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The Habitable Planet: A Systems Approach to Environmental Science 

Many Planets, One Earth

Astronomers have discovered dozens of planets orbiting other stars, and space probes have explored many parts of our solar system, but so far scientists have only discovered one place in the universe where conditions are suitable for complex life forms: Earth. In this unit, examine the unique characteristics that make our planet habitable and learn how these conditions were created.

Interactive Labs

Carbon Lab (Units 1-3, 13)

Throughout this course, the carbon cycle is featured as one of the most important planetary systems. This lab uses a robust model of the carbon cycle to give you an intuitive sense for how the system works.  It also allows you to experiment with how human inputs to the cycle might change global outcomes to the year 2100 and beyond. One especially relevant human impact is the increase in atmospheric CO2 levels. Between 1850 and today, atmospheric concentrations have risen from 287 ppm (parts per million) to over 380 ppm a level higher than any known on Earth in more than 30 million years (see Unit 12 to find out how scientists measure ancient atmospheric carbon levels). You will experiment with the human factors that contribute to this rise, and see how different inputs to the carbon cycle might affect concentrations of the greenhouse gas CO2.   launch lab


Unit 1: Many Planets, One Earth // Glossary

The fraction of electromagnetic radiation reflected after striking a surface.

A major division of microorganisms. Like bacteria, Archaea are single-celled organisms lacking nuclei and are therefore prokaryotes, classified as belonging to kingdom Monera in the traditional five-kingdom taxonomy.

Microscopic organisms whose single cells have neither a membrane-bounded nucleus nor other membrane-bounded organelles like mitochondria and chloroplasts.

Cambrian explosion
Between about 570 and 530 million years ago, when a burst of diversification occurred, with the eventual appearance of the lineages of almost all animals living today.

An ion with a positive charge.

A phylum of Bacteria that obtain their energy through photosynthesis. They are often referred to as blue-green algae, although they are in fact prokaryotes, not algae.

A single-celled or multicellular organism whose cells contain a distinct membrane-bound nucleus.

Microorganisms belonging to the domains Bacteria and Archaea that can live and thrive in environments with extreme conditions such as high or low temperatures and pH levels, high salt concentrations, and high pressure.

geochemical cycling
Flows of chemical substances between reservoirs in Earth’s atmosphere, hydrosphere (water bodies), and lithosphere (the solid part of Earth’s crust).

An organism that requires organic substrates to get its carbon for growth and development.

negative feedback
When part of a system’s output, inverted, feeds into the system’s input; generally with the result that fluctuations are weakened.

An array of reactions involving several different types of chemical conversions: (1) loss of electrons by a chemical, (2) combination of oxygen and another chemical, (3) removal of hydrogen atoms from organic compounds during biological metabolism, (4) burning of some material, (5) biological metabolism that results in the decomposition of organic material, (6) metabolic conversions in toxic materials in biological organism, (7) stabilization of organic pollutants during wastewater treatment, (8) conversion of plant matter to compost, (9) decomposition of pollutants or toxins that contaminate the environment.

The largest generally accepted groupings of animals and other living things with certain evolutionary traits.

plate tectonics
A concept stating that the crust of the Earth is composed of crustal plates moving on the molten material below.

Organisms without a cell nucleus, or any other membrane-bound organelles. Most are unicellular, but some prokaryotes are multicellular. The prokaryotes are divided into two domains: the bacteria and the archaea.

radiometric dating
A technique used to date materials based on a knowledge of the decay rates of naturally occurring isotopes, and the current abundances. It is the principal source of information about the age of the Earth and a significant source of information about rates of evolutionary change.

Snowball Earth
Hypothesis that proposes that the Earth was entirely covered by ice in part of the Cryogenian period of the Proterozoic eon, and perhaps at other times in the history of Earth.

stratigraphic record
Sequences of rock layers. Correlating the sequences of rock layers in different areas enables scientists to trace a particular geologic event to a particular period.

The process in which one plate is pushed downward beneath another plate into the underlying mantle when plates move towards each other.

Beyond the Habitable Planet: Detecting Earth-like Planets

by Lisa Kaltenegger


Earth: Seen as an Extra-Solar Planet

This “super-Earth,” 13 times the mass of the Earth (shown in this artist’s conception with a hypothetical moon) is orbiting a red dwarf star 9,000 light-years. Discovered by microlensing events, in which the gravity of a foreground star distorts the light of a more distant background star, scientists think such worlds are three times more common than Jupiter-sized planets.

