Resonant Constellation

]Michel Mayor and his exoplanet hunting team have done it again. Gliese 581e weighs in with a minimum mass of . For the technical details, followthis link to the pre-print paper. Of course, this is a pre-print (though accepted for publication in A&A), so it’s not in publication state yet, just out to show that this group discovered it first. The paper will likely be revised before it is finally published, but in my (admittedly short) experience, this is the only copy most people will read.

Most of the time, the inclination angle for these systems isn’t known (unless the planet transits the host star, in which case it can be derived from the impact parameter of the transit, b via a relatively simple formula), so the masses of most known extrasolar planets are listed as lower limits. In this case, however, the system has three other low mass planets, and since it is a stable system, it can be dynamically modeled and upper limits can be placed on the planet masses. In this case, the upper limits on all planet masses in the system is at 1.6 times the minimum, so the maximum mass for this newly discovered planet is only -still a significant discovery. The planet is, however, too close to the star to support life at 0.03 AU and has an orbital period of roughly days (3.14942 to be more exact).

The other planets in the system are worth mentioning as well. Gl581b, the first discovered is at least 15.62 earth masses with a semi-major axis of 0.04 AU, Gl581c is only 5.36 earth masses with a semi-major axis of 0.07 AU, and Gl581d is 7.09 earth masses and orbits at a distance of 0.22 AU. This last is particularly interesting because the planet lies in the habitable zone around the host star.

This remarkable system is only 20 lightyears distant from earth (it is actually the 87th closest known star system), and the host star is likely visible in relatively small (say, 6-7 inch) amateur telescopes. You can find it in the constellation approximately 2 degrees north of beta libra (’commonly’ called Zubeneschamali or Zuben el Chamali). For those of you who want to look for it yourselves, here’s a starchart centered on the location that should help you find it. Of course, libra isn’t up high in the sky until late these days, so you’ll have to stay up late to catch it.

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.