This means our solar system may have formed differently than other solar systems did, the research team suggested, although more observations are needed to learn what the different mechanisms were.
"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," Lauren Weiss, the study's lead author and an astrophysicist at the University of Montreal, said in a statement.
The research team examined 355 stars that had a total of 909 planets, which periodically transit across their faces (as seen from Earth). The planets are between 1,000 and 4,000 light-years away from Earth.
After running a statistical analysis, the team found that a system with a small planet would tend to have other small planets nearby — and vice-versa, with big planets tending to have big neighbors. These extrasolar systems also had regular orbital spacing between the planets.
"The similar sizes and orbital spacing of planets have implications for how most planetary systems form," researchers said in the statement. "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."
In our own solar system, however, the story is very different. The four terrestrial planets (Mercury, Venus, Earth and Mars) are very widely spaced apart. The team pointed to evidence from other research that Jupiter and Saturn may have disrupted the structure of the young solar system. While the statement did not specify how, several other research studies have examined the movements of these giant planets and their potential impact on the solar system.
Each of the exoplanets examined in the study was originally found by Kepler, which launched in 2009 and continues to send data today. But more-detailed information was obtained with the W.M. Keck Observatory in Hawaii; Weiss is a member of the California-Kepler Survey team there, which is examining the light signatures of thousands of planets discovered by Kepler.
Weiss said she plans a follow-up study at Keck to look for Jupiter-like planets in multiplanet systems. The aim is to better understand if the presence of a Jupiter-size planet would alter the position of other planets in the same system.
"Regardless of their outer populations, the similarity of planets in the inner regions of extrasolar systems requires an explanation," researchers said in the statement. "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."
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