solar systems Flashcards
What is the best idea of how the solar system formed?
The nebular model
What uncertainties come with the nebular model?
e.g. angular momentum and the formation of the planets
What evidence supports the nebular model?
observational evidence from the Hubble telescope
Explain how to nebular model works.
- Collapse of a (normally stable) molecular cloud into the solar nebula with a central proto-Sun
- Flattening of the nebula into a circustellar protoplanetary disc (to conserve angular momentum)
- Planet formation sweeps up gas and dust (high melting point mateial forms near sun)
- Elimination of remaining gas and dust via accretion and high velocity T Tauri solar winds
- Sun enters half way through the Main Sequence and begins fusing hydrogen

What is a molecular cloud?
A molecular cloud is a large volume of cold, dense gas in interstellar space/a galaxy, consisting of mostly hydrogen H2 (98%) and helium and some metals, that capable of collapsing to form new stars

Outline the evidence that the collapse of a molecular cloud and the formation of the solar system were driven by a supernova
Meteorites contain decay products of short-lived isotopes such as 26Al and 60Fe; These isotopes must have formed by nucleosynthesis in a supernova shortly before the formation of the solar system.
What is the weight, size and density of a molecular cloud?
Typically a million solar masses
50 ly across
desnity of 109 particles per cubic metre
How many molecular clouds are there in the Milky Way and what form do they take?
2000 known
typically form a ring halfway between the Sun and the galactic centre
What determines if and how fast a molecular cloud will collapse?
- Gravity is the driving force, but many clouds are sable - held open by gas pressure
- The relationship between T, P and collapse rate is described by the Jeans intsability
- The Jeans mass is the mass at which the gravity of the dense cloud overwhelms its internal pressure (therefore allowing for collapse to take place)

What triggers the collapse of a molecular cloud?
Increasing the mass via addition of new gas
- collsion of clouds
- passage through a galactic spiral arm (denser part of galaxy)
Changing the distrobution of mass within a cloud
- supernova shock slams into cloud
What does 26Al and 60Fe decay to and what are the half-lives of these decay products?
26Al decays to 26Mg with a half-life of 700kyr
60Fe decays to 60Ni with a half-life of 2.6Myr
(Excess 26Mg is found in CAIs in meteorites dated to 4568Myr)
What happens to a molecular cloud as it collapses?
- A collapsing cloud tends to fragment as the Jeans criterion is reached in various regions
- These fragments are typically a few ly across
- Contraction results in heating
- Angular momentum forces a spherical nebula to flatten into a 200-AU circumstellar disc with a central proto-star over about 100kyr (preserving angular momentum)
What is a protoplanetary disc?
- A circumstellar disk with planets forming
- Observed around a very high fraction of stars in young clusters
- Lasts about 10Myr before being cleared by T Tauri stellar winds and accretion
- Angluar momentum is shed via viscous drag (collsiions dissipate angular momentum); T Tauri winds and possible magnetic braking.
- Initially hot; later cooling allows formation of solids in the inner disk
- Outer disk retains (methane and ammonia) ices
In the image “empty” rings represent locations of new born planets

What do the “empty” rings represent in this protoplanetary ring?

The locations of new born planets
Sketch a cross-section through a protoplanetary disc, labelling significant features
- Proto-star at the centre;
- very hot gaseous inner disk;
- beyond this is a colder outer dust disc;
- the disc thickens further away from the proto-star;
- frost line marked at a sensible point in the dusty outer disc.

What happens in the hot, gaseous inner disc of a protoplanetary disc?
- Material blown away from the proto-star by radiation pressure
- cooling allows solids to condense out of the vapour phase
What makes up the colder, outer disc of a protoplanetary disc?
- A mixture of gases and solid matter (dust);
- Ices, organic matter and hydrogen
(Organic matter comes from the molecular cloud)
Outline the significance of the frost line to planetary formation
A planet forming at the frost line would have benefited from a large amount of mass in the form of water ice. H2O is stable and doesn’t evaporate.
Therefore, the concentration of ices beyond the “frost line” aids planetary formation
Outline the evolution of a protoplanetary disc
- Angular momentum is shed via viscous drag
- Cooling of inner disc allows condensation of micrometre-scale metal and silicate dust/smoke
- Condensation sequence of (i) metal and silicates, (ii) water, and (iii) ices
- Disc lasts around 10Myr before destruction via solar wind and accretion (so planetary formation must occur within 10Myr)
- Proto-Sun enters Main Sequence and begins stable fusing of hydrogen after 50-100Myr
Explain the concept of the condensation sequence with regard to the protoplanetary disc
The condensation sequence describes the sequence in which solid materials formed via condensation of the hot gas of the protoplanetary disc
What are the strengths of the nebular model?
- Well supported by observations
- Capable of explaining the formation of dust that can ultimately form planets
- Capable of producing planetary systems that share a common direction of orbit
- Broadly supported by the variations in planetary cheistry
- Refractory silicate planets in the inner solar system
- Gas giants and ice giants further out
What are the outstanding questions that negate the nebular model?
- How exactly does the inner disc shed its angular momentum?
- Viscous drag? T Tauri wind? Magnetic braking?
- Why are other star systems so different?
- Hot Jupiters and Hot Neptunes
What is a Hot Jupiter?
A hot Jupiter is a gas giant orbiting very close to its star, heated by its star to over 1000 K
Why is Jupiter not a Hot Jupiter?
This is the Grand Tack hypothesis:
Inwards migration of Jupiter and Saturn began via interactions with the disc, then Jupiter and Saturn entered a 1:2 orbital resonance, forcing inwards migration to halt and reverse.












