Since the demotion of Pluto, many people question “How many planets are there in our solar system?”. In 2005, Mike Brown discovered Eris, an icy object beyond Pluto and relative in size. Due to skepticism, before Eris could be considered the tenth planet, the term ‘planet’ must be defined.
In 2006, the International Astronomical Union debated the question, “How many planets are there in our solar system?”
On one side, the total number of planets would be increased to twelve to include Pluto, its moon Charon, the Asteroid Ceres and the newly discovered Eris. On the other, Pluto would be demoted and the planets in order would end at Neptune.
Ultimately, the IAU decided a planet must meet the following criteria:
- Orbits the Sun
- Has sufficient mass to be round
- Is not a satellite of another object
- Has removed debris from its orbit
As a result, Pluto and the remaining smaller bodies were defined as Dwarf planets.
Thus, the answer to ‘How many planets are there in our solar system?” is: 8 planets and 5 official dwarf planets, Ceres, Pluto, Haumea, Makemake, and Eris.
Konstantin Batygin and Michael Brown at the California Institute of Technology proposed that a new Planet Nine is hanging out beyond the orbit of Pluto. Further discoveries in dwarf planets have also been found. A contender called 2014 UZ224 has been spotted 8.5 billion miles from the Sun. While the discoveries are significant, these potential planets haven’t been written into textbooks just yet.
Here’s a bit about all 8 planets in order:
Mercury is the innermost terrestrial planet in our solar system. At a distance of 29 million to 43 million miles away from the Sun, it takes a mere 88 Earth days to complete an orbit. Lacking an atmosphere, temperatures on the surface reach 800 degrees Fahrenheit during the day and can drop to -290 degrees at night.
Although it is one of the brightest visible objects in the night sky, it is often regarded as one of the most difficult to see.
Also a terrestrial planet, Venus’ year spans 243 Earth days at an average distance of 67 million miles from the Sun. Venus is only slightly smaller than Earth, however, surface temperatures can reach 880 degrees Fahrenheit due to its thick atmosphere.
Like Mercury, the best time to view the planet is just before the Sun rises, or just after the Sun sets.
Orbiting at one Astronomical Unit (AU) (93 million miles) away from the Sun, Earth is the fifth largest planet that harbors 7.5 billion people. Unlike Mercury and Venus, which lack natural satellites, Earth’s Moon is moving away 1.6 inches per year while the planet’s rotation slows.
Mars orbits around 1.5 AU from the Sun and is the last terrestrial of the planets in order. Mars, also known as the red planet, appears red due to oxidized iron in the soil.
Similar to Earth and Venus, Mars is home to volcanoes. In fact, Mars has the tallest volcano in the solar system, Olympus Mons, which stands at 15 miles high – three times the height of Mount Everest.
Apart from the Sun, the mass of Jupiter accounts for two-thirds of the mass in the entire solar system. Jupiter is the third brightest object in the night sky behind the Moon and Venus. The gas giant was recently visited by NASA spacecraft Juno which was designed to analyze Jupiter’s atmosphere, magnetosphere, and magnetic and gravitational fields.
At 9.5 AU, Saturn is the second gas giant in the solar system and takes just 10.7 hours to complete one full rotation. This speed combined with its low density makes Saturn the flattest planet in the solar system.
Of its 150 moons, Titan has been considered as a potential vessel for life due to its composition consisting primarily of water ice and rock.
Composed of water, methane and ammonia, Uranus is considered an “ice giant”. The planets 98-degree tilt on its axis makes orients the planet on its side compared to Earth’s tilt of 23 degrees. Researchers theorize that this tilt is the result of successive impacts. Although there is no true surface on Uranus, the core can reach temperatures of up to 9,000 degrees Fahrenheit.
Neptune is the last of the planets in order. Despite being the fourth largest planet in terms of diameter, Neptune has the second greatest gravitational pull behind Jupiter, due to its thick composition (similar to Uranus). However, the surface gravity of Neptune is only 1.14 times the gravity on Earth.
With new discoveries happening all the time, the definite answer to “How many planets are there in our solar system?” remains uncertain. However, firm classifications for planets keeps this list at eight until sufficient evidence forces us to rewrite the books.