Resonant Constellation

There’s a new smallest exoplanet in town, COROT-Exo-7b. Even better, it’s a transiting exoplanet (a type of planet outside of our solar system that moves across the disk of their host star from our point of view to block some light), a special type of object that I actually know quite a bit about (I’ve been on two projects observing these).

This is actually quite a lucky find, it is difficult to impossible to find out much about most extrasolar planets (which don’t transit their host stars). Since the technology to directly image them hasn’t really developed yet (notable exceptions are the recently imaged Fomalhaut B and HR 8799 B, C and D which are special cases), all we know about them comes directly from the changes they can induce in the host star. This includes the wobble back and forth caused by the planet’s mass (this method of finding planets can be called astrometry or radial velocity-they both exploit this motion in different ways), the sudden magnification and brightening of a background star caused by gravitational microlensing, and a few other less frequently used methods. These methods allow you to approximate a mass and orbital solution for the planet but nothing else. Transiting, however, not only helps to narrow down these parameters, but it also allows you to measure the radius of the planet, the eccentricity of the orbit, and in one special circumstance, map the surface of the planet.

COROT-Exo-7 is a K0 type main-sequence dwarf star about 140 parsecs away (that’s about 460 light years). It has an apparent visual magnitude of 11.7 (to put this into perspective for non-astronomers, the absolute dimmest thing a human can see in the best conditions is about magnitude 6.5. Magnitude is a wacky scale invented by the ancient Greeks, so it’s backwards and logarithmic, all of which is to say that you can’t see this star). It’s a relatively young star at an age of 1.1 billion years (+1.1 -0.4).

The planet itself is about 1.7 times the diameter of earth but about 11 times more massive. Since we know both of these quantities, we can figure out the density of the object and the density can tell us something about the composition. It has been suggested that this planet could actually be half rock and half water, which is a very interesting mixture indeed. Unfortunately, this planet has an orbital period of 20 hours and is practically burning up at nearly 2700 degrees Fahrenheit because it is exceptionally close to the host star. So even though it’s the first planet you could actually walk on, you probably wouldn’t want to.

This discovery is just the beginning of a whole new era for extrasolar planetary science. One of my dreams when I began studying astronomy was to discover an earth-like planet and COROT-Exo-7b is a major step on the road to this goal.