Bizarre Planet Acts Like A Comet

Artist's impression of HD 20782b as it passed by its star. NASA

Astronomers have detected a flash of light coming from the close encounter between the most eccentric known planet and its parent star. Eccentricity, to astronomers, is measured not by odd behavior, but how elongated an orbit a planet has. In the case of HD 20782b, that is very elongated indeed, making it the most eccentric planetary orbit known.

A planet with a perfectly circular orbit has an eccentricity of zero, while an infinitely stretched orbit is defined as 1. Earth's eccentricity is just 0.016, and in the Solar System, only the dwarf planets beyond Neptune are highly eccentric.

HD 20782b, on the other hand, has an eccentricity of 0.96, completing an orbit every 1.6 Earth years. If it orbited the Sun its path would take it from well past Mars to far inside Mercury, with Helliconian-like effects on the climate. "It's around the mass of Jupiter, but it's swinging around its star like it's a comet,” said Dr. Stephen Kane of San Francisco State University in a statement.

In The Astrophysical Journal (preprint at arXiv.org) Kane reported on a change in the light seen from HD 20782, the parental star. The extra brightness occurred while the planet was at its closest point to HD 20782, suggesting it is a result of increased reflection. HD 20782b's mass, at least double Jupiter's, makes it a gas giant, implying a surface large enough to reflect a lot of light.

HD 20782, located 117 light-years from Earth, is a very stable star, older than the Sun and almost identical in mass, so the extra light is unlikely to be a result of any internal variability.

Just detecting this reflected light is a considerable achievement, but Kane hopes to learn about the composition of HD 20782b's upper atmosphere using what we can collect.

HD 20782b's orbit compared to the inner Solar System. Stephen Kane

Details on the chemical composition will have to wait for observations taken using more powerful telescopes during future HD 20782b close approaches this September or April 2018. Nevertheless, Kane is confident the scale of the increase indicates a highly reflective atmosphere.

Most “hot Jupiters” – gas giants orbiting very close to their stars – appear to be quite dark, probably because they lack the ice crystals that make more distant gas giants reflective. HD 20782b spends so much of its time being cold that Kane said when it does approach its star, “The atmosphere of the planet doesn't have a chance to respond. 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”

So far, however, observations of HD 20782b have not helped answer one question that has long troubled astronomers – how do planets get into such eccentric orbits?

HD 20782 is an interesting object in its own right. Part of the only binary star system in which each star is known to have planets, it has a suspiciously high metal concentration, which astronomers suspect is a result of having swallowed several rocky planets whose orbits got disrupted by HD 20782b.

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