PLANETPLANET

“I don’t think I’m alone when I say I’d like to see more and more planets fall under the ruthless domination of our solar system.” — Deep Thoughts by Jack Handey

---

This series is about how planets die — it is introduced here.

Getting old sucks. Not only am I (mostly) bald but I’m becoming grumpier, dumber and more injury-prone by the day….

Just like people, stars age. Their planets go along for the ride and are often killed or maimed in the process.

Small stars and big stars age at different speeds.  It’s like rock stars and farmers.  Rock stars live like crazy people then die within a few years die a drug-fueled blaze of glory. Farmers just keep farming for decades and decades until they’re all farmed out.  (Apologies for the generalization if you’re a farmer or rock star).

Massive stars — born with at least 8 times as much mass as the Sun — are the rock stars.  They burn super bright, then quickly explode. We obsess over their remains (black holes, neutron stars).  And, like rock stars, massive stars are very rare.

“Normal” stars are the farmers.  They burn at an appropriate level and evolve slowly.  After using up all their fuel, which typically takes a few billion years or more, they burn out.  They pass on some of their experience to the next generation as they shed their outer layers, then they end up as run-of-the-mill white dwarfs.

The most common type of star are red dwarf stars.  They don’t have a good analog among humans because they burn so slowly that they basically live forever.  To be annoyingly precise, they don’t actually live forever, it’s just that their lifetimes are much longer than the current age of the Universe (about 14 billion years).

What happens to planets in the habitable zones of different types of stars?

I’ll chop it into three parts. Part 1 is about puny red dwarf stars. Part 2 is about stars similar to the Sun (farmers). We’ll end — with a bang! — with the massive (rock) stars in Part 3.

Part 1. Planets orbiting red dwarf stars: dehydrated and toasted

Red dwarf stars basically live forever.  And they don’t change much.  So, you might think that any planets orbiting them are safe…

Think again!

It takes a while for red dwarfs to really become “stars”.  The process of collapsing into a star and triggering nuclear fusion is slowed down by their low gravity.  While stars like the Sun take perhaps ten million years to settle onto the main sequence, red dwarfs — and especially the puniest among them — can take upwards of 100 million years.

Why do we care?  Because, before settling down stars are brighter. This means that the habitable zone — where a planet might have liquid water if it has the right conditions — is farther away.

Imagine an Earth-sized planet around a red dwarf star.  Once the star has settled down, the planet will be in the habitable zone.  But for 100 million years or more the planet is on the wrong side of the habitable zone.  It is too hot.

During this too-hot phase the planet could lose all its water! 

First, the planet’s atmosphere would undergo strong greenhouse heating.  This would transport water up into the stratosphere, where could be broken apart (into hydrogen and oxygen) by high-energy radiation from the young star, and slowly lost to space (see planet death by climate catastrophe for more on this).

Here is a cartoon of this process, specifically with regards to the roughly Earth-mass exoplanet Proxima b, which orbits a puny red dwarf star:

If the planet loses all of its water then, as the star settles down the planet will enter the habitable zone completely dried out.  It will be in the right place for liquid water but all of its water is gone!

We don’t know whether this process dries out planets completely or just strips off a few outer layers of ocean. If a planet has enough water trapped in its interior (Earth is thought to have a few times its surface water in the mantle) then it could withstand losing its oceans by later out-gassing new ones.  It’s a complex interplay between geology and astronomy and the outcome is….unknown (for now).

Part 2. Planets around Sun-like stars: roasted in a slowly-heating oven then deep-fried

Stars like the Sun evolve faster than red dwarfs.  They reach the main sequence too quickly to worry about water loss early on.

But while they are on the main sequence they don’t remain perfectly constant.  The Sun is slowly brightening over billions of years.

As the Sun converts hydrogen to helium in its core (via the proton-proton chain), the average density in the core increases, because atoms with just one proton are being converted into atoms with two protons. This causes a very slight increase in the local temperature, which increases the strength of fusion.

That means that life on Earth lives under a constantly-brightening Sun.  Sure, the brightening is extremely slow (only about 0.00000000003% from one day to the next).  But over billions of years it adds up.

Rewind the clock and we have the Faint Young Sun paradox (first pointed out by Carl Sagan and George Mullen in 1969).  There is evidence for life on Earth 3-4 billion years ago (the earliest evidence is always debated, but it’s at least 3.5 billion years ago).  However, most climate models calculate that the Earth should have been frozen solid under a Sun that was only 70% as bright as it is today. This paradox remains heavily debated; solutions may involve the greenhouse effect or a number of other effects.

Project forward and the future looks bleak. Earth is already close to the inner edge of the habitable zone.  In a billion years the Sun will be 10% brighter and the Earth will cross over and no longer be in the habitable zone.

When that happens, Earth will become hot enough to trigger a runaway greenhouse. That means that there is a strong positive feedback that keeps making the planet hotter and hotter (this includes vaporizing the oceans).  Heating increases the amount of water vapor in the atmosphere, which increases the rate of heating, and so on.

