An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 2:30
Lecture 46: Are We Alone?
Life in the Universe
Basic requirements/conditions for life
- Source of Energy
- Complex Chemistry (liquid water and carbon)
- Benign Environments (esp. low UV)
Criteria for Habitable Planets
- Distance from its parent star (Habitable Zone)
- Size of the Planet
Searches for Earths and Life
Basic Requirements for Life
The first question we must address is what conditions are necessary for
life to exist in the first place.
Based on what we know about life on Earth, the only place we know
life exists for sure, we can determine at least 3 major requirements:
- Warmth to allow liquid water to exist (or liquid methane?)
- Energy is needed to fuel chemical reactions (metabolism)
- Complex Chemistry
- Elements heavier than Hydrogen & Helium
- Carbon as building blocks for complex organic molecules
- Protection from harmful UV radiation
- UV light can damage or break complex molecules, causing
mutations that may inhibit the emergence of complex life.
- Protection from UV is afforded by the Ozone Layer, underwater, or
Extreme Life on Earth
In addition to the more familiar life we see around us, life on Earth
is often found in surprising and extreme environments. These
In other words, life can be pretty tough, so it might thrive in a broad
range of conditions.
- Dark Life
- Bacteria that thrive many kilometers beneath the Earth or deep
inside polar ice
- Hot Life
- Microbes surviving in boiling water in geyser pools (e.g., Grand
Prismatic Spring in Yellowstone National Park)
- Deep ocean life near very hot thermal vents (e.g., thermophilic
microbes and Pompeii worms)
Life Elsewhere in the Solar System?
Could life exist elsewhere in our Solar System? So far we haven't
found it, but some places people have suggested we look are:
- Evidence it had liquid water and maybe a heavier atmosphere in
the distant past. Life might have briefly arisen there, and
might survive underground (like terrestrial geobacteria).
- This is a big driver of present and future Mars exploration.
- Europa (Icy Galilean Moon of Jupiter)
- One model is that it has a liquid ocean under its ice that
is warmed by tides
- The outer shell of ice protects it from UV radiation and cold
- Enceladus (Icy Moon of Saturn)
- Warm water geysers seen by the Cassini spacecraft, suggest
reservoirs of warm liquid water below the ice (heated by tides), as
well as signs of organics.
- Like Europa, shielded by the outer ice layer.
- Titan (giant moon of Saturn)
- Titan has a thick methane atmosphere, and liquid methane chemistry
- Complex molecules are seen to be present
- Maybe too cold for water-based life, but methane-based life???
The Habitable Zone
What would happen if we moved the Earth closer to the Sun?
- Temperature would increase due to the greater solar heating.
- At a distance of 0.84-0.95 AU, extra solar heating is enough to
trigger a Runaway Greenhouse Effect.
- Earth would become like Venus is today.
What would happen if we moved the Earth away from the Sun?
In between, where water can be liquid at normal atmospheric pressure,
is called the Habitable Zone.
- Temperature would decrease because of the reduced solar heating.
- At a distance of 1.4-1.5 AU, water would begin to freeze out (depends
somewhat on how well the greenhouse effect can keep the Earth warm).
- Get a frozen "Snowball Earth".
There are two estimated ranges for the Habitable Zone in our Solar System:
- The Sun's Habitable Zone today
- [Click on the image for a full-size version]
The more conservative estimate is based on the assumption that a runaway
greenhouse effect starts at a lower temperature, and that catastrophic
freeze-out occurs just before the orbit of Mars.
- Conservative: 0.95-1.4AU
- Optimistic: 0.85-1.7AU
The more optimistic estimate has a higher temperature found closer in,
and that the greenhouse effect helps keep a heavier atmosphere like
Earth's warmer further away from the Sun.
A Question of Size
What happens if we kept the Earth in its current orbit within the Sun's
Habitable Zone, but made it larger or smaller?
Make the Earth too small
An example of a "too small" planet is Mars. Mars is almost within the
Sun's habitable zone, but it is too small, and is a frozen desert world
with a very thin atmosphere, solidified interior, and virtually no
- Too small to retain a warm atmosphere
- Interior would cool rapidly, and it would lose its magnetic field,
making the atmosphere vulnerable to loss to the solar wind.
Make the Earth too big
- It can now retain H and He in its atmosphere, and so can build
a very heavy atmosphere.
- Leads to an atmosphere that is too hot and too high pressure
for liquid water.
- Abundant Hydrogen shifts the basic chemistry from oxidizing chemistry
to reducing chemistry.
This implies that there is also a mass limit within which a planet
in the Habitable Zone is hospitable to life. A rough estimate is within
the range of 0.2-10 Earth Masses.
This leads to a classic Goldilocks Problem:
To be hospitable to life, a planet cannot be too hot or too cold, or too
big or too small. Conditions have to be "just right".
