S14 - Formation of planetary systems 2 Flashcards
What is a planet?
A large body that is not sufficiently large for fusion to ever consume a majority of their deuterium (M < 13 Jupiter masses).
What are brown dwarfs?
Bodies large enough for deuterium fusion but not massive enough to sustain hydrogen fusion.
Why is it hard to distinguish between stars and brown dwarfs or brown dwarfs and planets using only luminosity?
Because stars, brown dwarfs, and planets all shrink and cool as they age, so there is not a unique relationship between luminosity and mass. For example, an old brown dwarf can have the same luminosity as a young planet.
For what planets is the transit photometry planet detection method particularly effective?
Large planets, orbiting cool stars (as relative luminosity dip is greater) and in short-period orbits (where overlap is more common).
For what planets is the radial velocity technique particularly sensitive?
For short-period and large planets.
What is the predominant size range of exoplanets?
Between Earth-size and Neptune-size.
Planets are more likely to be found orbiting stars with…
…high metallicity as the disk that helped form the star likely contained lots of dust material (e.g. metals, silicate) required to form planets.
What are hot Jupiters?
Massive planets with almost circular orbits at radii, a < 0.1au and very hot temperatures ∼1500K.
What is the relationship, if any, between the number of planets and planet mass?
The number of planets decreases with planet mass. Large planets are rare.
When does gravitational instability arise in a disk?
The formation of a massive disk during protostellar core collapse.
Cooling of a disk from a stable to an unstable state.
Slow accretion of mass which causes a buildup of surface density in the disk.
Perturbation by a binary companion.
Close encounters with other star/disk systems.
Accumulation of mass in a magnetically dead zone.
Describe the core accretion scenario for gas-giant planet formation.
(1) Core formation: a solid protoplanet core grows via a succession of two-body collisions until it becomes massive enough to retain a significant gaseous atmosphere.
(2) Hydrostatic growth: initially the envelope surrounding the solid core is in hydrostatic equilibrium. Over time, both the core and envelope grow until the core exceeds a critical mass.
(3) Runaway growth: once the critical mass is exceeded a runaway phase of gas accretion ensues. The rate of growth is supply-limited and defined by the hydrodynamic interaction between the growing planet and the disk.
(4) Termination of accretion: eventually the supply of gas is exhausted, either as a consequence of the dissipation of the entire protoplanetary disk, or more likely, due to the planet opening up a local gap in the disk. Accretion tails off and the planet commences a long phase of cooling and quasi-hydrostatic contraction.
Why do we find hot Jupiter-sized planets in close orbit of their star when we need to be beyond the snow lone to have enough solid material available to form a massive giant planet?
Planetary migration. The presence of a planet in a protoplanetary disk modifies the distribution of gas in the planet’s vicinity. Gravitational interaction between the planet and the non-uniform arrangement of gas generates torques that alter the planet’s orbit. This causes a planet to migrate toward or away from the star and also dumps or excites the orbital eccentricity and inclination. The direction and rate of migration vary depending on the mass of the planet and the local properties of the gas disk.
How do disk-planet interactions lead to inward planetary migration?
A gap is formed in the disk by planet due to tidal interaction and mass accretion by planet. The gap prevents material flowing to the inner disk so it shrinks. Angular momentum is transferred from the inner disk to the planet and from the planet to the outer disk, causing the planet and gap to move inwards. If the disk is viscous, the outer disk also moves inwards. Migration continues until the inner disk has disappeared (as there is no longer anything to exchange angular momentum with) and the tidal interaction of the planet with the spinning star stabilises orbit.
Which planet formation model is favoured, core accretion or disk gravitational instability, and why?
The probability of finding planets is a strong function of metallicity, so the core accretion method is favoured.
