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Why is there no Planet B? (commentary)

Earth. Image courtesy of NASA.

Amazing image of the Earth. Original from NASA. Digitally enhanced by rawpixel.

  • A popular slogan at climate change demonstrations is a play on humanity ‘not having a Plan B’ after Earth, so for Climate Week, two astrophysicists explain why that is.
  • There are around 300 billion stars in our galaxy, with 60 billion planets similar to Earth, but none of them is quite the way we need it to be in order to survive.
  • Not even Mars is workable, they say: it has 100 times less air and it’s primarily carbon dioxide. It’s also as cold as the Antarctic, so we cannot escape our ecological problems: “Earth is our unique and only home in the cosmos,” they write.
  • This post is a commentary. The views expressed are those of the authors, not necessarily of Mongabay.

Everyone is throwing this catchy slogan around these days, whether it be activists, writers, presidents, or the former Secretary-General of the United Nations. It gives strong impetus to address our planetary crisis. But they don’t actually explain why there isn’t another planet we could live on. So, is it true? What do Earth sciences and astronomy tell us?

Next time you’re out on a clear night, look up at the sky and pick a star. It most likely has planets around it. Could we live on one of them? The data show that 1 in 5 stars host an “Earth-sized”, “temperate” planet. Astronomers deem a planet “Earth-sized” if it has a radius between 0.5-1 times that of Earth, and “temperate” if it receives between 30-100% of the heat that Earth receives from the Sun. There are around 300 billion stars in our galaxy, which adds up to 60 billion planets with sizes and temperatures similar to Earth!

Here’s the catch: while there are billions of planets out there, none of them is quite the way we need it to be in order to survive on it. Every planet has its own history and character, much like people do. There are almost eight billion people alive today; and yet everyone is different. Even identical twins each have their own unique personality and life story. The same goes for planets. We have no reason to expect any two planets to be alike. Thinking we could live on a planet just because it is a similar size and temperature to Earth is a bit like deciding that someone is our best friend after finding out their shoe size and favorite color.

The Martian surface as photographed by NASA’s Curiosity Rover. Would you really want to live there? Photo courtesy of NASA/JPL-Caltech/MSSS.

Take Mars. It is half the size of Earth and receives 40% of the heat that we get from the Sun, so technically speaking, it is “Earth-sized” and “temperate”. The photos sent back by NASA’s Curiosity rover reveal familiar landscapes: we could be on Earth, perhaps somewhere in the Atacama desert. But don’t be fooled! Look at the sky: it’s a mucky brown-yellowish color. That’s because Mars has 100 times less air to breathe and it is full of dust. What little air it has is primarily carbon dioxide, which would immediately suffocate us. Oh, and it’s as cold as the Antarctic. Mars may have similar properties to Earth by astronomical standards, but it is nothing like Earth to us humans.

Any semi-reasonable plan to establish a long-term presence beyond Earth requires a planet to which we are fit. Earth was not always so friendly to humans as it is today. For around 90% of its 4.6 billion-year history, it had conditions completely incompatible with our survival. We have only been a part of our planet for less than 0.05% of its existence. Geological evidence from ancient rocks shows that Earth has had many faces. Earth’s earliest life was welcomed onto a world that would have been entirely inhospitable and alien to us: at the time, early Earth had green oceans and a red sky. It had no continents to stand on, no oxygen to breathe, and no ozone layer to protect us from the Sun’s harmful UV rays. And yet life took hold and thrived, slowly shaping Earth over hundreds of millions of years, eventually transforming it into a world capable of supporting complex organisms such as ourselves. For example, billions of bacteria worked over billions years to shape our atmosphere into air we can breathe.

Earth is the home we know and love not because it is “Earth-sized” and “temperate.” No, we call this planet our home thanks to its billion-year-old relationship with life. Just as people are shaped not only by their genetics, but by their culture and relationships with others, planets are shaped by the living organisms that emerge and thrive on them. Over time, Earth has been dramatically transformed by life into a world where we, humans, can prosper.

The relationship works both ways: while life shapes its planet, the planet shapes its life. Present-day Earth is our life support system, and we cannot live without it.

Despite impressive advances in technology, transforming Mars into a planet capable of supporting humans is complete science fiction. It took hundreds of millions of years to shape Earth into a world capable of supporting us. And that was with a 3.7 billion-year head start from the billions and billions of organisms that preceded us! Let’s be clear: when people talk about terraforming Mars, they’re talking about replicating this very same process, except without billions of planet-construction workers and on timescales of just a few human lifespans.

Is there definitely no Planet B? Astrophysicists believe so. Image by M. Vokser via Wikimedia Commons.

Another issue to consider is that other worlds are at unimaginable distances from us. Mars, our neighbor, is on average 225 million kilometers away. Imagine a team of astronauts traveling in a vehicle similar to NASA’s robotic New Horizons probe, one of humankind’s fastest spacecrafts – it recently flew by Pluto! With New Horizons’ top speed of around 58,000kph, it would take at least 162 days to reach Mars.

Beyond our solar system, the closest star to us is Proxima Centauri, at a distance of 40 trillion kilometers. Going in the same space vehicle, it would take our astronaut crew 79,000 years to reach planets that might exist around our nearest stellar neighbor. That’s 79,000 years one way.

Living on a warming Earth presents many challenges. But these pale in comparison to the challenges of converting Mars, or any other planet, into a viable alternative. Astronomers study Mars and other planets to better understand how Earth and life formed and evolved. We are not looking for an escape from our problems: Earth is our unique and only home in the cosmos. There is no planet B.


Dr. R. D. Haywood is an Assistant Professor in Astrophysics and Ernest Rutherford Fellow at the University of Exeter, UK. Her research is on planets that orbit other stars than the Sun. She has formal training in sustainability science and has been invited to talk about Earth as our unique home in the Cosmos at over a dozen institutions worldwide. Dr. A. E. Nicholson is a Leverhulme Research Fellow in Astrophysics at the University of Exeter, and holds a PhD in Geography. Her research focuses on understanding life-environment feedbacks and how these impact the long-term habitability of planets. Both authors are members of Exeter’s Global Systems Institute.

Editor’s note: A previous version of this commentary stated that there are 63 billion planets similar to Earth, when the number is actually 60 billion. The text has been updated accordingly.

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