Planets on tilted orbits tend to pass over the poles of their suns

Planets on tilted orbits tend to pass over the poles of their suns
A planet orbiting the star WASP-79 (illustrated) orbits above and below its sun's poles. Credit: NASA, ESA AND L. Hustak/STSCI
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The Earth is following an orderly orbit around the sun, nearly in the same plane as our star’s equator. However, in 2008, astronomers discovered worlds in other solar systems that sail far above and below their star’s equatorial plane.

Now, an unexpected discovery about these off-kilter worlds may eventually reveal their origin: The majority of them travel in polar orbits. If Earth had such an orbit, we would pass over the sun’s north pole every year, dive through its equatorial plane, and then pass below the sun’s south pole before rising again.


Aarhus University in Denmark astronomers Simon Albrecht and Marcus Marcussen, along with colleagues, studied 57 planets in other solar systems to determine the true tilt between a planet’s orbit and its star’s equatorial plane. The team discovered that two-thirds of the planets have normal orbits that are no more than 40 degrees tilted. The remaining 19 planets are misaligned.

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But those misaligned planets’ orbits don’t just make any old angle with their star’s equator. Instead, they form a 90-degree angle. In fact, all but one of the misaligned planets are on polar orbits with tilts ranging from 80 to 125 degrees, according to a paper published online on May 20 at arXiv.org.

“It’s very strange,” says Amaury Triaud, an astronomer at the University of Birmingham in England who has discovered several misaligned planets but was not involved in the new study. “It’s a beautifully executed idea, and the outcome is very interesting,” he says. “It’s so new and strange.”


The findings may shed light on the planets’ most vexing mystery: how they came to be. Planets form inside pancake-shaped disks of gas and dust orbiting in the equatorial planes of their stars, which surprised astronomers. Planets should also be near the plane of their sun’s equator. In our solar system, for example, Earth’s orbit tilts only 7 degrees from the solar equatorial plane, and Pluto — which many astronomers no longer consider to be a planet — has an orbit that tilts only 12 degrees from that plane (and 17 degrees from Earth’s orbital plane).

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Albrecht admits, “At the moment, we are not sure what the underlying mechanism” or mechanisms are for creating misaligned planets. Whatever it is, he claims, it should account for the newly discovered slew of perpendicular planets.

According to Albrecht, the single exception to the rule: the one misaligned planet in the sample that is not on a polar orbit, provides a possible clue. This planet is also the most massive in the sample, weighing between five and eight Jupiters. That could be a coincidence, according to Albrecht, or it could reveal something about how the other planets became misaligned.


Astronomers hope to learn more about how these strange worlds got their odd orbits in the future. All known misaligned planets orbit close to their stars, but are these worlds more likely to have giant planets nearby than normal, close-in planets? The scientists don’t know yet, but if they find such a link, those strange companions may have flung these strange worlds onto their strange planetary paths.

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