Imagine, for a moment, standing on the surface of Gliese 581g, and you’ll quickly appreciate just how unearthly it is. Its sun, though much smaller than our own, would loom far larger in the massive planet’s sky due to its close proximity, and light would filter down through a thick atmosphere to fall upon a landscape flattened by the world’s stronger gravitational field. But half the globe would never see the sun at all; tidal forces raised by the nearby star would quite likely have sapped the planet’s rotational energy until it spun once for every orbit, so that it perennially showed the same face to its star. One hemisphere would be bathed in light, sputtered by solar flares and harsh ionizing radiation, while the other would be forever shrouded in darkness. Perhaps only the thin ribbon of twilight encircling the planet from pole to pole would be hospitable.
Driven by the temperature difference between the two sides, high-altitude winds would whip around the planet in an eternal circulating storm. Or, if the atmosphere somehow lacked sufficient amounts of greenhouse gases like carbon dioxide, it could conceivably freeze out into a massive ice cap on the dark side, rendering the entire planet uninhabitable. A global ocean could act as a heat reservoir, preserving the atmosphere, but only if sufficient amounts of water were somehow delivered to the planet during or after its formation. Any way you slice it, this isn’t exactly home sweet home; you wouldn’t want to live there.