Putting the science in fiction - Dan Koboldt, Chuck Wendig 2018
Exoplanets and habitability
Star wars and the far future
By Jim Gotaas
Why do writers want to know about exoplanets?
Perhaps you want to write hard science fiction, in the spirit of Hal Clement, Poul Anderson, Gregory Benford, or Stephen Baxter, stories that fit in the magazine Analog Science Fiction and Fact. The discovery of exoplanets provides new worlds unlike any in our own solar system. If you’re more interested in space opera, these new planets give the chance to build stories with modern, exotic backgrounds. They may even trigger ideas for different types of stories.
On to goldilocks exoplanets!
The Goldilocks zone (more scientifically the habitability zone) is the range of distances from any star that allows liquid water to exist on an “ordinary” planet, similar to Earth. This concept centers around the idea that life as we know it requires liquid water at some point.
In our solar system, given the Sun as it is now, the Goldilocks zone stretches from inside the orbit of Venus to outside the orbit of Mars. This tells us that things aren’t always equal, since neither Venus nor Mars currently has liquid water. This is probably due to factors such as a runaway greenhouse effect for Venus and the smaller mass and lack of a magnetic field for Mars. (This is covered in more detail by Lynn Forrest in chapter 44, “Habitable Atmospheres.”)
Outside the Goldilocks zone, we’re fairly certain liquid water exists under the icy surface of Europa, one of Jupiter’s Galilean moons. Here, tidal heating provides the energy to support liquid water. We don’t know whether life can arise under these circumstances because we don’t know exactly how our sort of life arose, or precisely what conditions it needs.
The exoplanet menagerie
We’re getting a glimpse of the kinds of planets orbiting other stars. As of November 2017, we’ve identified more than thirty-five hundred definite exoplanets, with almost five thousand more still to be confirmed, with varying sizes and orbits around other stars. It turns out our old familiar solar system, with rocky planets close to the sun (Mercury, Venus, Earth, and Mars) and gas giants (Jupiter and Saturn) and ice giants (Uranus and Neptune) further out, isn’t at all typical.
Instead, we have hot Jupiters, hot Saturns (also known as puffy planets, because their density is so low that they would float in water—if you had a big enough bathtub!), hot Neptunes, Super Earths, water worlds, and even Chthonian planets (Chthonian describing inhabitants of the mythological Greek underworld, their version of Hell). The latest models of solar systems require radical movements of planets during the evolution of such systems.
With existing techniques, we’re more likely to find massive exoplanets that are close to their star, so the current catalog is biased toward these types. If you’re interested in detection methods, check out the webpage Exoplanet Detection Methods Visualized at the Planetary Habitability Laboratory (PHL).
Some newly discovered exoplanets are not new to science fiction, such as Super Earths (rocky planets that are larger and more massive than Earth) and water worlds (planets that are completely covered by oceans). These are still exciting because there’s now a scientific basis for stories about these worlds. But for our purposes, we’re going to concentrate on the exoplanets that are really new and different from our own solar system and traditional science fiction.
What are these new worlds like? Planets are usually characterized in terms of their mass (often expressed as multiples of the Earth’s mass), their radius, and their elemental composition. Beyond that, their physical state depends on the radius of their orbits (often expressed using astronomical units (AU), the average radius of the Earth’s orbit about the Sun), and the type of star they orbit. The last two basically determine the star’s habitability zone. Whether or not a planet in that zone can be considered Earth-like depends on the first three.
Astronomers have discovered a huge range of exoplanets. We’re going to look more closely at just a few of them that are most unusual and unexpected.
Hot Jupiters
Like Papa Bear’s porridge, these are just too hot. Roughly the size of Jupiter, their orbital radius ranges from 0.015 to 0.15 AU. For comparison, Mercury orbits at 0.39 AU. This means they’re very hot, which affects their atmospheric composition, but if they’re massive enough, they remain gas giants. (If they’re not that massive, they can become Chthonian worlds.) Orbiting so close to the star, they are tidally locked, meaning that they always show the same face to the star (just as our Moon is tidally locked to Earth, always showing us the same face).
