08/09/2026
Our next feature is ‘D’ for ‘Dwarf’ ✨
The Solar System contains various types of space objects: comets were once thought to be an atmospheric phenomenon, and asteroids, first discovered in the 19th century. Planets have been observed for thousands of years and when the telescope was invented, it showed they are different to stars, revealing disks of varying appearances instead of just points of light. An extra layer of complexity was added to the Solar System when Galileo discovered moons around Jupiter, showing that not everything orbited the Earth or the Sun.
A greater understanding of the Sun’s family came from discoveries of new planets - Uranus and Neptune in the 18th and 19th century, and the 20th century brought the discovery of Pluto and planetary exploration by spacecraft, further expanding our knowledge of the Solar System. Still, it was all pretty straightforward, in terms of the known classes of object, but that changed in the 21st century.
3 objects were found beyond Neptune in 2005 and were named Eris, Haumea, and Makemake. After studying these objects, it was thought that Eris was larger and more massive than Pluto, but this created a problem... Nobody was going to call Eris a planet with its tilted and very elliptical orbit, but if Eris wasn’t a planet, what was Pluto?
The word planet originally meant wanderer (from the Greek, planetes), and even Jupiter’s moons were sometimes referred to as planets. Despite all the study of the planets though, nobody had formally defined what a planet was. In 2006, the outcome was that ‘planets’ referred to the eight large objects from Mercury to Neptune, with a new category, dwarf planet, being created to include Pluto, Eris, Haumea, Makemake, and the first object discovered in the asteroid belt, Ceres.
The formal definition for dwarf planet is a celestial body that is in orbit around the Sun, has sufficient mass that it is or almost is a round shape, has not cleared the neighbourhood around its orbit, and is not a satellite. Meaning, a dwarf planet is a round object, larger than about 1,000 km, orbiting the Sun, but may share its orbit with other large objects. In the special case of Haumea, it is rounded, but not spherical.
