We love stories that take us to alien planets and let us explore whole new environments. But not every alien planet is totally realistic, especially given how much we've learned about exoplanets lately. So we asked six experts to tell us the biggest mistakes they see in fictional habitable worlds — and here's what they told us.


Until recently, science fiction creators had to imagine alien planets, without knowing that much about real-life exoplanets. But in the past 20 years, we've discovered over 1000 confirmed exoplanets, and learned a lot more in the process. Here's what our six experts told us bugs them about alien planets in science fiction:

Not being weird enough

Given the amazing variety of exoplanets we've been finding out there, and "the remarkable range of orbital and physical characteristics they seem to have," writers and creators really should be more bold in the kinds of planets they choose to portray, says Andrew Fraknoi, chair of the Astronomy Department at Foothill College in Los Altos Hills, CA.


"The diversity of exoplanets is apparently so great that nearly any rendering in science fiction could occur in cosmic nature," adds Geoff Marcy, Professor of Astronomy at UC Berkeley, who's credited with discovering 70 out of the first 100 exoplanets we found.

In fact, the best candidates for supporting life at the moment appear to be tidally locked planets orbiting red dwarfs, says Seth Shostak, Senior Astronomer with the SETI Institute. "And if you have biology on planets around such stars, it will be mostly confined to a thin strip along the terminator. That would be interesting, of course, and quite unlike most of the planets you see in films."

Assuming exoplanets must have ecosystems as diverse as our own

People often criticize science fiction for having planets with a single ecosystem — we've leveled that criticism in the past, in fact. Science fiction is full of "desert planets" (like Tatooine) or "ice planets" (like Hoth.) But in fact, there's nothing wrong with aplanet with a more homogenous ecosystem than Earth, says Greg Laughlin, Professor and Chair in the Department of Astrophysics at UC Santa Cruz.


"I've often thought that both Hoth and Tatooine are quite convincingly presented as representative habitable 'worlds,'" says Laughlin. "Hoth could credibly be an Earth-like planet in the grip of a "snowball earth" geologic period, similar to what we know that Earth has experienced at least twice in its history, and recent research suggests that desert planets such as Arrakis (Dune) or Tatooine might be better suited to resist long-term greenhouse warming than a moist planet such as Earth."

"The binary Sun for Tatooine was a brilliant visual shorthand that immediately telegraphs that the planet is extraterrestrial," adds Laughlin. If anything, the biggest mistake in imagining fictional alien worlds might be "to envision planets that are too similar to Earth. I have a sense that Earth is somehow less monochromatic than many worlds that present habitable (or marginally habitable) environments," Laughlin says.


Picking the wrong star

There are two ways that happens, which irritate Stanford University physics professor Bruce A. Macintosh:

[The first problem is picking] a star that's highly unlikely to ever host a habitable planet. A good example is Epsilon Eridani. It's got a good name, very popular choice. But it's also far too young a star to have a planet that is likely to be habitable - there's good astronomical evidence that the star is only about 500 million years old - far too young to have really built up a ecosystem that would build up an oxygen atmosphere. In part because it's young, the system has a huge number of comets and asteroids (so the planet is probably getting massively bombarded with meteors), etc. Another very common example (though less common these days) is to pick a massive hot star (like say Vega), because massive stars are almost always young (since they have short lifetimes.) My second pet peeve is picking a star based on recent and trendy discoveries - a star which has hints of a habitable planet from current astronomical measurements. All the possibly-habitable planets we have now are all marginal and uncertain and the odds that any given one will ultimately prove to be habitable are very slim.


Not having enough oceans

For a long time, most alien planets "used to all look like the landscapes around Los Angeles, for obvious reasons," says Shostak, but at least that's no longer the case. Still, Shostak says one thing is missing: "How often do you see oceans on these alien worlds? They seem to be 'all continents,' as far as I can tell."


In fact, Marcy says that continents may be the rarity:

The Earth may be very rare in one attribute few people realize. The Earth has 0.03% of its mass in ocean water. Yet, the continents, stemming from plate tectonics, just barely poke above sea level. Many "Earth-like" planets may receive twice as much water, or even 5 or 10x as much. Why not? If so, such planets would be more like a bad Kevin Costner film. Without continents, the development of technology would be slowed and compromised, if not impossible. How can an intelligent species invent fire, the printing press, or iPhones on a water world? It seems difficult. Perhaps planets enjoying a thin veneer of water, with land barely poking above sea level, are actually rare.


Shoving planets too close together

One thing that bothers Abel Méndez, Associate Professor of Physics and Astrobiology with University of Puerto Rico at Arecibo is when you see "multiple planets or moons close to each other, when viewed from space." It may look cool, but it's a recipe for planetary collision. "Planets could not be that close in reality," he warns.


Alien creatures that don't make sense

That could be a whole other article — but Shostak has a few major gripes about the inhabitants of alien worlds. For example, "megafauna on planets that look barely able to sustain bacteria."


There's a huge problem with "assuming native complex life or even intelligent life in desert planets when there are no traces of primary food source (e.g. plants) to support these life forms," adds Méndez. "This has problems with how the food chain works and evolution."

Shostak also points to the fact that "intelligent inhabitants are (1) generally very similar to us in appearance (four appendages, etc.), and (2) more or less at our level of technical sophistication, which means we can take them on in dogfights, etc."