Bad Astronomy

Weather sizzles on a planet that kisses its star

Still suffering from the heat, the planet reaches the peak of its orbit, momentum carrying the bizarre world far, far from its star.

From that distance the star, slightly less massive and cooler than our own Sun, would gently heat the planet, giving it an almost Earthlike temperature. And during the days and weeks it spends out there, the planet cools. But it’s not enough. It’s never enough.

HD 80606b, the Icarus planet.

The planet has pulled away as far from the star as it can, but gravity cannot be denied. Slowly, inexorably, it begins the long fall back. Growing ever-larger as the planet draws ever-nearer, the star swells hugely. Just 55 days after reaching its farthest distance from the star, the planet drops hellishly close to the star’s surface, and a furnace blast of heat bears down on it. The planet’s atmosphere roils and churns, reaching temperatures hot enough to melt copper.

Accelerated by the star’s gravity to a speed hundreds of times faster than a rifle bullet, the planet whips around the star and begins the long climb back. It will cool down as it puts over 100 million kilometers between itself and the fierce light of the star… but the cycle will repeat itself, and the planet will burn once again.

orbit diagram of HD 80606b
The orbit of HD 806060b. The small
circles represent 1 hour intervals,
the four positions separated
by 19 hours each.
Such is the life of HD 80606b, a gas giant planet four times the mass of Jupiter that orbits a star 190 light years from Earth. The planet’s orbit is incredibly elliptical, with a whopping eccentricity value of 0.927 – meaning the orbit is elongated like a rubber band being fought over by jealous children. It’s posited that gravitational interaction over time with a distant binary stellar companion to the star may have forced the orbit into this shape; it peaks at a distance of 125 million kilometers (75 million miles) from the star, but the planet’s 111 day orbit drops it to a mere 4 million kilometers (2.4 million miles) from the star’s surface. In the 55 days it takes to drop, it sees the disc of the star swell to 30 times its previous size, flooding the planet with nearly 800 times the amount of heat it felt at greatest distance.

During the close approach in late November, 2007, astronomers used the Spitzer Space Telescope to observe the planet. They couldn’t resolve the planet; in fact they couldn’t even see it at all. All they could detect was the slight increase in infrared light emitted by the warming planet adding to the star’s light; a tiny fraction to be sure, but detectable by Spitzer. In just six hours, the planet’s upper atmosphere heated from about 500 C to over 1200 C (980 to 2,240 degrees Fahrenheit).

Using very sophisticated computer models, the scientists could make an image representing what the planet’s upper atmosphere would look like after its dip into hell. The image above is a simulation, a computer model of this Icarus planet 4.4 days after closest encounter. If you had infrared eyes and hovered over the planet, it might look something like this.

The blue light is the day side of the planet, lit by reflected light of the star. Most of the planet we see here is facing away from the star, so it’s night. But it’s hardly dark: the red glow is actually the heat from a massive storm, blasted into life by the intense energy absorbed during the close passage. It’s almost an explosion, with supersonic winds whipping air at 5 kilometers (3 miles) per second from the day side around to the night side. The planet’s rotation causes a Coriolis effect which wraps the storm into curls as it screams away from the heat of the star.

Think on it: this map shows violent weather on a distant planet we’ve never actually seen.

While this isn’t a real image of the planet, it represents a huge step in our understanding of these hot gas giants which orbit so close to their stars. Remember, that’s not an artist’s drawing; it’s the output of a computer model of the atmospherics of the planet. The observations using Spitzer give astronomers critical information on how much the planet is heated, and how. Armed with that information, and the ability to understand how atmospheres behave, they can generate models like these.

While no other planet we know of makes such an extreme path around its star, there are plenty of other planets out there to observe. And we may find another bizarre world like this one; in fact, it’s almost certain we’ll find many. We’ve only detected the presence of about 300 planets, and there must be tens or even hundreds of billions of planets orbiting stars in the Milky Way.

I wonder: what’s out there that will make this planet seem almost normal by comparison?

Planet image credit: D. Kasen, J. Langton, and G. Laughlin (UCSC). Orbital diagram from their paper in Astrophysical Journal.