One common fallacy even among astronomers is to only consider the
Habitable Zone part of the argument (too hot or too cold), and forget
that the size of the planet also plays a crucial role. A number of
nonsensical statements have been made by astronomers in the press about
discoveries of new planets in the Habitable Zones of those stars to the
effect that they might have liquid water, ignoring the fact that the
large sizes of those planets almost certainly precludes them from being
places with liquid water or comfortable atmospheres.
Where to Look?
So, to find other habitable planets, where should we look?
To review, the basic conditions for life, at least as we understand it,
may be summarized as follows:
The best bet is to look for stars with rocky planets in their
- Stable, long-lived source of energy
- Basic Elements of life (water, carbon, etc.)
- Benign environmental conditions
- A location for life to arise
What to Look For?
The first thing is to find other Earth-like planets around Sun-like stars.
Easier said than done.
search strategies have not yet found Earth-mass planets, for a number
- Radial Velocity (Doppler Wobble)
- Most sensitive to massive planets close to their parent stars
- Required sensitivity to find Earths in the Habitable Zone is the ability
to measure speeds of a few centimeters/second, while currently the best
precision is 1 meter/second.
- Transit Method
- Current searches are also only sensitive to close-in, massive planets.
- Future high-precision spacecraft missions (e.g., Kepler), might be able
to find Earth-size planets, but it's right at the limits.
- Microlensing Method
- In principle this can find Earth-like planets in Earth-like orbits now,
but only around distant stars, precluding follow-up studies to search for
- This would be good for a census of such planets (estimate fraction of
stars with Earth-like worlds).
The best future hopes seems to be direct imaging searches around nearby
stars using techniques of interferometry and coronography, with follow-up
spectroscopy to study any likely candidate habitable planets.
Two proposed (but not yet fully funded) missions being designed to accomplish
The main goals of these projects are:
- Darwin: Multi-satellite ESA interferometer/planet finder
Planet Finder (TPF) - NASA imaging interferometer and coronographic
- Find Earth-mass planets in the Habitable Zones of nearby Sun-like stars.
- Follow-up spectroscopy to search for spectroscopic biomarkers
of atmospheres and life.
These are spectral signatures in the atmospheres of habitable planets,
based on what see on Earth, that are signs of life.
The primary spectroscopic biomarkers are:
- Molecular Oxygen (O2)
- Generated by planet photosynthesis from sunlight, CO2 and
- Number of strong absorption bands, especially at visible wavelengths,
but they can be easily confused (false-positives).
- Ozone (O3)
- This is a photolytic product of O2, so its presence also
requires life, if at second hand
- Has a strong, distinct Infrared absorption band that makes it easier
to spot and less prone to confusion than O2.
- Carbon Dioxide (CO2)
- Shows a planet has an atmosphere (secondary indicator).
- Has a number of strong, distinct infrared absorption bands.
- Water Vapor (H2O)
- Essential for life. Would be a sign that liquid
water is possible, but it is not foolproof.
- Methane (CH4)
- In oxidizing atmospheres, Methane is a byproduct of anaerobic
chemistry associated with certain kinds of bacteria
(methanobacteria), either arcaeobacteria in the pre-biotic Earth,
or methanobacteria living in the guts of ruminant animals like sheep and
cows (and humans, too).
- Strong infrared absorption band that is easily visible even with
relatively small (fraction of a percent) concentrations in an atmosphere.
Other markers are more difficult to employ, for example, the Red Edge,
the sudden increase in the reflectivity of chlorophyll in plants in the
near infrared above 700nm wavelength. This is a natural property of
plants that helps them keep cool in full sunlight, and it is seen in the
reflectance spectrum of the Earth, but it may be hard to distinguish
definitively from other spectral features (this is a debatable point).
The Last Word
There is no last word on this question yet, and it is one of the leading
emerging scientific questions of this new century.
So far, the only place where we know life exists is right here on Earth.
We have some good ideas of where to look, and have started developing
the technologies to carry out that search, but we have not found
anything else anywhere yet.
What do I think?
I believe that we'll find Earth-mass planets, perhaps even in the
Habitable Zones of their parent stars, before this decade is out (2010),
and certainly before the first generation of space-based planet finders
like TPF or Darwin become a reality.
I do not know if we will find Earth-like planets with good enough data
to establish the presence or absence of spectroscopic biomarkers in my
lifetime (I'm 46), but perhaps it will happen during the lifetime of
most of my students.
If we do find biomarkers on a planet circling a star within a few 10s of
light years of us, then subsequent generations, if we do not do
something stupid and self-destructive to our civilization before that,
will be irresistibly drawn to go there. It might even help our
civilization survive, by giving us a common goal, nutured by a common hope
that in the vastness of the Universe we are not alone.
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Updated: 2007 November 28
Copyright © Richard W. Pogge, All Rights Reserved.