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Exoplanet With Most Eccentric Orbit Flashes With Reflected Light

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The planet Venus is one of the brightest objects we see at night because its thick clouds reflect the sunlight that reaches it. The planet's orbit is nearly circular just like that of the rest of the planets, so it's still quite "normal."

Now, scientists have detected an exoplanet with the most "eccentric" orbit about 117 light-years away from Earth. At the same time, the exoplanet is capable of flashing reflected starlight as it moves closer to its host star.

Meet HD 20782

Astronomer Stephen Kane led the team that detected exoplanet or extrasolar planet HD 20782. Its orbit is eccentric because it is highly elliptical, unlike that of the planets in our solar system.

In fact, the exoplanet revolves in an almost flattened ellipse, moving a long path from its host star. The exoplanet then makes a quick slingshot around the star at its closest approach, researchers said.

Orbits similar to that of HD 20782 are not seen in our own solar system, so the exoplanet's detection provides a "particularly lucrative" opportunity to study its planetary atmosphere.

In particular, astronomers may learn more about the structure and composition of a planetary atmosphere that can endure a brief but blistering exposure to its host star.

Swinging Around Like A Comet

At the furthest point in its elliptical orbit, Exoplanet HD 20782 is separated from its star by 2.5 times the distance between our planet and the sun.

But when HD 20782 is closest to its star, it ventures as much as 0.06 of the same Earth-sun distance, making it much closer than when Mercury orbits the Sun, researchers said.

"It's around the mass of Jupiter, but it's swinging around its star like it's a comet," said Kane, who is a professor of astronomy and physics at San Francisco State University.

Reflective Atmospheres

Kane and his team were able to confirm the exoplanet's eccentric orbit as part of a project called the Transit Ephemeris Refinement and Monitoring Survey (TERMS). This was designed to detect exoplanets as they pass in front of their stars.

Scientists applied these new parameters to time their observations, while using a satellite-based telescope to collect light data from the planet as it orbits closest to its star. They detected a change in luminosity, which appears to be a signal of reflected light bouncing off the exoplanet's atmosphere.

Kane said the reflected light could tell astronomers more about how the atmosphere of an exoplanet like HD 20782 responds when it spends most of its time away from its star and then has a very close approach, where the planet is flash-heated by the star - a sudden burst of light and heat.

How bright a star appears in the sky is determined partly by its atmosphere's composition. For instance, Jupiter and Venus are shrouded in clouds full of icy particles and are very reflective. But if Jupiter were able to move too close to the sun, the heat would remove the icy material.

Kane said that in some exoplanets that are the same size as Jupiter and tread short, circular orbits, the phenomenon does appear to strip the atmosphere of reflective material, rendering it to appear dark.

In the case of HD 20782, however, the planet's atmosphere does not have an opportunity to respond.

"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," said Kane.

Meanwhile, scientists have yet to determine the exact makeup of HD 20782's atmosphere, but their observations suggest that the exoplanet might have an atmosphere with highly-reflective cloud cover.

Exoplanets such as HD 20782 also brings a trove of questions for astronomers, Kane said. He compares it to people who examine blood spatter patterns on the walls of a murder scene.

"You know something bad has happened, but you need to figure out what it was that caused it," added Kane.

The team's findings are featured in the Astrophysical Journal.

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