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.