orbit of most debris.
By this definition, Pluto isn’t a planet because it hasn’t cleared most of its orbit. Titan isn’t a planet because, even though it is massive and round, it orbits as a moon of Saturn.
Other scientists define a planet according to whether or not is has moons (which would exclude Mercury and Venus), by its position in the solar system (usually excluding Ceres and all other asteroids, only by its mass (which greatly expands the list and includes dwarf planets and large moons), or by other criteria.
How Many Planets Are in the Solar System?
If you exclude the dwarf planets, smaller asteroids, and icy trans-Neptunian objects (TNOs) in the solar system, you have eight bodies that are universally accepted as planets, plus a ninth undiscovered planet.
In order moving away from the Sun, the planets are:
- Planet Nine (Planet 9)
Mercury, Venus, Earth, and Mars are the terrestrial planets. They are rocky bodies. Jupiter and Saturn are the gas giants. Uranus and Neptune are the ice giants.
Are There Undiscovered Planets? Planet Nine
Planet Nine or Planet 9 is the temporary name of an undiscovered planet whose existence is predicted based on the gravitational effects on bodies at the extreme edge of the solar system. According to these effects, some astronomers estimate the undiscovered planet is about the same size as Neptune. If it exists, Planet Nine could be a captured rogue planet, the ejected core of a gas giant planet, or something completely new. Other scientists assert that the gravitational effects are not real, but are the result of observational bias. Whether or not Planet 9 exists is an unanswered question.
Since we know so little about the edge of the solar system, it’s possible there are other undiscovered planets out there.
How Many Exoplanets Are There?
Planets orbiting stars besides the Sun are extrasolar planets or exoplanets. Since the first confirmed discovery of an exoplanet in 1988, astronomers have discovered over 5000 more. So far, about 20 percent of stars like our Sun have Earth-sized planets within the “Goldilocks” or habitable zone. Based on what scientists have discovered so far, there are an estimated 100 billion planets in the Milky Way galaxy, with over 10 billion potentially habitable Earth-like planets. Other galaxies also have stars that can have planets, so there are countless planets in the universe (something like one septillion or 1,000,000,000,000,000,000,000,000).
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