brown dwarves

[neufer]

Planet sized-- 8)
Two moons in our solar system are larger than Mercury; Ganymede and Titan.
How big does a moon have to be to be called planet sized?

Actually a planet is more defined by its mass than its volume:

• A planet must have sufficient mass:
  
  1) for self-gravity to overcome rigid body forces such that
  it assumes a hydrostatic equilibrium (nearly round) shape, and
  
  2) that it is able to clear the neighborhood around its orbit.

If it were my decision I would make the cutoff at 10^23 kilograms
such that Ganymede, Titan & Callisto are "planet sized"

Code: Select all

Mass in
10^22 kilograms

33.022   Mercury 	
--------------------
14.82    Ganymede 	
13.45    Titan
10.76    Callisto
..................
8.932    Io
7.348    Moon
4.80     Europa
2.14     Triton 
--------------------
1.67     Eris  

[/b]

• ----------------------------------
  Midsized planets:
  
  Neptune ~ 1,000 × 10^23 kg
  COROT-7b ~ 300 × 10^23 kg
  ----------------------------------

At the other end of the spectrum, planets must have less mass than about 13 Jupiters
(< 250,000 × 10^23 kilograms) in order to avoid turning into a brown dwarf
by fusing deuterium in their cores:

http://antwrp.gsfc.nasa.gov/apod/ap991120.html
http://antwrp.gsfc.nasa.gov/apod/ap990603.html
http://antwrp.gsfc.nasa.gov/apod/ap990324.html
http://antwrp.gsfc.nasa.gov/apod/ap951204.html
http://antwrp.gsfc.nasa.gov/apod/ap950920.html
http://antwrp.gsfc.nasa.gov/apod/ap050510.html
http://antwrp.gsfc.nasa.gov/apod/ap010220.html
http://antwrp.gsfc.nasa.gov/apod/ap000713.html