As with many things in astronomy, the term planet has been ambiguous for many years. Depending on when you were born, you may feel differently about how many planets there are. This begs the question – just how many planets are there in the Solar System?
What Is a Planet?
The ancients were the first to notice that particular objects in the night sky behaved differently than others. Since they moved differently than stars, they called these objects planets, or “wanderers” in Greek.
In just 2006, the International Astronomical Union (IAU) adopted a more concrete definition for a planet. This definition says that a planet must do three things:
- It must orbit a star.
- It must have enough gravity to be spherical in shape.
- It must be large enough to clear its neighborhood of similar objects near its orbit.
While these are the standing requirements for planethood, these criteria are not set in stone.
History of Objects Called Planets in Our Solar System
As mentioned earlier, the ancient Greeks had discovered five wanderers in the night sky, and Renaissance astronomers were able to expound on these findings after the invention of telescopes in the early 1600s.
In March of 1781, William Herschel discovered the planet Uranus when surveying for distant comets. This marked the first planetary finding in nearly 2000 years. Uranus was quickly labeled the seventh planet in the Solar System.
In 1801, a Sicilian monk located an unknown body in the heavens between Mars and Jupiter. He named the object Ceres and at first believed the thing was a comet. Upon closer inspection, Ceres was labeled a planet.
Unfortunately for Ceres, such a declaration wouldn’t last. As more objects were found around Ceres, William Herschel took to calling these rocks asteroids. Ceres was a planet no more.
Credit for the discovery of Neptune goes to mathematics and the astronomers who determined its location. After observing oddities in the orbit of Uranus, these astronomers correctly calculated the location of an eighth planet. Neptune was observed for the first time via telescope in 1841.
In a similar way, scientists realized that some other object was affecting the orbit of both Uranus and Neptune. In 1929, Clyde W. Tombaugh began combing the skies one small section at a time for yet another unknown object.
Finally, on February 18th, 1930, Tombaugh located the tiny object through new astronomic techniques. There was no way to tell the size of the object at the time, but Tombaugh decided to give it the name Pluto. Pluto would stay a planet for over 75 years before being declassified.
2005 brought about a new discovery, this time an object presumably larger than Pluto out in the far reaches of the Solar System. Named Eris, the object sits almost three times further from the Sun than Pluto. Initially hailed as a planet, things changed when the term planet was redefined in 2006.
Planetary Naming Conventions
The first planets to be discovered were named after gods of the time, up through Saturn and including Earth. Although Herschel originally wanted to name his discovery “Georgium Sidus” after King George III, astronomers eventually settled on Uranus.
Each planetary discovery since, including Ceres, has been named after a god, albeit not always from Greek culture.
Planetary Categories in Our Solar System
At present, we have eight objects in the Solar System that meet the three criteria set by the IAU. Each of our eight planets falls into one of two categories:
The four planets closest to the Sun – Mercury, Venus, Earth, and Mars- are known as terrestrial planets because each has a rocky, hard surface. These planets vary wildly in composition and characteristics, including surface temperature and atmosphere.
Each terrestrial planet houses a molten metallic core responsible for volcanic activity. On the surface, it’s possible to see distinct features such as mountains, volcanoes, valleys, and craters. None of these worlds have rings, but Earth and Mars do have satellites.
Jupiter, Saturn, Uranus, and Neptune represent the gas giants in our Solar System. Unsurprisingly, they are called gas giants because they are comprised almost entirely of hydrogen and helium. These planets do not have a well-defined surface, and it’s unclear where one may even begin.
If these planets do have a solid surface, it is likely rocky or metallic as each has a strong magnetic field. While Jupiter and Saturn likely have layers of liquid hydrogen before their cores, Uranus and Neptune contain a mixture of water, rock, methane, and ammonia. As a result, these two planets are often called “ice giants.”
These four planets are sometimes called Jovian planets because each one has similar characteristics to Jupiter. Each planet has rings and several known moons.
Why is Pluto Not a Planet?
Those of us born before the turn of the 21st century likely remember a time when Pluto was our ninth planet. After 76 years as a planet, Pluto’s fate changed when the IAU redefined what it meant to be a planet.
Pluto does orbit the Sun and has enough gravity to be spherical in shape, but the ex-planet’s declassification came because it has not cleared its orbit. To meet this criteria, the planet must be the dominant gravitational force, which Pluto does not.
Charon, Pluto’s satellite, is nearly half its size, and the two have a binary relationship. The two objects orbit around each other, rather than Pluto holding all the power in the relationship. This alone is grounds for dismissal from planet status.
Not all hope was lost when the hammer fell on Pluto in 2006. A new category was created for objects resembling planets but don’t have a clear orbit – dwarf planets.
This new designation was a blessing for a few other bodies in the Solar System as well. Once Pluto was reclassified as a dwarf planet, scientists looked to every other object that orbits the Sun to see what else made the cut.
Another name that once tasted planet status resurfaced – Ceres. With a round shape thanks to its size as the largest asteroid, it was upgraded to dwarf planet status as well.
At the same time, our very distant tenth planet Eris had to be reclassified because it does not clear its orbit either. To date, it’s the furthest known dwarf planet from the Sun.
Although discovered a few years earlier, two other rocky worlds were added to the list of dwarf planets in 2008. Both Haumea and Makemake have roundish shapes and orbit the Sun from well beyond the orbit of Pluto. These were never classified as planets and left somewhat in limbo until given this title.
What Else Is Out There?
There are likely several other large objects in the Kuiper Belt and beyond that could at least be given dwarf planet status. Scientists currently estimate upwards of 200 more out there, just waiting to be discovered.
The biggest planetary question revolves around the mysterious planet X. Mathematical evidence supports the possibility of a giant planet deep in the Solar System, explaining the unique orbit of many smaller bodies in the Kuiper Belt.
Planet X could have mass up to ten times that of our Earth. If it does exist, it likely sits some 20 times further from the Sun than Neptune and could take over 10,000 years to complete one orbit around the Sun.
Are There Other Solar Systems?
We’ve still very limited in our ability to see all that lies outside of our own galaxy, but astronomers have already discovered over 4,000 worlds within the Milky Way galaxy alone. These planets all orbit stars of their own, although many look very different than our own.
Just 35 light-years away, one particular solar system has some interesting characteristics that mirror our own. This particular system has at least four rocky planets closest to their star and may be within the range that could sustain life. The third planet in this system even appears to have oceans!
Given all the strict criteria humans need to survive, there are likely few planets out there that we could live on. Even if we needed to travel somewhere new in a hurry, it would take us nearly 700,000 years to reach this other system!
How Many Planets Are There in the Universe?
Although we currently have no way of knowing for sure, we can extrapolate an answer from what we do know. With an estimated 100 billion planets in the Milky Way and 200 billion galaxies in the universe, there could easily be 20 sextillion planets out there somewhere.
The number has changed over the years, but at present, we have eight objects that meet the three criteria that allow them to be called planets. Each is unique, but only Earth has the resources we need to survive.
We still know very little of what exists in the far reaches of our Solar System, let alone what else is hiding beyond our borders. At the very least, the five planets closest to us make for some amazing photographs with even just a bit of magnification.