What is a Planet?

While it’s quite easy to name a few of the planets in our Solar System, it’s a much harder task to define what a planet actually is. In fact, until quite recently when the planetary status of Pluto was called into question, the definition of what makes a celestial body a planet was quite arbitrary.

In today’s post I will cover what defines a planet (using the current definition) and the discoveries that led to development of this definition over time. I will also give a brief overview of the vast diversity of planets and other celestial bodies in our Solar System.

Note: For those that don’t know, I am currently taking a Planetary Formations class and am using this post as fulfillment of a public outreach assignment. If you find this article helpful, or if you still have questions left unanswered, please leave your comments below! Feedback from YOU is a critical part of this assignment. I hope you find it insightful!

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Ancient Wanderers

Let’s start at the very beginning shall we? (At least as far back as human history can go…)

Astronomy is perhaps the oldest of the natural sciences, dating back to ancient times. Back then, early cultures identified celestial bodies and their movements with the gods and connected them with key events such as the coming of rain or drought. But even back then, peoples could distinguish between the static background of stars and planetary bodies which seemed to wander across the night sky (see picture below). In fact, the word “planets” means “wanderers” in Greek.

image, Saturn and Mars wandering against the stellar background.

As ancient peoples tracked the motion of these bodies, they were each given names of Greek (or Roman) gods. Jupiter, Saturn, Mars, Venus, and Mercury were given their names thousands of years ago in this manner. While Uranus, Neptune, and Pluto were named after their discoveries in the following centuries.

I’m always fascinated to find ways in which we are connected with our ancestors across time. And many times we don’t have to look far to find such connections. Take the days of the week for instance…

• Sunday: Sun’s Day (Associated with Sol, the Roman god of the Sun.)

• Monday: Moon’s Day (Associated with Luna, the Roman goddess of the Moon.)

• Tuesday: Tiu’s Day (Tiu is the English/Germanic god most closely associated with Mars, the Roman god of war.)

• Wednesday: Woden’s Day (Woden is the Anglo-Saxton god most closely associated with Mercury, the Roman god of commerce, travel, thievery, and science.)

• Thursday: Thor’s Day (Thor is the Norse god most closely associated with Jupiter, the Roman god of thunder and lightening.)

• Friday: Freya’s Day (Freya is the Norse god most closely associated with Venus, the Roman goddess of love and beauty.)

• Saturday: Saturn’s Day (Assoiciated with Saturn, the Roman god of agriculture.)

• Also related…how Earth got its name.

image, The seven Roman gods associated with the seven planets of the ancient night sky.

For centuries ancient peoples looked to the heavens and tried to make sense of our place is this universe. And it was believed for a very long time that the universe revolved around us, with Earth at the center (known as Geocentrism). But over time and with continued advancements in the field of astronomy, the Copernican Revolution happened and brought with it our currently held model of the Solar System, whereby the planets orbit around the Sun.

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Notable Modern Discoveries

With the advent of the telescope in the 1600s brought great strides and advances in the field of astronomy, as well as a fundamental shift in human’s perception of our place in the universe. Below are a few notable discoveries that helped to bring us into the modern age.

image, Galilean Moons of Jupiter–named after their discoverer Galileo Galilei. (My personal favorite objects to look at!)

• Early 1600s: Jupiter’s Moons | Galileo Galilei observed Moons circling Jupiter. This became some of the most damaging evidence against the Geocentric model of the universe at the time (everything orbits the Earth), and led to the more current understanding of our Solar System, whereby planets orbit the Sun.

• 1781: Uranus | The planet Uranus was discovered by William Herschel. This was the first planet discovered since ancient times. How exciting! This discovery reinvigorated astronomers to see what other celestial bodies and potential planets they could find. Uranus was in fact discovered by accident and had been mistaken for a star in the past. More information here.

• 1801: Ceres | The largest known object in the asteroid belt, Ceres, was discovered by Giuseppe Piazzi. At the time, it was dubbed a planet and remained that way for nearly half a century! However, as other objects were found in its vicinity (in the gap between Mars and Jupiter), Ceres was recast as a new type of object known as an “asteroid” (“star-like”). And while the differences between objects such as Ceres and the major planets was widely accepted, no formal definition of a planet existed at this time. More information here.

• 1846: Neptune | The planet Neptune was discovered by John Couch Adams and Urbain Le Verrier. The existence of Neptune had actually been predicted from unresolved irregularities in Uranus’s orbital motion. So its discovery was made possible by both mathematical predictions and astronomical observations. Neat, right?! You can read all about it here.

• 1930: Pluto | Pluto was discovered by Clyde Tombaugh. After discovery of Neptune and continued monitoring of Uranus’s orbit, it was postulated that another planet might exist farther out than Neptune. After continued observation, Pluto was discovered and dubbed the 9th planet of the Solar System. More information here. Nothing close to Pluto’s size was found for two generations. (Read on for why Pluto was demoted to being a dwarf planet.)

• Early 2000’s: Trans-Neptunian Objects | Several trans-Neptunian objects, or icy bodies beyond the orbit of Neptune, were discovered starting in the early 2000’s. Mike Brown was a notable leader in these discoveries, his most significant findings include Quaoar, Sedna, Haumea, Eris and its moon, Dysnomia, and Makemake. While finding objects in the outer Solar System was not surprising, the size of the objects found was! Eris, for instance, was even more massive than Pluto! This led to debates on whether Pluto should remain a planet or be demoted to a Kuiper belt object. (The Kuiper belt is similar to the asteroid belt, but lies in an orbit beyond Neptune’s location.)

image, Dwarf planets compared to size of Earth’s Moon.

