Lecture 18: Habitability

Question 1

what is the habitable zone?

Answer 1

the range of distances from a star within which we could move our planet without fundamentally changing its ability to support life

generally, the zone in which it is possible for a planet to have liquid water on its surface

Question 2

how do planets lose internal heat?

Answer 2

convection
- hot material expands and rises while cool material contracts and falls

conduction
- hot material transfers heat to cooler material

radiation
- energy can be lost by radiating light into space

Question 3

how do you determine surface temperatures?

Answer 3

when given stellar temperatures, radius, planets orbital radius and albedo:

???

Question 4

how do planets gain their atmospheres?

Answer 4

volcanic outgassing where water and gases trapped in the interior during formation

evaporation or sublimation of surface liquids and ices

micrometeorites and high energy particles

Question 5

what is thermal escape from an atmosphere?

Answer 5

when gas has enough energy to simply escape into space

v_esc= √2GM_pl/R_pl

Question 6

how does potential habitability change around stars different to the sun?

Answer 6

for example, proxima centauri

surface temperature is 3042K

the orbital radius is still within the potentially habitable zone

planet will likely be tidally locked- like the moon around earth

Question 7

how do planets lose their atmospheres?

Answer 7

condensation- gases condense forming surface liquids and solids

chemical reactions- gases incorporated into surface metals or rocks

impacts- atmospheric gases ejected into space

solar wind- magnetospheres

thermal escape- gas has enough energy to simply escape into space

Question 8

why are atmospheres important?

Answer 8

existence of liquid water depends of atmospheric pressure and temperature

we breathe the oxygen that’s in our atmosphere

it absorbs and scatters light, protecting us from harmful radiation

greenhouse effect makes planetary surfaces warmer than they would be without an atmosphere

Question 9

how do planets acquire internal heat?

Answer 9

heat of accretion
- accreting planetesimals release gravitational potential energy

heat from differentiation
- dense material sinks and releases gravitational potential energy

heat from radioactivity
- decaying radioactive isotopes in rocks and metals

Question 10

why is internal heat important for a planet?

Answer 10

it drives geological activities such as:

-volcanic outgassing producing atmospheres

• tectonic motion (plate tectonics)
• semi-molten core drives currents producing a magnetic field, which protects earth from harmful cosmic rays