About 260 light years away from Earth, there is a wild exoplanet about the size of Jupiter -- but with double its mass. Known as WASP-43b, this huge planet orbits its host star, an orange dwarf, in just 19 short hours, meaning its “years” are shorter than Earth’s days.

The recipe for creating a habitable planet turns out to be surprisingly simple: Just add water—and atmospheric gases. Mars has both, relics from four billion years ago when the planet was warm and wet. “When it comes to Mars, and only Mars, the notion of terraforming is no longer in the realm of science fiction,” says NASA astrobiologist Chris McKay. Humans could warm the planet and restore a thick atmosphere in a matter of decades, but producing breathable levels of oxygen would take 100,000 years with today’s best technology: plants. New inventions could, in theory, speed that along too. “Living off the land is going to be essential for long-term human explorers beyond Earth,” says Laurie Leshin, a geochemist on the Mars Curiosity team. “We have to figure out how to do this stuff.”

Given that we are unlikely to be visiting an exoplanet any time soon, astronomers have been contemplating whether it might be possible to detect indications of simple life - a biosignature - from a distance. Many think that the strongest case for extraterrestrial life would be the discovery of oxygen and methane on the same body. They also think that the likelihood of finding such a biosignature is greatest on an Earth-like planet that is orbiting a sun-like star.

In their infancy, maybe even into their adolescence, Earth and Mars were more alike than they were different. Each rocky world was pockmarked by volcanoes, and waterways carved both surfaces. Atmospheres blanketed both planets, and each harbors rocks that could only have formed in lakes or oceans. Four billion years ago, alien visitors standing on each planet’s surface might have looked up and beheld an identical blue sky.

What would a habitable, watery Mars look like? Four billion years ago, the dry, dusty planet may have had enough of an atmosphere to support bodies of water. Perhaps it looked like this beautiful, cloud-covered lake scene that animators at NASA's Conceptual Image Lab imagine in their latest video.

Despite its gusty reputation as a "gas giant," Jupiter's blood-red clouds hide a dense, rocky core that's perhaps 20 times as massive as Earth. That core blocks any spacecraft's passage through the centre of the planet, but even a detour through the clouds would be a disaster.