In 1954, a high school physics teacher wrote a science fiction story set on a highly oblate exoplanet, one so squished the gravity varied dramatically from equator to pole. Now researchers are contemplating how to detect such bizarre, alien worlds in our ever-expanding search for exoplanets.
Artist's concept of an oblate spheroid of an alien world. Image credit: Shivam Sikroria.
Gravity pulls. As it pulls, it can distort. We know this concept from how the sun and moon drive tides on Earth, and are constantly drawn to how the ramifications of tidal stresses play out on icy moons around the gas giants of our solar system. But we aren't as practiced thinking about how gravity distorts the shape of entire planets.
In the afterward of the 1954 science fiction novel Mission of Gravity, author Hal Clement set out the calculations behind his alien world. He outlines the orbital dynamics of Mesklin, an exoplanet 16 times the mass of Jupiter. The gravitational extremes of this highly oblate spheroid, its chunky rings and the thermodynamics of its methane seas are all key to the plot of its native fauna heading to places us squishy humans can't to retrieve misplaced equipment.
Planetary science from a fiction book published in 1954 is suddenly relevant again.
The story itself is fantastic and an object lesson in how incorporating real science into storytelling is an excellent way to extend the plausibility of world-building, but the afterword is where all that science is explicitly defined. In it, Clement hypothesizes that an observed dimming in the star Cygni C is attributable to this close-set world, a world heavily distorted by the star's gravity into a distinctly squished spheroid. Alas, in the more than half-century since publication, we've learned that Cygni C is actually a binary system (explaining the periodic dimming) with no planets yet detected, pushing Mesklin firmly into the list of fictional worlds. Yet here we are, six decades later, coming back around to the idea of contemplating squished, not-so-spherical planets.
From the literally hundreds (nearly thousands!) of exoplanets we've detected so far, we know that planets tucked in tight to their stars experience tidal distortion. It's relatively easy to spot in hot Jupiters where the atmosphere is stretched and distended by the star's pull (and the star-snuggling planets wreck havoc on their stars), but it gets more challenging when contemplating small, rocky planets with more subtle distortion.
Astronomers Prabal Saxena, Peter Panka and Michael Summers recently published a paper on the theoretical possibility of detecting and identifying solid planets experiencing severe tidal distortion from close, synchronous orbits around large stars. In the ongoing game of figuring out that everything we thought we knew about planetary formation is flawed, the researchers tackle the assumption that any distortion would be too small to detect with current instrumental capacities and find it lacking.
Light curves of assorted planets transiting stars. The theoretical distorted iron planet is the purple dash-dot line. Credit: Saxena et al.
While so far it's just a theory, the research team suspects that for M-class stars, planets within a given orbital distance may experience observationally significant tidal distortions. Both in-transit bulge signatures and ellipsoidal variations might be detectable in the data now, and even more likely detectable once the next-generational planet-hunters roll online.
The orbit of Mesklin has significant impact on its methane seas. Image credit: Mika McKinnon
In the author's afterward, Clement set out science-inspired fiction as a game between author and reader, where it was left to us to ferret out any inconsistencies in the science of the story. While Cygni C is a binary system devoid of squished planets and bug-like alien lifeforms, the science behind Mesklin is looking even better now than it did a few years ago. I can't wait until we start finding these planets and calculating out their varying gravity and extreme weather. Once again, we're living in a universe where if you can imagine a planet — a world of pure desert, a world of pitch blackness, a world of continuous sunsets — it probably exists, somewhere.
Read the The observational effects and signatures of tidally distorted solid exoplanets in the February 2015 volume of the Monthly Notices of the Royal Astronomical Society.