PLANETPLANET

Not too long ago we used the tools from the Building the Ultimate Solar System series to build a Hulk of a planetary system.  Our mega-system boasts 16 stars, spans 1000 Astronomical Units, and is host to more than 400 habitable worlds!  It looks like this:

I have a story to tell you about this titanic system. But be warned: it is the most tragic story you have every heard. In fact, it is the most tragic story that anyone has ever heard, not only on Earth but in the history of the entire Universe!  This may sound overly dramatic but I can prove it.  I’ll explain at the end.

The story has a cheerful start.  Within a giant swirling cloud of molecular gas, a marvelous planetary system was born.  The system was more bountiful, more beautiful, more fertile than any other.  It was perfect in every way.

This spectacular system was home to 480 worlds capable of hosting life (half of the stars hosted what we called Ultimate Solar System 1 and half hosted Ultimate Solar System 2; see here for details).  They blossomed into a diversity of life-bearing planets and moons.  Some were covered in oceans while others were mostly land. Some had thick atmospheres and some had thin atmospheres. Some had broad ice-covered plateaus and mountains, and others were hot with expansive deserts.

Life developed on some planets and moons and spread through the system. Every time a life-bearing planet was hit by a stray comet or asteroid, pieces of rock containing microbes were launched into orbit.  These microbe-infested rocks landed on other planets and moons in the system and delivered the seeds of life.

Within just a few hundred million years all 480 worlds had life.  Some only hosted single-celled creatures but others had complex life like plants and animals.  A handful of planets and moons even developed intelligent life, advanced life that started to observed the heavens.  Space travel and colonization of other planets were not far off.

But trouble was brewing.  Big trouble.

The villain in this story is the Galaxy itself. But it is an unwitting villain. The Galaxy is not evil, it just can’t help itself. What does the Cookie Monster do if you put Cookie in front of it?  He eats it.  That’s just what he does, it is his nature. The same goes for the Galaxy.

What did the Galaxy do that was so terrible, that caused such tragedy?  The same thing it always does and is doing right now.  It torqued.

Let me explain.  Our Milky Way Galaxy looks like a giant pancake with a golf ball in the middle. The Sun is located within the pancake, about two thirds of the way out from the center.

Most nearby stars live within the Galactic pancake.  That’s why at night (in a dark place far from city lights) the Milky Way looks like a band stretching across the sky.  That band is the combined light of countless millions of stars that are all in the same pancake as us.

The Galactic pancake is not perfectly smooth. It has the most stars in a very thin layer (the thin disk) and fewer stars above and below. It also has clumps and spirals. This matters because this non-smoothness equals differences in gravity.

Imagine a comet orbiting a star within the Galaxy. Because the Galaxy is not perfectly smooth, the star and planet feel slightly gravity from the surrounding stars and gas in the Galaxy. This difference in gravity kicks the comet’s orbit around the star. Sometimes a star whizzes kind of close by (maybe only half a light year away) and gives an extra kick.

Galactic kicks are pretty wimpy. The planets orbiting the Sun don’t even feel these kicks. Only comets on very wide orbits do. Why does the size of the orbit matter?  Because to change an orbit requires torque, a measure of twisting force. To unscrew something you need torque.  When you get stuck trying to unscrew a nut, what do you do?  You get a longer wrench. The longer the wrench, the stronger the torque.

It’s the same for comets orbiting the Sun. The size of the orbit is like the size of the wrench. The Galaxy only kicks very weakly, so it only torques very wide orbits.

The orbits of Oort cloud comets around the Sun are shaped by kicks from the Galaxy.  It is these kicks that transform their orbits into a cloud rather than a thick pancake.  As you can see in this image, the division between pancake orbits (the Kuiper belt) and cloud orbits (the Oort cloud) happens at about 1000 Astronomical Units away from the Sun.

When the Galaxy torques an orbit, the orbit’s shape changes.  The average size of the orbit does not change, but the shape does.  The inclination of the orbit can change — this is why the Oort cloud is a cloud and not a pancake.  More importantly for us, the the eccentricity of the orbit changes.  Eccentricity is a measure of how stretched-out an orbit is.

 

Galactic torques transform circular orbits into stretched-out orbits, and stretched-out orbits back to circular ones. When an orbit becomes stretched-out, its closest approach to the star shrinks. This is the dagger the Galaxy will use to slay our beautiful planetary system.

So let’s take another look at our 16-star system but to make things simpler let’s take the point of view of one 8-star clumps and put it in the center.  Here is what that looks like.

