HIT PLAY, ABOVE, TO SEE A GALLERY OF WEIRD WORLD IMAGES. The mother planet would loom large in the sky of habitable moon.
Credit: Jamie Tufrey
Binary planets might become tidally locked, with the same hemisphere of each forever facing the other. Gravity would cause vast oceans to pool on the facing sides.
Credit: Jamie Tufrey
A carbon planet is swaddled in a thick shell of dark graphite and hazy hydrocarbon atmosphere. A meteor impact has revealed a sparkling crater of pure diamond, formed below the surface under great pressure.
Credit: Jamie Tufrey
An artist's rendering shows a massively bloated 'hot Jupiter' transiting very close to the face of its star.
Credit: ESA
Keeping a stable climate on a desert world is problematic, but planets like Dune might exist.
Credit: iStockphoto
Not even the highest mountains would break the surface of waterworlds.
Credit: NASA
IN THE 1990s, a clever method was developed to detect distant planets by measuring the minute wobble their gravitational pull causes in their parent star.
Since then, using this method and others, astronomers have discovered a haul of more than 340 alien worlds orbiting other stars in our galaxy; and that list grows every week. As far as solar systems go, we are most definitely not alone.
Some of these extrasolar planets, or exoplanets, are freakishly bizarre by the standards of what we find in our own neighbourhood. Planets have been found orbiting the violently spinning pulsar remnants of supernovae explosions; others whirl around their stars so tightly that their surfaces are scorched to thousands of degrees, and one year lasts just a few Earth days.
One exoplanet is a hugely bloated world five times larger than Jupiter – and yet another orbits within a triple system of stars, bestowing upon it spectacular sunsets not unlike those depicted on the fictitious Tatooine, home world of Star Wars hero Luke Skywalker.
Though the list is impressive, there's a suspicion among planetary scientists that we're only scratching the surface of the incredible variety of worlds that could exist. Theories about planet building hint at some exotic possibilities.
Imagine the double planet of Vulcan, home of Star Trek's Mr Spock; or the forested moon of Endor, where the ewoks of Star Wars are found; or the desert world of Arrakis in Frank Herbert's Dune. These were all dreamed up in fiction, but could they really exist?
THE EARTH FORMED in a region of the Solar System's protoplanetary disc that was relatively rich in the element oxygen. So on top of an iron-rich core, our planet is mostly built out of oxygen-containing silicate rocks. But further out in the protoplanetary disc, the ratio of the elements carbon and oxygen was probably different.
A class of meteorites found on Earth, called enstatite chondrites, may have formed in this region – they have a ratio of carbon to oxygen that is a thousand times larger than the ratio found on Earth.
"If an entire planet were to have condensed from this kind of raw material, it would have ended up enormously different from the Earth," says Marc Kuchner, an expert on exoplanets at NASA's Goddard Space Flight Centre near Washington DC.
Built out of relatively more carbon than oxygen, such a planet would still have a metallic iron core, but the outer layers could be composed of ceramics – silicon and titanium carbides – with a shell of pure carbon on top.
These ceramics and graphite would make the entire planet extremely hard and heat-resistant, and it could survive much closer to its star. Even more bizarre is that the high pressure beneath the surface would convert the bottom of the graphite layer into an entire shell of diamond that would be many kilometres thick.
The surface of such a gem planet would be equally alien looking. The atmosphere would likely be rich with carbon monoxide and methane, which would react in the sunlight to turn the skies hazy with hydrocarbons. Long-chain hydrocarbons would condense into clouds and fall as rain, filing the oceans with oil and smearing the ground with sludgy tar.
"Actually, a surface of tar and the air choking with carbon monoxide and hydrocarbons sounds a lot like Los Angeles," quips Kuchner.
Any life emerging on such a world might be just as exotic. Earthly life takes carbon-rich molecules such as sugar and reacts them with oxygen in the air to release energy. But on a diamond world, chemistry would be reversed, with oxygen-containing materials flammable in the hydrocarbon atmosphere.
"Organisms on carbon planets would be like life through the looking glass: eating oxygen-rich food and 'burning' it in carbon from the air," he says.
Diamond worlds could actually turn out to be pretty common, as carbon-rich dust grains might always get concentrated in some regions of protoplanetary discs, says Kuchner. "It's even been suggested that the core of Jupiter is essentially a carbon planet."
New planetary systems form out of the gas and dust blown into interstellar space by previous generations of exploding stars. The more rounds of star birth and death there have been, the larger the ratio of carbon to oxygen.
