Researchers have observed an extrasolar planet, about 117 light-years from Earth, that showed the most eccentric orbit ever seen. The planet, called HD 20782, was also capable of flashing reflected starlight as it moves closer to its host star.

HD 20782’s eccentricity was measured in about .96, which means that the planet moves in a nearly flattened ellipse, different that any planet in Earth’s solar system. The gathered data also showed that the planet travels along a path far from its star and then makes a quick slingshot around it in the planet’s closest approach, as reported by

Astronomers Catch a Glimmer of Starlight from ‘Most Eccentric’ Exoplanet. Credit: Mother Board

Most of the known planets have nearly circular orbits, making the recently studied exoplanet the most eccentric yet. The eccentricity was confirmed as part of the Transit Ephemeris Refinement and Monitoring Survey (TERMS), the project which aims to study similar planet’s orbits.

At the furthest point in its orbit, HD 20782 is separated from its star by 2.5 times the distance between the sun and Earth. In its closest approach, the planet is as close as .06 the same Earth-sun distance.

“It is around the mass of Jupiter, but it is swinging around its star like it is a comet,” said Stephen Kane, a San Francisco State astronomer who led the study.

The observation of HD 20782 offers a particularly lucrative opportunity for studying the planetary atmosphere of an eccentric orbit planet, which is not seen in our solar system, Kane said. By studying the reflected light from the planet, astronomers may learn more about the structure and composition of a planetary atmosphere that can stand a close exposure to its star.

This graphic shows the orbit of the planet HD 20782 relative to the inner planets of our solar system. HD 20782’s orbit more closely resembles that of a comet, making it the most eccentric planet ever known. Image: Kane et al

A reflective atmosphere

Using a satellite-based telescope, scientists collected light data from the planet as it orbited closest to its star. The team used new parameters to time their observations and detect a change in brightness that appears to be a signal of reflected light bouncing off the planet’s atmosphere.

The reflected light may offer researchers an approach to studying how the atmosphere of a planet like HD 20782 responds when it spends most of its time far away from the star. The percentage of the reflected light or its brightness is determined in part by the composition of the atmosphere.

It is known that planets like Venus and Jupiter, shrouded in clouds full of icy particles 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 diminishing its brightness.

According to Kane, it looks like in the case of HD 20782 its atmosphere does not have a chance to respond. The time it takes to swing around the star is so quick that there is not the time to remove all the icy material or clouds that make the atmosphere so reflective, he added.