A new picture returned from Nasa's New Horizons spacecraft shows the little world to be two objects joined together - to give a look like a "snowman".
The US probe's images acquired as it approached Ultima hinted at the possibility of a double body, but the first detailed picture from Tuesday's close flyby confirms it.
New Horizons encountered Ultima 6.5 billion km from Earth.
The event set a record for the most distant ever exploration of a Solar System object. The previous mark was also set by New Horizons when it flew past the dwarf planet Pluto in 2015.