PLANETPLANET

For mad scientists who keep brains in jars, here’s a tip: why not add a slice of lemon to each jar, for freshness? — Deep thoughts by Jack Handey

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This series is about how planets die (introduced here).

Where would we be without the occasional asteroid crashing into the Earth?

Asteroids and comets are like last week’s meatloaf: leftovers.

They are the stuff that didn’t end up as planets.  As planets form, they grab onto as much nearby stuff as they can. But as they grow their gravity gets stronger and they end up launching things all over the place, like babies throwing food on the floor.

When planets get really big (think, Jupiter and Saturn), they gravitationally toss around small bodies rather than eat them (and bring water to planets like Earth).

In the Solar System there are two reservoirs for planetary leftovers (also called planetesimals): the asteroid belt and the Kuiper belt. 

The asteroid belt is located between the orbits of Mars and Jupiter. It contains less than a thousandth of Earth’s mass.  We think that the asteroid belt was not friendly to planet formation — maybe because of Jupiter — and these days it serves as a cosmic refugee camp.

The Kuiper belt is a vast collection of icy objects past the orbit of Neptune.  Like the asteroid belt, it contains surprisingly little mass — less than one tenth of Earth’s mass.  We think that, so far from the Sun, there was just not enough time for planets to form.

But things weren’t always like this. There is good reason to think that the Kuiper belt was born with at least a hundred times more mass than it has now (20-50 Earth masses in total).  The belt was cleared during an instability in the giant planets’ orbits sometimes called the Nice model.

Computer simulations suggest that the instability went something like this:

The giant planets started on orbits much closer to each other than their current orbits.  There was an outer disk of icy leftovers: the primordial Kuiper belt.  The giant planets’ orbits became unstable and one ice giant was ejected (this simulation started with an extra ice giant such that two survived).

During the instability most icy leftovers were cleared out. A few collided with the Sun. Most were ejected into interstellar space (maybe to pass through another Solar System one day, like ‘Oumuamua).  And a small fraction crashed into planets.

During the instability, asteroids and comets crashed down on the surface of every planet and moon in the inner Solar System.

Asteroids dominated the impacts on Earth — even though there were far fewer asteroids, they have a much higher collision probability with the inner planets.

A lot more small objects hit the Earth than small ones.  But each large impact had a stronger effect: it melted the surface to a deeper depth, created a larger crater, and took longer to cool off.  Animals — if there were any on Earth at the time — would have been completely wiped out.

But the bombardment did not completely sterilize Earth. 

Some microbes called hyperthermophiles can survive, and even thrive, at high temperatures.  Heat-loving organisms are actually found at the root of the tree of life, and one can imagine that other ancient organisms existed but only the hyperthermophiles survived the bombardment. It’s a compelling story.

[Side note: The timing of the Solar System’s instability is uncertain. The instability was originally invoked to explain the “late heavy bombardment“, thought to have happened a few hundred million years after the planets formed. But new analysis of craters and meteorites suggests that there may have been no late heavy bombardment at all. More likely, the instability happened shortly after the Sun’s planet-forming disk evaporated. This would have been before Earth finished forming, and may even explain the structure of the terrestrial planets (including stunting Mars’ growth).]

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What causes bombardments to happen and what are the possible triggers?

There are three flavors of bombardments.

1. Bombardment from local planetary leftovers.  Like a baby who throws food up in the air that splats down on its head.
2. Bombardment from planetary leftovers from a different part of the same planetary system.  Like a baby throwing its food across the room and splatting on someone else’s head.
3. Bombardment from an external source. A drive-by baby splat!

Bombardment from local leftovers is just the tail end of planet formation. But this flavor of bombardment can vary in length and in strength.  In some situations the planet can be continually impacted for hundreds of millions of years.  In others, the bombardment can be very short.

Can bombardments from local leftovers (flavor 1) cause mass extinctions?

For an extinction to happen a planet must already have stabilized enough to host life.  It must have a solid surface with water. This type of bombardment is happening as planets form, so it may instead cause impact frustration.  This means that impacts are more frequent than the time needed for life to evolve.  So life keeps getting close and then … splat!

Any bombardment eventually runs out of juice.  At some point life may take hold and then be wiped out by an impact.  Still, life emerging slowly from a dwindling bombardment is quite different than a sudden flood of sterilizing impacts.

Our Solar System’s bombardment was from a distant belt of planetary leftovers (flavor 2). 

The inner planets were pelted by an outer belt of planetary leftovers that were destabilized by the giant planets.  [To be annoyingly precise, they were pelted by both nearby objects (asteroids) and more distant ones (comets).]

It was the giant planets’ instability that transported leftovers inward, from beyond Neptune toward the inner Solar System.

Instabilities are very common in systems of giant planets. But as we saw in when good Jupiters go bad, instabilities are often much stronger than our Solar System’s.  Instead of causing bombardments, many instabilities destroy their rocky planets!

But not always.  Sometimes instabilities destabilize belts of leftovers without destroying their planets. And sometimes the instabilities happen very early — probably leading to impact frustration instead of extinction — but sometimes they don’t happen for a hundred or more million years after the planets form.

That is the sweet spot for planet death: late instabilities that are not strong enough to toss their Earths into their Suns. 

This kind of bombardment-causing instability is more likely in systems with ice giant planets but no gas giants.  Being lower in mass, ice giant systems have a higher probability of kicking around belts of planetary leftovers without destroying small rocky planets.

