TRAPPIST-1b, also known as 2MASS J23062928-0502285 b, 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. The planet was detected using the transit method, where a planet dims the host star's light as it passes in front of it. It was first announced on May 2, 2016, and between 2017 and 2018, more studies were able to refine its physical parameters.
The planet is about the same mass as Earth, but about 12% larger. Its relatively low density, along with spectroscopic observations, have confirmed that it has an extremely thick and hot atmosphere. Observations published in 2018 showed that the atmosphere of TRAPPIST-1b was much larger than that of Earth or Venus, as well as being very hot and potentially rich on CO2. Another study, published in February 2018, determined that TRAPPIST-1b has a water-rich atmosphere with up to 10,000 times the surface pressure of Earth's atmosphere. It is noted for being quite similar to Venus. Observations by the James Webb Space Telescope announced in 2023 suggest that the planet does not have any significant atmosphere. It is also known for being the 'head' leader of all the TRAPPIST planets due to the rough weather it faddists throughout the days on the surface.
Characteristics[]
Mass, radius, and temperature[]
TRAPPIST-1b is very close in both mass, radius and gravity to Earth. It has a radius of 1.121 Earth radii, a mass of 1.02 Earth mass, and about 81% Earth's surface gravity. However, the density of the planet indicates that it is not entirely rocky. With a density of 3.98 g/cm3, about ≤5% of its mass must be water, likely in the form of a thick Venus-like atmosphere due to its high stellar flux being nearly 4 times higher than Earth's. The planet's extremely thick atmosphere has likely risen its temperature far above its equilibrium temperature of 391.8 K (118.7 °C; 245.6 °F). Its surface temperature is estimated to be between 750 K (477 °C; 890 °F) and 1,500 K (1,230 °C; 2,240 °F), potentially as high as 2,000 K (1,730 °C; 3,140 °F). This is much hotter than the surface of Venus and can melt certain metals.
Orbit[]
TRAPPIST-1b orbits very close to its parent star. One orbit on this planet lasts about 1.51 Earth days, or about 36 hours. It orbits about 0.0115 AU from its star, just 1.2% the distance between Earth and the Sun. The close proximity to its host star means that TRAPPIST-1b is likely tidally locked. It also has a very circular orbit, with an eccentricity of 0.00622, significantly more circular than Earth's orbit.
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 planet's atmosphere was better examined with the Spitzer Space Telescope and found to be quite large and hot. The planet's transmission spectrum suggested two main possibilities for the atmosphere: one rich in carbon dioxide, and one with water vapor. The more likely CO2 atmosphere would have a scale height of approximately 52 km (with Earth's being at 8 km, and Venus's at 15.9 km) and an average temperature in excess of 1,400 K (1,130 °C; 2,060 °F), far higher than its equilibrium temperature. A water vapor atmosphere would need to have a scale height of >100 km and a temperature of >1,800 K (1,530 °C; 2,780 °F) to produce the variations seen in the planet's transit depths and its transmission spectrum. Other sources for the effects seen, such as hazes and thick clouds, would require an even larger atmosphere. TRAPPIST-1b will have to be studied further to confirm its potential large atmosphere.
The refined density of TRAPPIST-1b, along with the planet's transmission spectrum, indicate that its atmosphere is made mainly of vaporized water. The atmosphere has likely risen the planet's surface temperature to about 750-1500 K, which would require an atmospheric pressure between 101 (10) to 104 (10,000) bar, potentially thicker than Venus's atmosphere. TRAPPIST-1b's atmosphere is likely very similar to that of Venus.
Observations by the James Webb Space Telescope announced in 2023 suggest that the planet does not have any significant atmosphere, with a surface temperature of about 503 K (230 °C; 446 °F).