Source: David A. Aguilar

Hundreds of extra-solar planets have been detected; it is only a matter of time before we have the capability to search methodically for Earth-sized planets orbiting distant stars. Each of these planets will be a small unresolved spot on the sky, but if we keep observing it long enough, we can collect enough light to examine its spectrum in an attempt to unlock some of the secrets of its atmosphere. When we do, one of the first questions will be: Is it is habitable? Is there life?


Extra-solar Planet Search – What is the Challenge?

Over the next decades, space-based constellations of telescopes, like ESA’s Darwin Interferometer and NASA’s Terrestrial Planet Finder (shown here in artist’s conception), will enable astronomers to identify Earth-like planets orbiting distant stars.

Source: NASA/JPL-Caltech

We collect the light from planets using space telescopes. The key is to become better able to separate the light reaching us from the planet from the much brighter source that is the star that it’s orbiting. In the case of the Earth and Sun, that means for every one photon of the Earth, 10 billion (visible) or 10 million (infrared) photons of the Sun arrive at the same time. Astronomers like to compare this to “trying to see a firefly flying around a lighthouse from a thousand kilometers away.”

Once we build a new generation of space telescopes, we will collect enough light from the planet under study to use a prism to split the light into its several components, creating a spectrum that shows the amount of light at different frequencies.

Looking Closer to Home

The spectra of the three terrestrial planets, Venus, Earth, and Mars, reveals information about the composition of their atmospheres.
Source: European Space Agency


Even the kind of low-resolution spectra that we would expect from distant extra-solar planets can reveal some of the secrets of these far away and unknown worlds. Will they reveal if the planet is habitable? If there are bacteria or dinosaurs roaming around? Let’s look at the planets in our solar system. The differences in their spectra can give us some indication of what to look for.


Signatures of Life

Earth’s spectrum as viewed from space, superimposed on an image of Earth.
Source: European Space Agency


Earth’s spectrum as viewed by the Mars Global Surveyor.
Source: NASA/JPL-Caltech 

Light microscope view of cyanobacteria and other organisms in a microbial mat from Baja California. Photosynthetic bacteria like these may have contributed to the rise of oxygen in the Earth’s atmosphere.
Source: Lee Prufert-Bebout/NASA

Particular gases leave highly visible signatures in a planet’s spectrum, like fingerprints or DNA markers. By spotting those fingerprints, we can learn about an atmosphere’s composition. In a famous paper, Carl Sagan and colleagues analyzed a spectrum of the Earth taken by the Galileo probe, searching for signatures of life. They concluded that the large amount of O2 and the simultaneous presence of traces of methane are strongly suggestive of biology.

Numerous spacecraft have looked back to the Earth from space and measured its spectrum. The light it reflects shows peaks and valleys at different infrared wavelengths, which are caused by the absorption of light by the different components of the atmosphere. For example, a dip in the amount of reflected light at around 10µm indicates ozone, or O3, which indicates the presence of oxygen. Carbon dioxide is indicated by increased absorption at around 15µm. Water vapor is indicated by changes in absorption below 8µm and above 20µm.

We know that life on Earth depends on the presence of water, so if we are looking for an Earth-like planet, one of the signatures we would be looking for would be water vapor. In addition, we know that plants and photosynthetic bacteria release oxygen, a highly reactive gas that is usually found only in chemical compounds formed with other elements. If we find the signature of free oxygen (or ozone, a compound consisting of three oxygen atoms) we would assume that it came from living organisms.

On Earth’s spectrum, we detect the signatures of ozone (O3), water vapor(H2O) and carbon dioxide(CO2). On the spectra of Mars and Venus, only the CO2 can be seen.


The Snowball Earth

This view of Jupiter’s Moon, Europa, taken by the Galileo spacecraft hints at what the Earth’s surface may have looked like during one of its snowball periods. At 2220 million years ago (MA), 735 MA and at 610 MA ago, the Snowball Earth hypothesis infers that the entire planet was frozen, from the poles to the equator. The Earth’s atmosphere under these conditions must have had a very different composition than it does today.
Source: NASA/JPL-Caltech

During its evolution, Earth has gone through very different stages – from being an extremely hot place to being a snowball. The Sun was very dim at the beginning, about 30% less bright than today. At some point in its history, there had to be much higher concentrations of greenhouse gases in Earth’s atmosphere to keep the planet from freezing and, if already frozen, from becoming a snowball forever. All these different evolution stages would have produced very different spectra.

