8.3. THE NEBULAR HYPOTHESIS 113
8.3 THE NEBULAR HYPOTHESIS
e basic outline of a plausible scenario for the formation of the solar system has been known
for centuries. e basic concept behind the modern solar nebular model was first put forth by
Immanuel Kant in 1755, and in more detail about 40 years later by the French mathematician
Pierre–Simon Laplace.
A large cloud of gas and dust was pulled together by its own self-gravity, becoming hotter
and denser toward its center as it contracted. Most of the contracting mass became the Sun; we
will explore that process in more detail in Section 9.3.
Even a slight initial rotation of this contracting cloud would have been greatly amplified
because of the conservation of angular momentum. One of the fundamental principals of physics,
it is why an ice skater spins faster when they pull their arms inward, closer to their rotation
axis. But for a cloud of gas and dust contracting due to its own self-gravity, there is another
consequence—some of the cloud would flatten out into a thin disk of material orbiting the
newly-formed star.
e planets then formed out of this protoplanetary disk. e nebular hypothesis explains
in a natural way the fact that the planets orbit in the same direction, in very nearly the same
plane. Furthermore, there is evidence of such protoplanetary disks around at least some newly-
forming stars. See Figure 8.8 for an example—the star HL Tauri as imaged by the Atacama
Large Millimeter Array (ALMA), in Chile. e gaps in the disk are just what one expect if
there are proto-planets—too small to see—clearing out spaces in the disk as they orbit.
But this all begs the question of just how do we end up with planets? What causes them
to form out of the gas and dust? We take up this issue in Section 8.4.
8.4 THE CONDENSATION SEQUENCE
e inevitable consequence of a cloud contracting because of its own gravity is that the densest
parts, closer to the center, are hotter. As the proto-planetary disk cooled, there was a gradation
of temperature, with the hottest parts being near the newly-formed Sun, the coolest parts in the
outer regions of the disk.
As the temperature decreased, tiny solid particles and liquid drops could condense out
of the solar nebula. Little pieces could then stick together to form bigger pieces. e bigger a
clump is, the more efficiently it grows by sweeping up new material. And so larger clumps grow
at the expense of smaller clumps. If a clump becomes massive enough, then its own self gravity
will hold it together and gravitational attraction greatly accelerates this accretion process.
But the chemical composition of these condensing solid or liquid particles depends crit-
ically upon the temperature and pressure. e details are complex, but the basic trend is fairly
simple:
114 8. EVOLUTION OF THE SOLAR SYSTEM
Figure 8.8: e protoplanetary disk around the star HL Tauri, as imaged by the Atacama Large
Millimeter Array, in Chile. (Image credit: ALMA ESO/NAOJ/NRAO, CC BY 4.0.)
Molecules made of heavier elements tend to condense out at higher temperatures,
and so condensed out of the solar nebula first. Molecules made of lighter elements
condensed out later, and further from the Sun where the solar nebula was cooler.
ere is another issue: as molecules were condensing out of the solar nebula, what was
there to work with? What were the most common elements capable of making solids or liquids?
We already have this answer—it is the roughly 2% of “metals” that makes up the Sun and in-
terstellar clouds of gas and dust today. ese elements can potentially combine with each other
and with hydrogen to form molecules that can take solid or liquid form. Helium does not enter
into the calculation, as it does not chemically combine with other elements.
And we have already seen that the “metals” are made of mostly lighter elements; carbon,
nitrogen, and oxygen are the most prominent (see Section 5.4.8). After that, the elements silicon,
aluminum, magnesium, calcium, iron, nickel, and sulfur are important.
ere are four basic categories of materials that can be used to form planets. I list these
basic building blocks as follows, in order of decreasing abundance and increasing density:
1. Gases: the bulk of the solar nebula was hydrogen and helium. Under the right condi-
tions, the hydrogen will combine with itself to make H
2
molecules. Helium combines
..................Content has been hidden....................
You can't read the all page of ebook, please click here login for view all page.