Science writer Pat Brennan has published a great article on the NASA exoplanets website titled “Life in the Universe: What are the Odds?”. In the article, he published a diagram showing the habitable zones of our solar system, and the TRAPPIST-1 system. The amazing thing is how cramped the TRAPPIST-1 system: the orbits of the TRAPPIST-1 planets (seven in total) could fit into the orbit of Mercury!

So, probably, all these planets are tidally locked to their star (an ultra-cool red dwarf star), making the existence of the extraterrestrial life highly unlikely on the TRAPPIST-1 system, despite at least three of these seven planets (e, f, g) are in the habitable zone (the area around a star where it is not too hot and not too cold for liquid water to exist on the surface of surrounding planets).

Up to six of these seven TRAPPIST-1 system planets could be in the optimistic habitable zone (c, d, e, f, g, and h), with estimated equilibrium temperatures of 170 to 330 K (-103 to 57 °C or -154 to 134 °F)

TRAPPIST-1 System: a very-cramped planetary system

All seven of TRAPPIST-1’s planets orbit much closer than Mercury orbits the Sun. Mercury’s orbit is not a perfect circle, so its distance from the sun varies. The minimum distance from the sun to Mercury is 28.5 million miles or 45.86 million kilometers. The maximum distance is 43.5 million miles or 70 million kilometers.

TRAPPIST-1 system - solar system habitable zones
The comparison of the habitable zones of the TRAPPIST-1 system and the Solar System. All seven planets of the TRAPPIST-1 system could fit into the orbit of Mercury, the closest planet to the Sun. Green bands represent the orbital distance from a star at which temperatures could allow liquid water on the surface of a planet. While this depends on other factors, including the presence of a suitable atmosphere, it’s a useful way for astronomers to begin sorting among potential targets in the search for possible signs of life.

The planets of the TRAPPIST-1 system are also very close to each other: for example, the distance between the orbits of TRAPPIST-1b and TRAPPIST 1c is only 1.6 times the distance between the Earth and the moon (which is roughly 238,855 miles or 384,400 kilometers). According to NASA, “if a person was standing on one of the planet’s surface, they could gaze up and potentially see geological features or clouds of neighboring worlds, which would sometimes appear larger than the moon in Earth’s sky.”

Even the star in the center, the TRAPPIST-1 (designated 2MASS J23062928-0502285) has a radius slightly larger than the planet Jupiter. Its mass is 84 times greater than that of Jupiter’s, though. It is an ultra-cool red dwarf star of spectral class M8.0±0.5 that is approximately 8% the mass of and 11% the radius of the Sun.

TRAPPIST-1 vs our Sun size comparison
TRAPPIST-1 vs our Sun size comparison. By ESO –, CC BY 4.0, Link


  • TRAPPIST-1 on Wikipedia
  • Life in the Universe: What are the Odds? by Pat Brennan. March 9, 2021. On the NASA exoplanets website
M. Özgür Nevres

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