La Lettre de l'Académie des sciences n°37-38

350

YEARS

SCIENCE

Georges-Louis Leclerc de Buffon

(1707-1788)

Pierre-Simon de Laplace

(1749-1827)

A sketch of the formation of the solar system, from the collapse of a

molecular cloud and the formation of the Primitive Nebula : formation

of a protostar and a protoplanetary disk, and formation of big solid

bodies and structures in the disks. The solids called planetisimals

are the building blocks of telluric planets and the solid cores of giant

planets.

The formation of the solar system

Once the sun and Earth freed from their specific status, it became possible

to think about an origin that did not need the intervention of the hand

of God. The first truly scientific theories appeared in Europe in the 18

Century with, notably, Buffon on the one hand, and Kant and Laplace on

the other hand. Buffon suggested that Earth and the other planets are

debris from the sun, teared off when a comet collided with it. In 1785,

Laplace propounded the theory of the Primitive Nebula, a contracting and

rotating gas cloud in which planets would form. He thus laid the foundation

of the modern theory of the solar system formation. This scenario would

be sharply refined and detailed in the course of the 20

Century, especially

in what regards the initial physical conditions inside the protosolar nebula

and the processes leading to the emergence of the two great groups of

planets: the rocky planets, such as Earth, and the giant gas planets, such

as Jupiter. We owe these improvements to progress achieved in ground

observation tools, to space exploration, progress in terms of rock dating

and to laboratory experiments designed to reproduce and thoroughly study some of the processes involved.

We also owe them to the development of computers which have made it possible to simulate – although

in an simplified way, obviously –

the major steps of the solar

system formation. The scenario

that explains best the properties

of the objects in the solar system

predicts the formation of rocky or

icy elements of some kilometers

wide (planetesimals, comets)

from the gas and sub-micronic

grains of the interstellar medium.

Those planetesimals then would

agglomerate into cores, of a few

Earth masses, in the cold part

of the future solar system; such

cores would then very quickly

accrete enough gas to form giant

planets. Closer to the sun, the

rocky planets would form directly

from planetesimals in some tens

of million years. Some of the

steps described here are still ill-

understood : it is not possible to