Every planet in our solar system journeys around the Sun in a circular orbit. But that’s not the case for at least one planet outside our solar system. Exoplanet HD 20782 b has the most eccentric orbit astronomers have ever seen. Its orbit resembles more a comet than a planet.
When astronomers observed its orbit, they found an eccentricity measurement of 0.96. HD 20782 b’s orbit is almost a flat ellipse. Its furthest orbit point takes it 2.5 times the distance between the sun and Earth. But at its closest point, the planet slingshots around the star at just .06 of that same distance. Much closer than Mercury orbits the sun.
Here’s a graphic showing the orbit compared to planets in our solar system.
The exoplanet, located 117 light-years away, takes about 597 days to orbit its sun.
You would think the planet’s orbit is the craziest piece of news. Nope. This exoplanet also boasts an atmosphere despite its close encounter with a scorching sun along its orbit.
San Francisco State University astronomer Stephen Kane, along with his colleagues, observed a bright ‘flash’ of starlight bouncing off the exoplanet’s atmosphere. This ‘flash’ could help astronomers figure out what kind of atmosphere can survive “a very close approach where it’s flash-heated by the star,” Kane said.
The ‘flash’ seen from HD 20782 b hints at an atmosphere similar to Jupiter. A planet covered in highly reflective clouds.
Why HD 20782 b’s atmosphere can survive
The eccentric orbit is key. If you took HD 20782 b and placed it into a close, circular orbit around its star, the heat would remove the icy material, and the planet would appear ‘dark.’ Astronomers have seen this happen in Jupiter-sized planets with close orbits.
The exoplanet’s orbit brings it perilously close to its sun. But because it’s orbiting so fast, the potential ice atmosphere doesn’t melt completely. “The time it takes to swing around the star is so quick that there isn’t time to remove all the icy materials that make the atmosphere so reflective,” says Kane.
As for what HD 20782 b’s atmosphere is made of? That remains a mystery. And it’s not the only one. Kane explains:
“When we see a planet like this that is in an eccentric orbit, it can be really hard to try and explain how it got that way,” he explained. “It’s kind of like looking at a murder scene, like those people who examine blood spatter patterns on the walls. You know something bad has happened, but you need to figure out what it was that caused it.”
That doesn’t stop Kane and company from theorizing.
One theory suggests the system used to have more than one planet. This other planet’s unstable orbit could have brought the two dangerously close to one another. And ejected one planet from the system while pushing HD 20782 b into its new eccentric orbit.
Another theory points to a binary system. A second star could have pushed HD 20782 b into its unique orbit. Right now, astronomers can only explain its orbit with the help of another body – planet or star.
“A deeper understanding of the orbits and atmospheres of eccentric planets are key milestones towards unlocking the origin and nature of these mysterious objects,” the authors write in the study.
That ‘deeper understanding’ could come later this year. Kane and his colleagues encourage follow-up observations of HD 20782 b on (or around) September 3, 2016 and April 23, 2018.