From Google Webmaster Central Blog:
Posted: 17 Dec 2015 08:00 AM PST
As a natural continuation of this, today we’d like to announce that we’re adjusting our indexing system to look for more HTTPS pages. Specifically, we’ll start crawling HTTPS equivalents of HTTP pages, even when the former are not linked to from any page. When two URLs from the same domain appear to have the same content but are served over different protocol schemes, we’ll typically choose to index the HTTPS URL if:
- It doesn’t contain insecure dependencies.
- It isn’t blocked from crawling by robots.txt.
- It doesn’t redirect users to or through an insecure HTTP page.
- It doesn’t have a rel=”canonical” link to the HTTP page.
- It doesn’t contain a noindex robots meta tag.
- It doesn’t have on-host outlinks to HTTP URLs.
- The sitemaps lists the HTTPS URL, or doesn’t list the HTTP version of the URL
- The server has a valid TLS certificate.
Although our systems prefer the HTTPS version by default, you can also make this clearer for other search engines by redirecting your HTTP site to your HTTPS version and by implementing the HSTS header on your server.