An artist’s rendering shows the planet HD 20782, the most eccentric planet ever known, passing its star in close orbit. Credit: NASA
Extrasolar planet swings around its star like it’s a comet. Led by SF State Uni astronomer Stephen Kane, a team has spotted an extrasolar planet about 117 light-years from earth that boasts the most eccentric orbit yet seen. They detected a signal of reflected light from the planet HD 20782 – a “flash” of starlight bouncing off the eccentric planet’s atmosphere as it made its closest orbital approach to its star.
In this case, “eccentric” refers to how elliptical a planet’s orbit is around its star. While the planets in our solar system have nearly circular orbits, astronomers have discovered several extrasolar planets with highly elliptical or eccentric orbits. HD 20782 has the most eccentric orbit known, measured at an eccentricity of .96. This means that the planet moves in a nearly flattened ellipse, traveling a long path far from its star and then making a fast and furious slingshot around the star at its closest approach.
HD 20782 offers “a particularly lucrative observing opportunity” for studying the planetary atmosphere of an eccentric-orbit planet – a type not seen in our own solar system. By studying the reflected light from HD 20782, astronomers may learn more about the structure and composition of a planetary atmosphere that can withstand a brief but blistering exposure to its star.
At the furthest point in its orbit, the planet is separated from its star by 2.5 times the distance between the sun and Earth. At its closest approach, it ventures as close as .06 of that same Earth-sun distance – much closer than Mercury orbits the sun. “It’s around the mass of Jupiter, but it’s swinging around its star like it’s a comet.”
Kane et al confirmed its extreme eccentricity and the rest of its orbital parameters as part of the Transit Ephemeris Refinement and Monitoring Survey (TERMS), that detects extrasolar planets as they pass in front of their stars. Using these new parameters to time their observations, the scientists also used a satellite-based telescope to collect light data from the planet as it orbited closest to its star. They were able to detect a change in brightness that appears to be a signal of reflected light bouncing off the planet’s atmosphere.
The percentage of light reflected from a planet, or how bright it appears in the sky, is determined in part by the composition of its atmosphere. Planets shrouded in clouds full of icy particles, like Venus and Jupiter, for instance, 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. In some of the extrasolar, Jupiter-sized planets that tread short, circular orbits this strips the atmospheres of reflective particles, making the planets appear “dark.” But in the case of HD 20782, “the atmosphere of the planet doesn’t have a chance to respond,” he said. “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.”
This newest observation does suggest that it might have an atmosphere with Jupiter-like, highly reflective cloud cover. It could be that there was originally more than one planet in the system, and one planet developed an unstable orbit that brought the two planets too close together. This collision or near-collision might have ejected a planet from the system entirely and pushed HD 20782 on its eccentric path. The planet is in a binary star system, so it might also be the case that the second star in the binary made a close approach that threw HD 20782 off a more circular orbit.
Kane is a member of the science team for two upcoming satellite missions — NASA’s Transiting Exoplanet Survey Satellite (TESS) and the European Space Agency’s Characterizing ExOPLanet Satellite (CHEOPS) — that will have HD 20782 in their sights after they launch in 2018. http://news.sfsu.edu/news-story/most-eccentric-planet-known-flashes-astronomers-reflected-light
Recent Comments