Astrobiology 104: Looking for Life Beyond the Habitable Zone

This post was created as part of my crest project in 2020. You can read the original here or watch it as a youtube video!

Last week, we learnt about the possibility of life existing around the sun’s habitable zone. Although looking only within the typical habitable zone is an easy method of elimination when it comes to using precious telescope time, this narrow minded approach is becoming less relevant as we discover more about the conditions in the outer solar system. Read on, and find out how missions to the outer solar system have changed our views on how to search for life!

Gas Giants

It is highly unlikely that life will be found on the gas giants, due to the fact there is no real surface for life to live on! However, Saturn and Jupiter are surrounded by an array of moons, some of which (Ganymede, Titan) are bigger than our smallest planet, Mercury! Although some of these are little more than glorified asteroids, quite a few have very interesting features…


Europa is a tidally locked moon of Jupiter, and is approximately the same size as our moon, but in my opinion, it is much more exciting! Why? It has an icy surface, a thin oxygen atmosphere (too thin for humans to breath, and probably not biological in origin) and a saltwater ocean that could hold up to twice as much as all the water in Earth’s surface!

Its surface is crisscrossed with fractures and irregularities, suggesting that its icy surface is being broken apart and renewed as a result of convection in its subsurface ocean! This idea is reinforced by it’s relatively young surface- less than 100 million years old!

The reddish lines on its surface are possibly from Sulfate impurities in the ice- evidence that at least some of the compounds needed for life are present.. This, along with the fact it has a hot core and some sort of magnetic field means that it is one of the best possible places for life to exist!


Similar to Europa, but much smaller, Enceladus’ orbit of Saturn remains elliptical due to the gravitational tug of war with Dione, a nearby moon in orbital resonance. This means that Enceladus is stretched to different extents depending on its distance from Saturn, causing tidal heating!

It is thought that this heating is what keeps its subsurface ocean molten. What is really cool about Enceladus is that it has pretty much constant jets of water and gas erupting from its south pole. This then falls back on its surface as snow, keeping its surface fresh- the reason why it is so reflective! We can also see only a few craters from asteroid impacts, further evidence for it being continually resurfaced, partially by the jets.

By flying a probe through the jets, which we did with Cassini which orbited Saturn from 2004 to 2017, we can take samples of the inner ocean! These samples have since been analysed, showing that its ocean is made of nutrient rich salty water, and contains the organic compounds needed for life! Like Europa, Enceladus is a prime candidate for extra-terrestrial life!


Titan is the second largest moon in the solar system, one of the two that is bigger than Mercury! It is really interesting as it is the only moon with a substantial atmosphere! It’s atmosphere is Nitrogen based -like Earth’s- and has surface features reminiscent of Earths’ as a result of its methane cycle, which acts like our water cycle but at -180°C, to form a seasonal system of lakes and rivers made of methane and ethane!

It also may have cryovolcanoes- volcanoes that release torrents of water ice and ammonia from a subsurface layer of ammonia rich water.

All these processes and features result in Titan being a decent candidate for life, as we already know it has organic compounds on its surface, and that complex molecules like DNA could form under its current conditions. However, one of the biggest obstacles is that, at least on the surface, liquid water is practically non-existent, and all life on earth requires water as a solvent.

This might not be totally prohibitive, methanogenic life forms (life that uses methane as it’s base) has been proved hypothetically possible, at least for microbial life- now it’s just a case of proving its existence on Titan!

Callisto, Ganymede and Dione

There is evidence that these moons all have a subsurface liquid ocean, probably of very salty liquid water. So it is possible that if these oceans have an energy source from the interaction of its hot core and the bottom of its ocean (eg. volcanism/ vents) that salt loving (halophilic) organisms could exist here.


Triton is the largest unmapped solid body in the solar system, with only 40% of its surface mapped, with data from 30 years ago! From the small amount of information that Voyager 2 gave us, we can tell that Triton, the largest moon of Neptune, has active cryovolcanoes on its surface, suggesting an at least semi-liquid subsurface ocean, where life could exist. Personally, I don’t think it is likely that we will find life here, but it is still a moon worth visiting again, if only to gain information on the creation of the solar system!



Io’s chances of hosting life are quite small, but it is a reaally cool moon and I would like to talk about it, so: Io is the closest orbiting moon of Jupiter and this, combined with the gravitational pull of the other Galilean moons, causes the moon to be under immense tidal forces. This stress means that it is the most geologically active body in the solar system by far, with over 400 active volcanoes, some of them powerful enough to launch silicate lava and sulphur compounds 500 km above its surface!

These plumes are so large that after initial photographs of Io were studied, scientists thought there was an undiscovered moon hiding behind it!

Although most of its features are volcanoes, it also has mountains, some nearly double the height of Earth’s Mt. Everest! Another consequence of its proximity to Jupiter is that it has the least water of any solar system object, probably due to it all being boiled away by Jupiter’s heat when it was forming. In addition to its lack of water, it has basically no atmosphere, as most of what is produced is immediately stripped away by Jupiter’s magnetic field, resulting in a very uninhabitable environment!

An eruption leaves a scar on Titan

Objects of the Outer Solar System

I can hardly forget about Pluto! Although no longer considered a planet, and the minor fact that the light it receives from the sun is 1/900 the brightness of the sun from earth, the Pluto/Charon Binary System is quite interesting!

Our moon orbits around the Earth, with the barycenter (the point around which the moon and earth move), firmly beneath the Earth’s crust. But for Pluto and Charon, this point is slightly above pluto’s surface. On Earth, the tidal forces are only strong enough to move water, but for the Pluto-Charon system, this results in large tidal forces on both objects, which means that both probably have a liquid water subsurface ocean, which powers their cryovolcanism!

The orbit of Pluto (centre), and its largest satellite, Charon

There are lots of dwarf planets and asteroids further out in the solar system, including the possible planet 9, but it is highly unlikely we will find life so far from the sun. 

Then again, our perspectives regarding what places could be habitable broaden each time we venture out into the solar system and find out more about where we live! 

Do you think we will find life in the solar system? Where would be your number one place to look? Is there even any point in searching for life, regardless of its existence?

Habitable Zone- the area around a star where liquid water can exist on a, earth-like planet’s
Gas Giant- a large planet of relatively low density consisting predominantly of hydrogen and
helium, such as Jupiter and Saturn (Uranus and Neptune are a subtype of gas giants- the
ice giants)
Tidally Locked- When one side of a planet/moon always faces the body it orbits
Orbital Resonance- when orbiting bodies exert regular, periodic gravitational influence on
each other, usually because their orbital periods are related by a ratio of small integers.
Cryovolcano- a type of volcano that erupts volatiles such as water, ammonia or methane,
instead of molten rock
Tidal Forces- when a planet/moon is stretched by the difference in gravitational pull of a
nearby object between the near and far side.
Halophilic- an organism that thrives in salty conditions
Dwarf Planet- a celestial body resembling a small planet but lacking certain technical criteria
that are required for it to be classed as such. Eg. Pluto is a dwarf planet as it has not cleared
out the debris in its orbit