Planets around other stars like peas in a pod: Study

Toronto: Exoplanets orbiting the same star tend to have similar sizes and a regular orbital spacing, according to a study. This pattern, revealed by observations of planetary systems discovered by the Kepler Telescope, could suggest that most planetary systems have a different formation history than the solar system, researchers said. 

They used the W M Keck Observatory on Maunakea in Hawaii to obtain high-resolution spectra of 1,305 stars hosting 2,025 transiting planets originally discovered by Kepler.  From these spectra, the researchers measured precise sizes of the stars and their planets. Published in The Astronomical Journal, the study led by astrophysicist Lauren Weiss from University of Montral in Canada 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 - that exoplanets tend to be the same sizes as their neighbours.  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," said Weiss. The similar sizes and orbital spacing of planets have implications for how most planetary systems form, researchers said.

In classic planet formation theory, planets form in the protoplanetary disk that surrounds a newly formed star. 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 exoplanetary systems.

However, in our solar system, the inner planets have surprisingly large spacing and diverse sizes. Abundant evidence in the solar system suggests that Jupiter and Saturn disrupted our system's early structure, resulting in the four widely-spaced terrestrial planets we have today, researchers said.

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