HD 20782b’s orbit closely resembles that of a comet, making it the most eccentric planet ever known.

According to a team of astronomers led by Dr. Stephen Kane of San Francisco State University, HD 20782b is a giant exoplanet located 117 light-years away in the constellation of Fornax.

The planet has a minimum mass twice that of Jupiter and orbits a star known as HD 20782, which is part of a wide binary star system with HD 20781.

HD 20782b has an orbital period of 597 days and an eccentricity of 0.96. This means that the planet moves in a nearly flattened ellipse, traveling a long path far from HD 20782 and then making a fast and furious slingshot around the star at its closest approach.

“At the furthest point in its orbit, HD 20782b is separated from its star by 2.5 times the distance between the sun and Earth,” Dr. Kane explained.

“At its closest approach, it ventures as close as 0.06 of that same Earth-Sun distance – much closer than Mercury orbits the Sun.”

He and co-authors were able to detect a signal of reflected light from HD 20782b – a ‘flash’ of starlight bouncing off its atmosphere as it made its closest orbital approach to its parent star.

“The reflected light could tell researchers more about how the atmosphere of a planet like HD 20782b responds when it spends most of its time far away from its star, but then has a very close approach where it’s flash-heated by the star,” Dr. Kane said.

The percentage of light reflected from a planet is determined in part by the composition of its atmosphere.

Planets shrouded in clouds full of icy particles, like Venus and Jupiter, for instance, are very reflective.

But if a planet like Jupiter were to move too close to the Sun, the heat would remove the icy material in its clouds.

“In some of the extrasolar, Jupiter-sized planets that tread short, circular orbits, this phenomenon does appear to strip the atmospheres of reflective particles, making the planets appear ‘dark.’ But in the case of HD 20782b, the atmosphere of the planet doesn’t have a chance to respond,” Dr. 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.”

HD 20782b might have an atmosphere with Jupiter-like, highly reflective cloud cover, according to the team.

“Extrasolar planets like HD 20782b contain a wealth of questions for astronomers,” Dr. Kane said.

“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.”

“It could be that there was originally more than one planet in the planetary system, and one planet developed an unstable orbit that brought the two planets too close together,” he said.

“This collision or near-collision might have ejected one planet from the system entirely and pushed HD 20782b on its eccentric path.”

“It might also be the case that HD 20781, the second star in the binary system, made a close approach that threw HD 20782b off a more circular orbit.”

HD 20782b is the subject of a study accepted for publication in the Astrophysical Journal (arXiv.org preprint).

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Stephen R. Kane et al. 2016. Evidence for Reflected Light from the Most Eccentric Exoplanet Known. ApJ, accepted for publication; arXiv: 1511.08679