TRAPPIST-1e is the fourth closest planet to the star, and the first inside the habitable zone, orbiting in a near-circular orbit at a distance of only 4.21 million km (0.028 AU). It is approximately 62% the mass of the Earth, but only slightly smaller in radius (92%), making it a bit smaller than Venus. Its density is only 80% of Earth’s, suggesting an iron core and silicate rocky mantle composition similar to Earth, but with more water/ice or other lighter elements. It receives only 66% of the solar irradiance that Earth does, giving it a mean surface temperature of 251 K (-22°C). Some 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-1e may look like. Image credit: NASA/JPL-Caltech

The star’s size as viewed from the surface of TRAPPIST-1e would be 2.2 degrees across, more than 4 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 three 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 closer one, TRAPPIST-1d 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 other three 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-1f would cover 0.57 degrees, a bit 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 slower than they do on Earth. With a year only 6.10 Earth-days long, the entire celestial sphere would be observable every 146 hours!