March 19, 2016

Newly-discovered planet has the most eccentric orbit ever seen

A newly discovered exoplanet has the most eccentric orbit ever observed, moving in a flattened ellipse as it travels a long path far from its sun and then quickly swinging around its host star as if it were a comet, according to research published recently in The Astrophysical Journal.

The planet in question is located approximately 117 light years from Earth and is known as HD 20782, San Francisco State University astronomer Stephen Kane and his colleagues reported in their new study. They first discovered this extrasolar world by detecting a signal of reflected light while the planet made its closest orbital approach to its star, Kane’s team said in a statement.

Further analysis of HD 20782 revealed that it has the most highly elliptical orbit ever measured by researchers, with an eccentricity rating of .96. This means that the planet moves in an almost completely flattened ellipse, then makes a high-speed, highly-energetic slingshot around its sun upon its closest approach, they added.

Image of planet's orbit superimposed onto our solar system

Here's what the planet's orbit would look like if it was in our solar system. (Credit: University of San Francisco)

The newfound exoplanet provides “a particularly lucrative observing opportunity” for studying the planetary atmosphere of an eccentric-orbit planet – a kind of world which cannot be seen in our solar system, the researchers noted. By monitoring reflected light from HD 20782, the team believes they will gain valuable insight into the structure and composition of its atmosphere and learn how and why it is able to survive a short-lived close-encounter with its star.

So how does its atmosphere survive this stellar close-encounter?

At its most distance point, HD 20782 is 2.5 times further away from its host star and the Earth is from the sun, while at its closest approach, it is closer to that star than Mercury is to the sun, with just .06 Earth-sun distance separating the extrasolar planet from its star.

“It's around the mass of Jupiter, but it's swinging around its star like it's a comet,” said Kane, an assistant professor of physics and astronomy at the university. While previous observations of the planet suggested that it had a highly eccentric orbit, Kane’s team confirmed that fact as part of their work on the Transit Ephemeris Refinement and Monitoring Survey (TERMS), a project designed to detect exoplanets as they pass in front of their host stars.

They were also able to determine the planet’s other orbital parameters, and using this data, they were able to observe the planet using space telescopes to collect light data when HD 20782 made its closest approach to its sun. These observations led to the detection of a change in brightness that appeared to be a signal of reflected light bouncing off the planet’s atmosphere, which could help them better understand how the planet’s atmosphere behaves at different times.

Most of the time, Kane explained, the planet is far away from its star, but it then as “a very close approach where it's flash-heated by the star.” If a larger exoplanet moved too close to its star, the heat would likely remove the icy particles that are found in their atmospheres. In the case of HD 20782, however, “the atmosphere of the planet doesn't have a chance to respond,” said Kane.

“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,” he added. While they have not yet been able to discern the exact composition of the planet’s atmosphere, their observations indicate that it might have an atmosphere that is similar to Jupiter and has a highly-reflective cloud cover.


Image credit: University of San Francisco