Illustration showing two Saturn-sized planets crossing in front of their star, Kepler-9.

Two planets orbiting the Kepler-9 star (artist's illustration) have matching masses, a pattern found in other planetary systems as well. Credit: NASA/Ames/JPL-Caltech

Astronomy and astrophysics

Multi-planet systems come to order

Patterns emerge amid the wild diversity of exoplanets.

The planets surrounding a star tend to have roughly the same size and mass, suggesting that exoplanets might be arranged in systems that are more orderly than expected.

NASA’s Kepler spacecraft has discovered a cornucopia of planets, many of them bigger than rocky Earth but smaller than gassy Neptune. Earlier work had shown that planets around the same star tend to have similar diameters. Now, a team led by Sarah Millholland of Yale University in New Haven, Connecticut, has looked at whether they also resemble each other in mass.

The researchers studied 37 stars whose respective planets interact in such a way that their masses can be teased out. In each system, the planets were remarkably similar in both diameter and mass.

The work hints that astronomers might be able to predict the types of planet that might form around a given star.