Finding living planets

MOST PIECES in this series of briefs have stayed within the realm of the biological, examining life at the levels of the species, the individual, the organ and the cell. But the first dealt with a subject central to biology but not inherently of it: molecules. No molecule can be said to be truly alive, and not all molecules are biological. But some are, and they are biological in distinctive, illuminating ways, revealing how life gets things done and passes information down the generations.

This last brief takes on a similar, though far vaster subject. Like molecules, planets do not have to be biological. But at least one is, and in a distinctive, illuminating way that reveals how life has consequences far beyond the organisms embodying it. Earth’s past, present and future cannot be understood without appreciating the planet’s biological aspects. And that appreciation is, in turn, crucial to finding other planets on which biology plays a role.

The idea that Earth is in some way alive, or can be treated as if it were, is common to many mythologies and sensibilities, and has been a theme in science for centuries. Its modern form, though, dates from the 1960s and the insights of James Lovelock, a British scientist then working at JPL, a laboratory in California that is responsible for most of America’s planetary science.

In thinking about the detection of life on other planets Dr Lovelock turned to a broad definition of the phenomenon: one offered by physicists and based on thermodynamics, the science of heat, work and order. This is that life uses, or creates, flows of matter and energy that allow it to increase and maintain complexity within itself. In doing so it deals with the universe’s natural tendency to break down complexity, thus creating disorder (known in thermodynamics as entropy), by actively increasing the entropy of the rest of the universe while reducing its own—exporting disorder, as it were.

Armed with this definition, Dr Lovelock proposed detecting life elsewhere by looking for signs of order, particularly in the form of chemical disequilibria—namely, intrinsically unlikely mixtures of chemicals that would have to be maintained by the persistent export of entropy. He concluded that the most detectable such order, at a planetary level, would be found in the composition of the atmosphere.

On Earth, a variable amount of water vapour aside, 99% of the atmosphere is nitrogen and oxygen. Most of the last 1% consists of argon, helium and neon (“noble” gases that demonstrate their nobility by being inert and fundamentally pointless), and other trace gases such as carbon dioxide and methane.

As Dr Lovelock pointed out, this mixture is way out of equilibrium. Oxygen and methane react with each other. Such reactions take place constantly in the atmosphere. For these gases to be present simultaneously requires active sources of one or both. On Earth, life provides these. Plants, algae and some photosynthetic bacteria produce oxygen. Single-celled archaea called methanogens produce methane.

What is more, with energetic encouragement (as offered, for example, by bolts of lightning), oxygen and nitrogen will react with each other too, creating nitrogen oxides. Again, it turns out that life provides a countervailing process which lets the levels of both gases stay constant, despite the lightning. De-nitrifying bacteria produce the energy they need by turning nitrogen-bearing compounds like those oxides back into gaseous nitrogen, thus continuously topping up the level in the atmosphere.

The atmospheres of Earth’s neighbours, Mars and Venus, provided a stark contrast to this picture of biologically driven instability. They contained no pairs of gases that, at concentrations observed, would be likely to react. They were in equilibrium. This led Dr Lovelock to two conclusions: that there was no life on Mars and Venus; and that Earth’s atmosphere was, to a certain degree or in a certain sense, alive. It was not made of cells or enclosed in a membrane. Nor could it reproduce. But the flow of energy and matter through the living bits of the planet kept the atmosphere in disequilibrium and held entropy at bay. Life’s imposition of order and disequilibrium thus operated beyond the boundaries of cells, individuals and species.

The first of these conclusions was not popular. Many scientists wanted to send robots to Mars to look for life. To be told from the off that such searches would be fruitless served no one’s interests. But fruitless they have proved so far to be.