"So towards the centre of our galaxy, where the population of stars is more evolved, you'd expect to find more carbon worlds than silicate planets like Earth," he argues.
Similarly, over time, the gas clouds across the whole galaxy are getting progressively more carbon-rich. So in a few billion years, all new planets might turn out as carbon worlds, and possible life of the distant future would be carbon breathing rather than oxygen breathing.
OUR HOME PLANET is composed mostly of those oxygen containing silicate rocks, with the iron core taking up about 17% of the planet's volume. But planetary scientists believe there may exist a class of planets made almost completely of iron: cannonball worlds.
"The best way to make a cannonball world is with a 'hit-and-run' collision," explains Erik Asphaug, from the University of California, Santa Cruz. "A relatively gentle collision between two planetary objects blasts away the outer layers of the larger body, while the smaller body gets completely shredded."
All that remains is a denuded core. In fact, Asphaug believes exactly this process explains the curious case of Mercury, the planet closest to our Sun. Mercury is the smallest planet in the Solar System and yet is exceptionally dense. This is due to its large iron core, making up a massive 42% of the planet's bulk.
As well as stripping away the rocky outer layer of a world, a planet-cracking collision could produce the most exotic exoplanets of them all. About 4.5 billion years ago, a protoplanet the size of Mars is thought to have smacked into the young Earth, splashing off enough material to form the Moon.
Our Moon ended up about one per cent thetas of Earth (even though it has a quarter the diameter), but would a much more even distribution of material be possible, to produce a pair of twin planets orbiting each other?
This idea has already been explored in science fiction. Vulcan, the home world of various characters in the Star Trek universe, is said to be part of such a binary planetary system (with the sister planet named T'Kuht). Sci-fi author Robert L. Forward also based a novel on Rocheworld, a pair of planets orbiting so tightly that they distort each other into egg shapes and even share a drawn-out column of atmosphere.
Robin Canup at the Southwest Research Institute's Planetary Science Directorate, in Boulder, Colorado, uses computer simulations to study how the Earth and Moon were formed, and search for other planetary collisions in the early Solar System.
For example, Pluto and its moon Charon are pretty similar (Charon is about half the size of Pluto) and the pair is thought to have formed by a giant collision.
"The most likely scenario to produce a double planet is when the two colliding objects are close in size and they experience a near-grazing impact," she says.
Her computer simulations show how just such a glancing blow could cause the objects to essentially bounce off one another, and distort them into drawn-out blobs of rock. Under the right conditions, the smaller body is captured into orbit rather than being torn apart into decaying spiral arms around the surviving planet.
Once formed, the twin worlds would both orbit around point in between them, with the gravity of each planet strongly influencing the other. "The type of impact necessary to build twin worlds – a grazing impact between similar-sized objects – would leave them both spinning very fast," says Canup.
This means that the tidal effects they have on each other will cause their orbits to steadily spiral outwards and their spins to slow. Eventually, the double planets will become tidally locked and forever more keep the same face towards each other, like dancing lovers.
THE PLANET ARRAKIS, also known as Dune, is the setting for Frank Herbert's epic series of science fiction novels.
Dune resembles Mars in that it is now a parched desert world, but shows signs of large volumes of liquid water having gushed across its surface in the distant past. However, the fictional Arrakis is much warmer than Mars, and has a substantial atmosphere rich in oxygen.
Arrakis is also habitable and supports both giant sand worms and human tribesmen. The problem with a hot desert planet is that it would be difficult to keep its climate stable. If a terrestrial planet with liquid water receives too much warmth from its sun, the oceans would evaporate rapidly, loading the air with vapour.
Water vapour is a greenhouse gas, and so more of the star's warmth would be trapped by the atmosphere, causing even more evaporation, and so on until eventually the entire planet's surface boils dry. A double-whammy effect is that with rising temperatures, carbonate rocks break down to release carbon dioxide, another powerful greenhouse gas.
This process is called a runaway greenhouse effect, and is believed to be the reason why Earth's 'evil twin', Venus, turned into such a hellishly hot place.
Despite the fact that any oceans it may once have possessed have boiled away, the atmosphere of Venus is now surprisingly dry. Sunlight has broken down water molecules high in the atmosphere, and the hydrogen gas produced has blown away into space.
So how would it be possible to create a hot desert world like Dune, with enough moisture for life, but no catastrophic greenhouse effect? Kevin Zahnle, an expert on the evolution of planetary atmospheres at NASA's Ames Research Centre in California, thinks he knows.
"What could make this work", he suggests, "is if the water vapour is lost from the atmosphere quickly enough … once the equatorial regions have been baked very dry, the low humidity of the air can put a halt on a runaway greenhouse."
