## Saturday, September 17, 2011

### Planets By the Dozens

The past week has been one of the most stirring and notable in the history of planetary astronomy: no fewer than seventy -four new planets discovered orbiting other stars, including one planet (nicknamed "Tatooine") orbiting a double Sun or binary system (See, e.g. my blog:

http://brane-space.blogspot.com/2011/09/tackling-intermediate-astronomy_15.html

In the blog was included a brief overview of "spectroscopic binaries" and how their parameters can be computed. Similar methods were employed in the discovery of the double star system in Cygnus with its planet Kepler 16b (see top diagram), aka "Tatooine", after the frigid planet in Star Wars V, "The Empire Strikes Back" - with the double sun. In this case of Kepler 16b, deep analysis determined that it orbits its twin stars every 229 days. (Using similar methods to those discussed in the blog).

A kinematic view of the orbit of Kepler 16b relative to each of the member stars is shown here:

http://kepler.nasa.gov/Mission/discoveries/kepler16b/

And note its distance is given as 0.705 AU from the center of mass of the twin stars, or about what we'd expect using Kepler's 3rd law, given a period of 229 days. Thus:

P = 229 d = 0.626 yr

and a (semi-major axis) = [(P)^2]^1/3

The other orbital parameters are also given for the system in several tables, with the radial velocity of Kepler 16b of (-32.7 km/s) relative to the barycenter or center of mass of the twin stars, the key one to note - because this is what permits the total estimated mass of the system to be computed, as well as the planet (0.333 M(J) where M(J) = mass of Jupiter or 1.89 x 10^27 kg, hence Kepler 16b's mass is 6.94 x 10^26 kg or about 105 times more massive than Earth.

Note the binary star orbit itself has a period P = 41.07 days, and a semi-major axis of a' = 0.224 AU. The eccentricity is also high at e = 0.159. This information is enough to allow us to calclate the total mass of the 2 stars (A and B) in this system from (cf. my blog link to Intermediate Astronomy (7)):

m1 + m2 = (0.224)^3/ (0.112)^2 = 0.9 solar masses

And we see from the mass data given in the orbital link that star a has mass 0.68 M_s and Star B, 0.22M_s. This is almost in a 3:1 ratio so that we'd expect Star B to be traveling at a velocity of about 3x that of star A.

The other aspect to note is the two stars are relatively cool, probably spectral class K, given the primary has an effective (surface) temperature of 4450 K (4177 C) and given star B is much less massive, it must be cooler still. This explains why Kepler 16bis expected to have a frigid surface temperature of (-150 F), or colder than winter midnight in Antarctica.

The same week Kepler 16b and its star pair were found, so were 73 other systems, many thanks to the 'High Accuracy Radial Velocity Planet Searcher' or HARPS, based at the La Silla Observatory in Chile. One of the HARPS team's leaders, Michael Mayor of the University of Geneva, (while discussing some of the 50 explanets discovered using the HARPS spectrograph), estimated 40% of all stars like the Sun have at least one planet of Saturn size or smaller revolving around them.

The remaining question, of course, is how many - if any - of the 683 exoplanets found thus far are located in any of their star's habitable zones. So far, no one can say with any certainty that any habitable zones with planets exist beyond our solar system - though one exoplanet with 3.5 times the Earth's mass - may be within its star's habitable zone. The problem? It may be too massive, with too high a surface gravity for any life to have evolved.

But we will wait and see!