While it will be several years before astronomers can study the TRAPPIST-1 planets’ atmospheres to determine whether any biological activity is present, it is fun to imagine what life would be like on TRAPPIST-1e, f, or g - the planets in the habitable zone.

Planets that are tidally locked behave like our moon, rotating once per orbit (their year) while keeping the same part of the planet pointed at the star at all times. You’d need to decide whether to live in the constant sunshine of the daytime side, the constant darkness on the hemisphere facing away from the star, or a compromise. You’d likely take up residence somewhere along the terminator – the circle around the planet that separates the permanent day side from the permanent dark side. This way, the red dwarf star, your sun, would remain fixed in view near the horizon at all times. Or, you could dwell where the star is below the horizon and only the twilight glow is present – like a permanent sunset!

Cycles of day and night would not occur. The planets in the habitable zone orbit the star once every 6, 9, and 12 days respectively. If your planet has a tilt to its polar axis, you would see the sun shift north and south on a timescale of days. The differences in solar insolation would not last long enough to allow winter or summer to set in. The sunward side would be much warmer than the dark side, but planetary scientists predict that strong global wind systems would circulate to balance the temperatures.

Your sun’s light would be much redder and duller than Earth’s. Beings would likely evolve larger eyes that are sensitive to the infrared part of the electromagnetic spectrum, and their skins could be dark-coloured to absorb the meagre heat. Plant-life might have black vegetation. On the rocky, Earth-sized TRAPPIST-1 planets, gravity would be similar to Earth – ranging from between half and twice of ours. Complex life-forms would probably grow to similar heights and masses as Earth-life. If there is abundant surface liquid water, perhaps they’d be ocean-faring – or ocean dwelling!

If the TRAPPIST-1 planets have small orbital inclinations (tilts), they would frequently eclipse and occult one another. Instead of solar eclipses, you’d experience solar transits, possibly with multiple simultaneous transits repeating very often due to the short orbital periods of the seven planets. It appears that the TRAPPIST-1 planets’ orbital periods are close to resonance, meaning that their years differ by simple fractions, causing very frequent repetitions of their relative positions in space.

The small size of the TRAPPIST-1 system means that the planets are much closer together than those in our solar system, certainly allowing clear views of the surfaces of the other planets, even through a small telescope. In many cases, the neighbouring planets would appear several times the size of our moon viewed from Earth! A technically advanced civilization would find interplanetary communications and travel much easier than in our solar system. Astronomy would be a regional pursuit. Solar astronomers would be able to monitor the star constantly from sun-side observatories, while stargazers would set up on the dark side of the planets and gaze back at us for as long as they wish!