There are a bunch of orbits in the system.  8 tiny orbits of just 1 Astronomical Unit in size, 4 orbits of 10 Astronomical Units, 2 of 100 Astronomical Units, and finally one huge orbit that is 1000 Astronomical Units in size.

Galactic torques only affect the biggest orbit.  But the torques are so small that they act slowly. It takes about a billion years for the orbit to change shape, to be transformed from a pristine circle into a deranged ellipse.

From its birth, our beloved system was living on borrowed time.  A Galactic time bomb was hanging above its head and a billion-year fuse was already lit.

Yet this left a full billion years for our beloved system to flourish. A billion years for life to take hold, to envelop first one world then many. For intelligence to emerge and spread.  Animals and plants have only existed on Earth for the past 500 million years of our planet’s 4.5 billion year history. Life’s baby steps were a little bit quicker on some of the planets in our system so a billion years was plenty for intelligent life to emerge and thrive. Kingdoms rose and fell. The abundance of nearby planets and moons was a carrot dangling in the sky. Technological civilizations reached for the stars, so much closer in their sky than in ours. And they made it, they reached the other planets and discovered hundreds of hospitable oases in the sky. They colonized every last life-bearing nook among the 480 habitable worlds in the system. Empires rose, the largest claiming hundreds of planets under a single flag.  And empires fell in devastating interplanetary wars.

While these civilizations grew extremely adept with physics (and astrophysics in particular), they could not compete with the Galaxy’s tiny persistent torques.  And so, when a billion years were up, our system had shifted into this setup:

This was really bad. Our little angel of a system was doomed. While the Galaxy’s dagger had yet to pierce our dear system’s heart, all hope was lost. I hope that you, dear reader, enjoy gruesome scenes because I will lay out the details of the dismemberment of our fair damsel of a planetary system.

As the widest orbit became more and more stretched out, the closest approaches between the two clumps of 8 stars (which only happened every twenty thousand years or so) got closer and closer.  The clumps of stars starting giving each other stronger and stronger gravitational kicks every close passage.  This started to affect the shapes of the next-largest orbit in the system, the 100 Astronomical Unit-sized orbits of the clumps of 4 stars.  Those 100 Astronomical Unit orbits started to get stretched out as well.

This was a trickle-down process.  New fashion trends are often started by the uppermost classes of society and then spread down to the upper, middle and finally lower classes.  Likewise, the Galaxy only kicked the largest orbits in our beautiful system but its effects spread to smaller and smaller orbits.

Here is a snapshot of our marvelous system in the process of being stretched out:

This was getting bad for the orbits of the planets around their stars.  What would happen when the kicks could no longer trickle down to the orbits of clumps of stars?

Until that point the planets’ orbits had remained blissfully unaffected.  Of course, the night sky had changed a little, and the local astronomers may very have been aware of the impending doom, but the planets trucked on along their orbits like soldiers following orders until the end.

As the smallest orbits in the system — the 1 Astronomical Unit orbits of each pair of stars around each other — began the stretch, the planets’ orbits finally started to feel the Galaxy breathing down their back.

Ultimate Solar Systems 1 and 2 have habitable zones that are tightly packed with planets.  That is how we designed them. The bad thing is, you can’t kick those systems very hard without breaking them.

On our system’s last morning, she looked like this:

Let’s focus for a moment on one star within our system, with a habitable zone full of planets on carefully chosen orbits. There are two ways this system can break: 1) the delicate balance of the planets’ orbits can be lost, but planets (or some remnants) remain in orbit, or 2) the planets can be completely lost from the star. The first is like someone spitting in your beer; the second is like someone smashing your beer on the floor.

It started with a nudge.  A close passage of the companion star shifted the position of the outermost planet just a off of its trajectory.  This was just enough to destabilize the orbital layout of the whole system.  The planets’ elegant orbital configuration will be thrown off if a single planet’s orbit gets stretched out, because that orbit crosses the orbits of other planets. The ever-approaching companion star set a chain reaction in motion by disturbing the orbits of the outermost planet. Then the entire system went boom.

When the orbits of two planets cross, it means that the two planets can be in the same place at the same time.  You are probably thinking of a giant collision between planets.  Something like this:

There were truckloads of mammoth collisions during the death of our beautiful system. Full-grown, life-bearing planets — Earths, Naboos, Dagobahs, and Pandoras — crashed into each other and were pulverized. The plants and animals and civilizations on those planets were entirely obliterated, smashed into smithereens. Remember in Star Wars when Obi-Wan sensed a disturbance in the Force because the planet Alderaan was blown up?  Multiply that by a few hundred!

