New analysis by UK astronomers suggests our own planetary system may have formed in a very different way to those spotted orbiting other stars.

The findings suggest that one formation mechanism may not fit all planetary systems, as other astronomers have previously suggested.

The study appears in Monthly Notices of the Royal Astronomical Society.

In the past 10 years, over 100 extrasolar systems (planetary systems orbiting stars other than the Sun) have been discovered from the wobble in their host stars, caused by the motion of the planets themselves.

But none of them seem to resemble our Solar System very much. In fact, these exoplanets have several important attributes that are entirely at odds with the Solar System as we know it.

Lead researcher Dr Martin Beer of the University of Leicester's theoretical astrophysics group, pointed out that much of the modelling done on the formation of planetary systems is based on our own one.

"But existing data suggests that the planets in the Solar System are truly different from other planets," he told BBC News Online.

If this is the case, Beer and his colleagues argue in their research paper, it is unreasonable to base our understanding of all planetary systems on the one around the Sun.

They go on to speculate that if the Solar System is unique, then the search for Earth-like planets around other stars may be in vain.

Odd one out

When compared to all known planetary systems, say the authors, our own is something of an anomaly.

This appears to suggest that there might be two entirely separate mechanisms of planetary formation at work, or - at the very least - that there are two extremes of a single formation process.

Planetary size is one puzzle; most exoplanets are gargantuan, gaseous masses like Jupiter.

Smaller planets similar to the Earth's relatively humble proportions - and rocky composition - are noticeably absent, although the researchers admit that this may be because smaller planets are more difficult to spot.

Also, the large exoplanets are significantly closer to their stars than those in our own system are to the Sun.

They follow highly eccentric, or elliptical, orbits, which are more elongated than the largely circular orbits of the planets in the Solar System.