How do you define a planet?

The field of planetary science isn’t often plagued by drama, but there are occasional controversies. Perhaps the most famous was the 2006 reclassification of Pluto as a dwarf planet. Many people saw this as an unfair demotion of a full-fledged planet, but it wasn’t the first time humanity had narrowed our definition of a planet to account for new information. 

The earliest definition of a planet was simply any bright object that moved against the fixed backdrop of the stars. In ancient Greek astronomy, the planets included Mercury, Venus, Mars, Jupiter, Saturn, the Sun, and the Moon. Earth was not considered a planet; it was thought to be the stationary center of the Universe. 

The Copernican revolution of the 16th century introduced a new definition: Planets were objects that orbited the Sun, including Earth. This view held for centuries. 

The planetary family expanded with the discovery of Uranus in 1781, and as telescopes improved, the list continued to grow. Beginning with the discovery of Ceres in 1801, astronomers found a succession of small bodies between Mars and Jupiter, including Pallas, Juno, and Vesta. At first, these were all considered planets. But as dozens more objects were detected in the same region, it became clear they formed a distinct population. By the mid-19th century, the classification of “asteroids” or “minor planets” shrank the list of planets to only the major bodies orbiting the Sun — including Neptune, discovered in 1846. 

In 1930, astronomers thought they had found the ninth planet they had long predicted. They had spotted Pluto but had overestimated its size. The faint, fuzzy spot in their telescopes was actually Pluto and its companion Charon, blurred together by low telescopic resolution — an error not uncovered until 1978.

Decades later, improved measurements showed Pluto was much smaller than originally thought, and in the 1990s, astronomers began finding other icy bodies beyond Neptune. These Kuiper belt objects formed a large population to which Pluto clearly belonged, just as Ceres had belonged to the asteroid belt. 

The issue came to a head in 2005 with the discovery of Eris, a scattered disk object (another more distant group of bodies) about the same size as Pluto. If Pluto were a planet, many argued, Eris must be one too. Instead of expanding the list of planets to include many more Pluto-size worlds, the International Astronomical Union decided in 2006 to refine the definition of “planet” again. For the first time, a formal scientific definition was established: A planet must orbit the Sun, be massive enough for gravity to pull it into a roughly spherical shape, and have cleared its orbit as the dominant object in its region. Pluto met the first two criteria but not the third, reclassifying it as a “dwarf planet” along with Eris and Ceres. 

This decision sparked debate that continues today. But the IAU’s definition doesn’t limit what’s worth studying. The worlds of the Cosmos are all fascinating regardless of size. And, as you’ll learn in this issue, there is still much to be learned about the smallest objects in our Solar System.