The Strange Worlds Beyond our Solar System

Over the last two decades, astronomers have catalogued around 850 planets outside our Solar System. And the search for worlds orbiting other stars is turning up some weird and wonderful characters. From a scorched gas giant that's darker than coal, to a planet packed with diamond, here are some of the oddballs-in-chief.

In a memorable scene from the film Star Wars, Luke Skywalker looks out over the horizon while two suns set in the sky of his home planet Tatooine. Astronomers have already discovered several "Tatooine" systems, where planets orbit double stars. But this year, a team comprising volunteers and professional astronomers reported finding a planet illuminated by four stars - the first known of its type.

PH1

• Type: Giant planet
• Distance: 5,000 light-years
• Size: Radius is six times larger than Earth's (Neptune-sized)

The distant world orbits one pair of stars and a second stellar pair revolves around them. The planet is being tugged on by the gravitational forces from four different stars yet, despite this complicated environment, PH1 is able to maintain a stable orbit.

The discovery was made by volunteers using the Planethunters.org website along with a team from UK and US institutes. Named PH1 after the website, it is located in the constellation Cygnus.

At the time of the discovery, Dr Chris Lintott from Oxford University, said this was "absolutely not what we would have expected". He thinks that planets can form in the torus of dense gas that gives rise to planetary systems and then cling to stable orbits close to their parent stars.

TrES-2b

• Type: Giant planet
• Distance: 718 light-years
• Size: Mass and radius are about the same as Jupiter's

In 2011, a group of American astronomers announced that a Jupiter-sized exoplanet known as TrES-2b was the darkest known world, reflecting just 1% of the sunlight falling on it. TrES-2b is darker than black acrylic paint and blacker than any planet or moon in our Solar System.

The distance TrES-2b orbits from its parent star certainly has something to do with this. In our Solar System, Jupiter is shrouded in bright ammonia clouds that reflect more than a third of the sunlight that reaches it. But TrES-2b orbits its star at a distance of just three million miles. The intense energy from the sun heats the planet to more than 1,000C, which is much too hot for ammonia clouds to form. Its atmosphere also contains chemicals which absorb rather than reflect light. But these factors can't fully explain the planet's extreme lack of light.

However, TrES-2b is so hot that "it emits a faint red glow, much like a burning ember or the coils on an electric stove."

 

  55 Cancri e

• Type: Super Earth
• Distance: 40 light-years
• Size: Mass is eight times larger than Earth's; radius is twice the size of Earth

A nearby planet in the constellation of Cancer may have a rather peculiar composition. This body, known as 55 Cancri e, "is likely covered in graphite and diamond rather than water and granite," according to astronomer Nikku Madhusudhan from Yale University.

Belonging to a class of worlds known as diamond planets, 55 Cancri e is thought to be rich in the element carbon, which can exist in a variety of structural forms such as graphite (the material used as pencil lead), graphene, or diamond. Carbon-rich worlds contrast sharply with the Earth, whose interior is relatively poor in that element but rich in oxygen.

This year, Dr Madhusudhan and colleagues published the first measurements of the exoplanet's radius. This new data, combined with the most recent estimates of its mass, allowed the researchers to infer its chemical composition. Their results suggest 55 Cancri e is largely composed of carbon (in the form of graphite and diamond), iron, silicon carbide, and silicates. They estimate that at least a third of the planet's mass is diamond - the equivalent of three times Earth's mass.

 

Wasp 12-b is being cooked and contorted into an oval shape by its parent star

Located some 600 light-years away in the constellation Auriga, Wasp-12b is slowly being eaten by its Sun-like star. The giant planet is orbiting so close to its parent star that it is being superheated to a scorching temperature of 1,500C and distorted into a rugby-ball shape by its sun's gravity.

Wasp-12b

• Type: Giant planet
• Distance: 600 light-years
• Size: Radius is three times larger than Jupiter's

The planet's atmosphere has ballooned to nearly three times the radius of Jupiter and material is spilling on to the Sun-like star. "We see a huge cloud of material around the planet, which is escaping and will be captured by the star," said astronomer Carole Haswell .

Haswell and her colleagues used the Hubble Space Telescope to confirm earlier predictions about the planet, publishing their findings in the journal Astrophysical Journal Letters in 2010. They estimate that the planet may have just 10 million years left before it is completely obliterated.

Astronomers have found several other examples of worlds in the process of being devoured by their stars, or being heated up so much that their atmosphere escapes into space, forming a comet-like tail.

Glass rain may give planet blue hue

The turbulent alien world lies some 63 light-years from Earth

For the first time, astronomers have determined the true colour of a  discovered planet orbiting it's star. The world, known as HD189733b, has a deep azure hue - probably the result of silicate (glass) rain in the atmosphere, which scatters blue light. The discovery was made with the Hubble telescope.

Although it might resemble Earth from a distance, HD189733b is a huge gas giant which orbits close to its host star. The temperature of the planet's atmosphere is a scorching 1,000C, and it rains glass, sideways, in howling 7,000km-per-hour winds. Its atmosphere has been found to be dramatically changeable and exotic, with hazes and violent bursts of evaporation.

At a distance of 63 light-years from us, this turbulent alien world is one of the nearest exoplanets to Earth that can be seen crossing the face of its star.

There are countless types of alien worlds out there. All very strange and inhospitable but fascinating in their uniqueness. As for finding another earth....we get closer every day.