An early discovery in 2005 was the extrasolar planet HD189733b, about sixty-three light years away. With a mass about 13 percent larger than Jupiter’s, this blue planet orbits the star every 2.2 days at an orbital radius of about 0.03 AU. Its dayside temperature is 1,700º F (927º C), with a nightside temperature of 1,200º F (649º C).
Current theories about the origin of planetary systems require that gas giants must form out where hydrogen can easily condense into a planet (such as Jupiter or Saturn). At some point, they migrate into a close orbit about the star and heat up dramatically. During this migration, smaller planets such as Earth would be flung wildly around the system.
Hot Neptunes
More Papa Bear exoplanets! Hot Neptunes are similar in mass to Neptune or Uranus, but their orbital radius is normally less than 1 AU (compared with 30 AU for Neptune). One example orbits the star Gliese 436 every 2.64 days, trailing a sort of comet tail as it moves, the result of atmospheric hydrogen boiling off into space. If a hot Neptune loses all its atmosphere, it can end up being a rocky planet similar in size to Earth.
Chthonian Planets
Yet more Papa Bears: Chthonian planets are to some extent hypothetical. They are basically supermassive rocky or metallic worlds, although we think they have evolved from hot Jupiters whose atmospheres have been boiled off by radiation from the star, leaving behind a solid remnant core. One suspected example is CoRoT-7b, located 480 light years away, which orbits at about 0.02 AU, is 70 percent larger than Earth and almost five times as massive. With a dayside surface temperature of about 3,600º F (2,000º C), if it has any atmosphere at all, it will be a tenuous one of vaporized rock.
Potentially habitable exoplanets
Welcome to Baby Bear’s bowl of planetary porridge: These worlds are potentially “just right,” lying in the Goldilocks zone. The key word here is potentially, since our current detection methods can’t provide much detail about the masses or atmospheres of these worlds. You can find a graphic of some likely candidates at the PHL website, giving their astronomical designations and distances from Earth.
We don’t actually have direct images of any of these exoplanets. They’re all larger than Earth, but we have no estimates of their masses and we’re not certain about their composition and habitability. We think they’re roughly the right size and in the Goldilocks zone for their stars, but are subject to change as new interpretations or astronomical observations are made.
It’s difficult to detect atmospheres of exoplanets, as you have to see the effects against the backdrop of the star’s intense light. So far, all such examples are hot Jupiters or hot Neptunes. A variety of gases has been detected, including hydrogen, helium, carbon, oxygen, sodium, water vapor, carbon monoxide, and methane.
What about mama bear?
Current techniques for finding exoplanets aren’t accurate enough to give detailed descriptions of planets lying outside the habitability zone, which would make them too cold. Several hundred have been found, and astronomers have directly imaged four massive planets orbiting HR 8799, a young system (only 30 million years old, compared with the Sun’s age of 4.5 billion years) located 129 light years from Earth. These planets have estimated masses about five to seven times that of Jupiter, at distances ranging from 14.5 to 68 AU.
Watch these spaces!
The list of confirmed exoplanets will certainly grow, and new telescopes will provide more information about their physical characteristics, including their atmospheres. In a few years’ time, we may be able to describe many more of these planets much more completely. For exoplanet science fiction, these are exciting times!
Further reading
· For up-to-date information about exoplanets, visit the NASA Exoplanet Archive at https://exoplanetarchive.ipac.caltech.edu.
· If you’re interested in detection methods, check out the webpage Exoplanet Detection Methods Visualized at the PHL http://phl.upr.edu/library/media/exoplanetdetectionmethodsvisualized
· For the latest information about the habitable exoplanets that have been discovered, visit http://phl.upr.edu/projects/habitable-exoplanets-catalog
· A more leisurely explanation of most of these ideas can be found in The Planet Factory: Exoplanets and the Search for a Second Earth by Elizabeth Tasker (Bloomsbury Sigma, 2017)