How special is the solar system?

On the evidence to date, our solar system could be fundamentally
different from the majority of planetary systems around stars because
it formed in a different way. If that is the case, Earth-like planets
will be very rare. After examining the properties of the 100 or so
known extrasolar planetary systems and assessing two ways in which
planets could form, Dr Martin Beer and Professor Andrew King of the
University of Leicester, Dr Mario Livio of the Space Telescope
Science Institute and Dr Jim Pringle of the University of Cambridge
flag up the distinct possibility that our solar system is special in
a paper to be published in the Monthly Notices of the Royal
Astronomical Society.

In our solar system, the orbits of all the major planets are quite
close
to being circular (apart from Pluto’s, which is a special case),
and the
four giant planets are a considerable distance from the Sun. The
extrasolar planets detected so far – all giants similar in nature to
Jupiter – are by comparison much closer to their parent stars, and
their orbits are almost all highly elliptical and so very elongated.

‘There are two main explanations for these observations,’ says Martin
Beer. ‘The most intriguing is that planets can be formed by more than
one mechanism and the assumption astronomers have made until now – that
all planets formed in basically the same way – is a mistake.’

In the picture of planet formation developed to explain the solar
system, giant planets like Jupiter form around rocky cores (like the
Earth), which use their gravity to pull in large quantities of gas
from their surroundings in the cool outer reaches of a vast disc of
material. The rocky cores closer to the parent star cannot acquire gas
because it is too hot there and so remain Earth-like.

The most popular alternative theory is that giant planets can form
directly through gravitational collapse. In this scenario, rocky
cores – potential Earth-like planets – do not form at all. If this
theory
applies to all the extrasolar planet systems detected so far, then
none of them can be expected to contain an Earth-like planet that is
habitable by life of the kind we are familiar with.

However, the team are cautious about jumping to a definite conclusion
too soon and warn about the second possible explanation for the
apparent
disparity between the solar system and the known extrasolar systems.
Techniques currently in use are not yet capable of detecting a
solar-system look-alike around a distant star, so a selection effect
might be distorting the statistics – like a fisherman deciding that
all fish are larger than 5 inches because that is the size of the
holes in his net.

It will be another 5 years or so before astronomers have the observing
power to resolve the question of which explanation is correct.
Meanwhile, the current data leave open the possibility that the solar
system is indeed different from other planetary systems.

CONTACTS

Dr Martin Beer

University of Leicester, UK

Tel: +44 (0)116 2231802

Email: martin.beer@astro.le.ac.uk

Prof. Andrew King

University of Leicester, UK

Tel: +44 (0)116 2522072

Email: ark@astro.le.ac.uk

Dr. Mario Livio

Space Telescope Science Institute, USA

Tel: +1 410 338 4439

Email: mlivio@stsci.edu

Dr. Jim Pringle

University of Cambridge, UK

Tel: +44 (0)1223 337513

Email: jep@ast.cam.ac.uk

NOTES

1. Currently around 100 extrasolar planets are known which have been
detected through the wobble of their host stars caused by the motion of
the planets themselves.

2. The paper has recently been accepted by the Monthly Notices of the
Royal Astronomical Society but no publication date has yet been set.