The saving grace is that a dry atmosphere is very clear, and so a hot desert world is able to radiate heat back into space much more quickly than a wet world. The planet's climate would find a new stable state, with most of the world smothered by vast bone-dry deserts but the polar regions remaining cool and moist enough to be habitable.
"In fact, Dune represents a likely scenario for the Earth in its far future", predicts Zahnle. "In some two billion years time the Sun will have grown so bright as to dry up the oceans, but our planet may escape runaway greenhouse and remain partly habitable for another two billion years as a hot desert world."
What about the opposite possibility, though – how might a water world form, with a solid surface beneath deep oceans? In fact, water worlds ought to be pretty common in the galaxy, and could form in a very similar way to the Earth.
Because Earth formed inside the 'snowline' of the protoplanetary disc, where water was sparse (see "Making planets" at the end of this article), it would originally have been fairly parched.
The raw material for our oceans and atmosphere was later delivered by a barrage of asteroid and comet impacts during the early history of the Solar System.
To build a waterworld, all you need is a more efficient way of delivering water to the young planet from beyond the snow line. "The simplest way is to have a rocky planet forming in a more substantial protoplanetary disc," says Sean Raymond, an astronomer researching the formation of terrestrial planets at the University of Colorado in Boulder, USA.
"This would produce massive super-Earths, and the increased mixing within the disc would deliver great volumes of water from beyond the snowline." Giant water worlds formed in this way would probably be limited to about 10 times Earth's water content, he says.
An even more powerful process for building waterworlds might occur in solar systems quite different to our own. Many large exoplanets have been discovered in extremely tight orbits around their star – so-called 'hot Jupiters'.
Gas giants cannot form so closely to their Sun, and so must have migrated there soon after their birth. As these massive worlds spiralled inwards, they would have churned up the remaining dust and gas in the protoplanetary disc.
"These pile-up regions trigger the formation of terrestrial planets in orbits inside and outside the wandering giant," says Raymond. "The close-in planets would be scorched lumps of rock - 'hot Earths' - but the exterior worlds would be very water-rich indeed, containing up to 100 times the water content of Earth."
If these waterworlds formed at the right distance from their parent star, their thick shells of ice would thaw in the warmth. Found within the so-called 'habitable zone', these planets would possess great oceans of liquid water, hundreds of kilometres deep.
The pressure at the bottom of these planet-spanning oceans would be incredibly high, and might force the water into dense forms of ice. With such deep oceans, it is also difficult to see how any landmass, even the tips of volcanic mountains, could poke above the surface of the waves.
As a consequence, a planet with a global ocean unbroken by land would probably suffer from an unstable climate. On Earth, the interplay between the pumping of carbon dioxide into the atmosphere by volcanoes and it being scrubbed out again by the weathering of silicate rocks serves as a global thermostat.
"Without this carbonate-silicate cycle, the climate might be quite extreme, and susceptible to freezing over completely as a snowball world." So, ironically, for the possibility of life developing on waterworlds, too much of a good thing might turn out to be disastrous. Alongside an unstable climate, the lack of land surface and crushingly high pressures on the sea floor might make such waterworlds far too extreme to be habitable.
THE WEIRD WORLDS we've looked at so far have all been planets. But many of the most interesting bodies in our own Solar System are moons - in fact, Ganymede and Titan are both larger than the planet Mercury.
So how large could a moon grow? Could one ever be suitable for advanced plant or animal life, a world not unlike the forested moon of Endor imagined in Return of the Jedi?
"Most moons formed around the gas giant planets, born out of the disc of gas and dust swirling around the young giant like a mini solar-system," says Caleb Sharf of Columbia University in New York City.
"Computer simulations suggest that the total mass of moons always ends up around one 10,000th that of the gas giant. So a Jupiter-sized planet could in principle make a Mars-sized satellite, and a larger gas giant might even assemble a moon as large as the Earth."
If a gas giant, formed beyond the snow line, then shifted its orbit into the habitable zone of the star, its moons would thaw-out and may offer the perfect environment for life. "Any moons orbiting such a gas giant are likely to be very water-rich," says Raymond, "and when shepherded into the star's habitable zone, would produce oceans many kilometres deep."
As with planetary waterworld, such an Earth-sized watery moon might not leave much land area for forests; but moons would also experience some very weird effects from orbiting a massive planet.
The powerful gravitational tug of the gas giant would draw extreme tides in the moon's oceans, hoisting up an enormous bulge of water on either side of the world. As Earth does, these water moons would also feel a tidal tug from their sun.
