Astronomy190 - Topics in Astronomy

Astronomy and Astrobiology

Lecture 9 : Life’s Requirements

Ty Robinson

Questions of the Day

• What does an environment need to provide to be habitable?

• Why are astrobiologists so hung up on liquid water as being necessary for life?

• What is a habitable planet, and what factors affect whether or not a planet will be habitable?

…for what can more concern us than to know how this world which we inhabit is made; and whether there be any other worlds like it, which are also inhabited as this is?

Bernard le Bovier de Fontenelle, Conversations on the Plurality of Worlds, 1686.

What Is a Habitable Environment?

an environment which can support life

it does not HAVE to have life

What Is a Habitable Planet?a planet which can support lifeit does not HAVE to have life

Note: An Extrasolar Habitable Planet needs to have a habitable surface environment.

Questions?Why must an extrasolar habitable planet have a habitable surface environment?

subsurface environments are too dangerous for life

life is usually found at the surface of a world

liquids can only exists on a world’s surface

remote sensing of extrasolar planets is limited to studying surfaces and atmospheres

What Does a Habitable Environment Provide?

• Building Blocks– The environment must have a source of molecules

(including carbon) from which to build living cells

• Energy– The environment must have a source of energy to fuel

metabolism (photo or chemo)

• Liquid– The environment must have a liquid medium to

transport molecules needed for life.

• Of the three things provided by a habitable environment, which one is likely to be rarest in the Universe?

A Liquid

• life (metabolism) requires a liquid that can:– dissolve organic molecules and salts, making

them available for chemical reactions within cells

– allow for transport of chemicals in and out of the cells

– engage in metabolic reactions • e.g. water molecules are necessary for reactions

that allow for energy storage and release within the cell

Water’s Many Advantages

Water’s Many Advantages• liquid over a large temperature range• temperatures at which it is liquid are high

– increases chemical reaction rates

• water ice is less dense that liquid water– frozen water floats, insulating the liquid water beneath it

• water is a polar molecule– molecules and salts with charge separation dissolve easily – forms hydrogen bonds, which are easily made and broken, so are

absolutely crucial to life’s biochemistry (DNA uses H bonding).

• It’s the second most common molecule in the Universe!!

Water, Water, Everywhere….

Questions?Why are the mid-latitude oceans so devoid of life?

temperatures are too high

these regions lack an energy source

these regions lack nutrients

humans have over-fished these regions

There are very few natural environments on Earth where life is absent.

dividingbacterium

Transmitted light

10 µm

(Junge et al., 2001)

–15°C

Courtesy Jody Deming

Transmitted light

(no stain)

Epifluorescent light (DNA stain)

(Junge et al., 2001)

–15°C

diatoms

bacteria

Courtesy Jody Deming

What Factors Affect Planetary Surface Habitability?

• Parent star – lifetime, UV radiation. • Planet’s orbital parameters – distance, eccentricity• Planetary mass – tectonics, atmosphere, magnetic

field• Initial planetary composition• Atmospheric composition (albedo, greenhouse)• UV shielding• Dynamical stability of the planetary system• Impacts• Biology (CH4, rise of O2,albedo)

Parent Stars

Parent Stars

• to be a suitable “parent” a star must – live long enough for planets to form and life to

evolve• stars 1.5M (O,B,A) age too quickly

– be sufficiently “stable”• stars older than 1 Gyr preferred

– have high enough metallicity and/or mass to form terrestrial planets.

• habitable planets are more likely around F, G and K and M stars

The Habitable Zone “The region around a star in which an Earth-like planet could potentially maintain liquid water at its surface” (0.93-1.37AU for our Solar System)

H2O CO2

• CAVEAT: Finding a world in the habitable zone DOES NOT MEAN THAT THE PLANET IS HABITABLE!

Planetary Mass

• sufficient planetary mass (> 0.3 MEarth) enables– generation of a global magnetic field

• prevents atmospheric stripping by the solar wind

– generation and maintenance of plate tectonics• active volcanism can replenish atmospheric gases

like CO2 over geologic time

• plate tectonics is a crucial component of the carbon dioxide cycle which stabilizes the Earth’s surface temperature on long time scales.

Planetary Atmosphere• atmospheric pressure is required to prevent the loss of

an ocean• atmospheres provide surface UV shields• atmospheres provide buffers against large temperature

swings across the planet, and over time• greenhouse warming provided by atmospheres can raise

planetary surface temperature into a habitable range

Dynamical Stability

• the presence of a large Jovian planet in the outer solar system can act as a vacuum cleaner, to remove debris left over from planet formation

• on the other hand– Jovians on elliptical orbits can disrupt inner planets– Jovians are responsible for creating the debris in

the outer Solar System

• large swings in obliquity (changes in the axial tilt of the planet) can be avoided if the planet has a large moon– axial tilt can change climate, bringing on or ending ice ages

Obliquity

Life

• life can significantly alter the planet’s atmosphere and reflectivity, and thereby affect habitability.

• the dramatic rise of photosynthetically generated O2 about 2.3Gya probably wiped out most of life as it was then known on Earth, inadvertently creating an ice age!

Questions of the Day

• What does an environment need to provide to be habitable?

• Why are astrobiologists so hung up on liquid water as being necessary for life?

• What is a habitable planet, and what factors affect whether or not a planet will be habitable?

Quiz

1 - How is the ‘Habitable Zone’ defined?

2 - Is the ‘Habitable Zone’ closer to an M-type (dwarf) star or a G-type (dwarf) star? Why?

3 - What is one thing you did not understand from today’s lecture?