UP UNTIL the mid-1990s, the only planets whose existence we knew about for certain were those in our own solar system. This narrow worldview was changed forever when the first exoplanets were found orbiting around pulsars – the burnt cinders of exploded, monster stars – swiftly followed by the discovery of gaseous titans in searing proximity to their suns. In the best traditions of exploration, these foreign planets were unlike anything we had previously imagined.
The variety of exoplanets bagged since then has only pushed the envelope further. Exoplanets, we have learned, are happy to bask in the simultaneous light of four suns, wander the galaxy as starless outcasts from their home solar systems or orbit whipper-snapper stars barely 1 million years old.
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“What happened in our solar system hasn’t served us very well in predicting what we will find around other stars,” says of the University of California, Santa Cruz. “We expected planets like we have here, but we’ve been continually surprised.”
With more exoplanets discovered in the last two years than at any time before, and the haul fast approaching 2000, astronomers wager there are plenty more surprises in store. Naive theories of what extrasolar worlds should be like have been consistently outshone by the data, says of the McDonald Observatory at the University of Texas at Austin. “We really didn’t know the extent of the diversity of exoplanets and configurations until we actually observed them.”
In an attempt to steal a march on nature, researchers are now busy imagining weird new types of exoplanet that might turn up in future. Far from being a parlour game for bored astronomers, understanding nature’s ability to produce planetary bodies will be crucial to learning how our solar system compares to its galactic counterparts. Many of these proposed exoplanets challenge our understanding of planet formation, and could play havoc with our admittedly arbitrary criteria for determining what constitutes a planet in the first place. Furthermore, exotic new planetary types should expand our inevitably Earth-centric ideas about where habitable planets might form, aiding in the search for alien life.
Here are five of the zaniest exoplanet types that could shake things up in the years to come.
1. Binary worlds
Planetary double acts orbiting each other in a stately waltz
In our solar system, large planetary bodies are located far apart, orbited by moons of much smaller size. We think this familiar configuration emerges when bits of dust clump together in a protoplanetary disc encircling a young star, evolving into rocky hunks that hoover up any material in their orbital paths. Moons can then be crafted from leftover detritus orbiting the planet, or else be hauled in during the chaotic pinballing of objects thought to happen in developing solar systems.
There is a third option, however. Widely accepted models suggest our own moon formed when a Mars-like body – dubbed Theia – smacked into the primordial Earth, gouging out material that coalesced into the satellite we know today. But if those two bodies had undergone a less spectacular collision, they could have ended up in a stable partnership. “If you changed the nature of the encounter that led to our moon, then you might have gotten the binary planet outcome,” says of the California Institute of Technology in Pasadena.
Finding binary planets could therefore shed light on the rambunctious childhood years of fledgling solar systems. It would also prove that collisions of the kind that created our moon can be considered a viable route to planethood, and not just a way to form satellite hangers-on. “It would tell us that at least in some cases, planet formation proceeds by the close encounter of large, similar-sized bodies,” says Stevenson. Fortunately for astronomers, binary exoplanets should cast distinctive double shadows as they cross and partially eclipse the shining faces of their stars – so-called transit signals readily detectable by NASA’s and other observatories designed to look for new worlds.
Undoubtedly the most intriguing configuration would be two Earthlike worlds locked in a binary orbit. Imagine if Earth had a habitable twin in our sky, and if life, or even a technological, space-faring civilisation, arose there in parallel to our own. “Would the planets be at war?” asks Stevenson. “Makes for great science fiction.”
2. Party planets
Multiple worlds sharing the same orbital path
Although the worlds in our solar system stick standoffishly to their own orbital lanes, they do tolerate company beyond their faithful moons. Asteroids dubbed Trojans, for instance, hang out at Lagrangian points, sweet spots where the gravitational force of a planet and its star balance out. These points move around the planet’s orbit as it rotates, dragging their inhabitants along with them. Jupiter shepherds an army of Trojans around the sun, and Earth actually has a Trojan of its own, a tiny rock called 2010 TK7.
In theory, there is no reason why planet-sized objects couldn’t also arrange themselves in such complex configurations. According to simulations by Gregory Laughlin of the University of California, Santa Cruz, and Stefano Meschiari of the University of Texas at Austin, multiple Earth-sized, habitable worlds might plausibly share a “party orbit”, all at roughly similar distances from their host star. “It’s not just two to tango,” says Laughlin, “but three to four to five to six to tango.”
Arrangements of this sort can be stable for billions of years – so long as there are no gravitationally perturbing worlds on either side of the crowded orbital band to disrupt its delicate choreography. “Planets with very similar orbital periods are totally possible,” says Meschiari. What is less clear, however, is how the bulk of a solar system’s planet-making material would gel into a tight-knit batch of worlds. The hard part is not so much guaranteeing orbital stability, he says, “but whether nature actually allows the formation of these planets.”
Researchers on the Kepler telescope got ahead of themselves in 2011, when they reported spotting the signature of two planets with the same orbital period. Further analysis showed one of the pair in fact had a significantly longer year, consigning it to a completely separate orbit. Although continued Kepler analyses are still our best bet to spot planets of this kind, future transit missions such as NASA’s or ESA’s and Oscillations of Stars (PLATO) may get lucky.
The existence of co-orbital planets would upend the current doctrine that planets must keep their orbital backyards free of other large bodies – something that ousted Pluto from the full-planet club in 2006. These worlds could even cross-pollinate, thanks to meteorite impacts blasting out rocks harbouring hardy bits of genetic material. “The planets would share a genetic lineage,” says Laughlin, with their unique environments driving biology down alternate tracks. “Evolution would proceed differently on those two worlds.”
