The Reality of Close Proximity to Planets in Sci-Fi

In some sci-fi and fantasy drawings, characters are shown looking at planets in the sky that look close enough to touch. Could this ever actually happen or would the proximity cause chaos? Let's delve into the science behind these scenarios.

In our solar system, all the planets are so small that they appear from the surface of other planets as slightly bigger stars. They do not twinkle and are easily distinguishable from actual stars in the sky. However, in the realm of other types of solar systems, such as tight compact systems around red dwarf stars, like the Trappist system, planets are so close to each other that they appear much larger.

The book “Under Alien Skies” by Philip Plait has an interesting chapter that explains this phenomenon in detail. In the Trappist system, the planets are much closer to each other, making them appear significantly larger in the sky. The image below shows what the sky would look like, with the other two medium-sized planets having a slightly reddish tint.

Exaggerated Scenarios in Sci-Fi

While the images in sci-fi often exaggerate the size of the planets for visual impact, the actual reality is quite different. In NASA's image of what Earth looks like from Mars, the planets do not appear nearly as large as in many sci-fi depictions. If you get as close as those sci-fi images suggest, the two planets would either be orbiting each other as a binary or the smaller planet would become a moon of the larger planet. Get too close and you might pass the Roche limit, turning into an asteroid belt.

Possible Scenarios

There is one scenario in which something like this could be possible. By placing a gas giant, itself uninhabitable, in the Goldilocks zone, and giving it a moon the size of Earth, you could create a habitable planet (a Class M planet in Star Trek technobabble) with a very large planet in the sky, perhaps with rings as shown in the illustration. More realistically, the planet might be within the rings of the gas giant.

For example, consider the skies of Jupiter's moons. None of Jupiter's moons have more than traces of atmosphere, so their skies are nearly black. An observer on one of Jupiter's moons would be overwhelmed by the sheer size of the gas giant. On Io, the closest large moon to Jupiter, Jupiter's apparent diameter would be roughly 20° - 38 times the visible diameter of the Moon, covering about 5% of Io's sky. On Metis, the innermost moon, Jupiter's apparent diameter would be around 68° - 130 times the visible diameter of the Moon, covering approximately 18% of Metis's sky.

Conclusion

The concept of planets in the sky so close that they appear to touch is indeed scientifically intriguing. However, the reality of such proximity would have significant and potentially chaotic effects on the planetary system and the inhabitants of such planets. While it is not impossible, the scenarios depicted in many sci-fi works are usually exaggerations for artistic effect.