How planets die


Like people, planets are born and they die.

We know how people die. They are hit by buses, get cancer, are mauled by wild dogs, fall off of high places, and so on. But until recently we didn’t know how planets die.

Google “how planets are born” and you’ll get the right answer.  You’ll find links to the latest thinking about planet formation (including videos of me).

Google “how planets die” and you’ll get the wrong answer.

You’ll read that a black hole might pass too close to a planet and suck us into its gloomy depths. Or that an Earth made entirely of anti-matter might collide with Earth and completely annihilate it. Or that all of the protons that make up a planet may spontaneously decay and cause it to disintegrate.

Those ideas are fun but they never happen. In the history of the Universe not a single Earth-like planet has been impacted by its anti-matter equivalent. Nor has an Earth spontaneously decayed into nothingness. A few planets have been gobbled up by black holes and that is awesome! But only a tiny tiny number.

This series is about how planets are actually destroyed.

Planets die all the time right in our galactic neighborhood. Some are smashed to pieces by massive collisions with other planets. Some are tossed into the cold bleakness of interstellar space. Some are torn to pieces then tossed onto their stars. Some are swallowed by their stars when the star runs out of hydrogen fuel in its core and puffs up to become a red giant.

Life is more fragile than planets. Planets can lose the ability to host life without being destroyed. Some planets’ oceans evaporate into space. Some planets’ atmospheres are blown away by radiation from their stars or nearby supernova explosions. Some planets undergo such strong tidal heating that their surfaces turn into oceans of lava. Some planets freeze over into ice balls that never melt. And on some planets life is stunted or wiped out by bombardment of asteroids and comets.

Yet some planets can maintain life in places we didn’t think possible. On planets close to their stars with unchanging day- and night-sides, in a narrow band between blistering day and freezing night (“hot Eyeballs”). On planets covered in dense atmospheres on much colder orbits than Earth. On moons that orbit gas giant planets and are heated by tidally driven volcanoes (sort of like Pandora). Even on rogue planets that are not bound to any star but wander the Galaxy.

And some planets never die. They retain habitable conditions for ten times longer than the (current) age of the Universe.  I think of them as spooky, zombie-elf worlds.

Planetary death is not as black and white as for humans.  There are different ways to define a planet’s death:

  1. Destruction.  For example, when a planet falls onto a star.  There is no more planet left.
  2. Sterilization. For instance, when a planet loses its atmosphere and water.  Life as we know it is out of luck.  (You might be thinking that life as we don’t know it might survive — we’ll get to that later.)
  3. Mass extinction.  This can come from a dramatic change in conditions. For example, when a planet freezes over it, any species that can’t handle cold will die off.  Planetary life will not be wiped out as long as some species can handle the new conditions.

Destruction, sterilization and mass extinction.  Three ways to die.  But very different outcomes for the planets themselves.  I will use this planetary death scale to evaluate different causes.


How does Earth’s story fit in?

Our planet has undergone several mass extinctions.  The most famous was the asteroid that hit the Earth and killed the dinosaurs 65 million years ago.

Within the next billion years, Earth will be sterilized. The warming Sun will boil away our our oceans. That will be Earth’s sterilization.

In five billion years or so the Sun will swell into a red giant the size of Earth’s orbit. Mercury and Venus will be engulfed. Earth may be pushed outward or eaten up.  [Side note: Saturn’s moon Titan will be in the habitable zone during the Sun’s red giant phase.] When the Sun shrinks down into a white dwarf a few hundred million years later, the spectral signature of debris raining down will be the last glimpse of life on Earth.

The far future of our Solar System (note: it is uncertain whether Earth will also be engulfed, as it is in this image). See this article (much more in upcoming post about aging stars).

In this series we will take a stroll through the planet morgue.  Each post will explain a cause of planetary death and calculate how common it is.

The actors are stars, other planets, atmospheres, volcanoes, and even the Galaxy itself.  The most common weapons for planet assassination are pretty common: gravity and radiation. But just like tripping over a stuffed animal and falling down the stairs, everyday things like chaos and comets kill planets too.

And remember, our situation is unusual.  Most planetary systems are very different than our Solar System. 

For one thing, the Sun is rare.  Most stars aren’t yellow, they’re red.

Distribution of the types of stars within 30 light years of the Sun (21 white dwarfs are not included on the plot). Credit: Franck Selsis, with data from

Most potentially life-bearing planets orbit red dwarf stars because they represent 3/4 of all stars.  Red dwarf stars are small and faint and their habitable zones are much closer-in than Earth’s orbit around the Sun.  That means that some effects will be much stronger on these planets, like tides.

Over the next two weeks I’m going to roll out five causes of planetary death that cover the bases.  This is not a complete list so I’ll periodically add more.

Here is what is coming:

To avoid making everyone too sad, I’ll break out some Second Chance Planets afterwards.  These are planets that have opportunities for life late in the game, sometimes after a seemingly devastating past.

Final note: This series is at the source of the planetplanet blog.  As a kid (motivated by authors like Isaac Asimov, Douglas Adams, Ursula Le Guin, and Arthur C. Clarke) I had a phase during which I wanted to be a writer. I loved writing goofy or science fiction stories with surprise endings. About 5 years ago I came up with the idea of a book about how planets die. Caleb Scharf, astro-writer and scientist extraordinaire, suggested that I start a blog to practice writing for the general public. I never managed to persuade a publisher to make this its own book (or should I say, I haven’t yet).  So I decided instead to put it out there in blog form. These posts are longer than usual (2000-3000 words rather than 1000-1500) and include a lot of details and simple calculations to try to show you how much each cause of planetary death stuff matters.

As always, comments, questions and suggestions are welcome!

16 thoughts on “How planets die

  1. I want to learn more about those zombie planets! How are they able to exist as older than the universe???

    1. Well, they aren’t older than the Universe. It’s just that they orbit stars that are super well-behaved and won’t change for hundreds of billions or trillions of years. I’m planning to write a post about them soon!

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