Instabilities in ice giant systems also last longer than in gas giant systems.  This is because ice giants have weaker gravity, so it takes longer for them to kick leftovers out into space.  That means there is more time for leftovers to crash into planets.

Plus, ice giant systems are more likely to wait before going unstable, creating bombardments that are delayed until after life has taken hold.

The third flavor of bombardment is rare and unlikely to cause much damage. 

A bombardment from objects originating outside our Solar System is virtually impossible because they are just too rare.  But comet showers do happen from time to time.

Comets from the Oort cloud — at the edge of the Solar System — can be destabilized en masse and flood the inner Solar System.  The impact probability of these objects is very small, so even during a big comet shower the number of impacts on Earth only goes up by a little.  But the meteor showers must be spectacular!

There is evidence for a comet shower on Earth about 35 million years ago.  There are clear isotopic signs of an increase in cometary impacts, but there is no indication that it caused a mass extinction.

What does a bombardment look like?

During a bombardment impacts become more frequent. On Earth, small impacts happen all the time but big ones are rare.  For example: about 20,000 tons of space dust hit Earth every year, objects that are ~70 meters (~230 feet) across hit every couple thousand years, but kilometer-scale objects only hit every million years or more (see here).

During a bombardment it wouldn’t be a couple thousand years between 70 meter impactors but just a few years or much less.  Collisions of that size can have a noticeable effect on the atmosphere, so if they keep happening the effects may add up.

But the big danger is from the largest impactors.  Those impactors are the rarest but during bombardments they go from “once-an-eon-if-you’re-unlucky” events to “oh-crap-we’re-screwed“!

Here is a great animation of this type of mega-impact:

Imagine you live on one of those unlucky planets.  Some distant ice giants in your system go unstable and kick around the orbits of these planetary leftovers.  What would it feel like?

At first the night sky would get much more interesting, with all the comets and asteroids zooming through your planet’s neighborhood. And the meteor shower would be spectacular, with zillions of shooting stars.  The most dramatic would be fireballs from large objects that graze your planet’s atmosphere without colliding (like the Great Daylight Fireball of 1972).

Then the big impacts start. Once a large enough impact hit your planet it would likely throw a huge amount of dust and rubble into the atmosphere.  Apart from the devastation from the impact itself, dust would enshroud the planet and likely cause an Impact Winter.

The planet’s climate would cool but if the colliding object is small-ish then the impact winter is survivable.

But big impacts would keep coming and coming, creating stronger and stronger climatic events until a giant sterilizing impact roasted the planet’s surface.  Frowny face…

How many planets like Earth are pulverized by impacts of asteroids and comets?

Here is what we know:

• Most stars have planets. A large fraction (maybe a third or so) have Earth-like planets. About 10% of Sun-like stars (or a few percent of all stars) have gas giants, but a higher fraction (say, 10-50%) have ice giants.
• Planet formation is not 100% efficient. There are always leftovers.
• We can see the planetary leftovers.  About 20% of older Sun-like stars have “debris disks“, cold dust thought to originate in outer belts like our own Kuiper belt.

Let’s do the math.  To make the numbers simple let’s say that there are 400 billion stars in the Galaxy and that 1/4 of them have Earth-like planets.  That’s 100 billion Earths.

About 1/4 of those systems also have ice giants and outer belts of planetary leftovers (aka planetesimals).  [Note: some gas giant systems may also cause bombardments but since they are less common we’ll stick to ice giant systems.]

Computer simulations suggest that almost all of the ice giant systems should go unstable.  But only a small fraction will go unstable late enough to cause bombardments rather than impact frustration.  It’s tricky to estimate this — let’s say 1 in 25 (because it makes the numbers work out nicely but it could be less).

That makes about 1 billion bombardments on Earth-like planets in our Galaxy!

And remember: most of these bombardments would last longer than the Solar System’s, because ice giants take longer to clear out leftovers.

Let’s put bombardments on our planetary death scale

Bombardments are definitely bad for life.  But the extent of the devastation depends on how intense the bombardment is.  Some bombardments will completely roast habitable planets and sterilize them.  But others may be shorter in duration and only cause mass extinctions.

Story

A 7 year old girl has a new favorite computer game. It’s all about destroying planets. It’s called Planet Crash (not to be confused with Super Planet Crash, a real game in which you create planets).

The girl has gotten good at Planet Crash. Sometimes she drops planets onto their stars (often using Jupiters going bad) or bombards them with comets and asteroids.  Sometimes she uses tidal volcanoes or climate catastrophes to cause mass extinctions.  And sometimes she just lets the stars do the work by going supernova or brightening with age.

It’s a peaceful scene: the girl sitting sideways on a chair that is much too big for her, playing alone at the computer.

But guess what?  She isn’t just playing a game. Each time she clicks her mouse she is destroying a real planetary system.

The girl is killing real, living animals, planets and aliens. She is an eternal being.  Stars and planets are just her playthings.

Cue evil laugh…

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Resources

• Introduction to the how planets die series
• The “Nice model” instability in the Solar System.
• Stages of planet formation (MOJO video 1)
• Possible origin of interstellar objects ‘Oumuamua (MOJO video 11)
• Early instability model for Solar System formation
• A compilation of the effects and the frequency of impacts on Earth