Emoji

Emoji was a system of communication with the use of smileys and other pictograms or images. According to the Twelfth Doctor, no other species in the universe aside from humans used emojis.

It was popular on Earth during the 21st century, and remained a popular form of language centuries later, when Gliese 581 D was colonised. It was used by the Emojibots as their primary form of expression. The colonists also wore mood indicators that expressed their emotions in order to better communicate with the robots. (TV: Smile)

Known emoji
Known emoji included the following pictograms:
 * 😃 – smiling face (TV: Smile)
 * 💀 – a skull, used as a symbol for death (TV: Smile)
 * 😢 – crying face (TV: Smile)
 * 💡 – a light bulb, used as a symbol for new ideas (TV: Smile)
 * 😕 – puzzled face (TV: Smile)
 * ❓ – question mark, indicating confusion or a question being asked (TV: Smile)
 * 👍 – thumbs up, indicating approval (TV: Smile)
 * ☹️ – frowning face (TV: Smile)
 * 😒 – unamused face, indicating slight dissatisfaction (TV: Smile)
 * 💷 – Pound emoji, indicating profit (TV: Smile)

Behind the scenes
The 2016 Doctor Who Adventure Calendar introduced the Doctor Who emoji quiz, in which the goal is to guess which story is being told through emoji. This "festive edition" of the game included 3 stories to decipher.

While most of the emoji used in Smile are part of the Unicode emoji set in the real world, a few are not present in the real set. E.g. as of 2017 there is no emoji for a face with cross instead of its mouth. However, new emoji are still being regularly added to Unicode.