var f = 0.1; // speed up factor
var dx = planet.x - Sun.x;
var dy = planet.y - Sun.y;
var dist = Math.pow(dx, 2) + Math.pow(dy, 2);
var grav = 10 * f * Sun.mass / dist;
if (f2++ < 3) console.log(planet.x, planet.y, planet.velocityX, planet.velocityY);
planet.velocityX -= grav * (dx / Math.sqrt(dist));
planet.velocityY -= grav * (dy / Math.sqrt(dist));
if (f2 < 4) console.log(f / planet.mass);
planet.x += f * planet.velocityX;
planet.y += f * planet.velocityY;