Article curated by Holly Godwin
Following revolutionary steps in space exploration and a whole universe to explore, scientists and non-scientists alike are asking questions about distant solar systems and galaxies. Questions like... are there any other habitable planets? How many of them have atmospheres? And, most of all... is there extraterrestrial life? ...Are we, or are we not, alone?
How do we look for life on other planets?
Scientists keep a look out for ‘biomarkers’ when searching for alien life. Biomarkers are the characteristics of the Earth’s atmosphere that, when present on other planets, indicate the possibility of life. These include the presence of molecular oxygen, water, carbon dioxide and methane. While we don’t know that other planets would display all these signs, it again acts as a good starting place, and a logical indicator of possible life.
However, detecting biomarkers on distant exoplanets is hard because they're closer to their own star than Earth. This means the light we observe from these planets is often masked completely by the radiation emitted from their star, making analysis far more complex. Even when a planet is directly observable, an extremely high resolution telescope is required to distinguish between the light from the planet and the light from the star. To a lesser extent, radiation from other distant objects can also make it harder to distinguish what radiation belongs to the planet: even our own atmosphere can interfere and confuse results!
The resolution of a telescope is directly related to its size, and ground-based telescopes are limited by the fuzzy atmosphere. Technological progress has allowed us to improve images after capturing them, but still not enough to produce pictures of exoplanets that we can always draw information from. Space based telescopes require far more funding and, given the challenging environment, are far harder to implement.
Without progress in telescopic imaging, it is not always easy to detect these biomarkers and, consequentially, the search for life remains challenging.
Learn more about Biomarkers on Exoplanets.
The thickness of the ice covering the surface of Europa is still an unknown. Europa is thought to have either a thick layer of ice with areas where liquid water has reached the surface or a thin layer of ice with liquid water underneath. Water flows up through the cracks in the surface caused by asteroid impacts and gives the surface a very young appearance. Scientists have agreed that these large cracks along with evidence of movement of large areas of terrain and the water smoothing out the surface all suggest that there is a sheet of ice with liquid water underneath. Temperatures on Europa’s surface range between 53K and 111K but liquid water is enabled through a volcanic inner layer or tidal heating due to Jupiter's gravitational effects. We won’t know the thickness of the ice until a spacecraft can be sent to get a closer look.2
Methane has been detected in the martian atmosphere both by Earth-based telescopes and by the European Mars Express mission. The lack of a magnetic field around Mars means that this methane can be easily destroyed by cosmic radiation. Therefore the fact that its abundance seems to remain fairly constant implies methane production on Mars. But how?
Methane is usually associated with organic processes (as the output of processes involving life), however there are other means by which it can be produced. These include geochemical mechanisms such as a process known as serpentinisation, as well as the recently demonstrated release of methane from meteorites bombarded with UV radiation under Mars-like conditions.
More data are needed from the regions of the Martian surface where this methane has been detected to begin to determine it's source; however, the issue is clouded by the fact that these processes are not mutually exclusive, and so even if we can identify one source, it does not rule out the possibility of methane being produced simultaneously another way.
Is there life on Mars? Probably not, though if there's water at the poles and possibly underground, it remains a possibility. If there is, it's likely in the form of microbes rather than anything more advanced.
Perhaps a better question though, is was there ever life on Mars? Since we now believe there was once liquid water on the surface, this is a very strong possibility, but we have no evidence of it yet. This question is a driving force behind several Mars missions from Curiosity (NASA) to ExoMars (ESA).
Just as it has on Mars, methane has been detected in the atmosphere of Saturn's moon Titan. These observations have been confirmed both by Earth-based telescopes and the Cassini mission, which has also spotted shallow lakes of methane around the moons tropics. Calculations suggest that this methane ought to have been broken up by cosmic radiation within 50 million years or so (a very short time when compared to the age of the Solar System itself). Therefore it's present abundance seems to indicate that the methane in the atmosphere is being replenished from a source on Titan itself. Methane is usually associated with organic processes (as the output of processes involving life); however. there are other potential sources. These include cryovolcanoes (volcanic type features which release water and other gas/liquids with low melting points on colder worlds than our own). Titan has a thicker atmosphere than Earth, layers of which are almost entirely opaque to visible light. As a result, we have no good images of the Titanian surface, and are thus unable to say for sure what the source of the methane on Titan is.
