Squishier moons are better for life

My more eager readers will have noticed a sudden flurry of submissions to the arXiv since Christmas. I’ll try and bring you up to date with what I’ve been publishing recently, which is hitting a variety of topics.

The first is a return to a favourite area of research for me: exomoon habitability. As you can see from earlier posts, I’ve been looking at this subject for a while now, focusing in particular how an Earthlike object would fare orbiting a giant planet, which in turn orbits a star like our Sun.

I’ve been using a simple, 1D climate model to follow the temperature changes on this Earth copy, and discover what sort of orbital parameters might be needed for it to possess surface liquid water. But we’ve known from the start that these climate models have been overly simple, and in some cases they’ve missed out important physics that might affect our answers.

In our latest work, we took aim at two aspects of the model that we felt were lacking. In the first, we investigated the issue of atmospheric composition. Up until now, we had assumed a fixed composition, but we know that the Earth has adjusted its atmosphere over time. Sometimes, this is due to the presence of life (like the great oxygenation events that are responsible for the good stuff filling your lungs), but other processes play a role.

In fact, we can identify a complete cycle of processes that affect the total amount of carbon dioxide. CO2 is emitted into the atmosphere via volcanic eruptions. It then precipitates into rain, which falls into the sea, and gets incorporated into rocks and seashells on the ocean floor. This ocean floor is eventually sub ducted at a tectonic plate boundary, and returned to the mantle, where the whole cycle begins again.


It’s known as the carbonate-silicate cycle (because the rocks in play are carbonate and silicate rocks). What’s rather clever about this system is that it acts as a thermostat. If the planet starts to warm, then more Co2 rains out of the atmosphere and into the oceans than is expelled by volcanoes, which reduces the amount of Co2 in the atmosphere. As Co2 is a greenhouse gas, getting rid of it allows the planet to cool more easily. If the planet cools, less co2 rains out and is added to by volcanism. A little extra greenhouse gas helps the planet keep its heat. This is why scientists are worried about manmade co2 production, as we’re mucking around with the thermostat, and too much fiddling could break it.

We never considered this process before, so we changed our model to allow the co2 levels of our moon to vary to help keep the temperature warm and stable.

We then turned to the tidal heating of our moon, which until this point was done using a rather simple model. As you may already know, tidal heating is generated by the planet’s gravity stretching and squeezing the moon as it goes around its orbit. Crucially, the amount of heat the planet can generate in the moon depends on the material the moon is made of, as well as what state it’s in. If the heating is intense enough to allow melting, this can reduce the tidal heating, and stop moons from becoming too hot for life.

We found that when we added both effects, the habitable zone for the moon moves further away from the star.  It also gets wider around the planet as well – moons can orbit closer without being roasted, and changing CO2 levels lets the moon stay warm further away from heat sources by boosting its greenhouse effect.

We’re far from the final answer here: one day I hope to be telling you about fully 3D models of exomoon atmospheres.  Even 1D models like ours still need some extra physics, like investigating how changing the spectrum of incoming radiation affects this answer.  But every step is a step forward!


Galactic Habitable Zones Are Messier than You Think

In the search for life in the Galaxy, an important question to ask is: Can we predict which parts of galaxies are more likely to have habitable planets? If we knew the answer, we could target our search at the right parts of the sky, and we could assess the odds for life appearing in galaxies throughout the cosmos.

In an attempt to answer this question, the concept of the Galactic Habitable Zone was established. This is built from considering two factors that affect habitable planets: the availability of raw materials for planet construction (what astronomers refer to as metallicity, or “everything that isn’t hydrogen and helium”), and the chances of being hit by high levels of damaging ultraviolet radiation.

The Earth’s atmosphere screens the worst of the Sun’s ultraviolet rays, but if we were swamped by radiation from say a nearby supernova, that radiation could deplete the ozone layer, and cause a mass extinction. Some astronomers and geologists think there is some evidence that Earth’s geological and biological history bears the scars of such supernovae, but this is far from clear.

These two factors have a common link: star formation. Most of the heavy elements which make up planets like Earth, and all the life-forms along for the ride, are forged in the cores of stars. The elements heavier than iron are born in supernovae and other massive explosions that occur at the end of a massive star’s life. So local star formation is a blessing and a curse for habitable planets: we need several generations of star birth and death to give us the ingredients for planets, but vigorous star formation results in supernovae which can be deadly if too close.

In the classic galactic habitable zone picture, this means that the interior of galaxies are ruled out because the local radiation is too hazardous, and the outer parts are too metal poor to form habitable planets. Somewhere in the middle, things are best, a Goldilocks zone that is reminiscent of the Goldilocks zone around stars (although they exist for quite different reasons).  The Galactic Habitable Zone is therefore an annulus, a ring around the Galaxy’s centre, which the Sun happens to reside in.

The classic galactic habitable zone.  In the centre, there is too much hazardous radiation.  On the outskirts, there isn't enough heavy elements to form habitable planets.  Note the Sun nestled inside the zone
The classic galactic habitable zone. In the centre, there is too much hazardous radiation. On the outskirts, there isn’t enough heavy elements to form habitable planets. Note the Sun nestled inside the zone

However, this classical picture assumes galaxies are nice geometric shapes. And they ain’t. They grow through collisions with other galaxies, and tear up satellite galaxies into streams of stars and gas before gobbling them up entirely.

