Astronomers Uncover Another Mystery of the Universe’s Most Eccentric Planet

March 30, 2016 | Joanne Kennell

Artist's rendering of planet HD 20782 passing close to its star
Photo credit: NASA

“The notion that a giant planet could exist in such extreme conditions is difficult to imagine.”

Exoplanet HD 20782 is known as the universe’s most eccentric planet. It is located about 117 light-years away and was first discovered in 2006. However, a new study released online and published in The Astrophysical Journal, suggests that it’s even more extraordinary than previously thought.

The planet not only looks similar to a comet, but it also orbits its host star like one too, Dr. Stephen Kane, an astronomer at San Francisco State University and lead author of the study, told The Huffington Post (HP).

In terms of planets, the term “eccentric” does not refer to a state of mind. Rather, it describes how elliptical a planet’s orbit is around its star.

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“Most planets, including those in our own solar system, have orbits around their star that are shaped like a circle,” Kane said to The HP. However, HD 20782’s 597-day orbit is extremely unique — like a slingshot.

HD 20782 has the most eccentric orbit known, measuring at an eccentricity of 0.96. This means that the planet moves in a nearly flattened ellipse.

“This planet is about the size of Jupiter but has an orbit like an ellipse, very similar to what we see for comets. That means that the planet spends most of its time far away from the star, but once per orbit it swings around the star very closely and almost touches the surface,” Kane explained to The HP.

the orbit of the planet HD 20782 relative to the inner planets of our solar system

This graphic shows the orbit of the planet HD 20782 relative to the inner planets of our solar system.  Photo credit: San Francisco State University

But researchers are not sure how the planet ended up in such a bizarre orbit. It could have been affected by the gravity of, or collision with, another planet that was once in the star system, or maybe the second star found within this binary system got close to the planet, throwing it off course. Nevertheless, Kane was shocked to discover that the planet has such an elliptical orbit.

“The notion that a giant planet could exist in such extreme conditions is difficult to imagine,” Kane told The HP. “Myself and my collaborators spent several years studying the object further to find out if the situation is as extreme as we initially thought.”

At its furthest point, the planet is separated by its star by 2.5 times the distance between the sun and Earth, and at its closest approach, it gets about 0.06 of that same Earth-sun distance — much closer than Mercury orbits the sun.

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What’s more, Kane and his colleagues were also able to detect a “flash,” or signal of reflected light, from the planet’s atmosphere as it approached its star. “What we found is that, not only is the orbit the most eccentric ever found, the planet shows strong evidence of flashing reflected starlight at us whenever it makes its closest approach to the star,” Kane said to The HP. “It is a fascinating object!”

Scientists used a satellite-based telescope to collect the light data from the planet. The percentage of light reflected from a planet, or how bright it appears, is determined by the composition of its atmosphere. Planets covered in clouds full of icy particles, like Venus and Jupiter, are very reflective. However, if a planet like Jupiter was to move too close to the sun, the heat would remove the icy materials in the clouds.

HD 20782 gives astronomers the ability to study the atmosphere of such an eccentric planet — a type not seen in our own solar system. By studying the reflected light, they may be able to learn about the structure and composition of the planetary atmosphere capable of withstanding the brief but immense heat of its star.

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