I am assuming this is in C++.
When creating an array at run time it should be dynamically allocated. Like this:
http://www.cplusplus.com/doc/tutorial/dynamic/
So you would create a pointer in the class and then set the array up:
int * planet;
int planet_no;
Solar_system()
{
planet_no = 5;
planet = new int[planet_no];
}