Distant Asteroid provides clues as to how planets formed

[lpetrich]

That distant asteroid is the small Kuiper Belt Object observed up close on 2019 Jan 1 by the New Horizons spacecraft,  486958 Arrokoth

NH's operators found another object for it to observe up close after it observed Pluto in 2015: 2014 MU69, nicknamed Ultima Thule, and then formally named 486958 Arrokoth.

It has a distance of 45 AU, a nearly circular orbit, and a period of nearly 300 years. It looks like two roughly ellipsoidal asteroids attached along their long axes. Its dimensions are

• Overall: 36 × 20 × 10 km
• Ultima 20.6 × 19.9 × 9.4 km
• Thule 15.4 × 13.8 × 9.8 km

New Horizons: News Article?page=20200213

Their analysis indicates that the lobes of this "contact binary" object were once separate bodies that formed close together and at low velocity, orbited each other, and then gently merged to create the 22-mile long object New Horizons observed.

This indicates Arrokoth formed during the gravity-driven collapse of a cloud of solid particles in the primordial solar nebula, rather than by the competing theory of planetesimal formation called hierarchical accretion. Unlike the gentle, low-velocity process that is the hallmark of particle-cloud collapse, in hierarchical accretion, planetesimals slammed into each other at increasingly higher speeds to form larger bodies.

So Arrokoth's two parent bodies gently collided with each other.

Two other important pieces of evidence support this conclusion. The uniform color and composition of Arrokoth's surface shows the KBO formed from nearby material, as local cloud collapse models predict, rather than a mishmash of matter from more separated parts of the nebula, as hierarchical models might predict.

The flattened shapes of each of Arrokoth's lobes, as well as the remarkably close alignment of their poles and equators, also point to a more orderly merger from a collapse cloud. Further still, Arrokoth's smooth, lightly cratered surface indicates its face has remained well preserved since the end of the planet formation era.

However, more inward parts of the Solar System had likely gotten much more mixed up, because of the giant planets' gravity.