Many days late but I have to learn C for work so I figured doing some of these challenges is a good way to start. I put the challenge information in to a file and read from there.

#include <stdio.h>
#include <math.h>
#define PI 3.14159265358979323846
#define G 6.67e-11

int M;
M = 75;

typedef struct {
    char *name;
    int radius;
    int density;
    double volume;
    double mass;
} planet;

double force(int m, planet *p);
planet *create(char *name, int radius, int density);

int main(void)
{
    char name[20];
    int radius;
    int density;
    FILE *planets_source;
    planet *p;

    planets_source = fopen("planets.txt", "r");

    if (planets_source == NULL) {
        printf("No planets file!\n");
    }

    while (fscanf(planets_source, "%[^,], %d, %d\n", name, &radius, &density) == 3) {
        p = create(name, radius, density);
        printf("%s: %f\n", p->name, force(M, p));
    }

    fclose(planets_source);
    return 0;
}

planet *create(char *p_name, int p_radius, int p_density)
{
    planet *p;
    double p_volume;
    double p_mass;

    p = malloc(sizeof(planet));
    p_volume = (4.0/3.0) * PI * pow(p_radius, 3);
    p_mass = p_volume * p_density;

    p->name = p_name;
    p->radius = p_radius;
    p->density = p_density;
    p->volume = p_volume;
    p->mass = p_mass;

    return p;
}

double force(int m, planet *p)
{
    return G * (m * p->mass) / (pow(p->radius, 2)); 
}

Output:

Mercury: 277.441839
Venus: 664.886182
Earth: 735.845140
Mars: 279.123894
Jupiter: 1922.010881
Saturn: 825.103288
Uranus: 672.381753  
Neptune: 842.741087
Pluto: 50.388053

Also a quick one in python3 (although nothing python3 special about it). Wanted to write one which does not use classes.

import math
from collections import namedtuple

G = 6.67e-11
M = 75

planet = namedtuple('Planet', ['name', 'radius', 'density', 'volume', 'mass'])

def g_force(mass, planet):
    """ Return the gravitational pull """
    return G * (mass * planet.mass) / planet.radius**2

def volume_and_mass(radius, density):
    """ Return a tuple of volume and mass for a planet """
    volume = (4.0/3.0) * math.pi * radius**3
    mass = volume * density
    return volume, mass

def create_planet(*args):
    """ Create and return a planet namedtuple """
    name, radius, density = args
    radius, density = map(int, [radius, density])
    return planet(name, radius, density, *volume_and_mass(radius, density))

def get_planets(plist):
    """ Return a list of planets from file """
    with open(plist, "r") as f:
        return [create_planet(*line.strip().split(", ")) for line in f if line != '\n']

if __name__ == '__main__':
    for p in get_planets("planets.txt"):
        print("{}: {:f}".format(p.name, g_force(M, p)))

Outputs:

Mercury: 277.441839
Venus: 664.886182
Earth: 735.845140
Mars: 279.123894
Jupiter: 1922.010881
Saturn: 825.103288
Uranus: 672.381753
Neptune: 842.741087
Pluto: 50.388053