Study: Planets within Single Multi-Planet System are Similar in Size and Regularly Spaced

A study of 909 exoplanets in 355 multi-planet systems discovered by NASA’s Kepler Space Telescope reveals that, unlike our Solar System, other planetary systems are distinguished by strict regularity.

This illustration shows the TRAPPIST-1 multi-planet system. Image credit: NASA / JPL-Caltech.

Université de Montréal astrophysicist Lauren Weiss and co-authors used W. M. Keck Observatory in Hawaii to obtain high-resolution spectra of the Kepler planet-hosting stars.

From these spectra, they measured precise stellar and planetary properties for 355 multi-planet systems containing 909 transiting planets.

Using a statistical analysis, the team found two surprising patterns:

(i) exoplanets tend to be the same sizes as their neighbors: if one planet is small, the next planet around that same star is very likely to be small as well, and if one planet is big, the next is likely to be big;

(ii) exoplanets orbiting the same star tend to have a regular orbital spacing.

“The planets in a multi-planet system tend to be the same size and regularly spaced, like peas in a pod,” Dr. Weiss said.

“These patterns would not occur if the planet sizes or spacings were drawn at random.”

The similar sizes and orbital spacing of planets have implications for how most planetary systems form.

“In classic planet formation theory, planets form in the protoplanetary disk that surrounds a newly formed star,” the astrophysicists said.

“The planets might form in compact configurations with similar sizes and a regular orbital spacing, in a manner similar to the newly observed pattern in multi-planet systems.”

However, in our own Solar System, the inner planets have surprisingly large spacing and diverse sizes.

“Abundant evidence suggests that Jupiter and Saturn disrupted Solar System’s early structure, resulting in the four widely-spaced terrestrial planets we have today,” the scientists explained.

“That planets in most systems are still similarly sized and regularly spaced suggests that perhaps they have been mostly undisturbed since their formation.”

To test that hypothesis, Dr. Weiss and colleagues are conducting a new study at the Keck Observatory to search for Jupiter-analogs around Kepler multi-planet systems.

“The planetary systems studied by our team have multiple planets quite close to their star,” they said.

“Because of the limited duration of the Kepler mission, little is known about what kind of planets, if any, exist at larger orbital distances around these systems.”

“We hope to test how the presence or absence of Jupiter-like planets at large orbital distances relate to patterns in the inner planetary systems.”

“Regardless of their outer populations, the similarity of planets in the inner regions of extrasolar systems requires an explanation.”

“If the deciding factor for planet sizes can be identified, it might help determine which stars are likely to have terrestrial planets that are suitable for life.”

The study is published in the Astronomical Journal (arXiv.org preprint).

_____

Lauren M. Weiss et al. 2018. The California-Kepler Survey. V. Peas in a Pod: Planets in a Kepler Multi-planet System Are Similar in Size and Regularly Spaced. AJ 155, 48; doi: 10.3847/1538-3881/aa9ff6

Let's block ads! (Why?)