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Saturn from Cassini Astronomy 161:
An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 2:30

Lecture 46: Are We Alone?
Life in the Universe

Key Ideas:

Basic requirements/conditions for life

Criteria for Habitable Planets

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 underground.

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 "extremophiles" include:
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)
In other words, life can be pretty tough, so it might thrive in a broad range of conditions.

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?

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.
The Sun's Habitable Zone today
Current Habitable Zone of the Sun
[Click on the image for a full-size version]
There are two estimated ranges for the Habitable Zone in our Solar System:
Conservative: 0.95-1.4AU
Optimistic: 0.85-1.7AU
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.

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 magnetic field.

Make the Earth too big

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 Habitable Zones.

What to Look For?

The first thing is to find other Earth-like planets around Sun-like stars.

Easier said than done.

Current Exoplanet search strategies have not yet found Earth-mass planets, for a number of reasons:

Radial Velocity (Doppler Wobble) Method
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 life.
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 this are:

Darwin: Multi-satellite ESA interferometer/planet finder
Terrestrial Planet Finder (TPF) - NASA imaging interferometer and coronographic imager missions.
The main goals of these projects are:

Spectroscopic Biomarkers

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 water.
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.