"The interplay between these two tides would result in considerable sloshing across the surface," says Rory Barnes, an expert in gravitational effects at University of Washington in Seattle. "And if there aren't any landmasses, waves many hundreds of metres high would be possible."
Over time, these tidal forces would alter the spin of the moon so that it falls in-synch with the period of its orbit. "The moon would become 'tidally locked'," explains Scharf, "so that one side of the moon always faces the planet, just as we never see the far-side of our Moon from Earth." The day-night cycle of a tidally locked moon would be very complex and unlike that of any other world.
The gravitational field of a gas giant creates some interesting effects for the moons orbiting it, but this also extends far into space. Just behindhand ahead in the orbital path of a planet are two regions, called Lagrange points, where the planet's own gravitational effect is exactly cancelled out by that of its sun.
In Jupiter's gravity field, these exceptionally stable regions of space have collected clouds of asteroids, dubbed Trojans, which are a bit like marbles gathering in a dip in the ground. The intriguing possibility now being considered by planetary scientists is whether these gravitational traps could also serve as sweet spots for forming entire Trojan planets in alien solar systems.
"It seems likely that two gas giant planets could form in exactly the same orbit, sitting in each other's LaGrange points, or even Earth-like planets becoming captured in these regions," says Richard Nelson, a professor of mathematics and astronomy at Queen Mary, University of London, in England, who has been modelling such scenarios.
These trapped worlds could be pretty common in the galaxy too. "We're finding Trojan planets forming in about 10% of the planetary systems we have simulated," says Nelson.
If this were to have happened in our Solar System, looking up from the Trojan Earth, Jupiter would loom far larger in the sky – about half the size of the full Moon. And because the positions of both Earth and Jupiter would be locked in their shared orbit, the giant world would always appear as a half-crescent and not wander across the sky like the other planets.
The exciting thing about all these exotic planetary possibilities is that they may not remain purely theoretical for too much longer. For example, although an Earth-sized moon orbiting a gas giant would be pretty difficult to spot directly, it could be picked-up by the effect its gravity has on the orbital timing of its parent planet.
Cannonball worlds might even be identified with existing technology. "These are about the densest planets you could imagine, so if we discover an exoplanet that's much denser than the Earth, it's likely to have a very substantial heart of iron," says Kuchner.
And with such an abundance of iron on the surface, life on such a world might have developed some very exotic features. Iron-clad crabs anyone?
Making planets
There are essentially three kinds of planets in our Solar System: small rocky worlds orbiting close to the Sun; bloated gassy giants, like Saturn; and ice giants, like Uranus and Neptune, further out.
The theory explaining this pattern is that the Sun formed in the centre of a great swirling cloud of gas and dust – the protoplanetary disc. The inner regions of this disc were kept hot by the young Sun, and so only dusty grains of iron and silicate remained solid to clump together to build new worlds. This is where the rocky inner planets formed. Further out, the temperature was low enough to allow ‘volatile’ compounds to condense into solids. So in these outer regions of the disc, there was much more material available to build larger planets.
And once these outer planets had grown to a few times the size of Earth, their gravity was able to haul in thick atmospheres of gas from the surrounding nebula, creating massive gas giant worlds. The boundary between these warm and cool planet formation regions is called the ‘snow line’. Any moons forming around these gas giants are also very rich in volatiles. “For example,” says Caleb Scharf of Columbia University in New York City, “Jupiter’s satellite Europa is smaller than our own Moon, yet has more than twice as much water than all of Earth’s oceans and seas combined.”
Marauding giants
The standard model of planet building has it that rocky planets are born close to their parent star, with gas giants formed further out in the system (see “Making planets”, above).
But that doesn’t mean that gas giants always stay put once they have formed. If a giant forms quickly enough, there is still plenty of material left in the disc, and the planet’s gravity interacts with this diffuse gas as it ploughs through it. The giant marauding planet steadily loses orbital energy, spiralling inwards as it clears a path through the young planetary system.
This is thought to be the reason why many of the first exoplanets to be discovered orbit their star so tightly – far closer to their star than they should have been able to form. There is even some tentative evidence that Jupiter moved in towards the Sun slightly in the very early history of the Solar System. But now that the Sun’s disc has dispersed, Earth is safe from any further migration of our neighbouring giant.
Follow Cosmos on Twitter!
twitter.com/cosmosmagazine
Leiws Dartnell is an astrobiologist at University College London in England. He has written for Sky at Night and The New York Times and is the author of Life in the Universe: A Beginner's Guide.
