Hearth is the homeworld of the Pierson's Puppeteers with a population of around one trillion and is covered by arcologies, most over one mile tall. Its industries and population generate so much waste heat it no longer requires a star for warmth (the four other "farmworlds" use artificial orbital lights to grow food). It's the central planet in The Fleet of Worlds.
The Puppeteers had to make some drastic alterations to their home system, during their history, as waste heat due to overindustrialisation was rapidly making their planet uninhabitable. They moved their planet further from their sun, to lessen the effects of global warming, but overindustrialisation forced them to move four of the other planets in their system closer to their world and terraform them into "farming worlds", arranging the five planets into a Klemperer rosette.
Eventually, their sun converted from a yellow dwarf to a red giant, so the Puppeteers moved the "Fleet of Worlds?", the five planets, to their system's Oort cloud. This is one of the reasons the Puppeteers were so successful at keeping the location of their homeworld a secret—explorers would be looking for a yellow dwarf (as one could surmise that Puppeteers had evolved around a yellow dwarf from their biology and that they were comfortable on Earth-like planets without pressure suits) when their planet(s) were actually near a red giant.