TRAPPIST 1’s Habitable Zone Explained

Image Source: NASA/JPL-Caltech

We all know about NASA’s recent discovery. Just to recall, NASA discovered 7 planets 39.5 light years (12 parsecs) away that are potentially capable of hosting life. Located within the constellation Aquarius, each of these planets has sizes and masses similar to that of Earth and Venus.

These seven planets orbit a star named TRAPPIST-1, with a maximum orbit time of around 20 days.

TRAnsiting Planets and PlanetesImals Small Telescope made the initial discovery, hence the acronym TRAPPIST.


“Habitable zone” is a more commonly used phrase for the circumstellar habitable zone (CHZ). Moreover, it’s a name given to the planetary range of orbits that, provided there’s sufficient pressure, can sustain liquid water on the surface.

Water is life. It is necessary for the propagation and sustenance of life for a plethora of reasons. Most of all, it is required in most metabolic processes in the human body, it’s a key reagent in photosynthesis, it is the product of acid-base neutrality – the list goes on and on.

These seven aspects determine if the planets are likely to be habitable or not.

  1. Earth Similarity Index (ESI): The planet’s radius, density, temperature and escape velocity in comparison to Earth.
  2. Standard Primary Habitability (SPH): The planet’s suitability for the growth of vegetation.
  3. Habitable Zone Distance (HZD): The distance of the planet from the star’s habitable zone. This depends on the planet’s size, as well as the star’s temperature and luminosity.
  4. Habitable Zone Composition (HZC): The bulk composition of the planet, where the ideal composition is a mixture of iron, rock, and water.
  5. Habitable Zone Atmosphere (HZA): How likely it is for the planet to sustain a habitable atmosphere. This depends on the planet’s orbit size, radius, and mass, as well as the star’s luminosity.
  6. Planetary Class (pClass): The thermal zone of the planet.
  7. Habitable Class (hClass): The temperature of the planet itself. Ideally, it can be anywhere between 0°C to 50°C.

All seven of these planets are terrestrial, meaning they’re primarily silicate rocks or metal. However, of interest are three planets, TRAPPIST-1e, TRAPPIST-1f, and TRAPPIST-1g which are thought to be in its habitable zone.

Apart from this, the furthest planet – TRAPPIST-1h has an atmosphere that retains heat, similar to that of Venus. This planet also seems like it can host life.

Whilst every planet orbiting TRAPPIST-1 can potentially have liquid water sustained on the surface, it’s only on these four planets that life is a distinct possibility.

TRAPPIST-1 is an ultra-cool dwarf star around the size of Jupiter. Because of its smaller size, the orbital radius of the seven planets is much smaller than that of our solar system. Hence, each planet completes a full orbit in significantly less time as compared to our solar system.

Comparison of habitable zone of TRAPPIST-1 and our solar system
Scale rendition of our solar system’s orbital radius compared to TRAPPIST-1’s. Image Credits: NASA

The distance of farthest orbiting planet of TRAPPIST-1 is almost 6% that of the distance between Earth and sun. Not only that, Mercury, the closest planet to our sun, is almost 7 times further from our sun than TRAPPIST-1’s furthest known planet.

This means TRAPPIST-1’s habitable zone is much smaller than that of our solar system.

TRAPPIST-1 : Image result for earth
Comparison of TRAPPIST-1’s planet orbits and our solar system’s planetary orbits. Image Credits: NASA

More interesting facts and details about all of these planets can be found on the TRAPPIST website. There are some artistic representations of what the planets could look like.

The potential of finding extra-terrestrial life on one of the TRAPPIST-1’s planet is as ground-breaking as it is real.


Have you read?
NASA Released Travel Posters For Our New Sister Solar System TRAPPIST-1


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