If you’re looking for life beyond the solar system, there’s strength in numbers.

A new study suggests that systems with multiple planets tend to have rounder orbits than those with just one, indicating a calmer family history. Only child systems and planets with more erratic paths hint at past planetary sibling clashes violent enough to knock orbits askew, or even lead to banishment. A long-lasting abundance of sibling planets might therefore have protected Earth from destructive chaos, and may be part of what made life on Earth possible, says astronomer Uffe Gråe Jørgensen of the Niels Bohr Institute in Copenhagen.

“Is there something other than the Earth’s size and position around the star that is necessary in order for life to develop?” Jørgensen says. “Is it required that there are many planets?”

Most of the 4,000-plus exoplanets discovered to date have elongated, or eccentric, orbits. That marks a striking difference from the neat, circular orbits of the planets in our solar system. Rather than being an oddity, those round orbits are actually perfectly normal — for a system with so many planets packed together, Jørgensen and his Niels Bohr colleague Nanna Bach-Møller report in a paper  published online October 30 in the Monthly Notices of the Royal Astronomical Society.

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