Astronomers Find Giant Exoplanet with Extreme Orbit
Using data from several ground-based telescopes, astronomers found that a nearby star called HR 5183 hosts a giant planet with an orbital period of between 45 and 100 years. The planet’s orbit is highly eccentric, shuttling it from within the orbit of Jupiter to beyond the orbit of Neptune.
An artist’s concept of a giant exoplanet. Image credit: NASA’s Goddard Space Flight Center / S. Wiessinger.
HR 5183, also known as HD 120066, is a slightly evolved G0-type star approximately 103 light-years away.
Named HR 5183b, the newly-discovered planet is roughly 3.2 times the mass of Jupiter and is about 7.7 billion years old.
“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 Caltech graduate student Sarah Blunt.
“Other planets detected far away from their stars tend to have very low eccentricities, meaning that their orbits are more circular.”
“The fact that HR 5183b has such a high eccentricity speaks to some difference in the way that it either formed or evolved relative to the other planets.”
“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 Caltech Professor Andrew 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.”
For HR 5183b to be on such an eccentric orbit, it must have gotten a gravitational kick from some other object.
The most plausible scenario is that the planet once had a neighbor of similar size. When the two planets got close enough to each other, one pushed the other out of the planetary system, forcing HR 5183b into a highly eccentric orbit.
“HR 5183b basically would have come in like a wrecking ball, knocking anything in its way out of the system,” Professor Howard said.
“This discovery demonstrates that our understanding of planets beyond our Solar System is still evolving.”
HR 5183b was discovered using the radial velocity method, a workhorse of exoplanet discovery that detects new worlds by tracking how their parent stars ‘wobble’ in response to gravitational tugs from those planets.
However, analyses of these data usually require observations taken over a planet’s entire orbital period. For planets orbiting far from their stars, this can be difficult: a full orbit can take tens or even hundreds of years.
The astronomers used data from the High-Resolution Echelle Spectrometer (HIRES) on the Keck I telescope at W. M. Keck Observatory in Hawaii, the Lick Observatory in Northern California and the McDonald Observatory in Texas.
“The key was persistence. Our team followed this star with Keck Observatory for more than two decades and only saw evidence for the planet in the past couple years! Without that long-term effort, we never would have found this planet,” Professor Howard said.
The study will be published in the Astronomical Journal.
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Sarah Blunt et al. 2019. Radial Velocity Discovery of an Eccentric Jovian World Orbiting at 18 AU. AJ, in press; arXiv: 1908.09925