The Role of Atmospheric Compositions

Introduction

In the previous lecture, we examined the best candidates for past, present and future life in the Solar System: Mars, Europa and Titan. All three of these bodies are near enough to the Earth for physical visits by spacecraft to be possible. However, when we come to consider life on extrasolar planets, it is clear that spacecraft missions to such planets lie well beyond our technological, economic or political grasp. Therefore, we are restricted to indirect methods when it comes to deciding whether a given extrasolar planet harbours life.

Selection of Candidates

The number of known extrasolar planets is increasing at a rate of about one per month (see www.exoplanets.org). Therefore, we cannot hope to look for life on every single one; instead, we must first select candidate planets, which we believe to have the best chances of harbouring life. In making this selection, a number of criteria can be applied:

1. Does the planet contain water? As far as we know, water is an essential prerequisite for all life.
2. Is the planet situated within the habitable zone? For water to be useful, it must be in liquid form, which defines the habitable zone (see Lecture 3). However, we should remember that liquid water can exist outside the habitable zone, if mechanisms are present to prevent it from evaporating or freezing; in the case of Europa, these mechanisms are a surface crust of ice and the tidal warming caused by Jupiter (see Lecture 8).
3. Does the planet have an atmosphere? If a planet's gravitational field is too small to be able to retain an atmosphere, then many of the volatile compounds necessary to life (such as water) will evaporate into space. Again, Europa is an exception; the loss of volatiles is prevented by the surface ice crust.
4. Is the planet terrestrial sized? It seems unlikely that gas giants would provide suitable environments for life; their surface conditions are characterized by high pressures and low temperatures, and they would appear to be too hostile for life to develop.

It is important to remember that these criteria are based on our knowledge of terrestrial life. It may be the case that there are lifeforms which can survive in the complete absence of water, or exist in the harsh conditions found on gas giants. However, until such lifeforms are discovered, we must necessarily be guided by our Earth-based experience.

The Role of Atmospheric Compositions

Once we have selected those candidate exoplanets which appear hospitable to life, it is still necessary to determine whether life has indeed developed on them. Being restricted to remote observations, this determination is based around obtaining spectra of the planet. By measuring the strength of differing absorption lines in a spectrum, we can establish the atmospheric composition of the distant planet.

The atmospheric composition of a given planet reflects not only the material make-up of the planet; it can also be strongly modified by the presence of lifeforms. Take the case of the Earth; terrestrial life has left an indelible fingerprint on the atmosphere, by altering the concentrations of various elements and compounds. These include:

• free oxygen, which does not arise from non-biological processes, but is produced during oxygenic photosynthesis.
• carbon dioxide, which is usually abundant, but can be removed from the atmosphere via photosynthesis (either oxygenic or anoxygenic).
• pollutants, which are difficult to produce via natural means, but can arise as a by-product of the industrial activity of a technologically-advanced civilization (Mankind, in the case of the Earth).
• cloud cover, which can be controlled by living organisms, if one believes the Gaia hypothesis first put forward by James Lovelock. This hypothesis maintains that the physical environment of Earth is directly controlled by organisms, for their own benefit.

Of these, free oxygen is probably the most significant marker for the presence of life. If all oxygenic photosynthesis ceased today, then levels of free oxygen in Earth's atmosphere would decline rapidly (in geological terms), since the oxygen reacts readily with other compounds and is therefore quickly destroyed.

Evolution of the Earth's atmosphere

Clearly, then, by examining the spectrum of an exoplanet, and looking for certain absorption lines associated with biological processes, it is possible to determine whether life has developed on the planet. However, this in practice is a very difficult task, due to the fact that the parent star will outshine the planet by a factor exceeding 100 billion.

This difficulty can partially be avoided by examining the planet's infra-red spectrum, rather than its visible spectrum. In infra-red light, the contrast between the planet and star will be much better, with the star outshining the planet by a factor of only 10 million or so.

Infra-red spectra of Venus, Earth and Mars

As an illustration of the discussion, the figure above compares the infra-red spectra of Venus, Earth and Mars. All three spectra show absorption lines due to carbon dioxide. However, only Earth's spectrum shows additional lines due to water and ozone. The ozone (O3) is an indirect marker for life, being formed when free oxygen (O2) is hit by ultraviolet light.

Case Study : HD 209458

In 2001, observations by the Hubble Space Telescope led to the measurement of a spectrum for the Jupiter-sized planet orbiting the star HD 209458. This planet is unique amongst all known extrasolar planets, in that it eclipses its parent star once every orbital revolution. During an eclipse, starlight passing through the planet's atmosphere is selectively absorbed. By measuring this selective absorption, scientists were able to detect the presence of sodium in the atmosphere.

Schematic of HD 209458 and its planet

Measurement of the composition of a planet's atmosphere is much easier when, as with HD 209458, it eclipses its parent star. However, the eclipse scenario is rare, since it requires the orbital plane of the planet to be fortuitously aligned with the line of sight. As will be discussed in the following lecture, attempts to find more eclipsing planets are underway.

Rich Townsend
Last Modified Date: 13 March 2003