Detecting habitable planets
Each solar system has a theoretical 'zone' where the atmospheric pressure and temperature of the planets within is just right for water. These are known as Circumstellar Habitable Zones or, more colloquially, Goldilocks Zones. This is used as an indicator for possible alien life, because, for life as we know it, water is a necessity.
A planet is determined to be within or outside of the Goldilocks Zone depending on its orbit, its mass and the amount of power radiated by the star its orbiting. Being within the Goldilocks Zone does not guarantee the presence of water, just defines whether conditions are suitable. There are also other factors necessary for an ‘Earthlike’ planet such as atmosphere, and it rules out celestial bodies with water that are outside a Goldilocks Zone. The method is considered a good starting point for the search for alien life.
Many stars have a planet within their Goldilocks Zone. In fact, some have been observed with multiple planets within the zone. This again seems to support the idea that multiple ‘Earthlike’ planets exist.
Some doubts have been raised about the validity of Goldilocks Zones, as surface water is thought to have been detected on celestial bodies outside a zone, most commonly via absorption spectroscopy – a method of analysing the light from a body to determine what the body is made of. The water present on these planets can be maintained by processes such as tidal heating and radioactive decay, or pressurised by other means. It is also thought to be possible for water to be present on rogue planets or their moons. Because these planets are migrating through cold outer space, any water present is assumed to be frozen a lot of the time, and thus inaccessible for life (but we don't know).
So while these zones may be a good place to start the search for extraterrestrial life, maybe we should not limit ourselves to searching solely within their scope.
Learn more about Circumstellar habitable zones.
Scientists have been discovering planets outside of our own solar system since 1988, and the rate at which these have been found has increased exponentially across these years. The nature of planets is varied, and the possibility of any of them supporting life depends upon several factors.
Not all stars have the exact same elemental make-up, however, containing different quantities of heavier elements (than hydrogen and helium). The different composition of these stars directly affects the rate at which they burn, and so the lifetime of the star. This, in turn, affects the position and lifetime of the region around the star in which liquid water could exist. The composition of these stars may also affect the nature of the atmosphere and geological evolution of their planets, and thus how suitable these may be for life.
So far, only extremely large planets far bigger than Jupiter have been directly imaged. We are actually limited by our technology, unable to image most exoplanets directly.
Learn more about direct imaging of exoplanets.
Is all life dependent on water?
For life as we know it to be viable, water is a necessity. Humans are composed of 60% water and cannot survive without it for more than a few days. Some species need less, and are less dependent on water, but we don't yet know of any that don't need it at all. However, this does not necessarily mean that life is dependent on water universally. We just don’t know how life might have developed elsewhere. It is possible that other life forms may have evolved with alternative biochemistries, reliant on other elements altogether. Scientists are fairly certain that some form of liquid is necessary to life... but this isn't something they know how to prove or disprove.
A further puzzle we are yet to answer is how does life begin on Earth or other planets? While speculations have been made about how life begins, we are yet to find an overruling theory.
Learn more about dependency of life on water.
Life in the universe – the Fermi Paradox
The Fermi Paradox states that there is a contradiction between the estimates of probability of civilised alien life, and the lack of supporting evidence. Many people have attempted to come up with explanations for this paradox, but none have supporting evidence, and so the question still stands unanswered.
Given that there are billions of stars that are older than our own sun, with orbiting planets far older than our Earth, and assuming that our Earth is by no means extraordinary, we reach the conclusion that it is very likely there will be other Earthlike planets in the observable universe. The observable universe is the universe that can, in principle, be viewed from Earth at this point in time. For anything further away, there will not have been time due to the expansion of space (since the Big Bang) for the light to have reached Earth. It spans over 93 billion light years, giving us an estimate of at least 100 billion galaxies within observable range from Earth. These Earthlike exoplanets could've supported life for much longer than Earth, and thus the evolution of their life forms and civilizations could be leaps and bounds ahead of our own.
Extrapolating our own species' progress in space travel and exploration, we could colonise a galaxy in tens of millions of years, so if these planets are billions of years older, it follows that they could be far further along in the field of space exploration than us, and could quite possibly be fully aware of our existence. And yet... we have no evidence for their existence. It is also possible that intelligent life arises frequently in the universe, but invariably faces extinction before technology has advanced sufficintly that contact can be made.
Learn more about Fermi paradox.