The second conclusion led Dr Lovelock to hypothesise that Earth behaves, to some extent, as a living organism, in that biology-based processes provide it with a degree of self-regulation which the system as a whole uses in order to keep itself to life’s liking. This “Gaia hypothesis” was highly controversial in the 1970s and 1980s. The views of Dr Lovelock, his followers and his opponents have since evolved. The idea that life actively seeks to keep the environment to its liking, a crucial feature of the hypothesis in its early days, is not now widely held. It is, however, universally agreed that the composition of Earth’s atmosphere depends on biological activity and that various feedback mechanisms which maintain the planet’s habitability have biological components.

The idea of Earth’s environment being a creation of its evolving inhabitants, rather than a background against which they evolve, seems on the face of things quite unlikely. Earth’s living organisms are estimated to contain about 550bn tonnes of carbon. Add in the other elements and remember that living things are, by weight, mostly water, and you might get up to a few trillion tonnes all told. The atmosphere weighs 5,000trn tonnes. How could a thin green smear of life which weighs less than 0.1% of that be calling the shots?

The answer is that life is peculiarly energetic stuff. Expressed in terms of power (the amount of energy used per second), life on Earth runs at about 130trn watts. That is roughly ten times the power used by human beings, and three times the flow of energy from Earth’s interior—a flow which drives all the planet’s volcanism, earthquakes and plate tectonics.

Most of what life does with this energy is chemical: building molecules up, breaking them down and dumping some of the eventual waste products into the environment. And this chemical activity is persistent. The cycling which moves carbon from the atmosphere into living things (through photosynthesis) and back to the atmosphere (through respiration) has been fundamental to the planet’s workings for billions of years. The same goes for the cycling of nitrogen. The great biogeochemical cycles are older than any mountain range, ocean or continent. Their work mostly done by bacteria and archaea, they predate the dawn of animals and plants.

The composition of Earth’s atmosphere before the prokaryotes got their membranes on it is a subject on which there are few data. But the geological record makes one thing clear: it contained almost no oxygen. Significant amounts of free oxygen entered the air only after the relevant form of photosynthesis had evolved. At that point the level of bacterially produced methane—which, in the absence of oxygen, could be quite high—crashed. Because methane is a greenhouse gas, so did the temperature. Roughly 2.5bn years ago the “great oxidation event”, as it is known, plunged Earth into an ice age that saw ice sheets spread to the equator.

The subsequent history of the atmosphere, during which oxygen levels have increased episodically, is coupled to life in various ways. There seems to be a link between oxygen reaching a threshold level about 700m years ago and the evolution of the first animals—it is hard, perhaps impossible, to lead an energetically profligate animal lifestyle without the extra power that oxygen provides to a metabolism.

Vaster than empires, and more slow

The advent of trees, which were able to store carbon both in greater quantities than previous photosynthesisers and in forms that were hard for other organisms to break down, saw carbon-dioxide levels in the atmosphere fall far enough to trigger another global ice age. The recovery and utilisation of some of that hard-to-get-at carbon, now transformed to coal, is seeing life alter the atmosphere in yet another way a few hundred million years on—again with climatic consequences.

Over the decades which saw this new understanding of the role life had played in Earth history develop, Dr Lovelock’s associated insights about atmospheres as signals of life elsewhere made little progress. This was partly because of a lack of atmospheres. Aside from Earth’s, the solar system contains only seven thick enough to count. There have been reports of out-of-equilibrium trace gases on Mars (methane) and Venus (phosphine), but in both cases the evidence is shaky. The most recent orbital survey found no evidence at all for methane in the Martian atmosphere.

Happily, the past two decades have revealed thousands of planets orbiting other stars. Telescopes that can examine the atmospheres of some of these which look promisingly habitable should be available soon. One, the much delayed James Webb Space Telescope, is supposed to launch this Hallowe’en. If any of the orbs it studies suggest Gaia has been working her magic there, too, then the ideas these briefs discuss may one day undergo an intriguing examination. Some will probably turn out to be universal biological truths. Others, though, may just be chance properties of life on one particular planet. ■

In this series on the levels of life
1 Biology’s big molecules
2 Cells and how to power them
3 Making organs
4 The story of a life
5 What is a species, anyway?
6 Finding living planets*