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A few decades ago astronomers were pretty confident they knew how planets formed.

Based on our own solar system they thought small, rocky planets formed near their host star and larger, gaseous or icy planets formed further out.

So when they started finding planets that didn't conform to any of these theories they were confused: were their prevailing theories of planet formation wrong?

Theories of planet formation are being tested as astronomers find bizarre new worlds in the universe. This artist's illustration shows the planet designated HD 209458b orbiting close to its host star, despite being the size of Jupiter. Known as a 'hot Jupiter', this is a type of planet that was once thought not possible to exist

In the last 20 years, reports Nature, we have found worlds we never thought possible.

For example, some planets have been found so close to their star that they orbit in just days - but density studies indicate these planets are somehow made of ice.

Other rocky planets have been found so big that they have led astronomers to question exactly how planets can form.

The current model of planet formation is that they are born out of the dust and gas that also creates the star at the core of a planetary system, known as the core-accretion process.

As the central star rotates it spins the surrounding material and heats it.

Other time this material clumps together, with materials with high melting points like iron and rock forming nearer the sun.

Further out in the system it is colder, allowing ice to form, while planets also accumulate some of the gas in their vicinity, becoming 'gas giants' like Jupiter and Saturn in our own.

Why, then, have we found systems where there are gas giants in orbit a tenth the distance of Mercury in our own solar system (known as 'hot Jupiters')?

Why do some planetary systems have giant 'super-Earths', huge rocky planets devoid of a gaseous exterior, orbiting in their extremes?

And why, too, do some planets orbit in wildly elliptical orbits rather than in a flat 'plane' like those in our solar system?

The answer: we just don't know.

Previously it was thought our solar system was the norm for how planets form. But the discovery of worlds unlike any of the eight planets around our sun suggests the preconceived notion that gas giants form far out and rocky planets inwards (illustrated) is not necessarily correct

This graph shows the different sizes of planets discovered by Kepler as of January last year. Although the number of planets we know of has increased since then, it shows how many different types of planets we found, many of which are not even planets that exist in our own solar system

It's possible that, in some systems, planets are knocked into wild orbits by others, or they are captured by a star as it passes.

The process of planet formation itself might, too, be more chaotic than we once thought.

'The first detections of exoplanets revealed bodies which are utterly unlike any solar system planet,' says Nasa, 'and subsequent discoveries have shown that many exoplanet systems are very dissimilar from ours.

'In some exosystems, planets as massive as Jupiter orbit so close to their star that they are heated to high temperature and their upper atmospheres are swept into space.

'In other systems, planets follow elongated orbits (in contrast to the nearly circular orbits of the solar system).'

However, this is not necessarily a bad thing.

Finding planets that don't conform to our theories merely means that we haven't quite nailed down how planet formation works yet.

It may even be that our solar system is fairly unique when compared to other planetary systems.

After all, for one thing our system contains no 'super-Earths', whereas they seem to be common elsewhere in the galaxy.

Another mystery is the abundance of super-Earths (illustration of Kepler-62f shown) elsewhere in the universe. Why do other planetary systems host at least one of these giant rocky planets, whereas our own solar system has none? Astronomers will be hoping to answer this question in future with new theories

'Out studies of exoplanets are just beginning, and it is not possible to be sure what will prove to be "typical" planets among our neighbouring stars,' says Nasa.

'Will most planet systems prove to be much like our own, or are we exceptional in more ways than we can imagine? Only years of further study will tell.'

That is not to say there are not exoplanet systems like our own, though; the star 55 Cancri, 41 light years away, has a system of five planets with a similar distribution to our own.

But it may be that our theories for how these planets formed in the first place, and what sort of systems they inhabit, may need to be revised.

'Perhaps the most interesting question, and one of the most difficult to answer, concerns the uniqueness of the Earth,' concludes Nasa.

'Are there planets similar to the Earth around other stars and does life exist on any other planet beyond our own Earth?'