TRAPPIST-1g is the sixth closest planet to the star, the largest in radius, and the last one inside the habitable zone. It orbits in a near-circular orbit at a distance of only 6.74 million km (0.045 AU) from the star. It is approximately 34% more massive than the Earth and 13% larger in radius. Its density is only slightly lower than Earth’s, making it our distant doppelganger. It receives only a quarter of the solar irradiance that Earth does, giving it a mean surface temperature of 199 K (-74°C). Warmer regions could support surface water and conditions favorable to Earth-like life, since its gravity is sufficient to hold a substantial atmosphere.

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

The star’s size as viewed from the surface of TRAPPIST-1g would be 1.4 degrees across, about 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 five 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-1f would appear about one fourth larger than our moon does from Earth. On the opposite side of the planet, it would be perpetually dark, broken only by TRAPPIST-1h frequently racing across the sky. 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-1h would cover 0.21 degrees, less than half as large as the moon does from Earth. Meanwhile, the distant stars would rise in the east and cross the sky to set in the west, bit much slower than they do on Earth. With a year only 12.4 Earth-days long, the entire celestial sphere would be observable every 1.7 Earth-weeks!