The curious case of over-tilting planets

Astronomers have took on the challenge of explaining why planets have unusual tilts and what it would mean for their physical features


Yale researchers have discovered a surprising link between the tilting of exoplanets and their orbit in space. The discovery may help explain a long-standing puzzle about exoplanetary orbital architectures. Image credit: NASA/JPL-Caltech/S. Millholland

For almost a decade, astronomers have tried to explain why so many pairs of planets outside our Solar System have an odd configuration — their orbits seem to have been pushed apart by a powerful unknown mechanism. Researchers at Yale University in Connecticut, United States, say they’ve found a possible answer, and it implies that the planets’ poles are majorly tilted.

The finding could have a big impact on how researchers estimate the structure, climate, and habitability of exoplanets as they try to identify planets that are similar to Earth. The research appears in the 4 March 2019 online edition of the journal Nature Astronomy.

NASA’s Kepler mission revealed that about 30 percent of stars similar to our Sun harbour “Super-Earths.” Their sizes are somewhere between that of Earth and Neptune, they 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 these planets 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, according to Yale astronomers Sarah Millholland and Gregory Laughlin.

A similar situation between Earth and the Moon, which means the Moon is slowly getting further away. Image credit: NASA/Apollo 8

“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,” says first author Millholland, a graduate student at Yale. “This vigorous tidal dissipation pries the orbits apart.”

A similar, but not identical, situation exists between Earth and its Moon. The Moon’s orbit is slowly growing due to dissipation from tides, but Earth’s day is gradually lengthening.

Laughlin, who is a professor of astronomy at Yale, said there is a direct connection between the over-tilting of these exoplanets and their physical characteristics. “It impacts several of their physical features, such as their climate, weather, and global circulations,” says Laughlin. “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.”

Millholland said she and Laughlin already have started work on a follow-up study that will examine how these exoplanets’ structures respond to large obliquities over time.

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