Plotting the Earth’s Spectral Changes Over TIme

Looking for clues in the geologic record, we can trace the rise of oxygen on Earth. This information Earth’s past might help us learn more about present-day conditions on other worlds.
Source: Paul Hoffman

Are there habitable planets around other stars that are at different stages of their evolution than we are on Earth today? Geologic records show that the oxygen concentrations in Earth’s atmosphere have changed – in part, because of the developing life forms on our planet. Taking our planet as a test-case, as a first step to solving this intriguing puzzle, we need to go back and look at the geologic record to see how the composition of the atmosphere has changed.

Armed with this geological data, we can reconstruct what might have been spectral fingerprints of Earth’s atmosphere at different points along its evolution.

These fingerprints can then be compared to the spectral signals from a newly-discovered planet far away, enabling us to answer the question: What time it is there: the epoch of the methane bacteria, early single-celled photosynthesis, or complex life, even dinosaurs?

Dividing Earth’s History into Epochs

The spectral features of our atmosphere have changed considerably over Earth’s history. Because the first life forms that developed did not produce oxygen, not until Epoch 3 can we see oxygen/ozone; before that, the atmosphere was mainly CO2 and CH4. From Epoch 4, oxygen-producing bacteria evolved and oxygen accumulated in the atmosphere.
Source: Lisa Kaltenegger

We can divide the Earth’s past into six broad epochs: periods with very distinct spectral fingerprints. Numbered from zero to five, these epochs go back to the beginning of the planet’s 4.56 billion year history. These periods have such different atmospheric compositions that they could be distinguished using spectral analysis, even at distances of many light-years. They range from a newly formed, CO2–rich planet (Epoch 0) to one with a CO2’ and methane-rich atmosphere (Epoch 3) to the present-day atmosphere (Epoch 5).

Some of the differences we would be able to see in the different epochs are the spectral signatures of different chemicals in the visible part of the electromagnetic spectrum and in the infrared. For example, we would not see oxygen in the beginning half of Earth’s lifetime.

Epoch 0: 4.5 BYA to 3.9 BYA

During Epoch 0, one ocean covered our entire planet and absorbed bombardment from incoming meteors and comets.
Source: David A. Aguilar

At Epoch 0, from soon after the formation of the Earth 4.5 billion years ago (BYA) to 3.9 BYA, the young Earth possessed a turbulent steamy atmosphere composed mostly of nitrogen, carbon dioxide, and hydrogen sulfide. This carbon dioxide – and other greenhouse gases – helped warm our world since the infant Sun was a third less luminous than today. Although no fossils survived from this time period, isotropic signatures in Greenland rocks suggest the existence of life forms that may have been photosynthetic like modern-day plants.

Epoch 1: 3.5 BYA to 2.5 BYA

During Epoch 1, the planet landscape featured volcanic island chains poking out of the vast global ocean. See larger image
Source: David A. Aguilar

During Epoch 1, the planet landscape featured volcanic island chains poking out of the vast global ocean. The first life on Earth was anaerobic bacteria – bacteria that could live without oxygen. These bacteria pumped large amounts of methane into the planet’s atmosphere, changing it in detectable ways. Spectra from planets at this stage of evolution would have clear signals from the absorption of methane in the visible and infrared. If similar bacteria exist on another planet, future missions like NASA’s Terrestrial Planet Finder and ESA’s Darwin might be able to detect their fingerprint in the atmosphere – making the first signs of “E.T.” probably no radio or TV broadcasts but a methane spectral line from bacteria.

