The term “dwarf planet” has become increasingly relevant in our understanding of the solar system. In this article, originally published on The Conversation and written by Vahe Peroomian, professor of physics and astronomy at USC Dornsife College of Letters, Arts and Sciences, we explore what defines a dwarf planet and why this category matters in astronomy.
The word “planet” originates from ancient Greek, meaning “wandering star.” This term aptly describes how planets move across the night sky, unlike the seemingly fixed stars. Historically, five planets—Mercury, Venus, Mars, Jupiter, and Saturn—were known to early astronomers. Later discoveries included Uranus in 1781, Neptune in 1846, and Pluto in 1930, expanding our view of the solar system.
The Discovery of Dwarf Planets
As a space scientist with a focus on solar system exploration, I’ve witnessed the discovery of more objects beyond Neptune, especially in the Kuiper Belt—a region filled with remnants of the solar system. In the early to mid-2000s, astronomers identified Eris, Haumea, and Makemake, which are similar in size to Pluto. These findings led to the realization that the Kuiper Belt contains numerous icy bodies, potentially revealing more dwarf planets.
Defining Planet
In 2006, the International Astronomical Union (IAU) introduced the term “dwarf planet” to better categorize these celestial objects. According to the IAU, for an object to be classified as a planet, it must orbit the sun directly, be spherical in shape, and clear its orbit of other objects. If an object meets the first two criteria but not the third, it is classified as a dwarf planet.
Pluto’s Demotion and New Discoveries
Pluto was reclassified as a dwarf planet because it did not meet the “clearing its orbit” criterion. This decision remains controversial among scientists. Simultaneously, Ceres, previously considered an asteroid, was promoted to dwarf planet status. Located in the main asteroid belt between Mars and Jupiter, Ceres adds to the list of the planet, which now includes Pluto, Ceres, Eris, Haumea, and Makemake. This list is likely to grow as more candidates are identified in the Kuiper Belt.
Characteristics
Dwarf planets differ significantly from Earth in size and mass. For example, Pluto and Eris have less than one-fifth the diameter of Earth, and Ceres’ mass is about 6,400 times smaller than Earth’s. Additionally, dwarf planets are extremely cold, with Pluto’s average temperature around minus 400 degrees Fahrenheit (minus 240 Celsius).
The Potential for Life on the Planet
For life to exist, three key elements are needed: liquid water, an energy source, and organic molecules. While some dwarf planets like Pluto and Ceres might harbor subsurface oceans of liquid water and organic molecules, the lack of a significant energy source—especially given their distance from the sun—poses a challenge. The weak sunlight that reaches these distant worlds is insufficient for heating, and their small size prevents them from retaining internal heat from the solar system’s formation.
However, life on Earth thrives in extreme environments, such as deep-sea vents and volcanic regions, suggesting that life could potentially exist in similar extreme conditions elsewhere in the solar system. Thus, the exploration of dwarf planets remains a fascinating field of study with endless possibilities.
