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.
