Researchers reporting in the Astrophysical Journal Letters have found that the majority of circumbinary planets – planets that orbit two stars – were actually formed much further away from their binary stars and then migrated to their current locations.

There are few environments more extreme than a binary star system in which planet formation can occur.

Powerful gravitational perturbations from the two stars on the rocky building blocks of planets lead to destructive collisions that grind down the material.

To shed light on the formation process of circumbinary planets, lead author Dr Stefan Lines of Bristol University and his colleagues used a sophisticated model that calculates the effect of gravity and physical collisions on and between one million planetary building blocks.

They carried out three computer simulations: two of a circumbinary disk representative of the Kepler-34 system and one of a control simulation around a single star.

“Our simulations show that the circumbinary disk is a hostile environment even for large, gravitationally strong objects,” said second author Dr Zoe Leinhardt, also from Bristol University.

“Taking into account data on collisions as well as the physical growth rate of planets, we found that Kepler-34b would have struggled to grow where we find it now.”

Kepler-34b orbits at 1.09 AU (astronomical units), a likely scenario is that it formed beyond 1.5 AU and migrated inward.

“Based on these conclusions for Kepler-34, it seems likely that all of the currently known circumbinary planets have also migrated significantly from their formation locations – with the possible exception of Kepler-47c which is further away from the binary stars than any of the other circumbinary planets.”

“Circumbinary planets have captured the imagination of many science-fiction writers and film-makers – our research shows just how remarkable such planets are. Understanding more about where they form will assist future exoplanet discovery missions in the hunt for earth-like planets in binary star systems,” Dr Lines said.

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S. Lines et al. 2014. Forming Circumbinary Planets: N-body Simulations of Kepler-34. ApJ 782, L11; doi: 10.1088/2041-8205/782/1/L11