4.2: Planet Formation

Protostars spin up as they are contracting. Look again at the protostars in Orion. The flattened dark rings are a consequence of gravity and conservation of angular momentum and contain the left over debris from star formation. Remarkably, star formation theorists expected that stars would form flattened protoplanetary disks, long before astronomers were able to obtain these images. Planetary bodies are thought to form through a method called core accretion. Dust grains accrete to form rocky aggregates called planetesimals that range from kilometer size to the size of Earth's moon. Planetesimals interact with each other through gravity, colliding and building up to form planetary embryos that are Moon- to Mars-sized bodies.

Scientific theory has to agree with existing observations and the only example of a known planetary system in the 1980's was our own solar system. Any theory about how the planets formed had to explain some basic observations:

1. All of the planets revolve around the Sun in the same direction and most of their moons orbit their planets in that same direction; the rotation ("spin") of the Sun has the same direction as the orbital rotation of planets.
2. The orbital planes of the planets are inclined by less than six degrees with respect to each other. In other words, planetary orbits are nearly co-planar.
3. There are two types of planets: small rocky planets reside in the inner part of the planetary disk and gas giant in the outer part of the disk.
4. The solar system contains large numbers of asteroids and comets that have specific distributions in the disk.
5. There is some randomness in the solar system that a good model should accommodate: e.g., the spin axes of some planets are not well aligned with the spin axis of the Sun or the orbital plane; some planets have moons while others do not.

The solar nebula theory assumed that the same material that makes up the Sun was uniformly present throughout the protoplanetary disk. Because material close to the star will be hotter, there are differences in temperature and pressure in the protoplanetary disk. As a result, condensation of refractory elements (metals and silicates) into to grains and pebbles mainly occur in the hydrogen-depleted inner part of the disk. Volatile elements (ices, waters and gases) would not survive in the inner disk because it is too hot there, but at larger distances, beyond the so-called frost line (roughly the location of Mars), the volatiles could be incorporated into low density gas-rich planets like Jupiter. The solar nebula model posited that grains in the disk began to aggregate into larger bolder-sized objects called planetesimals. As the planetesimals plowed through the disk, they accreted more mass.

Figure \(\PageIndex{1}\): Artistic impression of the protoplanetary disk. Accretion of particles through collisions led to the formation of planetesimals, which then grew into planets. How does this model accommodate the existence of the two types of planets in our solar system?

Figure \(\PageIndex{2}\): This cartoon sketch of the protoplanetary disk illustrates that rocks and metals condense in the inner part of the disk where the temperatures are high. What is the approximate distance to the frost line in our solar system?

The solar nebula model helps to explain why planets should be expected to orbit in the same direction as the spin of the star - conservation of angular momentum results in a disk and since planets accrete in this disk, they will inherit that same rotation. The planetary orbits would also be constrained to the plane of the protoplanetary disk, explaining the co-planarity that is observed in the solar system.

The solar nebula model rather neatly explains the two types of planets we have: rocky planets are built from refractory material that can survive at the higher temperatures closer to the Sun, while hydrogen-dominated gas giant planets would necessarily form in the cool outer reaches of the disk. The large number of asteroids and comets? Those are left over crumbs that were not swept up into a planet or the central star. The random features that we observe, such as the tilt of Uranus, could be the result of collisions during the planet-building phase. The solar nebula model enjoyed widespread acceptance. The real test for this theory would happen when exoplanets were discovered: would they show the same attributes listed above for our solar system?

One unusual attribute of the Earth is that for our size, we have a very big moon. Our moon is similar in size to the moons of Jupiter, even though Jupiter is ~300 times the mass of the Earth. Such a large moon-to-Earth mass ratio results in ocean tides. Through conservation of angular momentum, the gravitational tug of the tides that cause the orbit of the moon to expand also slows the spin of the Earth. The Moon is slowly drifting away from the Earth and by tidal interactions, the spin of the Earth is decreasing - good news for those of us who need more hours in the day. Reversing the arrow of time, this means that long ago, the moon was much closer to the Earth and the Earth used to complete one day-night spin on its axis in roughly 5 hours. There would have been shorter lectures when the day was only 2.5 hours long.

The leading theory for the formation of the moon is that it formed when a Mars-sized object collided with a glancing blow to the Earth. The early solar system would have had a lot of debris that was not swept up in planet formation, so at first blush, this is plausible. This model for the formation of the moon is further supported by two quantitative lines of evidence. First, the Moon is compositionally the same as the Earth mantle. In particular the isotopic fractions of oxygen are the same on the Earth and the moon. This suggests that the collision must have occurred some tens of millions of years after the Earth formed, after iron and other heavy elements were already settling into the core.

One of the conundrums with the impact theory for the formation of the moon was that we were only able to establish a lower limit in time for the collision. Because time was needed for differentiation of heavy elements, the collision likely happened about 50 Myr after the formation of the Earth... but, could the moon-forming impact have occurred 200 Myr after the formation of the Earth? This turns out to be an important consideration for the origin of life on Earth. The energy from the moon-forming collision would have melted the surface of the Earth, raising the temperature to about 2000 K and sterilizing the surface of the planet. In a paper published in 2017, researchers describe how they have dated the age of the Moon using zircon crystals. This give an extremely precise age of 4.5 Gyr (with an uncertainty of only ±10 Myr). Since the age of the Earth is about 4.56 Gyr, this finding supports the theory that a moon-forming collision occurred 60 Myr after the formation of the Earth. This is an exciting result because it allows us to calculate a cooling time for the Earth that is needed in estimating the earliest times for the rise of life on the planet.

Current ideas about the evolution of the moon is nicely explained in the YouTube video below. Watch this and then answer the questions below.

What was the Late Heavy Bombardment? What evidence does the moon present for this event?

When did volcanic activity on the moon end?

Is the smoother surface of the moon geologically younger or older than the cratered surface?

Astronomers speculate that planet formation might be hampered by the presence of a second star. If a binary star system has a short orbital period, then the protoplanetary disk will be truncated, limiting planet formation around either or both stars. However, one of the surprises in the past 20 years is that planet formation is incredibly robust. In the case of the shortest-period binary stars, astronomers have found planets circling outside of the binary star orbits. For binary star systems with orbital periods of many years, planets have been found orbiting one or both stars individually. Therefore, it seems a good bet that planets exist in orbits around the three alpha Centauri stars, the likely first destination for space probes that venture out beyond our solar system, and indeed a planet has been discovered orbiting Proxima Centauri.