Astronomers Catch a Glimmer of Starlight from 'Most Eccentric' Exoplanet

Life on Earth has a whole lot of things going for it and many of those things have to do with our planet's proximity to the Sun. We're not close enough to cook but we're also not far enough away to freeze. When astronomers hunt for habitable zone planets or Earth-like planets, some large part of those classifications have to do with this magic distance, in which temperatures allow for the existence of liquid water.

Proximity is more than just a raw distance from here to there. That distance also has to remain reasonably constant. We orbit the Sun in an ellipse, but that bit of orbital "squish" isn't enough to really mess things up for us. Earth's perihelion (point closest to Sun) brings us within about 91,400,000 miles of the Sun, while its aphelion (point farthest from Sun) finds us some 94,500,000 miles from Sun. NBD.

It could be much, much worse. Take, for example, the planet HD 20782 b, an odd rock about 177 light years away that boasts the most eccentric orbit known to astronomers. With an eccentricity of .96, the orbit of HD 20782 b is a nearly flattened ellipse. At its farthest point, the planet orbits at about 2.5 times the distance from the Earth to the Sun, while its nearest pass has the planet at .06 percent of this distance. It's really more of a planet yo-yo.

With help from the satellite-based MOST telescope, Astronomers from San Francisco State University have made an intriguing new observation of HD 20782 b: the flicker of reflected starlight as the planet makes its closest-in pass. Their work is described in the Astrophysical Journal, with a preprint version available at arXiv.

The eccentricity of HD 20782 b has been documented several times since its discovery in 2006, but, as the authors behind the current paper explain, gathering further information about the planet has been difficult thanks to its only slight changes in radial velocity. (Observing distant planets by way of their varying radial velocities, aka Doppler spectroscopy, is a crucial tool/concept in the planet-hunting toolkit.) Snagging just a bit of reflected light opens up a whole new world of observational potential.

The reflection offers a potential way into questions about HD 20782 b, but also highly eccentric planets in general: What kind of atmosphere can even form in conditions like that? What happens to a planet that spends most of its life frozen with brief periods of flash cooking? How does a planet get knocked so far out of whack in the first place?

And so the paper concludes: "A deeper understanding of the orbits and atmospheres of eccentric planets are key milestones towards unlocking the origin and nature of these mysterious objects."