Except in A FEW RARE cases, we can’t directly see planets outside of our own solar system. Just about every time a new exoplanet is discovered, the announcement is accompanied by an artist’s rendering of the world.

The drawings are often impressive feats, managing the tricky task of being both beautiful and scientifically accurate, while depicting something that no one has ever seen before. But each time you see one, you might wonder how closely they represent reality and what part of them is pure fantasy?

“We’re in the job of telling a story with these pictures,” said visualization scientist Robert Hurt of Caltech, who works with NASA to make exoplanet renderings. “The science visualization process is always starting with one or two data points and trying to make an engaging illustration.”

Many new exoplanet discoveries have come from NASA’s Kepler space telescope, a space-based observatory that carefully watched the light of more than 150,000 stars to try and detect a tiny dimming of brightness (though sadly, it suffered a malfunction last year that has rendered it inoperable). This dip represents a planet passing in front of that star and eclipsing its light. Kepler can’t give astronomers much more than a few pieces of information about each exoplanet; its approximate size, the distance from its star, the length of its year, and an estimate of its surface temperature.

The Kepler science team takes this data to Hurt and his collaborator, animator Tim Pyle, and works with them to produce illustrations. Together, they decide what aspects of the exoplanet they want to highlight. Kepler-186f, a recently discovered Earth-sized planet that might have liquid water on its surface, has been designated “Earth’s cousin.” So the image from NASA’s visualization team needed to suggest the idea that this place was a like home, just not exactly.

NASA artists spent a good deal of time figuring out how to make the recently discovered Kepler-186f (a.k.a. "Earth's cousin") look simultaneously like Earth and totally alien.

Hurt and Pyle used a few tricks to do this. The illustration of Kepler-186f (seen in the first slide above) looks at first glance like Earth, with landmasses, an atmosphere, and clouds, but upon closer inspection starts to seem more and more alien. The ice caps, for instance, are larger than ours, and light from the star has a different quality than the white sunlight we receive.

Though Kepler’s data couldn’t say for certain if there’s water on the planet’s surface, it’s a reasonable idea given the distance at which Kepler-186f orbits around its parent star. Therefore, the planet in the drawing has oceans. But to make things less-Earth like, the team went with a 50/50 ratio of water to land, rather than our planet’s 70/30 split. They also discussed the possibility of plant life, talking with astrobiology experts about what color this vegetation might be. Because the star produces more light in the red range of the electromagnetic spectrum, plant life would likely not be the familiar green of Earth, but rather a shade of orange. But because it leaps quite far into speculation, the artists didn’t want to come out and say for certain that there would be plants on Kepler-186f, settling instead on a coppery tint for the landmasses.

“We wanted to be consistent with the idea that there was life or plants, without saying there’s plants there,” said Pyle.

Hurt said color is a powerful part of the artists’ storytelling toolkit, and the closer the overall palette matches Earth’s, the more Earth-like the world is implied to be. Painting an exoplanet’s oceans the same cobalt blue of our own world will make a viewer naturally feel that it’s more like home, while lighter or darker blue shades could imply exoticness. The artists went through several iterations for the Kepler-186f drawing, giving it tweaks to “de-Earth” it, until it represented exactly what they wanted.

This dramatic rendering shows Kepler-16b eclipsing in front of its two parent stars. Illustrator

“We know we’re going to have people running by a newspaper stand, and seeing a headline that reads ‘NASA finds another Earth’ with this graphic next to it,” said NASA’s public affairs officer Michele Johnson, who coordinates Kepler’s press releases. “So we wanted to be very smart about the little we do know, saying this is our best interpretation, with a healthy amount of imagination as well.”

There are of course deviations from what’s physically possible. Most of these artistic shots are positioned behind a planet, with the star’s light on the opposite side. In real life, this would make the planet’s surface pitch black, with only a small sliver of daylight, but the picture needs to give a good view for the reader, instead showing a brightly lit surface. In the case of the Kepler-186f image, four other exoplanets can be seen shining up close to the star (with one even transiting in front of it to remind viewers of Kepler’s method of detection). These planets are far brighter and larger than they would be to an observer at the exoplanet, but serve as embellishments for the sake of a good story.

Depictions of non-Earth-like worlds are where the artist’s imagination really gets to take over. Kepler has found a whole host of strange exoplanets: lava worlds tidally locked to their star, planets disintegrating like comets, and planets orbiting a binary system, like the fictional Tatooine in Star Wars. Because our solar system contains no such planets, Hurt and Pyle follow whatever scientific principles they can for these illustrations. They will use information about the star’s color, for instance, or the surface temperature of the planet to invent creative views that could still be within the realm of possibility and can present relevant information through visual means.

“Ideally what you’ve shown them in the picture gives them a leg up in understanding what they see in the text,” said Hurt.