Astronomers have found what is believed to be an extrasolar planet with the most eccentric orbit yet discovered. The planet, known as HD 20782, is about 117 light-years from Earth.
What's even more remarkable about the discovery is that researchers led by San Francisco State University astronomer Stephen Kane detected a "flash" of starlight bouncing, or reflecting, 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. With an eccentricity of .96, HD 20782 has the most eccentric orbit known.
In other words, HD 20782 orbits 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. 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. However, on its closest approach, the planet ventures as close as 0.06 of that same Earth-sun distance. This, Kane says, is 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 added.
By studying the reflected light from HD 20782, astronomers hope to learn more about the structure and composition of a planetary atmosphere that can withstand such a brief but intense exposure to its star, after spending most of its time so far away.
Using a satellite telescope, researchers collected light data from HD 20782 as it orbited closest to its star. This revealed a change in brightness that appears to be a signal of reflected light bouncing off the planet's atmosphere. Researchers can then determine the composition of the planet's atmosphere based on the percentage of light reflecting, or more simply, how bright it appears in the sky.
For instance, planets blanketed in clouds full of icy particles, like Venus and Jupiter, are very reflective. However, if a planet like Jupiter were to move too close to the sun, the heat would strip its atmospheres of reflective particles, making it appear "dark."
In terms of HD 20782, "the atmosphere of the planet doesn't have a chance to respond," Kane 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."
While the exact makeup of HD 20782's atmosphere is not yet known, the latest study suggests that it might have an atmosphere with Jupiter-like, highly reflective cloud cover. "When we see a planet like this that is in an eccentric orbit, it can be really hard to try and explain how it got that way," Kane explained. "It's kind of like looking at a murder scene, like those people who examine blood spatter patterns on the walls. You know something bad has happened, but you need to figure out what it was that caused it."
Their findings were recently published in The Astrophysical Journal.
