Habitable Zone ASTR 1420 Lecture 8 Sections

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 facts 1.range of distances 2.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! 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 CO 2 in the atmosphere Strong greenhouse effect due to 200,000 times more CO 2 in the atmosphere Why so much CO 2 than Earth? Why so much CO 2 than Earth? o Lack of CO 2 cycle  due to the lacking surface water (ocean) Most water on Earth were brought by impacts, and similar impacts must have happened in Venus 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 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!?! 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 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… 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 size o 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 atmosphere o 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). Inner boundary is not so hard to guess (somewhere near to the Venus’ orbit). Outer boundary is fuzzy. 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 clouds  break up of H 2 O by solar UV radiation  gradual lose of H 2 O 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. 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) Calculations show that the outer boundary is ~1.7AU (Mars is at 1.52AU) However, in cold atmosphere, CO 2 could condense on snowflakes and fall onto the ground  reducing greenhouse effect!  reducing temperature However, in cold atmosphere, CO 2 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 optimistic case : 0.84 to 1.7 AU conservative case: conservative case: 0.95 to 1.4 AU 0.95 to 1.4 AU

Different Kinds of Stars

massive (hotter) stars about 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 abundant  longer-lasting…

Habitable Zone around Different types of Stars HZ is closer to lower mass stars HZ is closer to lower mass stars HZ is further away from more massive 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. 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.) 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 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 Gyrs Sun will eventually die and white dwarf will vanish slowly over billions of years  hopping to other nearby young stars… Galaxy: 50+ Gyrs All gas will be used up in our Galaxy and no more new stars will be born  migrate to nearby galaxies Universe: 100s Gyr Universe 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 Zone o around different stars o Galactic Continuously Habitable Zone Chapter/sections covered in this lecture : 10.1 – 10.4