The outer planets are mostly formed of lighter elements because these three condense are a<u> lower temperature</u>.
<u>Explanation:</u>
The planets based on the location of the sun is divided into two, that is outer planets and inner planets. 
The Outer planet which is far from the sun consists of larger quantity of lighter substances such helium, hydrogen, methane which would condense at the lower temperature. And remains gaseous inside the frost line that would exit far away for the sun so the temperature is low and the lighter materials are easier to condense in lower temperature to form terrestrial  rocks.  
 
        
             
        
        
        
➜ <u>Mendel conducted breeding experiments with garden peas</u>:
[i] He studied plants (pure) of a tall/short varities.
[ii] He crossed them and obtained F1 progeny.
[iii] He found that F1 progeny was all tall plants.
[iv] He selfed the (hybrid) plants if F1 progeny.
[v] He found that in F2 progeny there were tall as well as short plants.
[vi] The three quarter plants were tall and one quarter was short.
 (or any other contrasting character may be taken).
<u>Note</u>: Here, F1 means <u>First fillial generation</u> and F2 means <u>Second fillial generation</u>.