Oddball exoplanet found on an eccentric, egg-shaped orbit
We tend to assume that planets orbit stars on more-or-less circular paths, but that’s not always the case – even Earth swings into a more oval orbit every now and then. But that’s got nothing on HR 5183 b, a gas giant exoplanet that astronomers have now found to be on an extremely eccentric, egg-shaped orbit.
Weighing in at about three times the mass of Jupiter, HR 5183 b circles its star once every 45 to 100 years. But it’s the path it takes that makes it such an oddball – if it was in our own solar system, it would get as close to the Sun as the asteroid belt, just past Mars, before whipping out beyond the orbit of Neptune.
"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," says Sarah Blunt, first author of the study. "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 team says that the most plausible scenario is that the planet inherited this eccentricity from a long-lost sibling. Basically, for it to get this out of whack it must have been given a gravitational kick by another planet of about the same size. But despite their best efforts, the astronomers couldn’t locate any such planet, leading them to believe that it must have been ejected out of the system.
The new world was discovered by the California Planet Search project, using data gathered by the Lick, Keck and McDonald Observatories since the 1990s. It was found using the radial velocity method, which watches stars closely and looks for the telltale “wobbles” that indicate an orbiting planet is tugging on it.
Unfortunately this kind of observation usually takes decades to notice a pattern, particularly if the planet’s orbit is long. Taking up to a century to complete one trip, HR 5183 b might have gone unnoticed – but luckily its extreme eccentricity sped up the process.
"This planet spends most of its time loitering in the outer part of the solar system in this highly eccentric orbit, then it starts to accelerate in and does a slingshot around its star," says Andrew Howard, leader of the California Planet Search and an author of the study. "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 research was published in the Astronomical Journal. An animation of the odd orbit can be seen below.