From the July 2026 issue

How do planets clear out objects on the other side of their orbit?

Over billions of years, a planet will have encountered all the small bodies in its vicinity enough times to clear its orbit of them.

By Astronomy Staff | Published: May 21, 2026 | Last updated on May 22, 2026

A diagram illustrating the outer solar system from an angled top-down perspective. Concentric orbital paths are shown around a central yellow Sun, with Jupiter, Saturn, Uranus, and Neptune labeled at increasing distances. Pluto is shown in yellow text outside Neptune's orbit, and the entire outer region is surrounded by a diffuse haze representing the Kuiper Belt, labeled in white text.

Pluto famously was demoted to dwarf planet because, with its location in the Kuiper Belt, it has not cleared the neighborhood around its orbit — one of the three requirements in the International Astronomical Union’s definition of a planet. Credit: Astronomy: Roen Kelly

To be a planet, NASA says an object must be big enough that its gravity has cleared away any other objects of a similar size near its orbit around the Sun. I can understand how something can clear objects close to itself, but how does it clear something 180° away in its orbit?

Rebecca Smith
Washougal, Washington

The solar system is a dynamic place, and objects located 180° from a planet don’t stay that far from the planet for long.