Ceres is the largest asteroid
The thing itself didn't change; its official category did.
TaughtBetween Mars and Jupiter lies the asteroid belt, and the biggest rock in it is Ceres.
NowThe same 2006 IAU resolution that demoted Pluto promoted Ceres. It is round under its own gravity, so it is a dwarf planet.
What actually happened
The 2006 vote is remembered as the one that took something away from Pluto. It also gave something to Ceres, which is rarely mentioned in the same breath.
Ceres had been called a planet when it was discovered in 1801, then demoted to asteroid as more objects turned up in the same region. The new definition asks whether a body is massive enough for gravity to have pulled it into a sphere. Ceres, at about 940 km across and holding roughly a quarter of the asteroid belt's total mass, is. So it became the only dwarf planet in the inner solar system, and the only one that is not out past Neptune.
NASA's Dawn spacecraft orbited it from 2015 to 2018 and found bright deposits of sodium carbonate left by briny water reaching the surface, plus evidence of a subsurface brine reservoir. Not a rock.
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Sources
- NASA Science: Ceresscience.nasa.gov
- NASA: Dawn missionscience.nasa.gov
Who was taught this
Still standard through 2007, so anyone who finished school between 1950 and 2006 learned the earlier version.