This will turn Earth into a Steamball planet, and water will eventually be lost from the stratosphere.  In time Earth will likely end up a dry desert wasteland of a planet, like Venus.  Frowny face…

This gradual brightening happens for all main sequence stars.  If Earth were on Mars’ orbit, it would remain in the habitable zone for longer.  But if Earth orbited a more massive star than the Sun then its lifetime in the habitable zone would have been shorter.

Things only get worse from there.

Stars like the Sun last about ten billion years.  At 4.5 billion years old the Sun is firmly in its middle age, slowly brightening and not doing much else.  But about seven billion years from now, watch out!

The Sun’s core will run out of hydrogen, its source of fuel. Fusion of hydrogen will continue in a shell and this will puff the Sun up into a red giant. Red giants are cooler than Sun-like stars (hence their redness) but are very bright because of their very large sizes. Betelgeuse, Orion’s bright right shoulder, is a good example.

The red giant Sun will grow to 100-200 times larger than its current size, about as big in size as Earth’s orbit.  Venus and Mercury will be swallowed whole, and Mars and the giant planets will be pushed outward.  Earth is on the cusp; it is uncertain whether it will be swallowed or pushed away.

A ray of hope is that, for a few hundred million years, Saturn will be in the habitable zone.  Saturn has several large moons and its largest, Titan, is the only world apart from Earth to have liquid on its surface (although in Titan’s case it is liquid ethane and methane).  Still, Titan might be a good place for life during the Sun’s red giant phase.

The Sun’s evolution (at least the interesting part) will finish with a series of explosions and the removal of its outer layers to make a pretty planetary nebula (no relation to planets, just a badly-chosen name).  The core — a white dwarf — will stick around and just passively cool off for eons.

White dwarfs are almost as massive as the Sun but only about the size of Earth. This means that they have extremely strong surface gravities, and anything heavier than hydrogen or helium settles out of their atmospheres and into the stars themselves in days to months, an astronomical blink of the eye.

But many white dwarfs are found to be “polluted”. Instead of looking like pure balls of hydrogen and helium, their spectra show signatures of rocks and other elements, sometimes including water.  These are signs of material that has very recently fallen onto their surfaces, probably asteroids and comets whose orbits were destabilized by the planets moving around as the Sun evolved.

What is amazing is that, because the outer layers of white dwarfs are just hydrogen and helium, we can measured the properties of these asteroids and comets with very good precision.  Remember, these objects are leftovers from the formation of the planets.  Being able to measure their compositions around white dwarfs whose habitable planets were destroyed is akin to analyzing the blood spatter at a crime scene.

Part 3. Planets orbiting high-mass stars: blown apart by a rock star!

Like rock stars, massive stars live fast and die young.  On the main sequence they are big and hot and bright.  Since they are so hot they appear blue.

But they don’t last long.  Within a few million years, very massive stars evolve into supergiants.  The most massive become blue supergiants and less massive ones because red supergiants.  Not long afterward, they go supernova!

Massive stars are born with ten to a hundred times more mass than the Sun.  Their huge luminosities are powered by vigorous nuclear fusion.  This creates a whole slew of elements — everything from carbon up to iron — in a series of onion-like rings, each of which is creating something different.

Supernovae blow their stars to pieces (note: one supernova, two supernovae). They are splattered all over the place.  Since stars form in clusters and massive stars die young, a large fraction of a massive star’s guts are immediately donated to the next generation of stars.

What is different about massive stars is that they are so energetic that they not only affect their own planets, but planets orbiting other stars too.

Can planets form around massive stars?  None has been found (yet), but that is probably because there are so few massive stars to search and planets are hard to find.  Massive stars form with massive disks around them, and there is reason to think that planets should form readily.  In fact, their disks are so massive that they may often form gas giants rather than rocky planets.

Planets may form around super high-mass stars but they wouldn’t last long. Supernovae are so energetic that they would likely vaporize their planets entirely.  Planets have been detected around a handful of a special type of neutron stars called pulsars (and these include the first confirmed exoplanets!).

The pulsar planets are probably not the original planets that formed around their host stars before they went supernova.  Rather, pulsar planets are likely to either be a) second generation planets that formed from leftover debris, b) the remnants of original companion star, or c) planets captured from another star.

Given that massive stars have such short lifetimes, a planet orbiting one is not a great candidate for life. 

Supernovae have a huge effect on their surroundings. The shock waves generated by supernovae can trigger other stars to form. Stars that are too close to a supernova can have their gaseous planet-forming disks completely evaporated.  This would stunt the growth of gaseous planets like Jupiter but might not preclude the formation of rocky worlds.

Supernovae also barf their guts all over the place.  There is evidence from meteorites that our Sun’s planet-forming disk was contaminated with highly radioactive material from a nearby supernova.

That radioactive material — specifically an isotope of Aluminum and one of Iron — provided a huge amount of heat in the baby Solar System.  It  played a central role in determining the water contents of the planets.  Indeed, without any of this radioactive Aluminum and Iron, it is quite possible that Earth’s oceans would be so deep that there would be no land!

