The TRAPPIST-1 System
Red Dwarf Star In Aquarius
Named for the small telescope located in Chile searching for planets around nearby stars, TRAPPIST-1, also registered as 2MASS J23062928-0502285, is a 2550°K 'cool', dim class M8 red dwarf star barely larger than Jupiter (but 84 times more massive), approximately 8.9% the mass and 12% the radius of the Sun. Its visual luminosity is 0.00000373 that of our Sun. It rotates around itself in 3.3 days and has a high radial velocity of -54km/s towards our Sun. Observations with Kepler K2 for 79 days revealed starspots and infrequent weak optical flares at a rate of 0.38 per day.
TRAPPIST-1 is located 39.6±0.4 light-years away in the constellation of Aquarius and, since Feruary 2017, known to host at least seven near-earth-sized rocky planets detected by transit, all orbiting closer to the star than Mercury to the Sun, in that they are probably tidally locked and exposed to strong radiation and flares.
The planetary system is extremely dense. The closest distance between TRAPPIST-1b and TRAPPIST-1c is merely 1.6 lunar distances at conjunction. The planets should appear prominently in each others skies some larger than our Moon. The light on the surfaces at noon should be no brighter than twilight on Earth. Three of the planets orbit within the star's habitable zone, the others close to it. Fortunately, the tilt angle of the orbits to the line of sight to Earth is near 90° allowing astronomers to observe planet transits in front of the star.
Star Chart Sky Survey
ESO/N. Bartmann/spaceengine.org
Imaginary surface of one of the inner planets
Planetary System
| Planet | Class | Mass[M🜨] | Radius[R🜨] | Period[d] | SMA[AU] | Ecc | Density[D🜨] | Gravity[G🜨] | Flux[F🜨] | Temp[°C] | ESI |
|---|---|---|---|---|---|---|---|---|---|---|---|
| TRAPPIST-1 b | Hot Terran | 1.374 | 1.116 | 1.51083 | 0.01154 | 0.00622 | 4.155 | 1.103 | 363.702 | -273 | 0.57 |
| TRAPPIST-1 c | Hot Terran | 1.308 | 1.097 | 2.42194 | 0.0158 | 0.00654 | 2.217 | 1.087 | 310.828 | -273 | 0.73 |
| TRAPPIST-1 d | Warm Subterran | 0.388 | 0.788 | 4.04922 | 0.02227 | 0.00837 | 1.116 | 0.625 | 261.811 | -273 | 0.91 |
| TRAPPIST-1 e | Warm Terran | 0.692 | 0.92 | 6.10101 | 0.02925 | 0.0051 | 0.647 | 0.818 | 228.447 | -273 | 0.85 |
| TRAPPIST-1 f | Warm Terran | 1.039 | 1.045 | 9.20754 | 0.03849 | 0.01007 | 0.374 | 0.951 | 199.147 | -273 | 0.68 |
| TRAPPIST-1 g | Warm Terran | 1.321 | 1.129 | 12.3524 | 0.04683 | 0.00208 | 0.252 | 1.036 | 180.546 | -273 | 0.58 |
| TRAPPIST-1 h | Cold Subterran | 0.326 | 0.755 | 18.7729 | 0.06189 | 0.00567 | 0.144 | 0.572 | 157.05 | -273 | 0.46 |
Data source: The Habitable Worlds Catalog (HWC), PHL @ UPR Arecibo (phl.upr.edu/hwc)
- Temp: Equilibrium Temperature is a function of star luminosity and distance to a planet assuming a 0.3 bond albedo, not tidally locked. It is generally lower than its actual surface temperature, which depends on any atmospheric greenhouse effect which is unknown for exoplanets.
- ESI (Earth Similarity Index) introduced by UPR Arecibo. A duplicate of Earth is ESI = 1, Mars = 0.8. As a function of merely stellar flux and planet radii it is simple and convenient but a rather fuzzy comparison.
NASA/JPL-Caltech
Line Up
Physical Properties
A study using several space telescopes published on February 5, 2018 reveals that the planets range in mass from 0.3 to 1.16 terran masses, and with the known radii, resulting in densities from 3.4 to 5.6g/cm³.
Compositions
Planets c and e are almost entirely rocky, while the others have a layer of volatiles composed of either a water shell, an ice shell, or a thick atmosphere. TRAPPIST-1 d appears to have a liquid water ocean comprising about 5% of its mass (250 times more than the oceans on Earth), while the water layers of TRAPPIST-1 f and g are likely frozen as orbiting beyond the arm zone. The density of TRAPPIST-1 e being slightly higher than Earth, indicates a composition of rock and iron.
Atmospheres
The atmosphere of TRAPPIST-1b was found to be over the runaway greenhouse limit with an estimated 101 to 104 bar of water vapor. Planets c, d, e, and f show no hydrogen-helium atmospheres such as on Uranus and Neptune. Observations of Planet g revealed only insufficient data for firm conclusions.
Received Solar Energy
Planet c receives about the star energy received by Venus in our solar system, Planet d is in a position similar to Earth, while Planet f is a bit further out than Mars, with Planet e in between d and f. Planets are named with alphabets in order of detection, not size or distance from star. In case of the TRAPPIST-1 system the distances align with detection times.
NASA/JPL-Caltech
Based on available data, here are scientists' best guesses about the appearances of the planets
- TRAPPIST-1 b, the innermost planet, is likely to have a rocky core. In 2023, with the help of the JWST, astronomers have measured the temperature and concluded that the planet's dayside has a temperature of about 500°K (over 220°C)) suggesting that it has no significant atmosphere. Source
- TRAPPIST-1 c also likely has a rocky interior, but with a thinner atmosphere than planet b.
- TRAPPIST-1 d is the lightest of the planets -- about 30 percent the mass of Earth. Scientists are uncertain whether it has a large atmosphere, an ocean or an ice layer -- all three of these would give the planet an "envelope" of volatile substances, which would make sense for a planet of its density.
- TRAPPIST-1 e is the only planet in the system slightly denser than Earth, suggesting it may have a denser iron core than our home planet. Like TRAPPIST-1c, it does not necessarily have a thick atmosphere, ocean or ice layer -- making these two planets distinct in the system. It is mysterious why TRAPPIST-1e has a much rockier composition than the rest of the planets. In terms of size, density and the amount of radiation it receives from its star, this is the most similar planet to Earth.
- TRAPPIST-1 f, g and h are far enough from the host star that water could be frozen as ice across these surfaces. If they have thin atmospheres, they would be unlikely to contain the heavy molecules of Earth, such as carbon dioxide.
Imaginary surface of one of the outer planets
