TRAPPIST-1b is the planet closest to the star, orbiting in a near-circular orbit at a distance of only 1.66 million km (0.011 AU). It is approximately 85% of the mass of the Earth (comparable to Venus), but almost 9% larger in radius, suggesting it contains more water/ice or other lighter elements than Earth. It receives 4.25 times more solar irradiance than Earth, giving it a mean surface temperature of 400 K (127°C) – too hot for surface water or Earth-like life to be present.

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

The star’s size as viewed from the surface of TRAPPIST-1b would be 5.6 degrees across, more than 11 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. On the opposite side of the planet, it would be perpetually dark, broken only by the other six planets racing across the sky, regularly passing near or in front of one another. Unlike our Solar System, the other six planets are close enough to show as small disks, even with naked eyes. At closest approach, the next planet out, TRAPPIST-1c would appear nearly three times 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 a little slower than they do on Earth. With a year only 1.51 Earth-days long, the entire celestial sphere would be observable every 36 hours!