Most stars are likely to get a Sun-like dose as long as there are enough stars in their birth clusters for there to be a supernova (statistically, at least ~1000 stars). The ones that form in puny groups may have much wetter planets.

So the proximity of a growing planetary system to a supernova can affect the growth of planets.  Too close and a star’s planet-forming disk is history.  Too far and your system forms with much less heating than the Solar System (although we don’t know how strongly this really affects planet formation).  The Solar System fell somewhere in between.

Supernovae may also harm fully-grown planets.  The blast of energetic particles from supernovae can destroy an Earth-like planet’s ozone layer if it is within about 20 light years.  Without an ozone layer the planet would be vulnerable to a lethal dose of ultraviolet light from its host star.

How many planets are killed by their aging stars? 

Let’s break it down for each type of star.  First, remember that most stars are red dwarfs (spectral type M).

How many stars dry out their planets before they even become full-fledged stars? 

A lot! This could in principle affect red dwarfs less than about half the Sun’s mass.  Sure, those stars live forever.  But they take a long time (100 million years or more) to settle down and become real stars, and during that time any planet in the habitable zone is vulnerable.

There are a few hundred billion stars in the Galaxy — let’s say 400 billion to make the numbers simple.  About three quarters are red dwarfs — that makes 300 billion.  At least one third of those have an Earth-sized planet in the habitable zone.  That makes 100 billion planets that may be affected by this type of water loss.

How many Sun-like stars fry their planets as they age, or kill them when they become red giants?

About ten percent of stars are Sun-like in that their main sequence lifetimes are less than the age of the Universe but they are too low-mass to go supernova. We don’t know exactly what fraction of these stars have rocky planets in their habitable zones.  Let’s say ten percent.

That makes 4 billion potentially rocky worlds. Each one of those planets will slowly heat up as its Sun slowly gets brighter. And each one is doomed when its Sun turns into a red giant, perhaps to be swallowed but at least to be thoroughly roasted.

How many planets are killed or sterilized by massive stars?

Only stars more massive than about 8 solar masses will become supernovae.  That is about one in every thousand stars.  Even though high-mass stars don’t last long, we can use our Milky Way’s 400 billion stars as tracers.  That means that about 400 million supernovae have gone off in our galaxy’s history.  And it is consistent with the current rate of a couple of supernovae per century.

If any of those massive stars formed their own rocky planets they were vaporized.  But since those stars go boom so fast, planets around massive stars are unlikely to host life when they are fried.

Each supernova affects nearby star systems: planets within about 20 light years are vulnerable to losing their ozone layers.  However, many nearby star systems are still in the process of formation and so may not yet have developed an ozone layer.  But some

The final tally: 100 billion planets are at risk of losing their water (red dwarf stars), 4 billion planets are doomed (Sun-like stars),

Let’s put these processes on our planetary death scale. 

We’ll break it down by star type: blue for the rock stars, yellow for Sun-like stars, and red for red dwarfs.

Red dwarfs can dry out their planets, but it takes a while for a planet to lose water.  Plus, many planets are likely to have extra oceans that remain stored in their mantles until their stars settle down.  So, this process is somewhere between sterilization and extinction.

Sun-like stars sterilize their planets as they get brighter with age and often destroy them when they turn into red giants.

Finally, any planets orbiting a massive star are likely vaporized by the supernova blast.  Planets around nearby stars would survive but might lose their ozone layers.  This would be bad for life and may cause extinctions or, in extreme cases, sterilization.  However, only a small fraction of stars are likely to suffer such a harsh fate.

Let’s use all this for a sci-fi story called A cosmic charity case…

Imagine a highly advanced civilization.  Maybe advanced enough to engineer their own Solar Systems.  They are master astronomers with extensive knowledge of the heavens.  They have mapped out all of the planetary systems in their galactic neighborhood.  They know which stars host habitable planets.  They have sent messages to thousands of planets in the hopes of getting an answer from another civilization.  But no luck.  They’ve found signs of life in the spectra of other worlds but no one has answered their calls.

Their government has a cosmic Search and Rescue division.  It is dedicated to identifying planets that are about to be destroyed or sterilized, then rescuing any creatures that live on such planets.  They frequently pat themselves on the back for being so noble. The Search and Rescue division has their own Noah’s Ark, a giant spacecraft that is filled up with specimens.  When it returns home, the creatures are moved to their own planet in a cosmic zoo, a planetary system with hundreds to thousands of planets teeming with life from across the galaxy.  (This may remind some of you of the Rama series by Arthur C. Clarke…).

But what happens when the animals in the zoo evolve, and revolt?

Dramatic Music!

---

Additional resources

• Introduction to the how planets die series
• Blog post (in poem form) about Proxima b and the effect of water loss
• Scientific article on water loss of planets orbiting red dwarfs (like Trappist-1, Proxima Cen): Bolmont et al (2017)
• Nautilus article: Reading Earth’s destiny in the blood spatter around other stars
• A nice animation of the discovery of supernova 1987A.
• Rendezvous with Rama by Arthur C. Clarke: my all-time favorite sci-fi book.