For several years, scientists have tried to explain why so many pairs of exoplanets have an odd configuration — their orbits seem to have been pushed apart by an unknown mechanism. Yale University astronomers Sarah Millholland and Gregory Laughlin have found a possible answer, and it implies that the exoplanets’ poles are majorly tilted.
Over-tilting exoplanets. Image credit: NASA / JPL-Caltech / Sarah Millholland.
NASA’s Kepler mission revealed that about 30% of solar-type stars harbor planets with sizes between that of Earth and Neptune.
These exoplanets have nearly circular and coplanar orbits; and it takes them fewer than 100 days to go around their star.
Yet curiously, a great number of them exist in pairs with orbits that lie just outside natural points of stability.
That’s where obliquity — the amount of tilting between a planet’s axis and its orbit — comes in.
“When planets such as these have large axial tilts, as opposed to little or no tilt, their tides are exceedingly more efficient at draining orbital energy into heat in the planets. This vigorous tidal dissipation pries the orbits apart,” Millholland said.
A similar, but not identical, situation exists between Earth and the Moon.
The Moon’s orbit is slowly growing due to dissipation from tides, but Earth’s day is gradually lengthening.
“There is a direct connection between the over-tilting of these exoplanets and their physical characteristics,” Professor Laughlin said.
“It impacts several of their physical features, such as their climate, weather, and global circulations.”
“The seasons on a planet with a large axial tilt are much more extreme than those on a well-aligned planet, and their weather patterns are probably non-trivial.”
“We already have started work on a follow-up study that will examine how these exoplanets’ structures respond to large obliquities over time.”
The study was published online this week in the journal Nature Astronomy.
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Sarah Millholland & Gregory Laughlin. Obliquity-driven sculpting of exoplanetary systems. Nature Astronomy, published online March 4, 2019; doi: 10.1038/s41550-019-0701-7
