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

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) 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.

Life on Tattoine Planets: More Complicated Than You Might Think

We all know that life on Earth depends on the Sun, and not just to keep the cold out. Plants convert sunlight into energy through photosynthesis, giving us oxygen to breathe, and in one way or another, the whole food chain relies on our parent star for support.

But what if our solar system had two stars? We know that there are several exoplanet systems out there that have the luxury of multiple host stars. In the case of the Kepler 47 system, there is a planet in the habitable zone (confirmed by several teams of astrobiologists, including myself). Kepler-47c orbits a star quite like our Sun and a cool red dwarf star (see graphic below), and so we refer to it as a circumbinary planet.

The Kepler-47 system, compared to our Solar System. Credit: NASA

But what would it be like to live on a planet like Kepler-47c, and gaze at a double sunset like Luke Skywalker? Would life be all that different? In a paper recently accepted for publication by the International Journal of Astrobiology, we explored this question.

Strictly speaking, it would probably be quite hard to stand on Kepler-47c, as it’s probably a gas giant comparable to Neptune, so we imagined that Kepler-47c was in fact an Earthlike planet – after all, chances are that there is a planetary system out there like this. We considered how the radiation from the two stars hit the planet’s surface, mapping patterns of light and darkness.

Because the two stars in the Kepler-47c system are so different in mass, they produce radiation at very different wavelengths – the sunlike star emitting a spectrum that terrestrial plants would happily photosynthesise, and the other star emitting much more red and infrared radiation, which some forms of anaerobic bacteria would photosynthesise (see more here). So depending on the time of year and time of day, different organisms would take the lead in converting starlight to energy.

But it’s not just the light patterns that are interesting. The darkness patterns show that above the polar circles (on earth, these are the Arctic and Antarctic circles), summer and winter become rather peculiar.

Above the arctic circle on earth, winter begins when the sun sets, and stays set until winter ends a few months later. On our Tattoine planet, there are two stars in the sky, so the arctic winter begins when both stars drop below the horizon. But the planet orbits the centre of mass of the system, as do the stars. This means that depending on the arrangement of all three bodies, some years have a winter that is a few days too short, and others have winters that are a bit too long. If you’re an animal counting on the end of winter to end your hibernation cycle, you need to know whether this year’s winter will be long or short!

Judging by life on earth, it seems likely that animals will be able to develop instinctive and biochemical rhythms to cope with these fluctuations, just as we have circadian rhythms to cope with day and night time. In fact, some organisms on Earth  already obey the influence of a second star – except it’s not really a second star, it’s just the Moon!

In short, life on circumbinary planets will be a slave to the rhythm, just like life on Earth. But there will be many more rhythms to choose from!

Life on Planets where 3 Spins = 2 Orbits

It’s been my pleasure to help on a recent paper to be published in the International Journal of Astrobiology, about how life might be on a planet with a peculiar spin.

Imagine a world where the planet’s spin was so slow, that one day took two thirds of a year.  Well, actually we don’t have to, as we can see a world in our Solar System that does this – Mercury:

Now, it’s immediately obvious that Mercury is extremely inhospitable, as it is so close to the Sun, and has no atmosphere to control its temperature (Mercurian days are 600 degrees C hotter than Mercurian nights!).  It is because Mercury is so close that it has this unusual relationship between its day and its year.  The Sun’s gravity causes tidal forces that twist and crush the planet, slowing down its rotation.  It just so happens that these tidal forces act in a rhythmic way, just as people do when they push each other on swings.  This rhythm allows the planet to enter what is known as a 3:2 spin-orbit resonance.  This means that there are 3 spins for every 2 orbits, 3 days for every two years!

Now imagine we take a planet like the Earth, and put it around a dim star.  For it to still be warm enough for liquid water, we have to put this pseudo-Earth closer to the star, close enough that it might fall into one of these 3:2 spin-orbit resonances.  What would it be like for life?

This is what we set out to discover.  Firstly, we had to think about how the sunlight would be distributed across our planet’s surface.  Now on a planet spinning quickly, it doesn’t matter whether you live in the West or the East, you get the same amount of sun.  Not on this 3:2 world:

2 years of sunlight on our planet’s surface. This planet’s orbit is elliptical, like Mercury’s.

When the planet’s orbit is elliptical, the sunlight tends to fall in hotspots.  This is because the star undergoes retrograde motion on the sky – this means that depending on where you stand on the planet’s surface, the star can rise in the east, change its mind, and set in the east! This happens because during an elliptical orbit, the orbital speed changes quite a bit, so sometimes the speed of spin outpaces the orbital speed, and sometimes it doesn’t.

Thanks to this (and the planet moving closer to and away from the star), the amount of light received at a point on the planet’s surface varies drastically, and according to a very unusual schedule.  Plants trying to use sunlight to carry out photosynthesis will need to take heed of this schedule, working frantically while the sun is up, and laying dormant for a very long time during prolonged periods of darkness.  The circadian rhythm for life on Earth is set to around 24 hours, and easily readjusted when it goes out of sync.  Imagine how complicated circadian rhythms would be on our imagined planet!

So what’s the point of all this? Well, we know that small dim stars are much more common than stars like our Sun, and we are getting closer to identifying Earth-sized worlds in the habitable zone of these stars.  So far, the only world we know of in a 3:2 resonance is Mercury, but that could soon change.  And when it does, we’ll continue our work, thinking carefully about how we might detect signs of life on these worlds.