Eccentric planet

Eccentricity is not (only) a strange word or a weird state of mind. Talking about conics it indicates how squeezed is an ellipse. Eccentricity equal to zero means circumferences. Eccentricity equal to one means parabola, i.e. an open curve.

Talking about planetary orbits, eccentricity indicates if the orbit is a circle-like oath or if it is a very elongated trajectory.

Looking in our solar system orbital eccentricity of planets is something between 0.2056 (Mercury, the most eccentric planet, generally speaking…) and 0.007 (Venus, the most circular planet). Actually Pluto, if still considered as a plant has the highest eccentricity of 0.2056. Earth’s orbit has an eccentricity of 0.0167, i.e. its distance from the Sun remains more or less constant (allowing us to stay here during all seasons). Ah, orbital eccentricity (may) change in time…

Looking outside our solar system things are different, much different.

There are several extrasolar planets with highly elliptical (i.e. eccentric) orbits.
Golden metal to HD 20782 with an orbital eccentricity of 0.96. This means that it is almost a coincidence that its orbit is closed around its star, and the planet is not flying away in the deep space. Just as comparison, in our solar systems, comets are among the most eccentric bodies, Halley’s Comet has a value of 0.967 (indeed it is a long period comet, but still bounded to our Sun).

HD 20782 moves along an along flat eclipse passing very close and fast near its star (0.06 times the Earth-Sun distance) and then moving far away and slowing down at the furthest point (2.5 times the Sun-Earth distance).

How is it possible that the plant reached such an orbit? It is probably the consequence of a close encounter (or even a small impact) with another body. Probably an ex-double planetary system. Or is is possible that one of the two star of the system (the planet moves in a binary system) passed close to the planet making its circular orbit to deviate from its nominal, regular, calm, relaxed, path. Indeed HD 20782 is also the first known case of a binary star system with planetary systems around both stars in the system.

A Jupiter size planet travelling like a comet. And this offers a unique opportunity to study the atmosphere of such a kind of plant. Usually the extrasolar Jupiter-like planets move rather close to the star and the heat removes the icy material in its clouds making them appearing dark (and hard to be analysed from satellite-based systems). HD 20782 is different; it swings around the star so quickly that there isn’t time to remove all the icy materials. As a consequence its atmosphere is so reflective.

Exotic from our point of view, but probably much more common that expected up there.

WU