3. Egg worlds
Rocky planets squeezed into extreme shapes
The gaseous giant WASP-12b orbits its star at such scorchingly close quarters that the strong stellar gravitational pull has warped it into a bulging oval. and his colleagues at George Mason University in Fairfax, Virginia, decided to explore how this tidal distortion might affect a rocky world such as Earth. They calculated that an exoplanet of this kind could stretch to be a fifth wider at its equator than from pole-to-pole before being torn asunder by its star (, vol 446, p 4271).
The discovery of rugby ball-shaped worlds could be a boon for planetary science. “Aspherical exoplanets have the potential to tell us a lot about planetary interiors,” says Saxena. The way a planet responds to a star’s gravitational pressure would provide an entirely new way of learning about its composition. Squeezing an object is a handy way to learn about its insides, says Laughlin: “Like at the grocery store, with a melon or a grapefruit.”
“A planet is like a melon – squeezing it can teach you about its insides”
The planetary equivalent of gauging a melon’s ripeness would be determining whether a planet is mostly solid or gaseous. As rocky planets are more likely to be habitable, measuring a planet’s flexibility would provide us with a way of determining its potential to harbour life, says Saxena. As a bonus, the atmospheres of ovoid worlds would experience different levels of gravity in different places, possibly making for intriguingly unpredictable climates.
Stay tuned, for these egg worlds might crop up in yet-to-be-processed data from NASA’s Kepler telescope, or even be detected by space-based instruments such as TESS or huge ground-based telescopes scheduled to be built over the coming decade.
4. Chthonian planets
Naked worlds whose atmospheres have been boiled away
As solar systems evolve, the gravitational force planets exert on each other means they can slowly move inwards or outwards from their shared star. Called migration, this process helps explain the Milky Way’s puzzling abundance of worlds the size of Neptune or Jupiter occupying star-hugging orbits. These massive exo-worlds must have formed further out, or else the star’s radiation would have prevented their constituent materials from ever coalescing into a planet. Nudged towards a stellar furnace by migration, starlight withers away their atmospheres, eventually leaving nothing but their rocky cores. This exposure of the planet’s hidden depths inspired the name chthonian, a reference to the deities of the Greek underworld.
There are broadly two types of scientifically valuable and potentially detectable chthonian planets. The first are habitable evaporated cores (HECs): cold, mini-Neptunes that migrate towards their stars’ temperate, habitable zones. The extra stellar radiation they receive can blow off their atmospheres and even melt exposed surface layers of their rocky cores, which are rich in water ice. These planets could accordingly transform into ocean-covered worlds with life-friendly air, opening up a new avenue for the rise of alien life. “The surfaces of HECs used to be the gas-solid interface deep in the planet’s interior – a rather hellish place indeed,” says astrobiologist of the University of Washington in Seattle. “But once the gas evaporates away, you’re left with a surface that’s potentially habitable.”
The other interesting type of chthonian planet starts out more like Jupiter, the iconic gas giant of our solar system whose origins are still shrouded in mystery. Studying the rocky core of such an exoplanetary gas giant stripped bare by its star would accordingly give scientists valuable insight into the sequence of planetary formation. “If you expose the core, then you see what happened first,” says Laughlin.
Catching the process of migration as it leads to atmosphere loss would be a valuable piece of evidence in favour of the chthonian planet theory. Intriguingly, of Northwestern University believes that the gaseous giant WASP-12b might be in just such an evolutionary phase: not only being squished by the gravity of its merciless star (see “Egg worlds”), but also having its atmosphere boiled away to nothing.
And as for the specific case of HECs, researchers believe their distinctively low densities suggests they migrated inwards from their solar system’s distant outlands of ice and snow.
5. Corkscrew planets
Prisoners of gravity ping-ponging between two stars
Even the craziest planets concocted by theorists still tend to trace conventionally near-circular orbits in a flat plane. Not so the corkscrew planet. Mind-bendingly, these worlds could exist in a sort of orbital limbo, spiralling about an axis between two stars in a binary system, pulled hither and thither by their competing gravities. The brainchild of theoretical physicist of Auburn University in Alabama, these whirligig worlds represent a completely new type of stable, albeit speculative, planetary orbit.
Oks ran the numbers for a corkscrew planet pulled between the orange and red dwarf stars comprising Kepler-16, a binary system 200 light years from Earth. The planet would complete a manic loop-the-loop in its cone-shaped orbit in under an Earth week, straining our conventional definitions of a year ().
Any life that managed to survive on such a permalit planet, where seasons change in the span of days, would experience one of the weirdest night skies in the cosmos. Upon reaching one end of the corkscrew orbit and heading back towards the other star, the closest sun would seem to suddenly reverse its direction in the sky. “I hope the would-be inhabitants of the planet would be accustomed to this,” says Oks, “and would not get scared each time it happened.”
It’s not yet evident how stars could force a corkscrew world to settle into such an unusual orbit, though snagging a passing starless “lone wolf” planet seems like their best bet. And while the transit method could spot these planets (see “Binary worlds”), their true screwball nature would be harder to identify.
According to Oks’s paper, a better way to find corkscrew planets is via gravitational waves, the ripples in space-time predicted by Einstein’s general theory of relativity. Corkscrew planets should generate telltale additional gravitational waves that would stand out against the background signals produced by interacting stars. Direct detection of gravitational waves from any source has so far proved elusive, but with instruments getting ever more precise, Oks believes the distinctive behaviour of corkscrew planets could one day make them easily distinguishable.
This article appeared in print under the headline “Worlds of pure imagination”
