Today, February 18th, 2020 is the 90th anniversary of the discovery of Pluto by Clyde Tombaugh at the Lowell Observatory in Arizona and a good time to remember the journey that the world has taken through our culture. Unlike Pluto, the visible planets (Mercury, Venus, Mars, Jupiter, Saturn) were known since humans first looked up and mapped the sky. They were called planets for the Greek word wanderer since they all wander across the sky relative to the more fixed stars. Once we realized that the planets all orbited the Sun, and the Earth did as well, Planet became the term for large bodies of rock, gas, and ice that orbit the Sun.
Uranus became the first new planet to be discovered by telescope in 1781 by William Herschel. Neptune was then predicted to exist based on the orbital path of Uranus in the 1840s by Urbain Le Verrier and discovered in 1846 by Johann Galle. Astronomers continued to search the sky for moving objects, mostly comets. In the early 20th century, photo technology would allow them to flash between two images of the sky from different nights to identify the moving objects. In 1930, Tombaugh, working at Lowell Observatory, discovered what would be known as Pluto. Pluto orbited the Sun, but was it a planet?
The exact nature of a planet was not well defined. A number of large asteroids (Ceres, Vesta, Pallas, Juno) were sometimes considered to be planets since they orbit the Sun, but have lost that status as the definition was tightened. Asteroids were not planets because they were too small. Comets were not planets since they were too small and had very elliptical orbits. Moons were not plants because they orbited other worlds. Pluto was discovered to orbit the Sun, to be larger than the largest asteroids, though smaller than many moons. Pluto had an orbit more circular than any comet, but more elliptical than any planet. Excitement for this discovery and the lack of a rigorous definition made Pluto into the 9th planet culturally, if not scientifically.
But the discoveries kept coming. In 2005, Eris was discovered. It was smaller and farther than Pluto, but more like Pluto than any other object. More Pluto-like bodies were discovered and named trans-Neptunian objects (TNO)s, these objects orbit the Sun past Neptune. There are now 528 numbered and more than 2000 suspected objects like this. Science had a choice. If Pluto is a planet, then there are hundreds (maybe thousands) more that should be planets. Some scientists wanted to keep the planets we think of as planets, but exclude everything else. In a controversial 2006 meeting, the International Astronomical Union (IAU) defined a Planet as having 3 characteristics.
1. The Planet must orbit the Sun. This counts out the many amazing moons in our solar system. They have volcanoes, oceans, and maybe life, but are not planets. Also since exoplanets do not orbit the Sun, they orbit other stars, they are not planets. Only our Sun has planets by this definition.
2. The Planet must be large enough to be round. This happens when gravity is stronger than the planet’s material. This is material dependent, so the smallest rocky planet is larger than the smallest icy planet. This keeps out most asteroids and comets.
3. The Planet must “clear out its neighborhood” of other materials (usually asteroids) or gravitationally dominate everything there. This is a weird one and mostly used to keep out Pluto and the other TNOs, comets and asteroids. However, other planets, Earth, Mars, Jupiter, Neptune have not cleared out their orbits, though they tend to more effectively gravitationally dominate what is left, but are still considered Planets.
These 3 rules were developed specifically and applied inconsistently to result in a universe with only 8 widely different planets. Pluto and the TNOs became dwarf planets. Scientists are still pushing for a more universal definition of a planet. While there are many alternatives, we consider our two favorites here. The first potential fix is to just use the second rule. If the object is too small to be round, then it is not a planet, it is an asteroid, a comet, or something else, but not a planet. If it is large enough to have fusion then it is a star, not a planet. In this definition, planets are round objects that don’t have fusion. Here Pluto is a planet, the Moon is a planet, Ceres is a planet as well as all of the classical planets. This option is independent of location. We don’t care what it orbits or clears. It can be floating alone in an open space and still be a planet. This would make dozens of planets in our solar system and billions beyond. The second option is to classify zones. Our solar system would have 3 zones. 1) Inner rocky planets, 2) outer gas and ice planets, and 3) the TNOs. This system works well for our solar system but may break down in others.
So, according to the IAU, Pluto was once a planet but is not anymore. However, a more interesting story is what Pluto actually is. In 2015, NASA’s New Horizons spacecraft finished a blistering fast but still a 9-year long journey to flyby Pluto and send back the stunning images and datasets of that world. Pluto is an ice world. Its surface is 98% nitrogen ice with traces of methane and carbon monoxide. Pluto has mountains, plains, and ridges, all made of ice. The complexity of these features shocked scientists who then went to work to learn how one of the coldest objects in the solar system created this diversity. Studies of craters show that the surface of Pluto is about 180,000 years old on average. This means that the entire surface would change in a blink of a geologic eye. While still debated, it is thought that the heat from radioactive decay of the heavy elements deep inside Pluto is enough to melt the ice from time to time. One of the most iconic features of Pluto is Tombaugh Regio, also known as the heart of Pluto. It is a 1500 km wide plain that featured prominently in some of the first New Horizon images. The plain has a diverse geologic origin with convection cells in parts that show the internal activity of the world. In the past 90 years, Pluto went from a dim spot on a photographic plate to a planet, a cultural icon, a dwarf planet, and a new world to explore. We will continue to dive into the spacecraft data and explore other icy worlds while watching Pluto from telescopes around Earth. Each observation will teach us more about this mysterious world.