Astrobiology: Trying to answer an age-old question

For the first time, recent advances in technology and space exploration have made the prospect of detecting evidence for life outside of our Solar System a foreseeable possibility. The pursuit of this evidence and the desire to learn more about the origin of life on Earth have led to the development of a new field called astrobiology. Astrobiology melds multiple disciplines including astronomy, chemistry, biology, geology and planetary science to answer one of humankind’s oldest questions: Are we alone in the Universe? Next generation telescopes, through their ability to detect signatures of life as we know it, can help answer this question.


Animation. Reflected star light off a planet can be broken up into individual wavelengths with a prism, allowing us to detect the specific molecules in an atmosphere. Source: NASA

The ultimate goal of astrobiology is to find evidence for life on planets outside of our solar system. To do this, we must gather as much information as possible about other planets including orbital parameters (the distance from a planet to its star and how circular its orbit is), bulk composition (its proportions of gasses, water, and land mass), and the atmosphere of the planet. Life has altered Earth’s atmosphere substantially from what we otherwise find on planets devoid of life, where chemical and physical processes alone shape the planet’s atmosphere. If life exists on other planets, it will likely have altered their atmospheres as well. Ultimately we need larger telescopes to characterize the atmospheres of extrasolar planets through a technique called spectroscopy (see Animation above and Fig. 1). Light interacts with molecules in very predictable ways, allowing us to determine what is in an atmosphere just by simply observing absorption and emission in either the starlight reflected off of a planet or in the heat radiated by the planet. Spectra measured with these improved telescopes are the key to detecting life on other planets as they will allow us to detect molecules in the atmospheres of planets and even test for surface features that may indicate life.

Figure 1. How to isolate and identify the spectrum of a planet. Step 1: Take spectra of the star and the planet. Step two: Take spectra of just the star. Step three: Subtract! Spectra of planet remains. Credit: NASA/JPL-Caltech

Biomarkers: Signatures of life

For these telescopes to be useful, we first need to know which molecules are indicative of life. Such indicators are referred to as biomarkers. Our knowledge of specific molecules produced primarily by organisms is limited by the fact that we only have one example of life — Earth-life. There could be very different forms of life in the rest of the Universe, but we simply don’t know what biomarkers might provide evidence for such exotic and hypothetical biology. We do know life evolved on Earth using carbon and water, so this is where we start. However, detecting water or carbon, in and of itself, does not indicate life as both are abundant in the Universe due to natural chemical and physical processes. In 1975, scientist James Lovelock hypothesized the main atmospheric compounds indicative of an active biosphere (the zone of life on a planet) are a combination of oxygen or ozone as well as another class of gases that include methane or nitrous oxide. Lovelock notes the combination of these gases is important because it shows the atmosphere is chemically out of balance from what would occur solely through natural geochemical and physical processes (e.g. volcanic activity). Perhaps many other biomarkers exist, but for now we are limited to chemical species we know are indicative of Earth-life.

When the space probe Galileo looked back at Earth, it detected large amounts of oxygen and traces of methane, which would strongly indicate life to an outside observer (Sagan, 1993). However, looking for life on other planets is not as simple as just looking for oxygen or methane. Many considerations must be taken into account when looking for individual biomarkers.

Figure 2. Habitable Zone around a star, highlighted in green. The Habitable Zone is defined as the region around a star where liquid water could be maintained at the surface. It is also referred to as the Goldilock’s Zone: too close to the star and it gets too hot for water, but too far and the whole planet freezes. For hotter stars, this region will be farther away from the star, and for cooler stars it will be much closer. Credit: NASA

Oxygen alone, though produced almost entirely produced by life on Earth, is not a sufficient biomarker for life because the build-up of oxygen can also be explained in the absence of life. For example, a planet on the inner edge of the Habitable Zone (defined as a planet1 where liquid water could be maintained on the surface – see Fig. 2) could conceivably have a large build-up of oxygen as a consequence of a runaway greenhouse effect that results in the evaporation of its oceans. Ultraviolet light from the planet’s star could then break down atmospheric water to produce hydrogen – which is very light and would escape the planet’s gravitational pull – and leave oxygen behind (Kasting 1988). At the same time, methane alone is not a sufficient biomarker because while atmospheric methane on Earth is also primarily of biological origin, abiotic sources such as hydrothermal vents (“geysers” of hot gas located on the ocean floor) can produce this gas in considerable amounts. Importantly, a gas like oxygen, which belongs to the class of chemical substances known as oxidizers, reacts very quickly with gases like methane or nitrous oxide, which are called reducers. Both would quickly be removed from the atmosphere in the presence of each other, unless they are being replenished by a steady source – such as life. Thus the combination of an oxidizing gas (oxygen) and a reducing gas (methane) indicates there is a steady source of both and points to Earth being inhabited.

