Tuesday, September 18, 2007

Tentative data on the moon

On request. Feel free to rip to shreds (and hopefully suggest ways to make it work), I'm not yet wedded to these numbers.

My model for the star and planet is HD 69830 d as it is currently known, but I'm not using it verbatim and I'm not going to say that the story is set in that system. My source on the physics is Stephen L. Gillett's World-Building. I'm no expert :)

The star has spectral class K5 (absolute magnitude 7,4, bolometric correction -0,8), which puts the habitable zone at roughly 67 million kilometers. A planet at that distance will have an orbital period of 134 standard days.

Assuming a Saturn-like planet, we have a rough density of 1600 kg/m³ and radius of 4 Earths, yielding a mass of 19 Earths. According to my numbers, the star-induced tidal forces on the planet are about 7 times as large as that of the Sun on Earth, which I'm hoping is small enough for avoiding tidal lock.

I've given the planet one Earth-like moon: radius 80 % of Earth giving a mass of 50 % of Earth and about 80 % surface gravity. I've currently assigned the moon a 24-hour orbital period, putting the moon at 0,1 million kilometers from the planet (center to center). The moon is well out of the Roche limit. The tidal force is enormous, and so the moon will be tidally locked to the planet.

Something I'm not sure about (in terms of feasibility) but would consider very cool is if the moon's axial tilt was nearly 90 degrees. This would mean that the moon's orbit would have a similar inclination relative to the plane of the planet's orbit. Please tell me it is feasible :)

I probably forgot some crucial parameters I have in my spreadsheet. Ask and I'll tell :)

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