Drupal 8, very simple hierarchical form doesn't work
You need change type $form["planet"] to fieldset or container. Don't use text field in this case:

$form["planet"] = array(
  '#type' =&gt; 'fieldset',
  '#title' =&gt; '1 : '.$this-&gt;t('planet'),
  '#collapsible' =&gt; TRUE, 
  '#collapsed' =&gt; FALSE,
);