Detecting Signs of Life

To discern if these gases and other biomarkers are in other planets’ atmospheres, telescopes measure the spectra of light radiated from the planet as heat (known as the infrared or IR region) as well as starlight reflected by the planet (known as the visible light region since this is the range of light our eyes can detect). In the IR region we can see spectral features indicative of molecules such as carbon dioxide, water, ozone, methane, ammonia and nitrous oxide. Methane, nitrous oxide and ammonia are produced on Earth primarily by bacteria. Ozone is produced higher up in Earth’s atmosphere when high energy light breaks apart oxygen gas, which then recombines to form ozone. So ozone can serve as a proxy for the presence of oxygen. In addition to the possible detection of atmospheric species, IR (which is radiated from objects with high temperature) also provide a measurement of the planet’s surface temperature, telling us if it can support liquid water, but only for planets with a thin atmosphere like Earth or Mars.

Figure 3. The transmission spectra for Earth for the visible (0.38μm – 0.76μm), near-infrared (0.76μm – 2.5μm) and infrared or IR (2.5 -20 μm) showing some of the biomarkers in each region. Image credit: Kaltenegger, L. and Traub, W. (2009) Transits of Earth-Like Planets. Astrophysical Journal.

Primary features of interest in the visible and near-infrared (the region inbetween the visible and IR) portion of the spectrum are water, ozone, oxygen, carbon dioxide, and methane (see Fig. 3 for a sample transmission spectrum of Earth). With new spectroscopy techniques, the visible region of the spectrum may also allow us to detect the presence of oceans and/or continents (Cowan et al. 2009; Palle 2010). Finally, global vegetation may also be detectable through something analogous to the Vegetation Red Edge on Earth (Kiang et al. 2008), which results from the fact plants strongly reflect red light.

Looking for Life

NASA and the European Space Agency have proposed the Terrestrial Planet Finder and Darwin missions, respectively, to detect these biomarkers, but both of these missions are just in the early planning phases. The James Webb Space Telescope (JWST), an international collaboration, is scheduled to launch in 2018 and may be able to detect biomarkers such as ozone, water, and carbon dioxide in the atmosphere of a nearby planet around a small star. To measure the spectra of planets with JWST, the planet must first transit, or pass in front of their system’s star. Not all planets transit from our point of view. In fact, only 0.47% of Earth-like planets around a solar type star would transit at all! And since a planet transits its star only very briefly and infrequently (remember, Earth only transits across the Sun once a year!), many hours of observing time, accomplished over the span of several months to years, will be needed for JWST to gather enough hours of transit observations to measure biomarkers on habitable planets (Kaltenegger & Traub, 2009). Future telescopes like the Terrestrial Planet Finder will have a higher resolution and not be limited to only transiting exoplanets, and thus will be able to greatly expand our search for life in the galaxy.

Figure 4. Artist conception of James Webb Space Telescope (left) and Terrestrial Planet Finder (right). Credit: NASA

When we are looking for other Earth-like planets, we must keep in mind that biomarkers change over geological time (Kaltenegger et al. 2007), for different masses of stars (e.g. Selsis 2000; Segura 2003, 2005; Grenfell et al. 2007), and for different types of microorganisms (see e.g. Kiang et al. 2007; Cockell et al. 2009). Due to potential false positives, any future remote detection of biomarkers will have to be scrutinized carefully in its planetary context. A convincing argument for remote life-detection will require multiple biomarkers with a strong context of a habitable planetary environment (deduced from factors such as the surface temperature, size and composition of the planet). Initial missions like the James Webb Space Telescope and eventually the Terrestrial Planet Finder will have low resolution and will only be able to detect very abundant biomarkers in optimal observing conditions. With improved resolution on future instruments, our ability to remotely search for biospheres and to characterize new planetary environments will steadily increase, pushing us ever closer to finding the first traces of life on other planets.

Sarah Rugheimer is a Harvard graduate student in astronomy modeling atmospheres of Earth-like planets.

1 In recent years scientists have made significant progress identifying planets that orbit other stars (called extrasolar planets or exoplanets) through space missions such as NASA’s Kepler mission. A recent SITN article introduced the Kepler mission and the search for planets in the Habitable Zones. Return

References

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