Threshold 4—The Earth and the Solar System ................................

LECTURE

Life, we’ll see, is too delicate to exist on stars, which are extremely

violent places. Planets, though, provide a perfect environment.

W

e’re beginning to see that everything, including chemicals, even
including the Universe itself, has a history. So, too, does our solar
system and our Earth. This lecture describes the fourth great
increase in complexity: the appearance of planets. To understand how planets
were formed, we must shift scales, from the cosmological scale to the scale
of a single star, our Sun. Our temporal scale shrinks slightly. We begin 4.5
billion years ago, when the Universe was about 9 billion years old.

Our spatial scale shrinks drastically as we focus on one tiny part of the
Universe. We have seen that there are as many stars as there are grains of
sand on all the beaches on Earth. Now we look at one of those grains of sand,
and the stuff around it. Yet from a human perspective, this is still a huge
scale. Remember, an airliner that takes ¿ ve hours to cross the U.S. would
take 80 years to reach Jupiter, 750 years to reach Pluto, and about 5 million
years to reach the nearest star, Alpha Centauri.

In the 1990s, improved observational techniques let astronomers observe
other planetary systems for the ¿ rst time. So far, 10% of all stars surveyed
have turned out to have planetary systems, which suggests that such systems
are very common. Soon we may know a lot more about solar systems, but in
the meantime most of what we know is based on our own system. Planetary
systems are created as by-products of star formation, but only in regions that
are chemically rich because of previous supernova explosions.

Our Sun was created like all other stars, by the collapse of a cloud of matter, a
“solar nebula.” Planets formed because this particular cloud was chemically
rich. Hydrogen made up about 70% of the matter in the solar nebula, and
helium another 27%. Carbon, oxygen, and nitrogen (all vital ingredients
of living organisms), accounted for about 1.5%, and the remaining 1.5%
included all the other elements of the periodic table.