Distance Learning Module: What is a Planet?
Our distance learning theme this week is planets. In this module, we’ll look at the official definition of what makes a planet, and put the current criteria into historical context by reviewing some of the ways that astronomical understanding has evolved throughout history. This content is most appropriate for middle- and high-schoolers, based on reading level.
You may think of outer space as a vast, empty void. There are plenty of empty expanses out there, to be sure—but space also contains lots of objects! Both within and beyond the bounds of our solar system, there are countless stars, nebulae, asteroids, comets—and, of course, planets. But what IS a planet, anyway? Why was Pluto “demoted” from a true planet to a dwarf planet, and what makes a planet different from other objects in space?
This star cluster, Westerlund 2, resides in the milky way galaxy, inside a stellar breeding ground known as Gum 29. It is located 20,000 light-years away from earth, in the constellation Carina. This image, called “celestial fireworks,” was selected as the special photograph to celebrate the 25th anniversary of the Hubble space telescope in 2015. [image credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA), A. Nota (ESA/STScI), and the Westerlund 2 Science Team]
Let’s start with some terminology.
A celestial object (aka astronomical object) is a natural object located outside of the Earth’s atmosphere. This broad definition includes a lot of materials that break up the void of space—including star clusters, galaxies, and nebulae. Some of these materials do not have clearly defined borders: for example, a nebula is basically a big mass of gas and dust without regular boundaries.
A celestial body (aka astronomical body) is one kind of celestial object, which has a cohesive structure and is not part of a group. Examples of celestial bodies include the Sun, the Moon—and yes, planets.
The two terms above are often used interchangeably to refer to “a thing in space.” Again, the term celestial object is somewhat more general: a celestial object may consist of multiple bodies or objects with substructures, while a celestial body has more definite boundaries. Think of it this way: an entire galaxy is a celestial object, but the individual planets and stars in that galaxy are celestial bodies.
A planet is one kind of celestial body (which means it’s also a celestial object). You’re probably familiar with the planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. What you may not know is that the official definition of what makes a planet comes from the International Astronomical Union (IAU), which is made up of over 13,000 professional astronomers from all over the world. (More on this later.)
A moon (aka natural satellite) is a celestial body that orbits (moves in a circular pattern around the outside of) a planet or asteroid. There are more than 200 moons in our solar system. Earth only has one moon (which is why we call it “the” Moon), but some planets have many natural satellites. By recent counts, Jupiter has 79 known moons, and Saturn has 82!
An exoplanet is a planet outside our solar system, which orbits a star other than our Sun. There are over 4,000 confirmed exoplanets, and an additional 5,000 candidates, as of the writing of this article. Check out NASA’s online catalog to learn more about them:
How has understanding of the planets has evolved over time?
The definition of a planet has changed over hundreds—even thousands—of years, as humans’ observational capabilities and understanding of the natural universe have changed and developed. Ancient peoples all over the world looked to the night sky and developed their own explanations for the multitudes of bright, twinkling objects that they found there—some of which stayed in place, and some that appeared to revolve (circle) around the Earth.
Babylonia was an ancient kingdom in the region of Mesopotamia, which was located in parts of modern-day Iraq, Kuwait, Syria, and Turkey. As long as three thousand years ago, the Babylonians recognized that astronomical phenomena are periodic (they happen over and over again, in a pattern) and began to apply mathematics to their recordings! Babylonian astronomers made planetary observations, predictions, and omens in the Enuma Anu Enlil, a collection of cuneiform tablets that dates to about 1595–1157 BCE. Tablet 63, the Venus Tablet of Ammisaduqa, recorded the movements of Venus over a period of about 21 years.
picture of the Venus Tablet of Ammisaduq, at the british museum in london (image credit: wikimedia commons)
About a thousand years later, in the first century BCE, Greek mathematicians developed their own theories to explain the movement of the planets and other celestial bodies in the sky. The great Greek scientist Ptolemy wrote a series of books called the Almagest in the second century CE (so, between 100 and 200 years after the imaginary “zero point”) which spread around the world and was widely accepted by many cultures for a long time. Ptolemy did account for several of the planets that we recognize in our solar system today: Mercury, Venus, Mars, Jupiter, and Saturn—plus the Sun and the Moon. Unfortunately, his geocentric model of the solar system, which placed Earth at the center of the universe, with all of the other planets revolving around ours—was not correct!
