A surprisingly gentle merger between two small primordial bodies formed the distant object Ultima Thule, a new study suggests.
These two progenitors themselves likely coalesced from the same cloud of icy material at the dawn of the solar system, billions of miles from the newborn sun. They initially circled a common center of mass but spiraled closer and closer, eventually meeting up in decidedly leisurely fashion.
https://www.space.com/ultima-thule-birth-new-horizons-first-science.html