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!


Search for Exomoons Around Kepler-22b Strikes Out (This Time)

It’s been a real pleasure to take my theoretical work on the habitability of exomoons out into reality, with the Hunt for Exomoons with Kepler (HEK) team. We’ve recently had this paper accepted for publication, where we used the Kepler Space Telescope’s data on Kepler-22b to search for an exomoon around it.

Image Credit: NASA/Ames/JPL-Caltech
Image Credit: NASA/Ames/JPL-Caltech

Kepler-22b was the first transiting exoplanet to be detected inside the habitable zone of its host star, a G type star very much like our Sun. As you no doubt know, the habitable zone is the region around a star where, if you placed a planet like the Earth, you would expect the surface water to be warm enough to be liquid, and potentially suitable for life. Sadly, Kepler-22b is not a planet like the Earth. We know this because its radius is 2.4 times that of the Earth (for comparison, Neptune is about 4 Earth Radii, and Jupiter is about 10 Earth Radii). We don’t know its mass for certain, but until now we knew that it couldn’t be any larger than 82 times that of the Earth(!).

It’s not clear at this point whether Kepler-22b is a really massive terrestrial planet, or a mini Neptune, but it’s clear that it’s not Earthlike, so its position in the habitable zone doesn’t mean very much for astrobiologists. However, if Kepler-22b had moons the same mass as the Earth, then those moons could themselves be habitable.

Exomoons are hard to detect directly, but they can be detected indirectly by studying their host planet in detail. If Kepler-22b has an exomoon of a sufficiently high mass, then we would be able to see the effects of the exomoon’s gravitational pull on the exoplanet in transit data. Transits rely on the planet obscuring starlight as it passes in front of it. The moon would show itself by subtly altering the time between individual transits, as well as how long the transits last for.

The HEK team have been searching the Kepler transit data for a few years without success (so far). This might sound a bit negative, but this lack of results still tell us something about how frequently moons of a given mass exist in the Galaxy. Kepler 22b is HEK’s first attempt to spot a moon in the habitable zone. A detection here would be pretty amazing…

Alas, we did not detect an exomoon around Kepler-22b – at least, no moon bigger than half an Earth mass. A non-detection is not as fun as a detection would have been, but a null result is a result nonetheless.  And don’t forget, the moons of our Solar System are nowhere near as big as half an Earth mass – Saturn’s moon Titan is about 0.02 Earth masses – so we can’t say for certain that Kepler-22b is moonless.  Also, this very in-depth analysis brings out extra info about Kepler-22b itself.  The upper limit for Kepler-22b’s mass is now a mere 53 Earth masses instead of 82, and its orbit is known to much greater certainty than it was before.

So why was I involved in this work? My job was to help the HEK team assess how well they can detect exomoons – in particular, how well they can determine if an exomoon is habitable. The team injected a fake moon into the transit data, and then used their software to see if they could find it. The algorithm they use doesn’t give data for one moon out as an answer – instead, it gives a whole selection of moons, with properties clustered around the “true” values. This is a way of expressing the uncertainty or error bars in the answer.

I took this selection of moons, and ran each one through the climate models I developed. This gives a series of climates, some warm, some hot and some cold. I also ran the “true” moon through the model, giving the “true” climate of the moon, which it turns out was warm.

Results from my analysis of the output of the HEK algorithm.  The "true" moon the HEK team injected into the data is warm, so the algorithm does pretty well!
Results from my analysis of the output of the HEK algorithm. In summary – the algorithm does a pretty good job!

I was pleased to find that the selection of moons produced by the algorithm produced climates that were on the whole very similar to the true climate. So, the algorithm does a very good job of determining an exomoon’s properties – good enough for us to be confident enough to determine if the moon could be habitable.

It’s just a shame there wasn’t a real moon for us to find…

What is the Habitable Zone for an Exomoon?

The search for life in the Galaxy will soon take on a new dimension.  We’ve been searching for exoplanets in the habitable zone of their parent star.  We’ve even begun figuring out how this habitable zone changes when more stars are involved.  Now, the promise of exomoons puts a new spin on the habitable zone concept.

What future exomoon hunters will find?
What future exomoon hunters will find?

The habitable zone for an Earthlike planet around a single star is conceptually simple.  Get too close to the star, and the planet gets too hot, losing its precious moisture and becoming a charred, lifeless rock.  Get too far away from the star, and even with a greenhouse effect turned up to maximum, the water on the surface freezes and animal life perishes.  Somewhere in between, the planet can boast liquid water on its surface, and provided the chemistry is right, a healthy atmosphere can be maintained.  This habitable zone is a shell around the star, which the planet needs to be inside (at least most of the time) for the planet to be “habitable”.

This habitable zone is determined entirely by the star’s radiation (and the planet’s response to that radiation).  Now if that planet is in fact a moon, then the situation gets more complicated.  There are extra sources of energy – the moon can be heated directly by the planet’s gravitational field, if the moon’s orbit is elliptical.  This is called tidal heating, and we see it in action in Jupiter’s moons.  Io is a volcanic nightmare thanks to this heating, but Europa seems to have a warm water ocean under its thick icy crust, that could support life (and it’s about time we visited).

How Tidal Heating makes Io the luxury resort destination it is
How tidal heating works.

There are also extra ways to lose energy.  As the moon orbits the planet, the planet will pass in front of the star, screening off all light on the moon for a short time.  If the moon orbits close to the planet, this will happen extremely often.

Exomoons are on the cusp of detection, as I’ve written about before, and their habitable zones could look very different to the exoplanet habitable zones.  There’s now a real drive for theoretical astrobiologists to describe these new habitable zones.  Some have taken an analytical approach – we took a numerical approach.  We took a climate model for a planetary system, and converted it into an planetary system with a moon like the Earth.  We looked at how the moon’s orbit affected its climate – just changing the direction of the orbit changes the average temperature of the moon, and causes large scale climate fluctuations (and that was without adding tidal heating).

Adding tidal heating often ruins the party.  Even making an orbit slightly elliptical will render an Earthlike moon a crispy Io…

The habitable zone without tidal heating.
The habitable zone without tidal heating…
And with tidal heating.  Blues are too cold, reds are too hot, greens are just right.  Purples are habitable but extremely variable.
And with tidal heating. Each dot is a simulation, run with a different planetary orbit (and the same moon orbit).  Blues are too cold, reds are too hot, greens are just right. Purples are just habitable but with extremely variable temperatures.

The habitable zone for an exomoon is not just about how close it is to the star, but how close it is to the planet.  And now that we are beginning to grasp this, we will be able to assess future detections of exomoons for habitability.  The number of Galactic holiday destinations just got a whole bigger…