Download U 7 Synopsis

Unit 7:
How did our earth form?
Synopsis:
Learn how the existence of new elements that could combine chemically to form a huge array of
new forms of matter allowed the appearance of new and more complex things, including … our
Earth.
Threshold 4: Planets and Solar Systems: The new elements formed in dying stars could combine
to form simple molecules such as water or methane. Around newly formed stars, these
elements were smashed together to form asteroids, comets and, eventually, planets. Planets
were an entirely new phenomenon because most of them contained many more chemical
elements than stars.
Simple chemistry: combining atoms: Atoms of different elements could combine to form new
and more complex chemical “compounds”. How they do so is studied in the discipline of
‘chemistry’. Atoms began to combine even in space, where we can detect simple “molecules”
with 10 to 20 atoms, including water, ethanol and acetic acid. This meant that new generations
of stars formed from clouds of matter that contained many different elements, and that made a
huge difference!
The formation of our sun: Unlike the very first stars, all later stars, including our sun, formed
from clouds of matter that contained, in addition to lots of hydrogen and helium, small amounts
of all the other elements of the periodic table. Like all stars, our Sun lit up when the centre of a
collapsing cloud of matter reached a temperature of about 10 million degrees C. That happened
about 4.56 billion years ago. Almost all of the cloud of matter (c. 99%) ended up inside the sun.
But wisps of matter continued to orbit the early sun and the planets of our solar system were
formed from them.
Making the planets: The cloud of matter from which the sun formed was spinning, so it
flattened out like pizza dough and the wisps of matter orbiting it spun around in a flat disk.
Energized by the heat of the sun and hurled around by its gravitational pull, tiny particles of dust
and ice joined together chemically or as a result of collisions or electric forces of various kinds to
form larger lumps, a bit like comets or asteroids. Eventually, within each orbit more and more
lumps combined to form huge ‘planetesimals’, until in most orbits all the matter was absorbed
within a single large object, a planet. This process of planet formation is known as accretion. It
may have taken 10-20 million years. (This is almost certainly how the inner, rocky planets such
as our earth formed, though it is possible that the giant planets, beginning with Jupiter, formed
in different ways.) Accretion was an extremely violent process. You can get some idea of how
violent it was by looking at the scars on the face of the moon. Indeed, the moon may have been
formed when an object almost the size of Mars collided with the young earth, gouging out a
huge chunk that eventually congealed to form the moon. (The surface of the earth has changed
too much to leave any traces of this violent era.)
The left-overs: Bits and pieces were left over. Today, they form comets and asteroids and
meteorites. The ‘asteroid’ belt between Mars and Jupiter seems to be a region in which a planet
failed to form, because of the huge gravitational pull of Jupiter.
Planets and increasing complexity: Studies of nearby stars suggest that planets formed around
most stars. Planets (particularly rocky planets such as our earth) represent a new threshold of
complexity, first, because they are chemically more complex than stars (which consist mostly of
hydrogen and helium); second, because they have a more complex structure than stars; and
third because some planets created the ideal Goldilocks conditions for the emergence of one
more level of complexity: life!