With the discovery of so many large celestial objects in the Solar System, and notably ones more massive than Pluto, it was evident that there needed to be a more concrete definition of what a planet was.

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What is a planet?

After two years of examining evidence and putting it to a vote by astronomers world-wide, a definition of planets was decided upon.

The International Astronomical Union (IAU) defines a planet as an object which…

1. Is in orbit around the Sun.
2. Has sufficient mass to assume hydrostatic equilibrium (a nearly round shape). Hydrostatic equilibrium refers to the balance between pressure forces outward and gravitational forces inward. If a body is massive enough, these forces will balance to create a rounded shape. (see asteroid variety of shapes in second picture below)
3. Has cleared the neighborhood around its orbit. As a planet forms, it becomes the dominant gravitational body in its orbit–either consuming smaller objects it finds or slinging them away with its gravity.

For Pluto, #3 of the definition is the problem. There are still several objects similar in size and mass to Pluto moving around in its orbit. While Earth has 1.7 million times the mass of other objects in its orbit, Pluto is only 0.07 times the mass of objects in its orbit. And so Pluto is not considered a planet under this current definition.

This means that there are now only 8 official planets in the Solar System–Mercury (2,440 km in radius,0.4 AU from Sun), Venus (6,050 km in radius, 0.7 AU from Sun), Earth (6,370 km in radius, 1.0 AU from Sun), Mars (3,390 km in radius, 1.5 AU from Sun), Jupiter (69,900 km in radius, 5.2 AU from Sun), Saturn (58,200 km in radius, 9.6 AU from Sun), Uranus (25,300 km in radius, 19.2 AU from Sun), and Neptune (24,600 km in radius, 30 AU from Sun)!

image, Current model of the Solar System, with the Asteroid and Kuiper belts illustrated as well. *not to scale

As a consequence of the great Pluto debacle however, the IAU also created a new definition for a dwarf planet.

A dwarf planet is defined to be a non-satellite object (not a moon of a planet) which satisfies #1 and #2 above, but has not cleared its neighborhood. Thus Pluto is considered to be a dwarf planet! Currently there are only five official dwarf planets recognized–Ceres (473 km radius, 2.8 AU from Sun, discovered 1801), Pluto (1,190 km radius, 39.5 AU from Sun, discovered 1830), Eris (1,160 km radius, 67.8 AU from Sun, discovered 2005), Makemake (715 km radius, 45.7 AU from Sun, discovered 2005), and Haumea (620 km radius, 43.2 AU from Sun, discovered 2004). See second image above for sizes of dwarf planets compared with Earth’s Moon.

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Smaller Solar System Bodies

As I was researching this post, I wondered if there were similar distinctions among smaller solar system bodies–namely, what’s the different between comets, asteroids, and meteors? After a little digging, I found the answer…Yes! There are definitely distinctions between these bodies; however, just as the definition of a planet was not concretely formulated until recently, the definitions I found seem to be somewhat vague. So bare with the somewhat arbitrary descriptions of each–it’s the best we’ve got right now!

image, Great variety in size and shape of asteroids in the Solar System.

What is the difference between comets, asteroids, and meteors? [Ref]

• Asteroid: a rocky fragment of solar system formation that is generally located in the Asteroid Belt (between Mars and Jupiter).

• Comet: a fragment of solar system formation that is generally both rocky and icy; they generally contain more ice, methane, ammonia, and other materials that create a cloud-like shell around the object (known as its coma), as well as its tail (when it gets close to the Sun). Long-period comets (>200 year period) originate from the Oort Cloud, while short-period comets (<200 year period) originate in the Kuiper Belt.

• Meteoroid: a rocky fragment of solar system formation that is larger than a dust grain but smaller than an asteroid (the line is fuzzy—generally objects bigger than boulders are asteroids, so think of a meteoroid as bread box-sized).

• Meteor (“shooting star”): when a meteoroid enters Earth’s atmosphere, we see a flash of light as the material burns up in the atmosphere. This flash of light is known as the meteor, not the debris itself.

• Meteorite: any part of the meteoroid that survives its decent through the atmosphere and makes it to Earth’s surface.

After looking up some of the major asteroids in our Solar System and comparing their sizes to our Moon, I next started to wonder about how moons are classified. Our Moon in fact is larger than each of the dwarf planets… So what does that mean for our Moon and how it’s classified?

What is a moon? [Ref]

It turns out that moons are not classified by their size, mass, or composition. They are in fact characterized by their motion. Moons are defined as bodies that orbit a planet but do not interfere with its orbit about the Sun. More information here. While Earth has only 1 Moon, other planets in the Solar System have several. Jupiter and Saturn both, for instance, each have over 50 moons! Woah! You can read all about the moons of our Solar System here.

image, Select moons of the Solar System, compared with Earth’s size.

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And that wraps up today’s post covering What is a planet?. I hope that it was informative, helpful, and easy to read for everyone! I also hope that after reading this post, you have a better appreciation for just how quickly our understanding of the universe can change–and that it is still (to this day!) evolving. Never be afraid to question what we think we know. That’s what science is ALL about. Only through questioning what we know and seeking truth through experimentation and observation can we find it!

What about you? What other questions do you have about planets?

You might also like…How To Use A Star Chart.