Beyond the Habitable Planet: Detecting Earth-like Planets

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. See larger image
Source: David A. Aguilar http://www.cfa.harvard.edu

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. See larger image
Source: NASA/JPL-Caltech http://planetquest.jpl.nasa.gov

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. See larger image
Source: European Space Agency http://sci.esa.int

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 what to look for.

Signatures of Life

Earth’s spectrum as viewed from space, superimposed on an image of Earth. See larger image
Source: European Space Agency http://sci.esa.int/

Earth’s spectrum as viewed by the Mars Global Surveyor. See larger image
Source: NASA/JPL-Caltech http://origins.jpl.nasa.gov

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. See larger image
Source: Lee Prufert-Bebout/NASA http://microbes.arc.nasa.gov

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. See larger image
Source: NASA/JPL-Caltech http://photojournal.jpl.nasa.gov

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. See larger image
Source: Paul Hoffman http://www.snowballearth.org.

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. See larger image
Source: Lisa Kaltenegger http://www.cfa.harvard.edu

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. See larger image
Source: David A. Aguilar http://www.cfa.harvard.edu.

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 green house 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 http://www.cfa.harvard.edu.

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. See larger image
Source: David A. Aguilar http://www.cfa.harvard.edu.

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. See larger image
Source: David A. Aguilar http://www.cfa.harvard.edu.

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 teaming with varied forms of animal life. See larger image
Source: David A. Aguilar http://www.cfa.harvard.edu.

800 million years ago, the Earth entered Epoch 4, with continuing increases in oxygen levels. This time period includes what is now known as “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. See larger image
Source: David A. Aguilar http://www.cfa.harvard.edu.

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. http://earthobservatory.nasa.gov.

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.