If our civilization survives for another decade or two, we may get our first glimpse of a planet remarkably like Earth. The red dwarf star Gliese 581 is about 20.5 light years away, practically next door. In 2006, two planets were discovered orbiting it. Both are giant worlds like Jupiter, detected by the subtle wobble they induce in their sun as they tug it slightly to and fro with their gravitational fields. Shortly afterwards, astronomers announced the discovery of a third planet, dubbed GL 581 c. Two things about it are intriguing: Its mass is only about 5 times greater than Earth's (as opposed to Jupiter's 300 times greater heft), making it one of the smallest extrasolar planets initially detected. If it is made of rocky material like earth, it would be only about 75 percent larger than our world. The second interesting thing is its orbit, which places it squarely in its parent star's Goldilocks Zone, where it is neither too hot nor too cold to sustain liquid water. This was the first exoplanet discovered that could conceivably look something like Earth, with white swirling clouds and vast oceans.
We should be reluctant to draw a graph using two data points, however, and all we know about this world is its mass and orbit. Gliese 581 shines with only 1.3 percent of the Sun's luminosity, so a planet would have to orbit 14 times closer than Earth orbits the Sun in order to receive the same amount of heat. GL 581 c does this, in fact, giving it a year that is only 13 earth-days long. In such a close orbit Gliese 851c has probably become tidally locked, so that its rotation period matches its orbital period. This means that the same hemisphere would always be turned toward the planet's primary. Our own moon does this, so that we see only one side of it.
Such a planet might have weather systems different than those on Earth. The subsolar point might be a steaming teakettle under a perpetual shroud of clouds. Climate might become more temperate as one moves farther from this zone. In the painting to the right I've imagined a scene near the pole of the planet at the terminator, the line separating perpetual day from perpetual night. The sun is fixed on the horizon, transited frequently by inner planets big enough and close enough to show detail to the unaided eye. In their quest for the unmoving sun's energy, plants struggle to grow out of shadows cast by their neighbors. Waves of trees march toward the horizon, leaving valleys of deadwood in their shaded wake in which new growth is sustained only by sunlight scattered by the sky.