What do we mean by “habitable planet”?
A habitable planet is a planet on which
life can exist
What does life require?
1) building materials – primarily H C O
N, which are widely available
Several slides from Wikipedia 2009 on molecules
found in interstellar clouds (interstellar medium or ISM) with specific
examples: http://en.wikipedia.org/wiki/List_of_molecules_in_interstellar_space
The current web site (as of June 25, 2010)
has more molecules (up to nmber of atoms = 24) and more pictures of some
interesting molecules. I didn't bother to update the lecture this
year.
What does life require?
Energy
Either external source (sunlight)
or internal source (internal heat)
Sunlight places restrictions on locations
where photosynthetic organisms
(and organisms that live off photosynthetic
organisms) can exist
Earth is 1 AU from sun. Jupiter
is ~ 5 AU – it receives 1/25th as much sunlight
Internal heat provides energy for volcanism and chemosynthesis and communities that survive on chemosynthetic organisms.
2 reasons a planet can have a hot interior:
size or tides
If a planet is large enough, it will have enough internal heat to be “active” over a long period of time
Sources for heat:
accretional heat
a) KE ?
heat
b) core
formation
(PE ? heat)
c) core
solidification
(exothermic
reaction)
2) decay of radioactive elements
The second source for internal heat is tidal energy, and can provide internal heat for even the smallest of planets.
To understand how it works, you need to
understand two things:
orbits are elliptical, not circular –
planets spend most of there time at the far point from their star
any object orbiting another object will
feel a gravitational pull from that object
For very small objects that are orbiting close to a massive object, the small object will wind up in synchronous rotation.
So what is all of this?
lunar tide
tides due to Moon’s gravity – a balance between gravity and centrifugal force
get two tidal bulges on either side of planet (facing towards and away from moon)
get body tide as well as ocean tide – body
tide is much smaller
get 2 high tides per day – occur approx.
50 minutes later every 24 hours, because of moon’s orbit (moon not stationary,
but revolving around Earth)
Spring tide =
highest high tide,
gavity of moon & sun work together
Neap tide = lowest high tide, gravity of
moon & sun work against each other
eccentricity (departure of
orbit from round
shape,
varies from 0
to ~1)
semi-major axis (size of
orbit)
revolution rate
so moving fastest when near sun – but that is when planet feels largest tidal pull
Synchronous rotation
Rotation = Revolution
Our moon is in synchronous rotation – why
we only see one side/face
Two slides with quotes and image from: http://oklo.org/?m=200609
Tidal Heating
Jovian system has four large moons close to a massive planet (all with synchronous rotation) – their interactions lead to complicated changes in tidal bulges
so in addition to all of the other sources of heat, Earth is heated by tidal forces as well
Information and image from University of
Arizona web site: http://uanews.org/node/21820
What does life require?
building materials – primarily H C O N,
which are widely available
energy – sunlight or a source of internal
heat
a liquid that will do the following:
dissolve organic molecules – making
them available for chemical reactions
transport
material into and out of a cell
be involved
in metabolic reactions
We think that water is the only good candidate.
Why water?
Heat capacity, heat of vaporization, surface tension, absorption of radiation
Density: water is one of the few substances (and the only common substance) where the solid is less dense than the liquid – prevents global freezing on Earth because ice floats to the top and insulates water below (prevents transfer of heat from ocean to atmosphere).
Solvent properties: water is a polar molecule
– means electric charges are unevenly distributed – affects way water dissolves
other materials – molecules that have charge separations (such as salt)
dissolve easily in water; molecules that don’t – such as cell membranes
do not
Other liquids that have been proposed as
useful for life:
ammonia NH3
methane CH4
ethane C2H6
do not have the solid/liquid density properties or polar structure of water
terrestrial cell membranes placed in liquid
ethane, methane, or ammonia dissolve!
Melting and boiling points:
Comparisons of liquids at 1 atm pressure:
Substance Freezes at Boils at Width
of liquid range
(degrees C) (degrees C) (degrees
C)
water 0 100 100
ammonia -78 -33 45
methane -182 -164 18
ethane -183 -89 94
of course, this depends on the atmospheric
pressure
So when we talk about habitable planets,
or planets in a habitable zone, we mean a planet that has abundant liquid
water on its surface. To a first order (ignoring the size of the
planet), this means distance from the planet’s star.
The sun's habitable zone today: http://ftp.astronomy.ohio-state.edu/~pogge/Ast161/Unit7/life.html But this is a snapshot of right now!
All stars change brightness while on the main sequence, so the location of the star’s habitable zone will move from closer to the star to farther out with time – a habitable planet has to be in the star’s continuously habitable zone
A more massive main sequence star has both
a larger habitable zone and one that is farther from its star – so shouldn’t
we be looking a really big stars??? NO
Massive stars don’t stay on the main sequence long enough for life to develop and evolve.
So we actually want to look at F G K stars
to optimize both the size of the habitable zone and the main sequence life
time of the star
But wait – we have two worlds in our sun’s habitable zone – the Earth and the Moon. So why don’t we have two habitable planets?????
Planetary size matters
For its surface gravity, the Moon’s surface
is too hot to hold onto an atmosphere
Although Mercury is smaller than Mars,
it is made of denser material. The surface gravities of the two planets
are almost identical. If you moved Mercury to Mars’ location, it
could hold onto the same amount of atmosphere as Mars.
Sense of scale: Jupiter versus the
rest of the solar system
What happens if you replace the Earth
with Jupiter?
Do we need a big moon?
It is known that variations in the Earth’s
axial tilt are related to ice ages and big climate changes. Our moon
prevents the Earth from having as large a change in axial tilt as Mars.
Recent studies of Mars suggest that it has experienced much greater global
climate upheavals as a result.
What happens if the planets orbit the star in a fairly elliptical orbit?
What would happen to the Earth if Jupiter
had a very elliptical orbit?
The Earth's Magnetic Field and Life - see
presentation at http://www.iugg.org/IAGA/iaga_pages/pdf/toulouse2005/david_loper.pdf
slides 26 through 31.
It now looks as if we need plate tectonics to have a habitable planet, which makes Earth close to the smallest possible habitable planet (if true).
BUT, all of this assumes you need a habitable
SURFACE, not an ice covered ocean as proposed for the Galilean satellite
Europa. Stay tuned.
Summary:
What do we really need to have for life to begin and evolve on a planet?
Answer – we’re not sure, but we suspect we need:
Right mass of star – long enough lifetime,
but wide enough habitable zone
Right mass of planet that is the right
distance from star and has plate tectonics and a magnetic field
The planet may need a large moon to stabilize
its axial tilt
Close to circular orbits for everything
with significant mass in the planetary system
I’m probably forgetting some things, but
these are likely to be the most important