How to get post image caption by index

I have multiple images in WordPress post, each image has different caption.
I want to get the image caption by the image index, but I can’t figure out how to print the caption.

Here is what I have tried:

//$index - index of the image (if index is 0, its first image)
function getImageCaption($postID, $index){
    $post_id = $postID;
    // get the post object
    $post = get_post( $post_id );
    // get the post thumbnail ID
    $thumbnail_id = get_post_thumbnail_id($post->ID);
    // we need just the content
    $thumbnail_image = get_posts(array('p' => $thumbnail_id, 'post_type' => 'attachment'));

    if ($thumbnail_image && isset($thumbnail_image[$index])) {
        return $thumbnail_image[$index]->post_excerpt;
    } else {
        return;
    }
}

How can I display image caption by index?