TRAPPIST-1f is the fifth closest planet to the star, and is well inside the habitable zone, orbiting in a near-circular orbit at a distance of only 5.55 million km (0.037 AU). It is approximately 68% the mass of the Earth and very slightly larger in radius. Its density is only 60% of Earth’s, suggesting it contains more interior water/ice or other light elements, similar to Jupiter’s moon Io. It receives only 38% of the solar irradiance that Earth does, giving it a mean surface temperature of 219 K (-54°C). Many regions could support surface water, conditions favorable to Earth-like life, if its lower gravity is sufficient to hold an atmosphere.

Artist concept showing what TRAPPIST-1f may look like. Image credit: NASA/JPL-Caltech

The star’s size as viewed from the surface of TRAPPIST-1f would be 1.7 degrees across, a bit more than 3 times larger than we see our own Sun. The planet is tidally locked to its star, keeping the same face towards it at all times. For a visitor to the sunward side of the planet, the dull red sun would hang in place, neither rising nor setting. The four inner planets would regularly cross the sky, probably transiting (but not eclipsing) the sun’s disk each time, and occulting one another. At solar conjunction, the closest one, TRAPPIST-1e would appear about as large as our moon does from Earth. On the opposite side of the planet, it would be perpetually dark, broken only by the outer two planets racing across the sky, regularly passing near or in front of one another. Unlike our Solar System, the other TRAPPIST-1 planets are close enough to show as small disks, even with naked eyes. At closest approach, the next planet out, TRAPPIST-1g would cover 0.69 degrees, one-third larger than the moon does from Earth! Meanwhile, the distant stars would rise in the east and cross the sky to set in the west, but much slower than they do on Earth. Still - with a year only 9.21 Earth-days long, the entire celestial sphere would be observable every 1.3 Earth-weeks!