Enormous Planet With Bizarre Orbit Is Unlike Anything Ever Discovered Before, Astronomers Say

A planet unlike anything ever discovered before has been identified by a team of U.S.-led scientists. The planet, named HR 5183 b, is about three times the size of Jupiter—the biggest planet in our solar system—and travels along a huge, egg-shaped orbit, slingshotting around its star once every 45 to 100 years.

Astronomers first started observing the planet's host star in the 1990s. A few years ago, they noticed a planet circling it. "This planet spends most of its time loitering in the outer part of its star's planetary system in this highly eccentric orbit, then it starts to accelerate in and does a slingshot around its star," Caltech's Andrew W Howard, one of the study authors, said in a statement.

"We detected this slingshot motion. We saw the planet come in and now it's on its way out. That creates such a distinctive signature that we can be sure that this is a real planet, even though we haven't seen a complete orbit."

The team's findings have been accepted for publication in The Astronomical Journal.

HR 5183 b was identified through a technique known as "radial velocity." This is where researchers look at how the star "wobbles" as a result of the gravitational pull of a planet passing it. After tracking the planet, they were able to determine its huge orbit. If the planet was in our own solar system, it would travel from a point beyond Neptune all the way into the asteroid belt that sits between Mars and Jupiter.

"This newfound planet basically would have come in like a wrecking ball, knocking anything in its way out of the system," Howard said.

Sarah Blunt, first author of the study, said in the statement: "This planet is unlike the planets in our solar system, but more than that, it is unlike any other exoplanets we have discovered so far. Other planets detected far away from their stars tend to have very low eccentricities, meaning that their orbits are more circular. The fact that this planet has such a high eccentricity speaks to some difference in the way that it either formed or evolved relative to the other planets."

Our solar system came together when a cloud of gas and dust collapsed, forming a rotating disk of material. As gravity pulled material into the center, reactions that released huge amounts of energy took place and the sun was born. The remaining material further from the center then started to clump together to form the planets. Jupiter took most of the material left over, becoming the biggest planet in our solar system.

It is not known exactly how other solar systems formed, but finding other giant planets like HR 5183 b—and its bizarre orbit—could provide clues. Because the orbits of most planets start off in a circular shape, the researchers believe HR 5183 b must have been pushed onto its current trajectory by another object. This could have been another massive planet—and when the two collided, HR 5183 b was placed into an egg-shaped orbit, while the other was flung from the solar system.

It could also mean that the formation of our solar system is not a model for other systems in the universe.

"This newfound planet is another example of a system that is not the image of our solar system, but has remarkable features that make our universe incredibly rich in its diversity," Howard said.

The researchers also note that one of the key goals of exoplanet science set out by the National Academy of Sciences is to find different planetary systems and the structures and patterns within them. "With this discovery, we continue to uncover the astonishing diversity of planetary systems in our the galaxy," they conclude.

HR 5183 b
Image showing the orbit of HR 5183 b if it existed in our solar system. The planet is three times the size of Jupiter and could give clues to the formation of other planetary systems. W. M. Keck Observatory/Adam Makarenko