Planets Around Other Stars Are Like Peas in a Pod

An international research team led by Université de Montréal astrophysicist Lauren Weiss has discovered that exoplanets orbiting the same star tend to have similar sizes and a regular orbital spacing. This pattern, revealed by new W. M. Keck Observatory observations of planetary systems discovered by the Kepler Telescope, could suggest that most planetary systems have a different formation history than the solar system.

Thanks in large part to the NASA Kepler Telescope, launched in 2009, many thousands of exoplanets are now known. This large sample allows researchers to not only study individual systems, but also to draw conclusions on planetary systems in general. Dr. Weiss is part of the California Kepler Survey team, which used the W. M. Keck Observatory on Maunakea in Hawaii, to obtain high-resolution spectra of 1305 stars hosting 2025 transiting planets originally discovered by Kepler. From these spectra, they measured precise sizes of the stars and their planets.

In this new analysis led by Weiss and published in The Astronomical Journal, the team focused on 909 planets belonging to 355 multi-planet systems. These planets are mostly located between 1,000 and 4,000 light-years away from Earth. Using a statistical analysis, the team found two surprising patterns. They found that 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. They also found that planets orbiting the same star tend to have a regular orbital spacing.

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

Read more at University of Montreal

Image: Kepler-11 is a sun-like star around which six planets orbit. (Credit: NASA/Tim Pyle)