So in our latest paper, we tried to look at how this messy picture of galaxy formation affects the galactic habitable zone. We took a cosmological simulation of the Local Group, our intergalactic neighbourhood which includes The Milky Way, Andromeda and Triangulum galaxies.  These simulations model the dark matter distribution on large scales, and the gas on small scales, with some of the gas becoming dense enough to form stars, which can then enrich the surrounding gas with heavy elements when they die.  We took the data from these simulations, and used standard galactic habitability criteria to measure the GHZ.

The cosmological simulation of the Local Group we used for our habitability calculations. The orange-red colour indicates high levels of gas density.

And it’s quite a mess! The GHZ isn’t nice and symmetric at all. Those big streams of stars and gas I mentioned? Quite suitable for habitable planets apparently.  As are those satellite galaxies before they’re gobbled up.  The best places to live in galaxies shifts quite a bit over time, as the whole edifice is assembled through all those collisions and mergers.  Each galaxy has its own path to tread towards producing habitable planets.

The habitability of Triangulum (M33).  The redder the colour, the more habitable planets present.  The centre looks very red, but this is only because the stellar density is high.
The habitability of Triangulum (M33) over time. The redder the colour, the more habitable planets present. The centre looks very red, but this is only because the stellar density is high.

In the animation above, it looks like the very centre is the most habitable, but really the centre has the highest concentration of stars.  Each star has far less habitable planets, but this makes a lot of habitable planets if we cram enough stars together.  The best spots are further away from the centre typically, but this changes quite a bit as the galaxy is built.

But this is only really the beginning.  What we discovered in this work is how inadequate our understanding of the problem really is.  We still have lots of questions about how supernovae can sterilise a planet, and we haven’t made a big enough census of exoplanets to really nail down the relationship between metallicity and producing habitable planets.  There are other things that can make parts of the Galaxy uninhabitable, which we weren’t able to look at in this paper – if stars are too crowded together, they can knock habitable planets away from their host (which would make the galactic centre very bad); gamma ray bursts can explode much more powerfully than supernovae, with potentially more devastating results.

What we do know is that whatever the true Galactic Habitable Zone of the Milky Way is, it ain’t an annulus.

The Fate of Top Down Planets

Avid readers may remember a few years ago I blogged about the statistics of objects formed by disc fragmentation. In our paper, we looked at tidal downsizing, to see if big fluffy disc fragments with masses bigger than Jupiter could form solid cores and lose their gas to become objects of Neptune, maybe even Earth masses.  We created what’s known as a population synthesis model – this is essentially a way of running lots of simulations very quickly to get lots of output data, in our case lots of fragments which have evolved to form objects (or not, as the case may be).

We discovered that making terrestrial planets is very hard – after producing millions of disc fragmentation events in our model, only one of those objects went on to make an Earthlike body. In fact, we lost about half of the fragments to tidal disruption – they were ripped to shreds by getting too close to their star before they had become fully formed. Most of the surviving fragments were giant planets or brown dwarfs orbiting at about Neptune’s distance from the Sun. 

Our one in a million Earthlike planet formed by tidal downsizing, all alone at the bottom of the plot.
Our one in a million Earthlike planet formed by tidal downsizing, all alone at the bottom of the plot.

But, our model was incomplete. We wanted to run millions of simulations, so we had to simplify the physics quite a bit. This is a common problem for population synthesis models, as they need to run to completion without it taking millions of years! The only way to beat it is by using clever algorithms and lots of supercomputer power. 

One of the most important pieces of physics we jettisoned early on was fragment-fragment interactions. Our discs were fragmenting to form multiple objects with relatively strong gravitational fields – they should be pulling on each other and changing their orbits.   Also, the systems were likely to still be in their parent star cluster. All those nearby stars would have a gravitational effect as well! 

Instead, our fragments coast along on nice circular orbits.  We didn’t add gravitational interactions to the model because it would have needed extra computational power, and would have taken much longer to produce results. Also, we knew that in the simulation, the disc would have been the biggest nearby source of gravity, and would have acted to smooth out these orbital changes. 

But eventually, the disc will disappear. Then what happens? We decided to run our models through N Body simulations to find out. With the help of Richard Parker, we placed our systems into his star cluster simulations, and we ran separate simulations to find out whether the planetary systems the model produced were stable. 

An example of an unstable planetary system produced by the population synthesis model.  Four bodies orbit the star initially - two bodies get launched away from the host star, leaving two in stable orbits.
An example of an unstable planetary system produced by the population synthesis model. Four bodies orbit the star initially – two bodies get launched away from the host star, leaving two in stable orbits.

So what did happen? One thing that happened quite a lot (about 25% of the time) is that a planet or brown dwarf gets ejected from the system. This means that if disc fragmentation happens regularly, we should see lots of free floating planets! Many of the bodies still orbit quite far from the star, but they now have very eccentric orbits. We know that some exoplanets have very eccentric orbits, like HD 8606b, so that is a sensible outcome.  Our simulations can now give us some predictions for what we should see with the next generation of direct imaging surveys, and we are working with observers to figure out whether our predictions match up with what they are now seeing.

Some, but not many, of the bodies get pushed very close to the star, probably close enough to become Hot Jupiters. This is where it gets interesting, because these will look very similar to Hot Jupiters formed by other planet formation mechanisms, like core accretion. How do we tell them apart?  This is something we’re working on right now, and I hope to tell you more about it soon.