Epoch 2: 2.5 BYA to 2.0 BYA

Stromatolites, masses of blue-green algae like the ones found today in western Australia, began releasing oxygen, which was quickly captured by ele.
Source: David A. Aguilar

At the time period 2.5 BYA (Epoch 2), the atmosphere reached its maximum methane concentration. The dominant gases were nitrogen, carbon dioxide, and methane. Scientists are holding ongoing discussions about the possibility that methane hazes could have darkened the skies during this stage. Continental landmasses were beginning to form. Rock formations that suggest masses of blue-green photosynthetic organisms similar to today’s stromatolites, were found in many parts of the Earth. While these suggest the formation of free oxygen, no oxygen was seen in the atmosphere yet, as the crust still was being oxidized and thus no residual oxygen could build up in the atmosphere. Nonetheless, big changes were about to happen.


Epoch 3: 2.0 BYA to 800 MYA

Brightly colored pools of greenish-brown scum created a sheen on the stench filled water. The oxygen revolution was fully underway.
Source: David A. Aguilar

In Epoch 3, from 2.0 BYA to 800 million years ago (MYA), the temperature on the surface was probably very hot, making heat-loving methane bacteria thrive. But photosynthetic organisms continued to evolve. They produced ever more oxygen, a highly reactive gas that cleared out much of the methane, while also suffocating the anaerobic bacteria that produced it The fight between the oxygen- and methane-producing bacteria ended luckily for us: the methane bacteria lost.


Epoch 4: 800 MYA to 300 MYA

After the Cambrian Explosion, the oceans were teeming with varied forms of animal life.
Source: David A. Aguilar


800 million years ago, the Earth entered Epoch 4, with continuing increases in oxygen levels. This time period includes what is now known as the “Cambrian Explosion”. Beginning 550-500 million years ago, the Cambrian Period is a significant marker post in the history of life on Earth. It is the time most major animal groups first appear in the fossil records. The Earth now would be covered with swamps, seas, and a few active volcanoes.


Epoch 5: 300 MYA to the Present

Beginning 300 million years ago, the vegetation on our planet could have been detected by a remote observer with a very large telescope.
Source: David A. Aguilar



Finally, 300 million years ago in Epoch 5, life has colonized the landscape. The Earth’s atmosphere has reached its current composition primarily nitrogen (78%) and oxygen (21%). This is the beginning of the Mesozoic period that will include the dinosaurs. Green plants now cover much of the surface of Earth.

Detecting Intelligent Life



Measurements of atmospheric ozone concentrations by NASA’s NIMBUS satellites documented the ozone hole over Antarctica, caused by CFC’s in the atmosphere. Nimbus observations began to point to a drop in ozone (blue areas) as early as 1980, with more extreme decreases developing in 1985.
Source: Paul Newman, Richard Stolarski, Mark Shoeberl, Arlin Krueger; NASA.

Human activity has altered Earth’s atmosphere by increasing atmospheric carbon dioxide concentrations, as well as adding manmade gases like chlorofluorocarbons (CFCs), which are used in refrigerators, aerosol sprays, and air conditioners. Could we identify the spectral fingerprints of this kind of byproduct of human civilization on other worlds? Learn from tens or hundreds of light-years away that refrigerators and, by implication, left-over take-out foods are common? Although Earth-orbiting satellites and balloon experiments can measure these trace gases here at home, detecting similar effects on a distant world are beyond the capabilities of satellites we will build in the near future. It will take flotillas of future space-based infra-red telescopes to be able to accomplish this. But even if we can’t detect fridges on other planets, there is exciting information that we can extract from low-resolution spectra: Water? Oxygen? Methane? Is the environment of that planet like ours? Is it in principle habitable or more like Mars or Venus?

We believe that in the next few decades we will know whether or not our habitable blue world is unique in the universe or if there are neighboring worlds like ours out there.



Lisa Kaltenegger

Is an astrophysicist at the Harvard-Smithsonian Center for Astrophysics. She received her Ph.D. in astrophysics at Karl Franzens University Graz. Her current research includes building a model for Earth’s evolution over geological time of Earth-like extrasolar planets and super-Earths; modeling detectable biomarkers on extrasolar planets; simulating surface and cloud signatures for Earth over its geologic evolution; establishing the design requirements for future space and ground-based missions to detect biomarkers on Earth and Earth-like extrasolar planets; and establishing guidelines and evaluation criteria for the reliability of biomarker detection.

Series Directory

The Habitable Planet: A Systems Approach to Environmental Science 


Harvard Smithsonian Center for Astrophysics in association with the Harvard University Center for the Environment. 2007.
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  • ISBN: 1-57680-883-1