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Last Updated: Friday, 6 August, 2004, 12:53 GMT 13:53 UK
Solar System could be 'unique'
By Jacqueline Ali
BBC News Online

Earth, Nasa
The search for other planets like our own could be in vain
The Solar System could be unique amongst planetary systems in the Universe, astronomers have announced.

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.

It is unreasonable to base our understanding of all planetary systems the solar system model
Dr Martin Beer
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.

Current thinking

Planets are thought to form from the aggregation of dust particles between stars into a rocky core. This core either forms a solid planet, or develops a gaseous layer to become a gas giant like Jupiter or Saturn.

Most theories of the formation of planetary systems are variations on this. But they do not account for the super hot and gaseous exoplanets that astronomers have been seeing around other stars.

Jupiter, AP
Most planets outside the Solar System are "gas giants" like Jupiter
However there is an alternative, more dynamic scenario.

Some researchers have proposed that giant planets can form directly through sudden gravitational collapse of the gaseous discs around stars.

At present, it is impossible to determine which of the theories is correct.

Much more observational work is needed before solid judgements about whether the Solar System is truly different can be made.

The current observational techniques rely on the gravitational pull that the planets exert on their parent stars. Since large, "hot Jupiters" as they are called, exert strong pulls on their stars, these planetary systems may simply be the ones that readily draw the attention of astronomers.

"It's like a fisherman deciding that all fish are larger than 5cm because that is the size of the holes in his net," Dr Beer commented.

"It will be another five years or so before we will be able to see systems like our own," he added. "At that point we will know whether the Solar System is truly different, or in fact very average."

"Nevertheless, the existing data leaves open the possibility that [our own planetary system] is quite unique compared to [others]. If this turns out to be true, then our current understanding of planet formation is unduly coloured by our intimate knowledge of the Solar System."

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