Quanta Magazine

Wandering Space Rocks Help Solve Mysteries of Planet Formation
Journey to the Birth of the Solar System 360 VR

Rebecca Boyle → Click here

In 2017, a weirdly shaped rock with a strangely erratic orbit swept through the solar system, leaving as quickly as it arrived. Astronomers soon realized that it was not from around here. It punched through the planets’ orbital plane from the top down, like a dart thrown at the concentric rings of a dartboard, and it moved super-fast, way too quickly to be caught in the sun’s gravity. It was also extremely dark and seemed oddly elongated, but because it was so speedy, many of its properties will remain forever mysterious.

But it turns out that the object, the first interstellar asteroid ever observed, was not unique. It may not even be all that rare. Astronomers are coming to realize that objects like it may pepper the galaxy, perhaps in such great numbers that they influence the formation of larger worlds, maybe even entire planetary systems including our own.

The appearance of the asteroid, which has been named ‘Oumuamua, was a dramatic demonstration of a sea change in astronomy: the recognition that the solar system does not exist in a vacuum, at least metaphorically. No planet is an island and no star forms in isolation. The cosmos is full of stuff that interacts across distances and time spans far greater than researchers have long appreciated, from unimaginably vast jets of gas flowing through interstellar space to ‘Oumuamua-style planetoid crumbs scattered like dandelion seeds in the wind.

This realization is changing the way astronomers think about how star systems form. Researchers who study the birth of planetary systems have not previously considered things like astrophysical gas flows, for instance. “Before, we could just stay in our lane,” said Sarah T. Stewart, a planetary scientist at the University of California, Davis. But as researchers reverse-engineer the recipe for solar systems, they’re now pulling ingredients together from all across the cosmos.

The new ideas sprouting out of this work have begun to offer potential answers to various exoplanetary mysteries. For example, planets appear to hatch much earlier in the life of a star than astrophysicists thought possible. In addition, enormous planets appear to grow out of relatively small amounts of gas and dust a loaves and fishes trick on an astronomical scale. And astronomers are trying to understand the paucity of planets just a little bit bigger than Earth.

All of which points to a stark fact: Planetary scientists still don’t have a bulletproof understanding of how planets get made. Their models are numerous and incomplete. But now, with the burgeoning combination of ideas from some of these disparate disciplines, researchers are starting to get a firmer handle on the planet-making process. “The fact that we haven’t settled on any one thing” for how planets get made, Stewart said, “is because we have learned so many new things.”

Dr. Edward Murphy University of Virginia → Click here