What Happens When a Planet Dies?

What Happens When a Planet Dies?

Whether or not we're there to ride it out until the end, all worlds – including our own – can die. So, what happens during a doomed a planet’s final days? And what exactly constitutes a dead planet, anyway?

For today, let’s say it’s a formerly habitable world, but one that can no longer have any hope of sustaining life. We all know that we haven’t yet discovered another planet which actually hosts life. But, we have found worlds that could be or could have been habitable – either in the past, present or future. These ‘habitable’ worlds are still fairly elusive, but some experts reckon that even Mars was once such a place – about 3.5 billion years ago. So, quite fortunately, we do have an example of a so-called dead planet fairly close at hand, galactically speaking that is.

Scientists have theorized that the Red Planet, was once very much like our own; with oceans, a dense atmosphere, and possibly even life (of some kind). The cold, barren vistas that Mars rovers beam back to us nowadays hardly resemble the “living” environment it might have been eons ago, but that’s what makes our nearest cosmic neighbour such a fascinating example. On the one hand, there’s ongoing speculation that it might not be ‘dead’ at all. On the other, it provides a potentially perfect outline of what happens to a dying planet.

Generally speaking, there are a number of reasons why a planet could be reduced to a lifeless husk. It may have been devastated by climate change, it’s nearest star could’ve burnt off its atmosphere, or perhaps it suffered an apocalyptic collision with another object. Theoretically, in Earth’s case, the molten core at the planet’s centre could cool – once the radioactive decay slows or stops. But, most theorists agree that a planet isn’t likely to ‘die of old age’. Rather, it’ll fade out slowly by some other means, or blaze out quickly after a cataclysmic event.

With Mars, its loss of atmosphere is key. Information gathered by NASA’s MAVEN orbiter points to the Martian atmosphere being destroyed by a combination of factors, all stemming from the sun stripping the planet of its magnetic field in the ancient past. In a kind of cruel twist, Mars was effectively slow-cooked by the very Sun that might’ve granted it life. If Martian oceans did exist, they evaporated. And the planet found itself with little defence against solar storms. It ultimately became an exceptionally hostile place.

But, of course, Mars’ fate (or at least, what we currently believe its fate was) is just one of many scenarios – each bringing its own brand of impending doom. We know for a fact that one day in the (thankfully) far-off future Earth, Mars, and every other object in our solar system will be destroyed when the sun goes supernova. Our central star’s fiery death will scorch everything orbiting around it, before engulfing and destroying the planets closest to it during a massive expansion.

However, Earth as a liveable location will’ve likely disappeared long before the Sun’s swansong, as drawn-out changes to the star will’ve already completely transformed our atmosphere. There would be no massive explosion, though, just millennia of increasingly inhospitable conditions setting in.
But, there are also ways in which a planet can meet a more dramatic end. And we don’t need to look as far as the Sun or Mars to find an example. Because Earth itself has had a number of close calls. There have been five mass extinction events in Earth’s history, brought on by various causes, each of which devastated the planet for thousands of years.

The Cretaceous–Paleogene event, or K-Pg extinction, is easily the most famous, wherein the dinosaurs (and more than 75% of all animal and plant species on Earth) fell victim to a massive asteroid strike. While the initial impact was catastrophic enough, what came afterwards was far worse, and gives a glimpse of what could happen on a dying planet. It’s theorized that the fallout from the collision, which was at least one billion times more powerful than the atomic bombs dropped on Hiroshima and Nagasaki, effectively created a nuclear winter. The sun was blocked out for over a year, dramatically altering the Earth’s atmosphere, cloaking it in darkness, and choking the life from it. Over the next few thousand years the species which had dominated the planet for close to 150 million years perished. Obviously, our planet did survive, but it was pretty lucky to do so.

And, incredibly, K-Pg was one of the least deadly mass extinctions on Earth, in terms of percentage of species lost. The closest that life has come to being completely wiped out was at the end of the Permian period, around 250 million years ago, as part of an event appropriately known as “The Great Dying.” It wasn’t quite the death of our planet, but it was a crippling time, unlike anything before or since. Global ecosystems were irreparably changed, as 96% of all marine species went extinct, and life on land took ten million years to recover the biodiversity that was lost. Even insects experienced a mass extinction, as part of the unprecedented disaster. As for the cause, there are a number of theories, covering many of our ‘planet death’ possibilities. Perhaps it too was the result of a massive asteroid impact, or perhaps a more gradual problem – such as rising sea levels, or depleting oxygen within the atmosphere.

Earth’s extinction events, including the reported sixth event that we’re currently experiencing – the Holocene Extinction – all offer some insight into what might happen to a perishing planet. But, clearly, we are still here, and the Earth has survived so far. So, the question isn’t fully answered. Beyond the improbable ‘old age’ death, the inevitable succumbing to a supernova, or the unpredictable collision with an asteroid (or something larger), there is one further way in which a planet could die.

Despite our tendency to envisage the end of all things as a hot and fiery spectacle, it could well be an icy apocalypse. And, while not likely for Earth, a planet could be plunged into impossible coldness if its orbit is altered – and it drifts further from its star, into interstellar space. Strip anything of its energy source – even a planet – and it will eventually die. Equally, while some kind of orbit shunt could send a planet into bleak and empty nothingness, it could also push it even closer to its star. In this case, a fierce and fiery end is definitely back on the cards – but it’d still be more of a drawn-out disintegration, rather than a sudden, sci-fi style explosion.

Either way, the inescapable truth for any planet is that they can only persist for as long as their star does. Once the sun goes supernova, everything else is toast – no matter how many mass extinction events it had survived in its past.

What happens when a planet dies? It usually loses atmosphere, often cools, and sometimes gets broken up by a careening cosmic rock of some kind. For as long as there’s a sun for the remnants of the dead world to orbit around, they will. But, say sayonara to the star, and you’re left with just a cold and aimless rock, so far removed from the potentially ‘habitable’ place it once was.