The origin of life
Despite the fact that we think life needs water, RNA is unstable in water unless the water has a high enough boron content. Boron stabilises ribose, the “R” out of RNA. However, it is believed that when life began on Earth, there was hardly any boron in the water. So this leaves us asking, how did life begin?
One hypothesis, Panspermia, considers the possibility that life in fact began outside our Earth. The most likely scenario for this hypothesis involves life originating on Mars. It is thought that boron was far more plentiful on Mars and this is supported by the fact that boron was found on a Martian meteorite. While this may seem far fetched, the idea that basic Martian life could have travelled over on an asteroid and evolved on Earth at least answers the RNA water problem. So are we in fact the long lost descendants of Martians, or is there something we’re missing?
Learn more about Location of the Origin of Life.
We can clearly identify polar ice caps on Mars using ground based telescopes, and measurements made by the Mars Express satellite and the Mars Reconnaissance Orbiter have been able to largely quantify what portion of these is water ice (the majority of the rest being frozen carbon dioxide). As for water elsewhere, the Mars Global Surveyor has made measurements of the amount of water in the Martian atmosphere. Any other water must be trapped in or beneath the rock. Measurements have been made of rocks and soil from the Martian surface which strongly suggest the presence of water in the ground at some point in the past, as do a number of geological features observed from satellites. The greatest challenge facing scientists when finding water on Mars is that water may be trapped not only in the permafrost (permanently frozen soil) to a significant depth, but also much deeper underground in large frozen reservoirs.2
Ganymede, Jupiters largest moon, may actually be composed of several layers of salty waters and ices – much like a club sandwich. Because of the huge pressures thought to exist on Ganymede's ocean floor, scientists had previously believed that the layer next to it's rocky core would most likely be ice. This was a problem for the idea that primitive life may have formed on Ganymede at some point in it's history. Rock/liquid water interfaces important for the potential development of life – many scientists believe that life on Earth most likely began at hydrothermal vents on our own ocean floor. The new findings from this study of Ganymede imply that instead of the core being next to water – because of the high salinity, denser fluid water would sink toward the core in Ganymeade allowing water and rocks to interact. By using computerised simulations and models, scienitists have concluded that a likely composition of Ganymede's ocean is water sandwiched between up to three ice layers, with a salty liquid water layer next to the socky sea floor.3
As part of the search for extra-terrestrial life, cosmochemists are exploring the chemical composition of other worlds – from interplanetary dust particles to samples returned from space missions. Asteroids with similar violent chemical pasts to our planet or high carbon contents are particularly promising as these have the potential to develop similar atmospheres and oceans to the Earth. By comparing the geology – or historical rock record of our planet – to meteorites, we can piece together a much further back Earth history which has since been destroyed by the rock cycle, but preserved in frozen meteorite samples. Cosmochemists use elemental analysis techniques, such as mass spec to determine the compositions of these space samples.
Maybe we are yet to come across alien life, because Earth is fundamentally unique. However, with probability in our favour, it’s likely we will stumble across alien life eventually. The next question will be, do we want to?
This article was written by the Things We Don’t Know editorial team, with contributions from Andrew Rushby, Ed Trollope, Jon Cheyne, Cait Percy, Grace Mason-Jarrett, Rowena Fletcher-Wood, and Holly Godwin.
This article was first published on 2017-11-26 and was last updated on 2021-05-16.
why don’t all references have links?
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 Kawahara, H., et al., (2012). Can ground-based telescopes detect the oxygen 1.27 μm absorption feature as a biomarker in exoplanets? The Astrophysical Journal 758(1):13. doi: 10.1088/0004-637X/758/1/13.
 Grossman, L., Webb, R., (2013). Martian chemistry was friendlier to life. New Scientist 219(2933):14. doi: 10.1016/S0262-4079(13)62173-9.
 Formisano, V., (2004). Detection of Methane in the Atmosphere of Mars Science 306(5702):1758-1761. doi: 10.1126/science.1101732.
 Clark, R,N., et al., (2010). Detection and mapping of hydrocarbon deposits on Titan. Journal of Geophysical Research 115(E10). doi: 10.1029/2009JE003369.
 Leshin, LA et al. Volatile, Isotope, and Organic Analysis of Martian Fines with the Mars Curiosity Rover. Science 341.6153 (2013): 1238937. doi: 10.1126/science.1238937.
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