Out there, across the Milky Way, other solar systems are very different from our planetary neighborhood. Four years of observations from NASA’s Kepler have provided a wealth of data that is now allowing us to better understand how planetary systems form, in particular "warm Jupiters".
A new study, published in the Astrophysical Journal, has discovered an interesting relationship between hot and warm Jupiters and their companion planets. Out of 45 hot Jupiters, which are gas giants that orbit around their stars in less than 10 days, only one has a companion. This suggests that these objects have migrated inwards over time.
The situation is quite different for warm Jupiters. These objects are still gas giants but they inhabit the more "temperate" region of a star system, completing an orbit every 10 to 200 days. Out of 27 warm Jupiters, 11 were found to have companions ranging in size from that of Neptune to that of Earth.
“Our findings suggest that a big fraction of warm Jupiters cannot have migrated to their current positions dynamically and that it would be a good idea to consider more seriously that they formed where we find them,” said lead author Chelsea Huang, from the University of Toronto, in a statement.
Gas giant planets are believed to have formed beyond the "snow line", the region in a solar system where the temperature is low enough for water molecules to freeze over dust grains. These icy grains merge and become the eventual seeds of gas giants.
In our Solar System, the snow line is found at the distance of the asteroid belt, and the gas giants are found beyond it. So, it has been thought that all giants form far away from their star and slowly move inwards. This is consistent for hot Jupiters but not for the warm Jupiters. A gas giant moving in would disrupt planetary companions too much. This means a significant fraction of warm Jupiters could have formed directly in their present location.
“And when we take into account that there is more analysis to come,” says Huang, “the number of warm Jupiters with smaller neighbours may be even higher. We may find that more than half have companions.”
The number of new exoplanets has grown exponentially, but there’s still so much that we don’t know about planetary systems. Every new discovery takes us a step closer.
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