Move object along spline(circle) in Three.js

I'm pretty new to Three.js (1 day experience lol) I want to create a model of Solar system so I got planets that should move along their trajectories (circles).

function render() { 
    requestAnimationFrame(render); 

    sun.rotation.y += 0.01;
    mercury.rotation.y +=0.03;

    renderer.render(scene, camera); 
} 
render();

I tried to use splines for this, but failed to animate because I don't get how to use requestAnimationFrame with variables (only this simplest incremental stuff like +=0.03)

mercury.position = spline.getPoint(t);

Also tried to do it with math, but same result. Don't know how to animate variables.

mercury.position.x = 20*Math.cos(4) + 0;

But I don't have any experience in animating anything in JS. So my mind is blown away by this requestAnimationFrame thing, that I got from some tutorial, it's like a black box to me.

Found a solution:

const sunGeo = new THREE.SphereGeometry(12, 35, 35);
const sunMat = new THREE.MeshPhongMaterial();
sunMat.map = THREE.ImageUtils.loadTexture("image/sunmap.jpg");
const sun = new THREE.Mesh(sunGeo, sunMat);
sun.position.set(0, 0, 0);
scene.add(sun); // add Sun

const mercuryGeo = new THREE.SphereGeometry(2, 15, 15);
const mercuryMat = new THREE.MeshPhongMaterial();
mercuryMat.map = THREE.ImageUtils.loadTexture("image/mercurymap.jpg");
const mercury = new THREE.Mesh(mercuryGeo, mercuryMat);
scene.add(mercury); // add Mercury

let t = 0;
function render() {
    requestAnimationFrame(render);
    t += 0.01;
    sun.rotation.y += 0.005;
    mercury.rotation.y += 0.03;

    mercury.position.x = 20 * Math.cos(t) + 0;
    mercury.position.z = 20 * Math.sin(t) + 0; // These to strings make it work

    renderer.render(scene, camera);
}
render();