The nearby ( d = 12 pc ) M8 dwarf star TRAPPIST-1 ( 2MASS J23062928 - 0502285 ) hosts a compact system of at least seven exoplanets with sizes similar to Earth .
Given its importance for testing planet formation and evolution theories , and for assessing the prospects for habitability among Earth-size exoplanets orbiting the most common type of star in the Galaxy , we present a comprehensive assessment of the age of this system .
We collate empirical age constraints based on the color-absolute magnitude diagram , average density , lithium absorption , surface gravity features , metallicity , kinematics , rotation , and magnetic activity ; and conclude that TRAPPIST-1 is a transitional thin/thick disk star with an age of 7.6 \pm 2.2 Gyr .
The star ’ s color-magnitude position is consistent with it being slightly metal-rich ( [ Fe/H ] \simeq +0.06 ) , in line with its previously reported near-infrared spectroscopic metallicity ; and it has a radius ( R = 0.121 \pm 0.003 R _ { \odot } ) that is larger by 8–14 % compared to solar-metallicity evolutionary models .
We discuss some implications of the old age of this system with regard to the stability and habitability of its planets .