L6 Formation of our solar system

What were the two main components of the spinning disk surrounding the forming Sun?

98.5% gas (99% H & He, 1% other elements) and 1.5% dust (mostly carbon, iron, and silica).

How long did it take the solar nebula to contract, flatten, and form the protostar and planetesimals?

Approximately 100,000 years.

What is 'Age Zero' of the solar system and how is it determined?

The point when nuclear fusion takes over as the primary source of luminosity, determined from the age of carbonaceous chondrites in meteorites, found to be 4.6 billion years ago.

What are the two key factors that determine where and how planets form in a disk?

Turbulence and temperature of the disk.

How does turbulence affect planetesimal formation?

Too much turbulence causes particles to move too fast and bounce off each other; less turbulence allows particles to collide and stick together.

What is the 'ice line' (or frost line) and why is it significant?

A critical distance from the star where it is cold enough for hydrogen compounds like water to condense into solid ice grains, allowing for more material available for giant planet formation.

What are the three temperature zones in the early solar disk and what do they mean for planet composition?

Inner zone (0.8-1.3 AU, ~1000K): too hot for water; Middle zone (2-5 AU, below 500K): volatile substances can remain solid; Outer zone (beyond 5 AU, below 150K): water-ice survives in solid form.

How long does it take for planetesimals larger than 1 km to form, and what affects the timescale?

With no turbulence, objects a few km across could form in tens of years; with turbulence, the timescale is 1,000-100,000 years.

What does the central plane of the disk contain after 100,000 years?

Approximately 100 billion objects of 1-10 km in size, surrounded by a thicker disk of nebular gas.

What is the difference between inner and outer planetesimals as they grow larger?

Inner planetesimals are rocky and retain dust but not gas; outer planetesimals contain water-ice, grow more massive, and can capture gas atmospheres.

What is the state of the inner and outer zones after about 1 million years of planetesimal growth?

The inner zone contains roughly 20 Moon-sized objects plus millions of smaller ones; the outer zone contains only a few large objects.

What happens to massive and smaller objects during the later phases of agglomeration?

Massive objects attract smaller ones, speeding up accumulation; smaller objects collide at high speeds, causing fragmentation.

What are the two types of planets produced in a solar system, and which planets belong to each type?

Gas Giants (Jovians): Jupiter, Saturn, Uranus, Neptune; Terrestrial (rocky) planets: Mercury, Venus, Earth, Mars.

What are the key properties of Gas Giant planets?

Composed primarily of hydrogen and helium, low densities, rapid rotation, deep atmospheres, rings, and many moons.

What are the key properties of terrestrial (rocky) planets?

Composed primarily of rock and metals, high densities, slow rotation, solid surfaces, no rings, and few or no moons.

What solid core mass is required for a gas giant to form, and why can this only happen beyond the ice line?

A core exceeding 10 Earth masses is needed to attract hydrogen gas, achievable only beyond the ice line where solid material is abundant.

What are the estimated core masses of the four gas giants?

Jupiter ~29 Earth masses, Saturn ~19, Uranus ~13, Neptune ~15.

What are the two key takeaways from Lecture 6 for the evolution of life?

Distance from the star determines whether a planet is rocky or a gas giant, influencing its chemical composition and evolutionary path; planetary atmospheres differ dramatically on either side of the ice line.