Astronomers Discover Massive Planet That Slingshots Around Its Star

This illustration compares the oval-shaped orbit of HR 5183 b to the more circular orbits of planets in the Milky Way galaxy. (Courtesy of Adam Makarenko/W. M. Keck Observatory)

PASADENA, Calif. (CN) – Using the combined power of three U.S observatories, astronomers discovered a planet three times the mass of Jupiter that orbits around its star on an unusual, oval-shaped path, according to a study released Tuesday.

Astronomers said in the study, published in The Astronomical Journal, that if the planet – called HR 5183 b – was placed in our solar system, it would swing from the asteroid belt to an area beyond Neptune. 

Sarah Blunt, a California Institute of Technology graduate student and lead author of the study, said in a statement Tuesday that other giant planets with elliptical orbits have been found near other stars, but none are located at the outer rings of their star systems.

“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,” said Blunt, referring to planets outside our solar system. “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.”

Other planets that are found long distances from their stars tend to have very low eccentricities, or orbits that are more circular, Blunt added.

CalTech said Tuesday that data leading to the new planet’s discovery came from three observatories: the Lick Observatory in Northern California, the W. M. Keck Observatory in Hawaii and the McDonald Observatory in Texas. 

Astronomers have been watching the new planet’s star, HR 5183, since the 1990s but have yet to collect data for one full orbital period for the planet, which revolves around the star every 45 to 100 years.

To detect the exoplanet, astronomers used the radial velocity method, a process in which new worlds are found by tracking the way stars “wobble” from the gravitational pull of a planet.

Analysis of a star “wobble” could take tens or hundreds of years since data must be collected across a planet’s entire orbit around a star.

The California Planet Search, led by CalTech’s Andrew W. Howard, is one of the few projects tracking stars over the decades-long periods required under the radial velocity method.

Howard said in a statement that astronomers found the star because of its unusual orbit path.

“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,” said Howard. “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.”

Findings from the study – titled Radial Velocity of an Eccentric Jovian World Orbiting at 18AU – indicate that it is possible to use the radial velocity method to detect exoplanets without having to monitor stars over decades.

Planets typically form along flat, circular paths made up of material left over from stars’ formation.

The study proposes that the new planet formed next to a similarly-sized planet, which it eventually pushed out of the star system, giving HR 5138 b its eccentric orbital path.

Howard said the discovery challenges astronomers’ understanding of how solar systems are formed.

“Copernicus taught us that Earth is not the center of the solar system, and as we expanded into discovering other solar systems of exoplanets, we expected them to be carbon copies of our own solar system,” Howard said. “But it’s just been one surprise after another in this field.”

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