Meet HD 20782: A Highly ‘Eccentric’ Planet That May Help Scientists Understand Planetary Atmospheres

Whipping around a star nearly 117 light-years from Earth is a planet whose orbit is unlike anything ever spotted by astronomers. This planet, named HD 20782, has the most “eccentric” orbit yet seen, meaning that it moves in a nearly flattened ellipse around its parent star.

The eccentricity of a planet’s orbit is measured on a scale of 0 to 1, with 0 representing a perfectly circular orbit. Earth’s orbital eccentricity, for example, is 0.017, while Mercury, which is the most eccentric planet in our solar system, has an eccentricity of 0.205.

HD 20782, on the other hand, has an eccentricity of 0.96. At the furthest point in its orbit, HD 20782 is separated from its star by 2.5 times the distance between our Sun and Earth. At its closest approach, meanwhile, it ventures as close as 0.06 of the Earth-Sun distance — much closer than Mercury's orbit.

“It's around the mass of Jupiter, but it's swinging around its star like it's a comet,” Stephen Kane from the San Francisco State University, who led a team of researchers that made the discovery, said in a statement.

The discovery also offers “a particularly lucrative observing opportunity” for studying planetary atmospheres of an eccentric-orbit planet, the researchers said in a study describing the findings.

The percentage of light reflected from a planet is determined, at least in part, by the composition of its atmosphere. For instance, planets shrouded in clouds full of icy particles, such as Venus and Jupiter, are very reflective. However, if planets like these were to venture too close to the sun, the heat would evaporate the icy material from its clouds. So, by studying the reflected light from HD 20782, astronomers hope to learn more about the structure and composition of a planetary atmosphere that can withstand a blistering exposure to its star, albeit briefly.

The latest observation also suggests that HD 20782 may be cloaked under a highly reflective cloud cover, much like Jupiter.

“The atmosphere of the planet [HD 20782] doesn't have a chance to respond,” Kane said. “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.”

And as to what could have caused the planet’s eccentric orbit, we can only speculate, Kane added.

It could be that there was originally more than one planet in the system, and one planet, for some reason, developed an unstable orbit that brought the two planets too close together. This collision, or near-collision, may have ejected one planet from the system entirely and pushed HD 20782 on its eccentric path.

“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,” Kane said in the statement. “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.”