Worlds in multi-planet systems have similar sizes, spacings
Contrast with diversity of our solar system could mean they have different histories and evolutionary processes.

By Laurel Kornfeld | 5 hours ago

A study in which solar systems discovered by NASA's Kepler Space Telescope were observed with the W.M. Keck Observatory in Mauna Kea, Hawaii, found the majority of these systems to be far more uniform than our own in terms of planet sizes and spacings.

Lauren Weiss of the Universite de Montreal led an international team of scientists in a project known as the California Kepler Survey, which looked at a total of 909 exoplanets in 355 star systems within a distance of 1,000 to 4,000 light years away.

The study's goals included drawing general conclusions on planetary systems as well as better understanding of individual ones.

Scientists conducting the study were surprised to find that unlike our own solar system, which has planets with a wide range of sizes and orbital spacings, most multi-planet systems in the study have planets that are all approximately the same sizes with very regular orbital spacings.

This means in any given system, if one planet is small, the others orbiting the same star are also small, and vice versa for large planets.

These arrangements could indicate that the exoplanet systems studied formed differently than our solar system did.

"The planets in a system tend to be the same size and regularly spaced, like peas in a pod," Weiss noted. "These patterns would not occur if the planet sizes or spacings were drawn at random."

According to conventional theory, planets form from protoplanetary disks, which surround infant stars. In systems like the majority of those observed in this study, they likely formed in compact arrangements with all planets having similar sizes and spacings between them.

Our solar system, in contrast, has inner planets of varying sizes with large spacings between them, gas giants of various sizes and spacings even further out, and numerous dwarf planets, all but one of which--Ceres--are located in the outer regions beyond Neptune.

Dwarf planets are currently too small to detect in exoplanet systems.

Based on the arrangement of our solar system, many scientists believe Jupiter and Saturn disrupted its early formation process, producing the configuration it has today.

Most of the systems studied by Weiss's team have planets that orbit close to their parent stars. Researchers are now searching for Jupiter analogs in these systems, with the goal of determining the effect a Jupiter-sized planet might have, especially when it is in a distant orbit around its star, and contrasting systems with Jupiter-type planets with those that do not have them.

Kepler operated for a limited time and therefore may not have detected planets orbiting at greater distances in these systems.

Findings of the study have been published in The Astronomical Journal.

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