There’s no planet B

Gathering this observation-based information is essential to counter an increasingly popular but flawed narrative that the only way to ensure our survival is to colonise other planets.

The best-case scenario for terraforming Mars leaves us with an atmosphere we are incapable of breathing

The scientific evidence is clear: the only celestial body that can support us is the one we evolved with. Here’s why

AEON, Arwen E Nicholson research fellow in physics and astronomy at the University of Exeter in the UK, Raphaëlle D Haywood senior lecturer in physics and astronomy at the University of Exeter in the UK. 3 Feb 23

At the start of the 22nd century, humanity left Earth for the stars. The enormous ecological and climatic devastation that had characterised the last 100 years had led to a world barren and inhospitable; we had used up Earth entirely. Rapid melting of ice caused the seas to rise, swallowing cities whole. Deforestation ravaged forests around the globe, causing widespread destruction and loss of life. All the while, we continued to burn the fossil fuels we knew to be poisoning us, and thus created a world no longer fit for our survival. And so we set our sights beyond Earth’s horizons to a new world, a place to begin again on a planet as yet untouched. But where are we going? What are our chances of finding the elusive planet B, an Earth-like world ready and waiting to welcome and shelter humanity from the chaos we created on the planet that brought us into being? We built powerful astronomical telescopes to search the skies for planets resembling our own, and very quickly found hundreds of Earth twins orbiting distant stars. Our home was not so unique after all. The universe is full of Earths!

This futuristic dream-like scenario is being sold to us as a real scientific possibility, with billionaires planning to move humanity to Mars in the near future. For decades, children have grown up with the daring movie adventures of intergalactic explorers and the untold habitable worlds they find. Many of the highest-grossing films are set on fictional planets, with paid advisors keeping the science ‘realistic’. At the same time, narratives of humans trying to survive on a post-apocalyptic Earth have also become mainstream.

Given all our technological advances, it’s tempting to believe we are approaching an age of interplanetary colonisation. But can we really leave Earth and all our worries behind? No. All these stories are missing what makes a planet habitable to us. What Earth-like means in astronomy textbooks and what it means to someone considering their survival prospects on a distant world are two vastly different things. We don’t just need a planet roughly the same size and temperature as Earth; we need a planet that spent billions of years evolving with us. We depend completely on the billions of other living organisms that make up Earth’s biosphere. Without them, we cannot survive. Astronomical observations and Earth’s geological record are clear: the only planet that can support us is the one we evolved with. There is no plan B. There is no planet B. Our future is here, and it doesn’t have to mean we’re doomed.

At the start of the 22nd century, humanity left Earth for the stars. The enormous ecological and climatic devastation that had characterised the last 100 years had led to a world barren and inhospitable; we had used up Earth entirely. Rapid melting of ice caused the seas to rise, swallowing cities whole. Deforestation ravaged forests around the globe, causing widespread destruction and loss of life. All the while, we continued to burn the fossil fuels we knew to be poisoning us, and thus created a world no longer fit for our survival. And so we set our sights beyond Earth’s horizons to a new world, a place to begin again on a planet as yet untouched. But where are we going? What are our chances of finding the elusive planet B, an Earth-like world ready and waiting to welcome and shelter humanity from the chaos we created on the planet that brought us into being? We built powerful astronomical telescopes to search the skies for planets resembling our own, and very quickly found hundreds of Earth twins orbiting distant stars. Our home was not so unique after all. The universe is full of Earths!

Deep down, we know this from instinct: we are happiest when immersed in our natural environment. There are countless examples of the healing power of spending time in nature. Numerous articles speak of the benefits of ‘forest bathing’; spending time in the woods has been scientifically shown to reduce stress, anxiety and depression, and to improve sleep quality, thus nurturing both our physical and mental health. Our bodies instinctively know what we need: the thriving and unique biosphere that we have co-evolved with, that exists only here, on our home planet.

There is no planet B. These days, everyone is throwing around this catchy slogan. Most of us have seen it inscribed on an activist’s homemade placard, or heard it from a world leader. In 2014, the United Nations’ then secretary general Ban Ki-moon said: ‘There is no plan B because we do not have [a] planet B.’ The French president Emmanuel Macron echoed him in 2018 in his historical address to US Congress. There’s even a book named after it. The slogan gives strong impetus to address our planetary crisis. However, no one actually explains why there isn’t another planet we could live on, even though the evidence from Earth sciences and astronomy is clear. Gathering this observation-based information is essential to counter an increasingly popular but flawed narrative that the only way to ensure our survival is to colonise other planets.

The best-case scenario for terraforming Mars leaves us with an atmosphere we are incapable of breathing

The most common target of such speculative dreaming is our neighbour Mars. It is about half the size of Earth and receives about 40 per cent of the heat that we get from the Sun. From an astronomer’s perspective, Mars is Earth’s identical twin. And Mars has been in the news a lot lately, promoted as a possible outpost for humanity in the near future. While human-led missions to Mars seem likely in the coming decades, what are our prospects of long-term habitation on Mars? Present-day Mars is a cold, dry world with a very thin atmosphere and global dust storms that can last for weeks on end. Its average surface pressure is less than 1 per cent of Earth’s. Surviving without a pressure suit in such an environment is impossible. The dusty air mostly consists of carbon dioxide (CO₂) and the surface temperature ranges from a balmy 30ºC (86ºF) in the summer, down to -140ºC (-220ºF) in the winter; these extreme temperature changes are due to the thin atmosphere on Mars.

Despite these clear challenges, proposals for terraforming Mars into a world suitable for long-term human habitation abound. Mars is further from the Sun than Earth, so it would require significantly more greenhouse gases to achieve a temperature similar to Earth’s. Thickening the atmosphere by releasing CO₂ in the Martian surface is the most popular ‘solution’ to the thin atmosphere on Mars. However, every suggested method of releasing the carbon stored in Mars requires technology and resources far beyond what we are currently capable of. What’s more, a recent NASA study determined that there isn’t even enough CO₂ on Mars to warm it sufficiently.

Even if we could find enough CO₂, we would still be left with an atmosphere we couldn’t breathe. Earth’s atmosphere contains only 0.04 per cent CO₂, and we cannot tolerate an atmosphere high in CO₂. For an atmosphere with Earth’s atmospheric pressure, CO₂ levels as high as 1 per cent can cause drowsiness in humans, and once we reach levels of 10 per cent CO₂, we will suffocate even if there is abundant oxygen. The proposed absolute best-case scenario for terraforming Mars leaves us with an atmosphere we are incapable of breathing; and achieving it is well beyond our current technological and economic capabilities.

Instead of changing the atmosphere of Mars, a more realistic scenario might be to build habitat domes on its surface with internal conditions suitable for our survival. However, there would be a large pressure difference between the inside of the habitat and the outside atmosphere. Any breach in the habitat would rapidly lead to depressurisation as the breathable air escapes into the thin Martian atmosphere. Any humans living on Mars would have to be on constant high alert for any damage to their building structures, and suffocation would be a daily threat…………………………………………………………………………………………….

Living on a warming Earth presents many challenges. But these pale in comparison with the challenges of converting Mars, or any other planet, into a viable alternative. Scientists study Mars and other planets to better understand how Earth and life formed and evolved, and how they shape each other. We look to worlds beyond our horizons to better understand ourselves. In searching the Universe, we are not looking for an escape to our problems: Earth is our unique and only home in the cosmos. There is no planet B. https://aeon.co/essays/we-will-never-be-able-to-live-on-another-planet-heres-why