The definition of “planet” has changed through time. In ancient times, a planet was any object that moved through the sky, that didn’t follow the motion of all the other stars.
A view of the Milky Way galaxy as seen from Earth.
To be clear, the stars are not really rotating. Over the course of one night the stars don’t move much at all. The stars are standing still, and the earth is rotating.
Detecting the motion of planets against the stars
One phenomenon that ancient astronomers had difficulty explaining was the retrograde motion of the planets.
Over the course of a single night, a planet will move from East to West across the sky, like any other celestial object near the ecliptic.
If observed from one night to the next a planet appears to move from West to East against the background stars most of the time.
Occasionally, however, the planet’s motion will appear to reverse direction, and the planet will, for a short time, move from East to West against the background constellations.
This reversal is known as retrograde motion.
Retrograde Motion in the Ptolemaic (Geocentric) System
Ptolemaic Explanation: The model of the solar system developed by Ptolemy (87 – 150 A.D.) was a refinement of Aristotle’s (384 – 322 B.C.) universe.
This model consisted of a series of concentric spheres, with the Earth at the center (geocentric). The motions of the Sun, Moon, and stars were based on perfect circles.
To account for the observed retrograde motion of the planets, it was necessary to resort to a system of epicycles, whereby the planets moved around small circular paths that in turn moved around larger circular orbits around the Earth.
This accounts for retrograde motion, as shown here:
In its final form, the model was extremely complicated, requiring many nested levels of epicycles, and with even the major orbits offset so that they were no longer truly centered on the Earth.
Despite all of this fine tuning, there remained significant discrepancies between the actual positions of the planets and those predicted by the model. Nevertheless, it was the most accurate model available, and it remained the accepted theory for over 13 centuries, before it was finally replaced by the model of Copernicus.
Copernicus replaced the geocentric universe of Ptolemy with one that was centered on the Sun (heliocentric), with only the Moon orbiting the Earth. His model was still based on circular orbits (and therefore still required further refinement), but it was able to achieve superior precision than the Ptolemaic model without the need for epicycles or other complications.
The explanation for retrograde motion in this system arises from the fact that the planets further from the sun are moving more slowly in their orbits than those closer to the sun. The retrograde motion of Mars occurs when the Earth passes by the slower moving Mars, as shown in the following animation.
What is a planet?
The definition of “planet” has changed through time.
In ancient times, a planet was any object that moved through the sky, that didn’t follow the motion of all the other stars. (The sun and moon were usually excluded from the list.)
Mercury, Venus, Mars, Jupiter, and Saturn.
Nothing was known about them – not their size, shape, composition, or how they were formed.
1781 CE – Uranus discovered, and is referred to as a planet.
1801-1845. Numerous small objects are discovered, and also referred to as planets, such as Ceres, Pallas, Juno and Vestra (today these are called asteroids)
1846 – Neptune discovered, and is referred to as a planet.
1846-1852. Many more small objects are discovered, and also referred to as planets – until in 1852 several are reclassified as asteroids.
Problem: Even up to this point in history, science had never created a specific definition of what a planet actually is1
1930 – Pluto is discovered. Initial measurements suggested that it may almost be as large as Earth (we later discovered that it was much smaller) Initial observations didn’t reveal any other planet-like objects in it’s region, so it too was named a planet.
1992 – A new era of astronomy begins with the first confirmed discovery of an exoplanet – planet’s around other stars in our galaxy.
Around the same time, astronomers began discovering new planet-like objects in our own solar system, near and beyond the orbit of Pluto, including Eris, Makemake, Sedna and and Huamea.
Which of these worlds should be called a “planet”? “dwarf planet”? “asteroid”? There was no way to decide, as there never had been any specific definition. So in 2006, the International Astronomical Union (IAU) studied the issue, and developed criteria:
“(1) A “planet” is a celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
(2) A “dwarf planet” is a celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.
(3) All other objects , except satellites, orbiting the sun shall be referred to collectively as “small solar-system bodies”
By this definition, Pluto, Eris, Makemake, Sedna and and Huamea are dwarf planets. This decision is an astronomical decision. But geologists think that there are other, equally valid ways to classify astronomical bodies. In 2017 a group of planetary geologists developed “A Geophysical Planet Definition”
“A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.”
This definition would bring Pluto back into the family of planets -but it would also include almost 100 more satellites and large asteroids in our solar system, into the same category. What is the rationale for this classification scheme? Kirby Runyon writes:
“The IAU definition is useful to planetary astronomers concerned with the orbital properties of bodies in the solar system, and may capture the essence of what a ‘planet’ is to them. The definition is not useful to planetary geologists. I study landscapes and how landscapes evolve. It also kind of irked me that the IAU took upon itself to define something that geologists use too.
“The way our brain has evolved, we make sense of the universe by classifying things. Nature exists in a continuum, not in discrete boxes. Nevertheless, we as humans need to classify things in order to bring order out of chaos. Having a definition of the word planet that expresses what we think a planet ought to be, is concordant with this desire to bring order out of chaos and understand the universe.”
But now see A geophysical planet definition
The International Astronomical Union (IAU) defines a dwarf planet as a celestial body that:
* is in direct orbit of the Sun
* is massive enough for its gravity to turn it into a nearly round shape
* has not cleared the neighborhood around its orbit.
It is estimated that there are hundreds to thousands of dwarf planets in the Solar System, most of which lie beyond the orbit of Pluto.
Poster showing a comparison of images from planetary surfaces ordered by increasing complexity of the surface processes. Image shows the surfaces of Asteroid Itokawa, the Moon, Venus, Mars, Titan, and Earth.
All images show a view of nearby rocks to the distant horizon. The amount of surface modification evident of each of the bodies increases roughly from left to right. From the the rubble pile asteroid of Itokawa, the cratered plains of the moon, the volcanic basalts of Venus, the basalt filled craters of Mars, the eroded icy cobbles of Titan to the great oceans of Earth, a variety of surfaces in our solar system is represented.
Image Credits: Asteroid Itokawa [Hayabusa]: ISAS / JAXA / Gordan Ugarkovic Moon [Apollo 17]: NASA Venus [Venera 14]: IKI / Don Mitchell / Ted Stryk / Mike Malaska Mars [Mars Exploration Rover Spirit]: NASA / JPL / Cornell / Mike Malaska Titan [Cassini Huygens]: ESA / NASA / JPL / University of Arizona Earth: Mike Malaska Composition by Mike Malaska
Understandings about the Nature of Science (Crosscutting Concepts): Science knowledge has a history that includes the refinement of, and changes to, theories, ideas, and beliefs over time.
Science Is a Human Endeavor: Scientific knowledge is a result of human endeavor, imagination, and creativity. Individuals and teams from many nations and cultures have contributed to science and to advances in engineering.
World History I Learning Standards: Scientific Revolution and The Enlightenment in Europe
WHI.33 Summarize how the Scientific Revolution and the scientific method led to new theories of the universe and describe the accomplishments of leading figures of the Scientific Revolution, including Bacon, Copernicus, Descartes, Galileo, Kepler, and Newton.
Next Generation Science Standards
Connections to Nature of Science: Science Models, Laws, Mechanisms, and Theories Explain Natural Phenomena.
A scientific theory is a substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experiment, and the science community validates each theory before it is accepted. If new evidence is discovered that the theory does not accommodate, then the theory is generally modified in light of this new evidence. (HS-ESS1-2),(HS-ESS1-6)