Troas is a fictional planet that serves as the setting for the science fiction novellas "Sucker Bait" by Isaac Asimov and "Question and Answer" by Poul Anderson, the books of an incomplete "Twayne Triplet" (proposed by Fletcher Pratt, editor of Twayne Press). It may have been created by John D. Clark, who is known to have created the scenario for the earlier Twayne Triplet The Petrified Planet.
Troas is part of the [fictional] Lagrange system, named after Joseph Louis Lagrange, who first worked out the existence of Lagrangian points. Located within the globular cluster Messier 13, the Lagrange system is a binary star system consisting of a blue-green star called Lagrange I and a red star called Lagrange II. (In standard astronomical nomenclature, the two stars would be called Lagrange A and Lagrange B.)
Troas (also known as Junior) is named after the Troad region of ancient Troy, because of its position at the system's L5 point, also known as a Trojan point after the Trojan asteroids that share Jupiter's orbit in the Solar System. The system's L4 point is occupied by a group of asteroids called Lagrange Epsilon, also known as the Puppies.
Troas is about the same diameter and mass as Earth, with a rotation period of 36 hours. The atmosphere of Troas has a sea level pressure of 106.6 kilopascals, and consists of 30% oxygen and 70% nitrogen, with less than 1% hydrogen, helium, and carbon dioxide. Troas is in the midst of an ice age, with large polar ice caps covering both poles, though both ice caps are slowly retreating. Troas has a single large continent 16,000 kilometers across straddling the equator, with mountain ranges near each coast and a large river system that empties into the sea through a gap in the western coastal mountain range. Troas has extensive photosynthetic plant life and considerable animal life.
Troas has a single satellite, Ilium, also known as Sister, with an atmospheric pressure of 13.3 kilopascals. Although there is no longer any free water on Ilium, there are large former sea bottoms that have formed salt flats.