TRAPPIST-1c, also known as 2MASS J23062928-0502285 c, is a mainly rocky, Venus-like extrasolar planet orbiting around the ultra-cold dwarf star TRAPPIST-1, located approximately 39 light-years (12 parsecs) away from Earth in the constellation Aquarius. It is the most massive and third largest planet of the system, with about 116% the mass and 110% the radius of Earth. Its density indicates a primarily rocky composition with a very thick Venus-like atmosphere, although it is expected to be thinner than that of TRAPPIST-1b.
Characteristics[]
Mass, radius, and temperature[]
TRAPPIST-1c was observed with the transit method, which enabled scientists to calculate its radius. Transit-timing variations and computer simulations were able to determine the mass, density, and gravity of the planet. TRAPPIST-1c is the third-largest planet of the TRAPPIST-1 system, with a radius of 1.095 Earth radii (R⊕). It is also the most massive of the system as well, with a mass of 1.156 Earth mass (M⊕), slightly higher than that of the next most massive, TRAPPIST-1g. Despite its Earth-like mass and radius, TRAPPIST-1c has a lower density (4.89 g/cm3) and gravity (0.966g) than Earth. This is consistent with a rock-based composition and a thick, Venus-like atmosphere of water vapor, similar to TRAPPIST-1b. TRAPPIST-1c's atmosphere is expected to be thinner than that of its inner sibling, but still large enough to raise its surface temperature far above the calculated 334.8 K (61.7 °C; 143.0 °F) equilibrium temperature.
Orbit[]
The orbit of TRAPPIST-1c is very close to its host star. One year on this planet lasts a mere 2.42 days (58 hours), a fraction as long as that of our Solar System's innermost planet, Mercury. The planet orbits at a distance of 0.0158 AU, which is about 1.6% the distance between Earth and the Sun. At this proximity, TRAPPIST-1c is most likely tidally locked. However, due to the small size of its host star, the planet only receives about 2.1 times the sunlight as Earth. Its orbital eccentricity is very low at 0.00654, similar to that of TRAPPIST-1b.
Host star[]
The planet orbits an (late M-type) ultracool dwarf star named TRAPPIST-1. The star has a mass of 0.089 Solar mass (M☉) and a radius of 0.121 Solar radius (R☉). It has a temperature of 2516 K and is anywhere between 3 and 8 billion years old. In comparison, the Sun is 4.6 billion years old and has a temperature of 5778 K. The star is metal-rich, with a metallicity ([Fe/H]) of 0.04, or 109% the solar amount. This is particularly odd as such low-mass stars near the boundary between brown dwarfs and hydrogen-fusing stars should be expected to have considerably less metal content than the Sun. Its luminosity (L☉) is 0.0522% of that of the Sun.
Atmosphere[]
The combined transmission spectrum of TRAPPIST-1 b and c rules out a cloud-free hydrogen-dominated atmosphere for each planet, so they are unlikely to harbor an extended gas envelope. Other atmospheres, from a cloud-free water-vapor atmosphere to a Venus-like atmosphere, remain consistent with the featureless spectrum.
In 2018, the composition of TRAPPIST-1c was determined, with turned out to be rock-based with a very thick, Venus-like atmosphere. The atmosphere of TRAPPIST-1c is likely thinner than that of TRAPPIST-1b.