Studies of transiting extra-solar planets are of key importance for 
understanding the nature of planets outside our solar system, because 
their densities can be determined which tells us of what the 
planets are made of.  In our solar-system we have basically two species 
of planets: gas or ice giants which have masses larger than 15 MEarth 
and densities  between 0.7 to 1.6 gcm-3, and rocky planets which have 
densities from 3.7 to 5.5 gcm-3.  It is thus natural to 
expect that planets outside the solar system should have the same 
properties: Planets with more than 15 MEarth should have a low density, 
and planets with the mass of the Earth should have a high density. 
The CoRoT satellite has now discovered more than 26 transiting extrasolar 
planets including the first rocky one, which allows to draw first 
conclusions. 
The first surprising results is  that planets of the same mass can 
have different densities but there are still some trends: 
Planets with more than 1000 MEarth have densities 
larger than 6 gcm-3, and are preferentially found in stars 
that are more massive than the Sun. All known planets in the 
mass-range between 15 and 600 MEarth have densities of less 
than 3 gcm^-3.  When going further down in the mass of the planets, the 
density of planets increases but there is no sudden transition from 
gaseous to rocky planets.