MadSci Network: Astronomy
Query:

Re: What causes the great variety in the planets' densities?

Date: Wed Feb 7 16:48:28 2001
Posted By: Randall Scalise, Faculty, Physics, Southern Methodist University
Area of science: Astronomy
ID: 981223118.As
Message:

A glance at a table (such as below) listing planetary densities reveals
two major divisions.  The inner or Terrestrial planets have a higher
average density than the outer or Jovian planets.

------------------------------------------------------------------
Terrestrial Planets    Specific Gravity or             Radius in 
                      Density in (grams/cm^3)        millions of km
     Mercury               5.43                             2.43
     Venus                 5.25                             6.06
     Earth                 5.52                             6.37
     Mars                  3.95                             3.37
                                                           
Jovian Planets                                             
     Jupiter               1.33                            69.9
     Saturn                0.69                            58.5
     Uranus                1.29                            23.3
     Neptune               1.64                            22.1
                                                           
Icy Planets                                                
     Pluto                 2.03                             1.5

Densities from  http://python.swan.ac.uk/~pydunbar/PH104/PH104_planets.html

Radii from Serway and Beichner "Physics for Scientists and 
Engineers", 5th ed.ISBN:0-03-031561-1
-----------------------------------------------------------------------

It is true that the planets and Sun formed from the same cloud of gas,
dust, and ice called the Solar Nebula.  The Sun did not begin to shine
at the time of planet formation -- that happened from 10 to 100 million
years later.

The main reason for the difference in planetary density is the change in
temperature with distance from the center of the Solar Nebula.  The
inner regions were warmer than the outer regions, but not because the
Sun was shining, rather because the particles in the cloud were moving
faster and colliding more often near the center of the Nebula.

Near the center of the dust cloud, the temperature was high enough to
keep some chemicals in the cloud such as methane, ammonia, and water
from becoming solid.  Consequently, these chemicals were not available
to build planets.  Only denser chemicals such as iron, nickel, and
silicates (sand) were solid and could form planets.

Just inside the orbit of Jupiter, the temperature was low enough for
water ice to form.  Since water was abundant in the Solar Nebula, water
ice provided a lot of planet-building material.  Jupiter and the other
outer planets therefore grew very large.  Notice from the table that the
planetary radii increase dramatically with distance from the Sun after
Jupiter.

The enormous gravitational pull of the large Jovian planets then held
onto the light hydrogen and helium gas copiously present in the Solar
Nebula.  When the Sun did begin to burn, the strong Solar Wind swept
most of the left over gas and dust from the Solar System.  The smaller
Terrestrial planets did not have strong gravitational fields that could
hold onto large amounts of hydrogen and helium.

The vast quantities of light gases trapped by the Jovian planets
accounts for their relatively low densities.  The planetary density is
an AVERAGE over the heavy rocky-metallic-icy core and the light gaseous
atmosphere.  The Jovian planets have such large atmospheres composed of
light gases that their overall density is low.  The rocky-metallic
Terrestrial planets have such thin envelopes of atmosphere that their
overall density is high.

Reference   http://zebu.uoregon.edu/~imamura/121/lecture-5/lecture-5.html
----------------------------------------------------------------------

I don't think that anyone can answer the question concerning the minimum
density necessary to support life because there could be forms of life
in the Universe which are not only undiscovered but unimagined.  Who
knows if life (not as we know it) can exist in densities as low as one
molecule per cubic meter of space?

Any estimate of the number of "life"-supporting planets throughout the
Universe will necessarily vary greatly with the assumptions one makes
about extraterrestrial life.  To demonstrate our lack of knowledge about
life off the Earth, remember that we don't even know if the other
planets of our own Solar System support life or not.
 
One can, however, make the following definite statement: The number of
planets capable of supporting life (whatever definition one is using for
"life") increases as the total number of planets increases.  This is
particularly interesting in light of the relatively recent (last five
years) discoveries of many extra-solar planets.  There are many web
sites devoted to this topic, for example http://cfa-www.harvard.edu/planets/




Current Queue | Current Queue for Astronomy | Astronomy archives

Try the links in the MadSci Library for more information on Astronomy.



MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci


MadSci Network, webadmin@www.madsci.org
© 1995-2001. All rights reserved.