The elliptical orbit of a newly discovered exoplanet is shown in relation to Earth’s solar system. Image: W. M. Keck Observatory/Adam Makarenko

Astronomers have discovered an exoplanet three times more massive than Jupiter that circles its star in an unusual elliptical orbit, one that would carry it between the Sun’s asteroid belt and the orbit of Neptune if it were transported to Earth’s solar system. It is the most extreme such exoplanet orbit yet found.

“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,” Sarah Blunt, a Caltech graduate student and first author of a study accepted by The Astronomical Journal.

“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.”

The planet orbits a star known as HR 5183. It was discovered by the radial velocity method, that is, by spectroscopically studying the subtle movement of a star as it is tugged back and forth by an orbiting planet’s gravity. Normally, astronomers need observations over an entire orbit to collect the necessary data.

That was not possible with the newly discovered HR 5183 b because it takes more than 45 years to complete one trip around its star. But the California Planet Search project, combining years of data from the W.M. Keck Observatory in Hawaii, the Lick Observatory in California and the McDonald Observatory in Texas, managed to trace the exoplanet’s path by studying the host star’s wobble as the giant world passed through the low point of its orbit.

“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,” Andrew W. Howard, a Caltech astronomer who leads the California Planet Search. “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.”

Planets are thought to form in rotating discs of dusty debris left over from star formation. Such worlds should start off in relatively flat, circular orbits. HR 5183 b’s unusual orbit may have been caused by a close encounter with another large planet that was then ejected from the infant solar system.

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

“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,” he added. “But it’s just been one surprise after another in this field.”