page of copernicus manuscript showing a heliocentric model, with sol in the center of the solar system (image credit: wikimedia commons)
There were scientists and philosophers during, before, and after Ptolemy’s time who proposed a heliocentric model (with the Sun at the center of the solar system)—particularly in India and the Islamic world. However, geocentism was the widely accepted paradigm in Europe for hundreds of years after Ptolemy. In 1543 Nikolaus Copernicus published a new heliocentric model in On the Revolutions of the Heavenly Sphere. Copernicus’ model did not catch on immediately, but work by other scientists in the 1600s and 1700s (including Johannes Kepler, Galileo Galilei, and Isaac Newton) helped eventually lead to the widespread understanding that the planets do orbit around the Sun. Today, we look back and call this long period of changes in Western scientific thinking the Scientific Revolution (but for people who lived through this period, they just knew it as “everyday life”!)
Clyde Tombaugh, who discovered pluto, standing here with his homemade 9-inch telescope (Image credit: wikimedia commons)
I mentioned above that the “classical planets” of Mercury, Venus, Mars, Jupiter, and Saturn were known for thousands of years, because they could be seen in the night sky with the naked eye. The remaining planets in our solar system required advances in technology to be observed by humans. In 1781, William Herschel found Uranus with the aid of a telescope, and in 1846 Johann Galle located Neptune based on mathematical predictions previously made by Urbain Le Verrier. Pluto wasn’t found until 1930, when it was observed by Clyde Tombaugh at the Lowell Observatory in Arizona!
Hold on—I thought you said in the introduction that Pluto wasn’t a real planet?
Yes, and that’s where we get back to the International Astronomical Union!
It should be clear this point in the blog post that scientific knowledge is always changing, based on new discoveries, the spreading of knowledge, and the development more sensitive and more accurate technologies.
This image of pluto was taken as New Horizons zipped toward Pluto and its moons on July 14, 2015, from a range of 22,025 miles (35,445) kilometers. (image credit NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker)
By the 1990s, increasingly advanced telescopes began detecting numerous icy bodies in the Kuiper Belt, beyond the orbit of Neptune—where Pluto resides. Evidence was mounting that Pluto looks and acts more like one of these Kuiper Belt Objects than the other planets. Pluto is much smaller than Mercury, and is even smaller than some of the moons of other planets in our solar system. Its largest moon, Charon, is close to Pluto in size and distance, and the two share an orbit around a point in space between them—more like the way binary stars act, than planets in relation to their moons.
In 2005, a team of astronomers announced that they had found a tenth planet—a KBO similar in size to Pluto. This prompted lots of debates in the astronomy community about what it means to be a planet. A year later, the International Astronomical Union (IAU) attempted to settle the debate by publishing a new definition of a planet. According to the IAU, a celestial body is only considered a planet if:
it is in orbit around the Sun
it has sufficient mass that the forces of self-gravity and outward pressure balance the object in a state of hydrostatic equilibrium (giving it a spherical or nearly spherical shape), and
it has “cleared the neighborhood: around its orbit—meaning it has become gravitationally dominant, and there are no other bodies of comparable size in its orbital path
A celestial body must meet all of the criteria to meet this definition of planet-hood. It’s the third point that got Pluto re-classified as not a true planet. Because Pluto shares its orbit with Charon and with many Kuiper Belt Objects, it has cannot be said to have cleared its neighborhood. The IAU created a new category, dwarf planets, to include celestial bodies, like Pluto, that are not satellites of other planets, are in orbit around the Sun, and have reached hydrostatic equilibrium—but are not gravitationally dominant (that is, they share their orbit with other celestial bodies).
image credit: cafepress.com
The IAU’s definitions have not been without controversy. Some astronomers believe the new criteria are too vague; some think there needs to be more room to consider an object’s location in the solar system; and some think the new restrictions are too stringent and exclude lots of celestial objects that should be considered planets. Some people even felt defensive, and are sad for Pluto that it’s no longer considered a “real” planet!
In conclusion…
You may have your own strong feelings about Pluto and the IAU’s 2006 definition of what makes a planet. Maybe you think it’s a great definition, or you have some ideas about how it could be improved. However you feel, hopefully you’ve learned today that science is a story that is always unfolding. The quest for knowledge is as old as time, and there will always be new discoveries and technologies helping humankind to improve our understanding of the universe around us. It’s up to you, the scientists of the future, to make your own discoveries and add to this growing body of knowledge!
model of the solar system, showing the eight true planets orbiting sol, our sun (image credit: universetoday.com)
