As our spinning globe of rock and metal tracks its steady path around the Sun, we find ourselves crossing once again through the winter solstice, the point at which Earth's northern pole is pointed as far from our fierce stellar parent as it can be (this year at a coordinated universal time of 5.30 am on December the 22nd, almost the same as 5.30 am Greenwich Mean Time). The chill that this brings to the northern hemisphere can make it seem a little confusing that a mere 12 days later, on January 3rd, the Earth also passes through its closest approach to the Sun (its periapsis point).
Why do we not get hot and bothered by this passage? The answer is that the Earth's orbit is very close to circular, and so periapsis is only about 3% closer to the Sun than the furthest point in our great elliptical orbital loop. The resulting increase in solar radiation (a few percent) does toast the planet a little more, but locally it's a small effect compared to that of our planetary tilt. Thus, for those of us in the north our closest passage to a star is still a bone-chilling affair.
Elsewhere in the universe this is definitely not always the case. So, while you're sipping hot cocoa, or flinging innocent antipodean crustaceans on the barbeque, spare a thought for a planet less fortunate than your own.
HD 80606b: a serious case of repeat sunburn
A mere 190 light years away, in the direction of the Big Dipper is a gas-giant world, some four times the mass of Jupiter. Every 55.5 days this planet switches between a moderate, temperate, distance from its ordinary, Sun-like, parent star to a ridiculously scorching place that receives more than 800 times as much stellar radiation. Welcome to HD 80606b, a planet with an orbital eccentricity of 0.93 and a closest approach to its star that is a mere 1/29th of its furthest distance - a startlingly near pass of only seven times the radius of the star.
As a result, HD 80606b experiences a rise in its outer atmospheric temperature of over 700 degrees Celsius in the space of 6 hours as it rockets through its periapsis with the star. We know this because, in a stroke of statistical luck, HD 80606b both transits its stellar parent (passing directly between us and the stellar disk) and is later eclipsed by the star immediately after its closest approach. The latter event allowing astronomers to catch a glimpse of the infrared radiation pouring off the planet as it dips its toes into the realm of Hades.
Subjecting a planet to this kind of brutal heating likely results in some spectacular atmospheric changes, with great hotspots driving colossal winds. Hydrodynamical simulations give us some idea of what might be going on.
Visualization of HD 80606b before, during, and after periapsis (NASA/JPL-Caltech/G. Laughlin et al.)
This movie shows HD 80606b from our Earthbound point of view. At first the planet has not yet passed through its periapsis, the blue color is light from the star reflected towards us. Four seconds into the animation is the closest approach, and now the red glow is the infrared radiation emitted from the planet itself - the equivalent of a peal of thermal thunder. As time passes, all we can see is the night side of the planet as it again climbs away from the star, but its atmosphere is still hot, roiling, and flowing from the massive radiation punch of its close encounter.
In this case nature has provided us with a wonderful experimental apparatus for understanding the effect of extreme variations of stellar radiation on a planetary atmosphere. Every 111 days (55.5 days out, 55.5 days back) HD 80606b goes through the same brutal experience, providing another opportunity for us to observe and refine our models.
While this is a truly extraordinary system, we expect that many of the "Earth-like" worlds on the cusp of being discovered, will have orbits that are significantly more elliptical than that of our homeworld. Our tentative investigations of what this would imply for life on such planets (for example, work by Dressing, Spiegel, Menou, Raymond and myself) yield some surprising answers. Quite strongly elliptical orbits (although much less so than HD 80606b) need not render an Earth-like, rocky, wet, world uninhabitable. While certain locations on such planets might indeed become uncomfortably hot or cold during the course of a year, the so-called thermal-inertia (the reticence of land or ocean to give up or absorb heat quickly) may carry these worlds through the worst times - smoothing out the extremes.
So, as we pass through our rather mild solstice and periapsis, there may well be another place, out there somewhere, bracing itself for its most appalling season, with only the comfort of the thought that better days are on the way.