As a guide to how to look for life on other planets, there is no need to look any further than your own back garden. Whether it sprawls over countless acres or is compacted into a small balcony, a garden artfully demonstrates the theories, methods and tools astrobiologists use in their search for alien life on worlds far beyond Earth.
A garden is a cultivated space, forever being nudged away from the wilderness it desires to become by the hand that shapes it. Wild spaces and gardens, regardless of the aesthetic you prefer, are easy for us to tell apart at a glance. They also provide the perfect analogy to the difference between an inhabited and an uninhabited planet. A living planet, like Earth, is significantly shaped by the life living upon it. Life tempers environmental extremes, cultivating and sustaining the balance of gases in the atmosphere to suit it best. If we were to remove all life from Earth, the planet, like an abandoned garden, would slide into a very different state. It is this extreme difference between a living and a barren planet that astrobiologists will use when they search the new worlds we are beginning to find beyond our Solar System for the first big clues about how prevalent life is in the Universe.
A close examination of other worlds for signs of life is a complicated, time-consuming and expensive procedure. This means we need to narrow down our lists of target planets from thousands to just a handful that have the best chance of hosting life. These planets fall within the habitable zone; the region around a star where a planet’s temperature will be comfortably between the boiling and freezing points of water. This is such a crucial selection criterion because all life (as we know it) requires liquid water; a fact that is clearly demonstrated in a garden. Watering and irrigation are unavoidable aspects of gardening. While the water requirements of any given garden will vary considerably, remove all sources of water and any garden will eventually wither and die. So to have a good chance of uncovering a living planet, alien-hunters follow the water. Yet water alone is not enough.
A garden is defined by its plants, but in turn, the character of a garden’s plants are determined by the availability of light from our nearest star, the Sun. The personality of a sun-drenched south-facing garden contrasts notably with that of shaded north-facing plot. Permanently remove access to light and a garden ceases to be. It would still be alive to an extent, because not all living things on Earth require light, but it would be so much less alive that to our eyes there would not be much to see. Life on Earth is largely underpinned by plants turning sunlight into food for the rest of the living world. Given that our planet is bathed in energy-rich sunlight, it is not surprising that this is the case – it would be more surprising if life had not evolved to exploit such an abundant free energy source. For this reason, astrobiologists expect that living things on other worlds would also want to exploit the starlight their planets receive. So on our hunt for a richly inhabited world we also want to narrow down the target list to planets that receive enough light to power photosynthesis on Earth-like scales. How plant life works under different light regimes on Earth, as demonstrated within our own gardens, allows astrobiologists to go a step further and speculate about how vegetation would express itself on brightly lit planets, dim planets or even planets that experience permanent day or permanent night. So in a way, the variations of plant life within the illuminated landscapes of our gardens offer us small windows into the appearances of other worlds.
Everything will not grow everywhere. Certain plants will only grow in the sunniest, well drained parts of a garden, failing in the shadier, wetter corners and vice versa. Yet more limitations are enforced by climate, as can be seen by comparing a garden in Scotland with one in Florida. With effort, some of these limitations can be overcome, but in general, the best results are most easily achieved by working with Nature, rather than struggling against it. These same rules apply to how we expect life to work on other worlds. We expect certain types of life to thrive on cold icy planets, while different forms would dominate humid jungle planets, for example. It is perfectly possible to imagine creatures suited to hot tropical environments finding a way to eke out an existence on a frozen snowball world, but they would likely not exist in great numbers. So, by figuring out the kind of climate a planet would have, just as we choose the plants for our gardens based on where in the world those gardens are, astrobiologists can shortlist the most likely types of life to exist there. By knowing that, we can determine what signs of life to look for on any given habitable world.
With the aid of technology in the forms of greenhouses, heating and well-rehearsed over-wintering regimes, gardeners in less forgiving climates can expand their collections to include plants that would not survive unaided in their parts of the world. While this approach does not help us on our quest to find life on other Earths, it parallels with another aspect of life beyond Earth: the colonisation of other planets. If humans were to establish a colony on Mars, centuries of terrestrial gardening habits would be called upon and pushed to their extremes in order to grow food and oxygen on the barren planet. When there is no Nature to work with, or against – something we do not yet know one way or the other about the Red Planet – bringing life to a lifeless world could be one of the purest expressions of gardening humans could achieve.
A garden is never complete and will constantly change form as it evolves through the seasons and through the years. Watching our gardens change provides a great fast-forwarded analogy for the way life on a planet changes, adapts and evolves over the timescales of millions, hundreds of millions and even billions of years. Just as some plants do well one year, while others dominate in another, so the balances of types and forms of life on a planet are altered over the course of a world’s lifespan. In our gardens, it is the annual variations of weather, successes and failures of self-seeders, or the waxing and waning attentiveness of the gardener that determine a garden’s composition at any given moment in time. For a habitable planet, it is changes in climate over geological timespans, the successes and failures of competition between different species, or the waxing and waning of the availability of energy sources that determine the compositions of a planet’s biosphere at any given point in the geological time. We can use the geological history of Earth and our knowledge about how stars – the ultimate energy providers for any Earth-like world – evolve and change over their tens of billions of year lifetimes to predict how conditions on any given planet will appear at different stages in its life. So, whether the planet we find is very old or very young, we can narrow down the likely appearance of the garden of life it could best support, giving us clues about what signs of life to search for there.
Gardening is a slow process. It forces the gardener to be patient, sowing seeds only when the time is right, waiting for a season, or even for years, to see the fully grown plant. Therefore, to be a gardener is to think seasonally, adopting a pace of life slower than the minutes, hours and days that tend to define our everyday world. The same is true for astrobiologists looking for living planets. A planet’s existence encompasses a lifetime that stretches over billions of years and (assuming Earth is a typical example) the evolution of life on such a planet takes place over stages measured in many 100s of millions of years. Timescales that are utterly alien to our usual way of thinking. Yet, spend enough time thinking over those timescales, and you, much like the diligently seasonal gardener, expand your perception of time.
A gardener never truly learns how to grow something until faced with the inevitable challenges of pests, diseases and the various other plant health issues that occur. These problems push the gardener to learn, growing their wisdom and expanding their understanding of how their garden grows as the years roll by. This parallels with our studies of Earth as a living planet, especially now as we face the challenge of a rapidly changing climate. Yet the more we study the Earth, the more our appreciation of how all the pieces of a living planet work together grows. This is crucial for navigating the current climate crisis, but also for imagining how other living planets work, increasing the odds of finding life beyond Earth.
However, there is arguably a greater wisdom to be gained from our gardens. The more time we spend in our gardens, the more we appreciate that, although they are enclosed, they are not isolated from the wider world. They are a part of it, bridging the imaginary gap between us and Nature. So too the discovery of other worlds beyond our own bridges the divide we have constructed between ourselves and the rest of the Universe.