Planets Found With Crisscross Orbits—A First

Published in National Geographic News


A “super Jupiter” and its sibling world have been found circling their parent star with steeply tilted orbits—the first time such a configuration has ever been spotted, astronomers say.

All eight planets in our solar system orbit the sun in roughly the same plane, an imaginary disk that extends from the sun’s equator.

But new data show that two of the three Jupiter-like planets known to circle the sunlike star Upsilon Andromedae have orbits that are tilted 30 degrees from each other.

The orbit of the third, innermost planet is still unknown. (Find out about a batch of planets recently found outside our solar system that have tilted, “wrong way” orbits.)

The unusual discovery suggests that astronomers can’t assume all star systems with multiple worlds will always be able to keep planetary siblings in line. (See “Our Solar System May Have Millions of ‘Twins.'”)

The Upsilon Andromedae system probably formed the same way as other planetary systems, including our own, said study leader Barbara McArthur of the McDonald Observatory at the University of Texas at Austin.

When a cloud of material condenses and collapses to form a star, some leftover material can circle the star’s middle in what’s called a protoplanetary disk. This disk, in turn, can coalesce to form planets.

In Upsilon Andromedae’s case, something knocked two of the outer planets into strange, crisscrossing orbits after the star system had been born.

The star has a binary companion, a dim red dwarf star that orbits Upsilon Andromedae at a distance of about 70 billion miles (112 billion kilometers). If that red dwarf has a very elongated orbit, it might have swooped in close to Upsilon Andromedae sometime in the past, disturbing the planets’ orbits with its gravity.

There’s also evidence that a “missing” planet might have been involved in a gravitational tug-of-war with its peers. The planets could have pushed each other around so much that the loser got thrown out of the system, while the remaining worlds were left askew.

(Related: “Mystery Space Object May Be Ejected Black Hole.”)

“Planet-planet scattering is a very likely cause for what we see,” McArthur said.


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