Habitable Zone

ASTR 1420

Lecture 8

Sections 10.1-10.4

Habitable Zone = Zone of liquid water

Goldilocks’ zone

This porridge is too hot…

This porridge is too cold…

Ahhh, this porridge is just right!!

Habitable Zone

The range of distances from a star at which a planet could potentially have surface temperatures right for liquid water!

Important facts1. range of distances2. is not a sufficient condition (e.g., Moon is in the H.Z.)3. HZ is changing over time

? Surface habitability only?While it is possible for us to find sub-surface life in our solar system, it is nearly impossible to find subsurface life around other stars. Therefore, in the search for life beyond solar system, we will only focus on worlds with possible surface water!

Free floating Earths

Ejected Earth-size planets during planet formation with a thick atmosphere can trap (or insulate) heat for billions of years can have surface water without a sun!

But, we cannot detect these! Likewise, subsurface water-worlds.

So, we will ignore these possibilities from now on.

Venus and Earth• Venus is only 30% closer to the Sun than Earth, but with surface

temperature of 880°F!• Strong greenhouse effect due to 200,000 times more CO2 in the

atmosphere• Why so much CO2 than Earth?o Lack of CO2 cycle due to the lacking surface water (ocean)

• Most water on Earth were brought by impacts, and similar impacts must have happened in Venus

Then, Venus once had an ocean but lost her water over time?

• Yes! From the fossil record of ocean from High Deuterium to Hydrogen ratio…

If we “move” Earth to the Venus orbit…

Runaway greenhouse effect?

Not so simple…• Higher temperature more evaporation more clouds more reflection

of incoming sunlight balancing effect!?!

So, the true inner boundary of Habitable zone should be b/w Venus and Earth orbits!

Ancient Venus : Lost Tropical Paradise?

• Young Sun was about 30% fainter than the current one (young fainter Sun)!

• Young Venus did not have high concentration of greenhouse gases from outgassing yet…

That 30% reduced sunlight + lesser amount of greenhouse gases in the past might have produced ancient oceans on the Venusian surface.

Changing Habitable Zone! Over time, Sun will get brighter

eventually engulfs the Venus

Surface Habitability Factors : Summary

1. distance from a central star o when we consider the minimum distance, in addition to the solar heating, we

must account other processes (e.g., greenhouse effect) also.2. role of planetary sizeo Mars is a good example…o loss of atmosphere loss of magnetic field loss of internal heat due to a

smaller size …o Is there a minimum size? Smaller than Earth, larger than Mars…

3. role of an atmosphereo sources of gases outgassing of trapped gas from ancient impacts.o possibility of forming rocky planets without gases? o importance of magnetic field slowly rotating planets..o planets around binary stars or even binary planets?

Habitable Zone around our Sun• Inner boundary is not so hard to guess (somewhere near to the Venus’

orbit). • Outer boundary is fuzzy.o Mars: Had it been in the HZ? Mars might have never had an extended period of

surface water.

Inner Boundary

Detailed calculations predict that if we “put” the Earth at around 0.84 AU, it will go through a runaway greenhouse effect.

However, a moderate additional warming stronger air circulation taller cloudsbreak up of H2O by

solar UV radiationgradual lose of H2O

Moist Greenhouse Effect

even at 0.95AU!!

Outer Boundary

The distance from the Sun where even a strong greenhouse effect could not warm the planet enough to above the freezing temperature.

• If Mars were larger and with a thicker atmosphere, Mars could have strong enough greenhouse warming to have surface liquid water.

• Calculations show that the outer boundary is ~1.7AU (Mars is at 1.52AU)

• However, in cold atmosphere, CO2 could condense on snowflakes and fall onto the ground reducing greenhouse effect! reducing temperature

Sun’s present Habitable Zone• optimistic case : 0.84 to 1.7 AU

• conservative case: 0.95 to 1.4 AU

Different Kinds of Stars

•massive (hotter) stars• about 30-40 times less

abundant than Sun-like stars.• ~1000 times shorter life

• Sun-like stars• about 10 billion yrs of life

• less massive (cooler) stars• ~10 times more abundant

and 100 times longer life times than Sun-like stars

Understanding Stars…

Less Massive Stars are better…

more abundantlonger-lasting…

Habitable Zone around Different types of Stars• HZ is closer to lower mass stars• HZ is further away from more massive stars

Life Cycle of Stars

As stars age, their sizes and temperatures are changing drastically!

cause a changing HZ with time

Changing habitable zone in our solar system over time

Changing Habitable Zone

• Continuously habitable zone : range of distances that remain always habitable over time.

Galactic Habitable Zone

• Inner boundary : threats to life (too high radiation such as Super Novae, gamma-ray bursts, etc.)

• Outer boundary : existence of heavy elements such as Carbon

How long can we survive?Phase of Giant star: 5+ Gyrs

Habitable zone will move outward over time due to more dumping of energy from the Sun

building a sunshade?slowly moving Earth outward

Planetary nebula & white dwarf phase: 6-7 GyrsSun will eventually die and white dwarf will vanish slowly over billions of years hopping to other nearby young stars…

Galaxy: 50+ GyrsAll gas will be used up in our Galaxy and no more new stars will be bornmigrate to nearby galaxies

Universe: 100s GyrUniverse is ever expanding faster … then?Isaac Asimov’s Last Question:

In summary…

Important Concepts• Runaway greenhouse effect• Surface habitability factors• Habitable zone is changing over

time due to the life-cycle of Stars• HZs are closer to less massive stars

and further away around massive stars.

• Best host stars for life = less massive stars. Why?

Important Terms• Habitable Zoneo around different starso Galactic

• Continuously Habitable Zone

Chapter/sections covered in this lecture : 10.1 – 10.4