The Planets. Andrew Cohen

the rains long before they could reach the ground. Venus had reached a tipping point: with the increasing temperatures feeding more and more water vapour into the atmosphere, a runaway greenhouse effect took hold, driving away the oceans. This led to the surface of the planet getting so hot that carbon trapped in rocks was released into the atmosphere, mixing with oxygen to form increasing amounts of another greenhouse gas – carbon dioxide. With no water left on the surface and no other means to remove it, carbon dioxide built up in the atmosphere, setting the planet on a course that would result in the scorched body that we see today.

      © NASA

      © NASA

      In 1977, in the days before computer-generated imaging, NASA commissioned artist Rick Guidice to paint illustrations of the surface of Venus, based on images received from Pioneer probes.

      And so Venus’s moment in the Sun came to an end. Earthlings take note: when it comes to the greenhouse effect, there is a precariously thin line between keeping a planet warm and frying it.

      THE END OF EARTH?

      Of the four rocky worlds, only one has managed to navigate through the instability and constant change of our Solar System over the last 4 billion years and maintain the characteristics needed to support life. Mercury lost its fight early as it was flung inwards towards the Sun, Venus flourished at first, before slowly coming to the boil, and Mars, the runt of the litter, became a frozen wasteland long ago. Only Earth, uniquely amongst the planets, has persisted with an adequate stability over the last 4 billion years to allow liquid water to remain on its surface and an atmosphere just thick enough to keep its climate calm – not too hot and not too cold. Events have rocked us and extremes of temperature have waxed and waned, but never outside of the parameters needed to harbour life. In a chaotic solar system, filled with planetary might-have-beens, Earth is a shining example of stability, and the evidence for this is to be found in every nook and cranny of the planet.

      Today Earth is dominated by life; the land and seas are teeming with millions upon millions of species, with thousands of new life forms discovered each year. Somehow, even when disaster threatened, the Earth has remained a living world; while endless species have come and gone, life has always persisted. It’s woven into the fabric of the planet – an integral part of every continent and every ocean. Life plays a crucial role in maintaining the balance of the atmosphere that keeps our planet temperate, but we know for certain it cannot last.

      In a chaotic solar system, filled with planetary might-have-beens, Earth is a shining example of stability.

      The Kamchatka Peninsula in Eastern Siberia is one of the most inhospitable places on Earth. A volcanic wasteland, peppered with thousands of hot springs, it’s here that we find some of the toughest living things. Extremophiles survive here that are able to withstand temperatures and pH levels higher than any other land-based life forms we have ever discovered. Kamchatka is part of the Pacific ring of fire, and despite its remoteness, biologists have long been enticed here to explore its toxic, bubbling cauldrons for signs of life. Complex life, animals and plants struggle to survive in temperatures above 50 degrees Celsius, so searching for life here is all about searching for single-celled life forms, bacteria and archaea – ancient microorganisms – that are somehow able to endure in this hostile environment. Life forms like Acidilobus aceticus, an archaea that can be found in a hot spring where the water is so acidic it reaches a pH of 2, and where temperatures rise to 92 degrees Celsius. In other parts of the hydrothermal field, bacteria like Desulfurella acetivorans have been discovered, which happily live in pools that are touching 60 degrees Celsius, but it’s these that are the real hotheads. In one of the biggest and hottest pools investigated by scientists, a large number of microbes have been found living in temperatures approaching 97 degrees – making it one of, if not the hottest environment ever studied for signs of life on land.

      But to find the greatest hotheads on Planet Earth you need to look not on land but deep beneath the sea. In the furthest depths of the Atlantic, around the black smoker hydrothermal vents blurting out of the ocean floor, we’ve found strains of archaea that can survive temperatures of 122 degrees Celsius, and perhaps even higher.

      These rare life forms live at the very edges of biology. Unique adaptations to their cellular chemistry enable the proteins and nucleic acids that create the structure of the microorganism to function, while the membranes that are protecting the cells utilise different fatty acids and lipids to keep the cell stable at the higher temperatures.

      © Igor Shpilenok / naturepl.com

      © Igor Shpilenok / naturepl.com

      Russia’s Kamchatka Peninsula is one of Earth’s most inhospitable areas; the volcanic landscape gives us an insight as to how planet Earth might appear when it becomes too hot for life.

      © DSS2 / MAST / STScI / NASA

      Arcturus, one of the brightest stars in the Northern Hemisphere, which in its early history would have had similar characteristics to Earth.

      Perhaps there are even tougher life forms that we are yet to discover, but the thermophilic microorganisms that we have so far identified and investigated in places like Kamchatka all point to the fact that life has its limits. Evolution by natural selection can only adapt so much, and even though it’s impossible to imagine what life on Earth will look like in a few hundred million or even a few billion years’ time, we know that biology is constrained by thermodynamics, and so we can say with some certainty that there will come a time when the Earth is too hot for any living things to exist. Natural selection will eventually run out of options as the laws of physics outplay it, and all life will come to an end.

      © NASA, ESA and G. Bacon (STScI)

      The blue white-dwarf star Sirius B (pictured to the right of Sirius A) has burned out to a core the size of Earth, giving us an insight into the future of our planet.

      When this will happen no one can be certain, but as the Sun ages and grows hotter, temperatures on Earth will rapidly rise. Today the average surface temperature on the planet is 14.9 degrees Celsius, but with just a 10 per cent rise in the Sun’s luminosity, the average temperature will rise to 47 degrees Celsius and climbing. The increased temperatures will raise great storms across the planet. The rains will remove carbon dioxide from the atmosphere and it will be locked away as newly formed sedimentary rock. Trees and plants will struggle as they are robbed of the gas that sustains them, until eventually photosynthesis will cease. The lungs of our planet will fail and the precious oxygen that green plants and algae produce will dwindle. With the primary food source gone, the food chain will collapse and the age of complex life on Earth will draw to a close.

      © Science History Images / Alamy Stock Photo

      The diamond-shaped constellation, Boötes, has been known to scientists for centuries, described by Ptolemy in the second century.

      Astrobiologist David Grinspoon on Venus as a window on Earth’s future

      ‘Left to its own devices, Earth will go the way of Venus. Now, this is nothing to lose sleep over right now because we’re talking at least a billion years, probably more like a couple of billion years in the future. We have more immediate concerns, but as we do compare the planetology and look at the