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ツア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/


Imaginary surface of one of the inner planets


Planetary System

 Planet bPlanet cPlanet d Planet ePlanet fPlanet gPlanet h
Period [days]1.512.424.056.109.2112.3618.76
Semi-Major [AU]0.01150.01580.02320.02930.03850.04690.0619
Distance [mkm]1.732.373.334.385.767.019.27
Radius [R🜨]
Mass [M🜨]
Density [D🜨]0.730.880.621.020.820.760.72
Gravity [g]0.810.960.480.930.850.870.55
Star Energy [F🜨]
Equilibrium Temp [°C]9340-9-43-72-91-119
ESI [Earth = 1]0.560.740.900.850.680.580.45
  1. Planet names in green orbit within the habitable zone (0.019 - 0.051AU) of the host star.
  2. 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.
  3. 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.


Stellar Flux


Line Up


Physical Properties

A study using several space telescopes published on February 5, 2018 reveals that he planets range in mass from 0.3 to 1.16 terran masses, and with the known radii, resulting in densities from 3.4 to 5.6 g/cm³.


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.


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.


Imaginary planet in the habitable zone


Based on available data, here are scientists' best guesses about the appearances of the planets



Imaginary surface of one of the outer planets