Gliese 581d

A planet was one of the first human colonies settled after Earth's population had been evacuated. The planet that supported the colony was 20 light-years from Earth and had twin suns.

When Bill asked the Twelfth Doctor travel to the future with her, he took Bill to this planet. (TV: Smile)

Astronomical data
This planet was the fourth planet of its star system and was located twenty light years from Earth, in orbit around twin suns. Following the terraforming of the planet by Vardies, the composition of the atmosphere became identical to Earth’s. It also had fertile soil and vast seas, rendering it suitable for human colonisation. (TV: Smile)

Landscape
This planet’s sky was blue, and it had two suns. The city that would serve as the home of the colonists was surrounded by vast fields of wheat on one side and was located at the edge of a sea on the other side. The architecture of the city was made up of futuristic and smoothly designed buildings with many windows, open spaces and an overall white colour. It had numerous plants, bridges and bodies of water flowing through the streets. (TV: Smile)

History
At some point in the far future, Earth was devastated and the human population was evacuated on spaceships. One of these was the starship Erehwon that contained millions of people in stasis.

The ship landed on this planet. Once it landed, a small number of the crew were awakened and deployed swarms of Vardies, "the worker bees of the third industrial revolution". The Vardies successfully terraformed the atmosphere and ground of the planet in order to make it habitable for humans. They then built an entire city out of Vardies, which would serve as the colonists' home.

However, the Vardy were programmed to keep the colonists happy at all times. When one of the colonists died of natural causes, her friends and family began to grieve. The Vardies, reading grief as the opposite of happiness, treated it like a virus and began killing the colonists as to them, eliminating sadness meant eliminating sad people. In just one morning, the entire set-up team was slaughtered for displaying negative emotions.

Sometime later, the Twelfth Doctor and Bill Potts visited the planet, only to find the city empty. They eventually discovered the remains of the set-up team who had been turned into fertilizer for the botanical gardens. The Doctor, assuming the colonists had not arrived yet, decided to blow up the city. They ventured into the starship Erehwon and the Doctor sabotaged the engine. However, Bill discovered the remaining population in stasis and quickly warned the Doctor, who undid his sabotage.

As the colonists began to awaken, the Doctor briefed them on what had happened and why. But the humans, believing the Vardies simply wanted to kill them, took up arms and an open conflict erupted between the humans and the Vardy, who had begun showing the first signs of sentience. The Doctor ended the battle by erasing the Vardies' memory up until the moment they made the error of perceiving grief as a virus to be eliminated. Though the Vardy thought the city belonged to them, they welcomed the humans onto their new world and the Doctor made sure they would not make the same mistake. (TV: Smile)

Technology
The colonists that populated this planet had made a number of technological advancements that participated in the functioning of the colony. The entire city was made up of Vardy, microrobots that interlocked in order to create solid and complete structures such as the buildings and the machines. They could easily break apart into swarms and attack anyone they perceived as a threat. They would envelop the target and could strip them of clothing and flesh in seconds, leaving nothing but bones behind. The Vardy also carried out daily tasks, such as scanning new arrivals or pollinating the wheat fields that surrounded the city. The city was also equipped with Emojibots, an interface between the human colonists and the Vardy that carried out maintenance.

Humans communicated over long distances through a wireless network that was implanted directly into their ears and used their nervous systems as circuitry.

Mood badges were handed out by Emojibots to the human residents and automatically put themselves on the humans' back. They monitored their emotions and communicated the data directly to the Emojibots.

The city contained a massive greenhouse where plants were grown. These included fruits and vegetables such as orange trees, olives, as well as rosemary and other plants. The Emojibots carried out pollination work and fertilizer was spread through large pipes above the crops. (TV: Smile)

Behind the scenes
The name of the planet is not stated on screen in Smile. However, it is said to be Gliese 581d on the BBC Doctor Who website,  and is named as such by the interviewer of the story' writer Frank Cottrell-Boyce in DWM 512 - though not by Cottrell-Boyce himself.