// If this planet is active...
Vector2 a = _body.position.normalized;
Vector2 b = (a + _body.velocity).normalized;

float angle = Vector2.Angle (a, b);
this._total_angle += angle * Time.deltaTime;

if (this._total_angle >= 360) {
    // If this planet has made a revolution...
    ++this._revolutions;
    this._total_angle = 0;
    this.OnRevolution.Invoke (this);
}

If reached minimum velocity
 next time a minimum velocity is reached (maybe not the same minimum value) an  orbit has  occurred
end if

If reached maximum velocity
 next time a maximum velocity is reached (not the same maximum) an  orbit has  occurred
end if

a= (distance(sunP ,minP) + distance(sunP ,maxP))/2