Two massive gas giants are locked in an unusual orbital danceS

Around the gigantic, dying star HD 200964, two gas giants bigger than Jupiter are locked in an orbital dance, closer together than any two planets of their size ever should be, hanging on only because their movements are perfectly synchronized.

HD 200964 is a massive star located about 223 light-years away. It is home to two gas giants that were detected using the usual method for locating extrasolar planets, by measuring the wobble in HD 200964's orbit. What's unusual is the nature of that wobble tells us the two planets are incredibly close together, roughly the same distance separating Earth and Mars.

That sort of distance is no problem for two small, rocky planets, but when we're talking about planets that make Jupiter look puny? Well, then things get complicated. The gravitational force each planet exerts on the other is unimaginably strong - 3 million times greater than what Mars and Earth place on each other, 700 times stronger than the Earth's exertion on the Moon, and even four times greater than the Sun's gravitational impact on the Earth.

One of the discoverers of this system, astronomer Eric Ford of the University of Florida, explains why this is so strange:

"A planetary system with such closely spaced giant planets would be destroyed quickly if the planets weren't doing such a well synchronized dance. This makes it a real puzzle how the planets could have found their rhythm."

The astronomers say the planets must be in a very unusual orbital resonance. An orbital resonance is where the length of time it takes for two planets to order the sun is described by the ratio of two small integers. The most common is a 2:1 resonance, in which for every one time the more distant planet revolves around its star, the closer planet orbits twice.

Caltech astronomer John Johnson explains how these resonances can change:

"Planets tend to get stuck in the 2:1. It's like a really big pothole. But if a planet is moving very fast it can pass over a 2:1. As it moves in closer, the next step is a 5:3, then a 3:2, and then a 4:3."

The two planets orbiting HD 200964 are locked in a 4:3 orbital resonance, which is completely unheard of for objects of their size. The next biggest pair of objects locked in a 4:3 resonance are a pair of Saturn's moons, as Ford notes:

"The closest analogy in our solar system is Titan and Hyperion, two moons of Saturn which also follow orbits synchronized in a 4:3 pattern. But the planets orbiting HD 200964 interact much more strongly, since each is around 20,000 times more massive than Titan and Hyperion combined."

Ultimately, Johnson chalks this mystery up to the unending strangeness of exoplanets:

"This is the tightest system that's ever been discovered, and we're at a loss to explain why this happened. This is the latest in a long line of strange discoveries about extrasolar planets, and it shows that exoplanets continuously have this ability to surprise us. Each time we think we can explain them, something else comes along."

[Astronomical Journal. Since there aren't any drawings of this planetary system yet, the image up top is an artist's conception of a different extrasolar planet in Star System CoKu Tau 4.]