Gliese 876 c

[toymaker]

Gliese 876 is an exoplanet within habitable zone of its star.<br />If any moons would exist around it then they could posses conditions for life.<br />I however found this information:<br />http://en.wikipedia.org/wiki/Gliese_876#Gliese_876_c<br />"However calculations of the Hill zone of the planet have found that the maximum possible mass of a moon in a stable orbit is far below that of Mars, killing any hope of Earth-like life-supporting moons."<br /><br />Is this correct ? And how is such calculation done ? Does anybody know if there is more info on this research ? What is the size of possible moons of Gliese 876 c ?<br /><br />

 

[thalion]

I'm betting whoever did that calculation was not an astronomer, but that's not important.<br /><br />The Hill radius is the radius beyond which tidal disruption makes stable orbits impossible. The formula for it is:<br /><br />Hill radius = a*(m_p/3M_sun)^0.33...<br /><br />From here: http://www.ifa.hawaii.edu/~jewitt/papers/2004/SJ2004C.pdf<br /><br />By the by, the Hill radius is not quite the same as the gravitational "sphere of influence", which is slightly larger, IIRC.<br /><br />If you use the data given here:<br /><br />http://exoplanets.org/esp/gj876/gj876.shtml<br /><br />--and assume that the radius of both gas giants is roughly equivalent to Jupiter (a valid assumption), then it should be easy to check the figures given in the Wikipedia article.