The Trappist System

Their star

TRAPPIST-1 is the red-glimmering dwarf star in the most studied planetary system aside from our own solar system, lying just about 40 light years away from us. It's mass is just 9% that of the sun, yet it is surrounded by 7 rocky planets, each of which holds the potential for water. Despite it being approximately twice as old as our sun, it is estimated to survive more than 100 billion more years, while our sun is expected to turn into a cooling white dwarf in about 7 billion years.

Habitability

As mentioned before, all of the 7 rocky exoplanets surrounding the Trappist's red dwarf could hold water while four of them are within the habitable zone of their star. This is especially interesting because not only is it extremely unlikely for a system to have over 55% of it's planets within the habitable zone but the fact they had a 3 billion year head start on our system makes them a possible oasis of life.

The expected longevity of their star shows just one thing: If we would have been born into the Trappist System instead of our solar system, we would have a much easier time going multiplanetary and any potentially intelligent lifeform in the Trappist System will be able to leverage that. Additionally, all of the Trappist Planets are so close to each others that the entire system easily fits in just half the orbit of Mercury, making it very easy to traverse between them.

There is one catch to all of them, though: They are all - some more, some less - suspected to be tidally locked, meaning one side permanently faces their star and the other permanently faces space, making only half of the planet hot enough to be habitable at all.

Trappist 1B

Trappist 1B is a rocky exoplanet being roughly 1.12 times the size of the Earth. The atmosphere on Trappist 1B is expected to be very hot and rich of CO₂. If this reminds you of Venus, kudos to you cause same, but Trappist 1B is expected to be too hot to allow the formation of sulfuric acid clouds, making it a really bad option for colonization since we learned that those clouds are one of the very few things coming in handy on Venus.

Trappist 1C

There is not much known about Trappist 1C apart from it being a bit bigger than Earth and a rocky exoplanet like all of it's neighbor planets. It is expected to have equal or similar conditions as 1B.

Trappist 1D

Trappist 1D is a small earthlike planet with around 0.78 times the size of Earth. It orbits around it's sun on the edge of the habitable zone, however according to model calculations it experienced a runaway greenhouse effect. A runaway greenhouse effect is an unstoppable and self-reinforcing greenhouse effect that eventually leads to the evaporation of all liquid water on a planet. Despite all that, a small amount of water might still be around, giving Trappist 1D an Earth Similarity Index (opens in a new tab) of 0,91 or 91%. This index is being calculated based on the radius, density, velocity and temperature of a planet.

Trappist 1E

Trappist 1E is the first planet in our observation that is not just on the edge of it but right inside the habitable zone. With a size 0.92 times the Earth, an equilibrium temperature of -27 °C and the possibility to be an ocean world, it has an Earth Similarity Index (opens in a new tab) of 0,85 or 85%.

If you are wondering why it isn't higher than the one from Trappist 1D, it's mostly because the mass is much lower than earth (just 69% of it, and no, this is not a joke) and we still lack information about the atmosphere and temperature of Trappist 1E, so it's Earth Similarity Index (opens in a new tab) might increase once we get our hands on more detailed information about it.

Trappist 1F

Trappist 1F has a size almost exactly like Earth, having a radius just 4.5% bigger than Earth's, with a mass just 68% of Earth's. Because of the latter, it is unlikely for Trappist 1F to be a very earthlike rocky planet.

Simulations suggested that 20% of Trappist 1F is composed of water. For most of it's surface area, liquid water is likely to only exist in clouds near the top of the atmosphere.

The most interesting part about Trappist 1F is that it is very likely an eyeball planet, tidally locked, with one side permanently facing the star and the other side permanently facing the dark void of space. One might harbor liquid water while the other is extremely likely to be covered in eternal icy rocks as it never faces the system's star.

At the small edge where these two sides meet, the so called terminator line, the temperatures may be suitable for liquid water and therefore life to exist, meaning life on Trappist 1F might be trapped to the ring around it's planet that is the terminator line, the only place habitable on their planet.

Trappist 1G

Trappist 1G is the second last planet orbiting it's system and is 1.15 the mass and radius of earth whereas the density is just 76% of it. This indicates that it might host oceans of water which would decrease it's average density to the level it appears to currently be.

On the 31st August of 2017, reports from the Hubble Space Telescope first delivered evidence of water on Trappist 1G, making it's sun-facing side a very realistic candidate for habitability and life, even though Trappist 1G orbits around it's star at the outer border of the habitable zone.

Trappist 1H

Being the outermost planet of it's system, Trappist 1H is expected to be cold yet very likely to have water: While being 0.77 times the size of Earth, it only has one third the mass of it, indicating lots of water making up the planet's composition. According to observations from 2021 and 2022, Trappist 1H most likely doesn't have an atmosphere, which could mean that water is trapped in a subsurface ocean, similarly to Europa in our solar system.

Summary

Trappist 1D, E, F and G are reasonable candidates for habitability and primitive life due to the fact they had much more time to develop it than our Earth did. It would be very surprising to find no traces of life on any spot on all four of them, so the moral question remains: Should we visit them or should we let them live and evolve in peace? Do we have any right to colonize their planet, even if they are primitive and have no desire to build a civilization?

Don't look at me, I'm not here to judge, I'm merely provoking the question.

Additional Resources

1.
Apart from the pretty decent Wikipedia pages for all of the Trappist System planets, this video by Melodysheep is a beautiful visualization of them.