Exoplanet's orbit is the most eccentric yet

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An artist's impression of HD 20782, the orbit of which is extremely eccentric, being more like that of a comet than most planets(Credit: NASA)

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Astronomers at San Francisco State University (SF State) have observed an exoplanet located just 117 light-years from Earth, which exhibits the most eccentric orbit yet found. The light reflected as the planet passed close to its parent star is providing researchers with clues as to the make up of the body's atmosphere, which is thought to be similar to that of Jupiter.

All of the planets in our home solar system have almost entirely circular orbits around the Sun, but some distant planets take a much less ordinary route around their own stars. Known as HD 20782, the exoplanet observed by the SF State astronomers has the single most eccentric orbit known, travelling along an almost flat path away from and towards its star, which it passes at extremely high speeds.

At its farthest point, HD 20782 is around two and a half times the distance from its star than the space between Earth and the Sun. But it was at its closest point, when it passes much closer to its star than the orbit of Mercury around the Sun, that the astronomers took their chance to take a peek at the object. Timing their observations, the scientists were able to watch the brightness of the exoplanet change as light from the star reflected off its atmosphere.

Planets with thick layers of clouds containing large amounts of icy particles, such as Venus or Jupiter, are usually very reflective. However, if they were to pass closer to a star, the heat would melt the icy particles and the planet would appear much darker when observed.

So, why doesn't HD 20782 dim as it passes by its parent star? Well, astronomers theorize that the high speed of the planet as it sling-shots around the star means that not all of the icy material has time to be removed, giving it the opportunity to reflect light out into space.

The observations provided a lot of new information about the planet, but the team wasn't able to work out the exact makeup of its atmosphere. At this point, they believe that the environment is probably similar to that of Jupiter's atmosphere, with thick, highly reflective cloud cover.

Similarly, the astronomers aren't exactly sure why the planet travels along such an eccentric orbit. Possible theories are that HD 20782 collided with another object and was ejected out onto its current path, or that the second star in the system – it being a binary system – might have passed close to the planet and thrown it out of an original, more circular orbit.

The findings of the research are published online in The Astrophysical Journal.

Source: SF State

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