Google made HTTPS a ranking signal in its algorithm about a year and a half ago. According to Moz, HTTPS URLs make up about 25% of page-1 results in the MozCast 10K. The 10K refers to a set of 10,000 queries across 20 industry categories.
Moz Marketing Scientist Dr. Pete Meyers tweeted (via Search Engine Roundtable):
HTTPS: URLs are currently right around 25% of page-1 results in the MozCast 10K.
— Dr. Pete Meyers (@dr_pete) January 12, 2016
Last month, Google announced its latest step in its "HTTPS Everywhere" initiative in that it now serves HTTPS versions of URLs by default in search results.
Google now crawls HTTPS equivalents of HTTP pages, even when the HTTPS version isn’t linked to from any page. When both versions are available and have the same content, Google will choose the HTTPS URL in most cases.
That is if it doesn’t contain insecure dependencies, isn’t blocked from crawling by robots.txt, doesn’t redirect users to or through an insecure HTTP page, doesn’t have a rel=”canonial” link to the HTTP page, doesn’t contain a noindex robots meta tag, and doesn’t have on-host outlinks to HTTP URLs.
Google says it will also choose to index the HTTPS URL if the sitemap lists the HTTPS URL or doesn’t list the HTTP version and/or the server has a valid TSL certificate.
Image via Google