Dawn (spacecraft) in the context of "Dwarf planet"

Ceres (minor-planet designation: 1 Ceres) is a dwarf planet in the main asteroid belt between the orbits of Mars and Jupiter. It was the first known asteroid, discovered on 1 January 1801 by Giuseppe Piazzi at Palermo Astronomical Observatory in Sicily, and announced as a new planet. Ceres was later classified as an asteroid and more recently as a dwarf planet, the only one not beyond the orbit of Neptune and the largest that does not have a moon.

Ceres's diameter is about a quarter that of the Moon. Its small size means that even at its brightest it is too dim to be seen by the naked eye, except under extremely dark skies. Its apparent magnitude ranges from 6.7 to 9.3, peaking at opposition (when it is closest to Earth) once every 15- to 16-month synodic period. As a result, its surface features are barely visible even with the most powerful telescopes, and little was known about it until the robotic NASA spacecraft Dawn approached Ceres for its orbital mission in 2015.

The number of dwarf planets in the Solar System is unknown. Estimates have run as high as 200 in the Kuiper belt and over 10,000 in the region beyond.However, consideration of the surprisingly low densities of many large trans-Neptunian objects, as well as spectroscopic analysis of their surfaces, suggests that the number of dwarf planets may be much lower, perhaps only nine among bodies known so far. The International Astronomical Union (IAU) defines dwarf planets as being in hydrostatic equilibrium, and notes six bodies in particular: Ceres in the inner Solar System and five in the trans-Neptunian region: Pluto, Eris, Haumea, Makemake, and Quaoar. Only Pluto and Ceres have been confirmed to be in hydrostatic equilibrium, due to the results of the New Horizons and Dawn missions. Eris is generally assumed to be a dwarf planet because it is similar in size to Pluto and even more massive. Haumea and Makemake were accepted as dwarf planets by the IAU for naming purposes and will keep their names if it turns out they are not dwarf planets. Smaller trans-Neptunian objects have been called dwarf planets if they appear to be solid bodies, which is a prerequisite for hydrostatic equilibrium: planetologists generally include at least Gonggong, Orcus, and Sedna. Quaoar was labelled as a dwarf planet in a 2022–2023 annual report, though it does not appear to be in hydrostatic equilibrium. In practice the requirement for hydrostatic equilibrium is often loosened to include all gravitationally rounded objects, even by the IAU, as otherwise Mercury would not be a planet.