Nov 14, 2009

So what range of Gravity is Habitable for Humans?

Low gravity (relative to earth) seems to give people no problems once they get used to it. We've had people in weightless environments for months. However the human body can easily adapt to those conditions, and gets rid of what it perceives as excess muscle, bone mass, and fluid volume. Which is fine, unless the person wishes to return to a higher gravity world-- like Earth. Astronauts use rigorous exercise programs to stay in shape enough to return to Earth, though the longer they stay in space at zero G, the longer it takes to return to full health on Earth.

How Heavy & how Big?

The next step is to figure out the size of your planet, and the strength of its gravity.  This information is derived from your planet's mass, radius & density. With only 2 of these variables you can figure out gravity, and anything else relating to your planet's size. Given the same mass, the denser a planet is, the higher the gravity.  Jupiter with over 300 times the mass of earth has a gravity of less than 3 times that of earth's (on the theoretical, unobserved surface of Jupiter) because it is very diffuse, made out of gas rather than iron and stone.

Mass can be pretty much whatever you want, within certain bounds.  Dole estimates (see Habitable Planets for Man, free PDF.) that a planet needs at least 0.4 Earth Masses to retain a human-breathable atmosphere.  Depending on who you ask, 5, 10, or even 14 earth masses is the upper limit for a terrestrial planet. Beyond that it will retain so much atmosphere that it crosses over into gas giant territory.

Radius can also be whatever you want, as long it doesn't force the other variables into something silly. Just be aware that radius has a strong effect on the surface area of your planet.  If you double the radius, the surface area increases four times.

Density is a little trickier. But here are the basics.  Density is primarily determined by what a planet (or moon) is made out of.  Gas, such as helium and hydrogen, tends to be least dense and then in increasing density are ice (water), rock, and iron.  These are the main ingredients of the known planets.  It is easy to find the densities for these materials at normal, surface of the earth conditions.  But as the mass of a planet increases, so does the pressure inside, and thus the density of various materials.

Nov 12, 2009

Freebie: The colors of Gas Giants

Of the lifeless planets, gas giants, or jovians are the most beautiful.  So i've wondered --the extrasolar giants we've been discovering but haven't yet got a good look at-- do they look like the our tan and striped jovians, or are they different?

Apparently they've been able to infer some things about how they appear.  See this Extrasolar Visions page.  Scroll down to "Color, Albedo, and Temperature".

I may have to swap out the first planet in this Orb's solar system for a Jovian.  I think it would be a Class II, white water-cloud jovian.

At closest it would be ~0.2 AU away, as compared to Jupiter's closest 5.2 AU, so at closest to Orb it would appear to have about half the diameter that the Sun does from Earth— assuming it is as big as Jupiter.  That would be quite a cool sight.

a water cloud jovian from Extrasolar Visions, apparently with a life-bearing moon.

Nov 10, 2009

Celestial Architect Spreadsheet

To simplify the effort for myself, & for others who may be interested i've created a spreadsheet for moderate control without much effort.

Download Celestial Architect Spreadsheet 1.2 below:

EDIT:   Updated to 1.1. Renamed the Spreadsheet and added another page dealing with the moon.
EDIT:   Updated to 1.2 with a better temperature calculation and a sheet for albedo, all thanks to Orion.

Using The Sheet

The "Sheet" is a spreadsheet-- a file you can open with Microsoft Office, or free alternatives such as OpenOffice or it's Mac port, NeoOffice.  All you need to do is enter numbers in the yellow boxes, & all the other figures will be instantly recalculated.