On the Age of the TRAPPIST-1 System

Abstract

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±2.2 Gyr. The star's colormagnitude position is consistent with it being slightly metal-rich ([Fe/H] +0.06), in line with its previously reported near-infrared spectroscopic metallicity; and it has a radius (R = 0.121±0.003 R ) 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.

Do the TRAPPIST-1 Planets Have Hydrogen-rich Atmospheres?

“…Our results show that all the TRAPPIST-1 planets used to have a hydrogen-rich atmosphere of 10 −2 − 1 wt% just after disk dispersal. All the accreted hydrogen-rich atmospheres of the TRAPPIST-1 planets, however, can hydrodynamically escape by a stellar X-ray and UV (XUV) irradiation from their central star in several 100 Myr, which corresponds to the lower limit of the age of TRAPPIST-1 based on Li I absorption and the rotation period (see Burgasser & Mamajek 2017). These imply that the TRAPPIST-1 planets, including TRAPPIST-1 g, have neither cloud-free nor cloudy/hazy, hydrogen-rich atmospheres.…”

“…The ratio of Lyman-α to X-ray emission from TRAPPIST-1 suggests that its chromosphere is moderately active compared to its corona and transition region (Bourrier et al 2017b). Considering that the age of TRAPPIST-1 is 7.6±2.2 Gyr (Burgasser & Mamajek 2017), we determine the saturated XUV luminosity of TRAPPIST-1; L 0 ∼ 4.7 × 10 −5 L ⊙ for L XUV /L bol = 5 × 10 −4 . Figure 1 shows the atmospheric growth of a migrating and a non-migrating planetary core with mass of 0.3 M ⊕ (for TRAPPIST-1 d, 1 h), 0.7 M ⊕ (for 1 e), and 1 M ⊕ (for 1 b, 1 c, 1 f, and 1 g) in a steady-state accretion disk.…”

A Reanalysis of the Fundamental Parameters and Age of TRAPPIST-1*

“…Table 2 contains our calculated fundamental parameters assuming two age ranges (1) 0.5 − 10 Gyr, the field age constraint from Filippazzo et al (2015) and (2) < 0.5 Gyr, to address the intermediate gravity classification. Also listed in Table 2 are literature values for derived fundamental parameters of TRAPPIST-1 from Filippazzo et al (2015), Gillon et al (2017), Burgasser & Mamajek (2017), and Van Grootel et al (2018). Our bolometric luminosity value differs from previous measurements by up to 2 σ, which can be accounted for by our use of the more precise Gaia DR2 par- To determine if TRAPPIST-1 exhibits similar or different features compared to the average field M dwarf, we constructed three comparative samples.…”

“…Because of the intermediate gravity (INT-G or β) classification that Burgasser & Mamajek (2017) found for TRAPPIST-1 when using the gravitysensitive indices, we calculated the indices for our entire comparative sample using our modified version of the ALLERS13_INDEX IDL code on both low-and mediumresolution spectra when available. The index values and final gravity scores are listed in Tables A6 and A7.…”

“…Section 10 makes concluding remarks on the age of TRAPPIST-1 after examining all samples. Section 11 examines LHS 132 and other sources from Burgasser & Mamajek (2017) comparing their overall SEDs to TRAPPIST-1. Lastly, Section 12 presents indices and gravity classifications for the entire comparative sample, absolute magnitude and fundamental parameters versus spectral type comparisons, and a comparison of the kinematics of TRAPPIST-1 to β-gravity sources.…”