But collisions were not the only face of our system’s death. Half of our stars hosted Ultimate Solar System 2, where most of the habitable worlds were moons of gas giant planets. When a system with gas giants goes unstable the outcome is different than for rocky planets. Gas giants are so massive, and their gravity is so strong, that they scatter instead of colliding.

When giant planets scatter, one planet usually receives such a strong kick that it is launched out of the system.  Surviving gas giants have stretched-out orbits. Here is a computer simulation of this process (by Eric Ford):

When giant planets scatter, smaller worlds are caught in the crossfire (see here for some animations on my research website).

Around the stars with unstable gas giants, small worlds were launched in every direction.  Most small worlds were doomed.  Some fell onto their stars and ended their lives in fiery blaze:

Other small worlds were thrown so high that they never came down. They were launched into interstellar space, destined to live out their days as Galactic nomads.  These free-floating or rogue planets had to endure the near-absolute zero temperature of empty space, far away from the campfire provided by a star.  Most rogue planets freeze over into iceballs.  But those with thick-enough atmospheres to use for blankets might be able to tough it out and remain habitable on their frigid journey among the stars.

Whether frozen-over or blanketed, rogue planets are so faint that they emit no visible light (and barely any radiation at all):

The end-result of scattering is a system with one or two surviving gas giants on stretched-out orbits.  On occasion, a surviving giant planet can hold on to one of its moons (but not all five, and the survivor may well have undergone a giant collision).

As darkness fell on our beloved system, 3 of the 8 stars with gas giants retained habitable moons. Two moons had been crushed by giant impacts other large moons.  But the third surviving moon was pristine.  Its orbit around the giant planet was changed, as were the orbits of the giant planet around the star, and of the star within the 16-star system.  Yet life, and hope, persisted on this one single moon.

Back to the big picture.  As the Galaxy kept torquing, the orbits in our 16-star system grew more and more stretched-out.  Eventually, stars came so close to each other that stars themselves dislodged some (but not all) of the remaining planets and launched them out into space.

Stars kicked each other hard enough to break the bonds that tied them.  The first to break was the widest orbit, and our 16-star system was split in two.  This turned off the Galactic torques and protected the 8-star systems. But too much damage was already done to one of the 8-star systems, and it broke into one 4-star system and two binaries (2-star systems). The four different systems went their separate ways in the Galaxy, never to meet again.

Our beautiful, precious system died.  It was born with 16 stars and 480 life-bearing worlds.  Each habitable zone was packed with habitable planets. It was as astrophysical paradise. Life flourished and spread over its all-too-short, billion year life.

But the Galaxy couldn’t help itself.  It torqued the largest (1000 Astronomical Unit-wide) orbits and those kicks trickled down all the way to the planets’ orbits. Hundreds of rocky planets crashed into each other. Systems with gas giant planets were scattered, launching worlds in all directions, some into interstellar space and others crashing onto their stars. The system itself was torn apart and split into four systems with 8, 4, 2 and 2 stars each.

Some planets survived in orbit around the stars, but most were in bad shape.  Almost all had been sterilized by giant impacts with other planets.  Only two had avoided large collisions, and their new orbits were stretched-out and only crossed the habitable zone instead of residing there permanently.  Still, planets on stretched-out orbits are good candidates for life, even if their orbits bounce around.

A single life-bearing moon remained in orbit around a gas giant.  The moon’s orbit around the gas giant was stretched-out, but since the gas giant’s orbit around the star (which was also stretched-out) remained in the habitable zone, this world was another ray of hope.

A hundred worlds were launched into interstellar space.  Most of these rogue planets froze over into permanent iceballs in the frigid emptiness between the stars.  But a handful — the ones with thick atmospheres — held on to their heat and maintained livable conditions on their surfaces or in subsurface oceans. The daughters of our lost system  spread throughout the Galaxy.

This marks the end of the most tragic story in the history of the Universe. Have you ever read a story in which more than 400 life-covered worlds were roasted, pulverized or completely frozen?  Imagine the diversity of plants and animals that was lost, the sheer number and diversity of living organisms.  I know stories where one or two planets are destroyed, but to be knowledge, there is more death and destruction in this story than any other in history, anywhere. (Let me know in the comments if you know of a more tragic tale).

It’s really sad when a good planetary system goes bad. Boom!

 

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Final note.  The Galactic torque idea in this story is due in large part to Nate Kaib, astrophysicist extraordinaire.  (See here for a summary of our 2013 study).