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Life

See also our review of planetary protection for how we might respond to the possibility of life. Extremophiles

Extremophiles are organisms that live in conditions that are harsh for most life. The recent discovery of extremophile organisms in the harshest ecological niches on Earth raise hopes that life might also be found elsewhere in the Solar System.

Extremophiles

Extremophiles are organisms that live in conditions that are harsh for most life. The recent discovery of extremophile organisms in the harshest ecological niches on Earth raise hopes that life might also be found elsewhere in the Solar System.

ExtremophileConditionsEarth HabitatRelevant Environment(s)
PsychrophilesLow temperatureSnow, ice, sedimentIce shells of Europa and Enceladus; poles of Mars
HalophilesHigh salinitySea ice inclusions, saline lakes, evaporation pondsSubsurface oceans of Europa, Titan, and Enceladus
PiezophilesHigh pressureHydrothermal vents of the ocean floorOcean floors of Europa (hydrothermal)
XerophilesLow water activityAtacama desert, rock surfaceSurface of Mars
Radiation-tolerant microorganismsHigh radiationNuclear reactor water, coresSurface of Europa
ChemolithotrophsLiquid hydrocarbon matrixPitch Lake, oil seepsHydrocarbon lakes of Titan

Source: 1.

Life in the Solar System

So far, there is no widely accepted evidence of life in the Solar System outside of Earth. The most promising candidates for the existence of such life are suggested as follows.

Celestial BodyMost Likely LocationEvidenceChallenges
VenusUpper AtmospherePhosphine gas, lacking clear abiotic production methods, was detected 2, though this is disputed 3.Lack of water 4
MarsSubsurface, where liquid water exists 1.Contemporary water and methane atmosphere 5.Radiation 6, toxic perchlorates 7, saltiness of water 8, low atmospheric pressure 9
Europa (moon of Jupiter)Subsurface Ocean 1Plumes of water and organic molecules found 1Radiation from Jupter 1
Enceladus (moon of Saturn)Subsurface Ocean 1Plumes of water found erupting from surface 1.
Titan (moon of Saturn)SurfaceBasic elements of life (C, H, N, O, P, S) found 1.Lack of liquid water 1.

Mercury 10, the asteroid Ceres 11, Jupiter's moon Ganymede, Saturn's moon Dione, Neptune's moon Triton 1, Jupiter's moon Callisto, the Kuiper Belt 12, and smaller icy objects with subsurface oceans 13 are also of astrobiological interest.

It has been suggested that NASA's Viking missions in 1976 found evidence of life 14, but this is considered unlikely 15, 16. There is also inconclusive evidence that life from Mars has been detected in the meterorite ALH84001 17. It may be possible for microbial life to move between planets via panspermia 18, 19, 20, which suggests that if life is found elsewhere in the Solar System, it might still be related to Earth life.

Life outside the Solar System

Even if the conditions for life are rarely met, the universe is vast, creating great uncertainty as to just how much life there exists. Much extrasolar astrobiological research focuses on the circumstellar habitable zone, which is the region around a star where Earthlike planets can retain liquid water on their surface 21. Even within the habitable zone, there are potential obstacles to life taking hold, some of which are reviewed below.

LocationChallenge for Life
Around M-drawf (red dwarf) stars, the most common type of starSevere solar flares and tidal locking 22.
Around large starsToo much UV radiation 23, short stellar lifetimes.
Large galaxiesSterilizing events and too many gas giants 24.
Too close to the galactic centerSterilizing events such as X-ray bursts and supernovae 25.
Too far from the galactic centerInsufficient heavy elements to form rocky planets 25.
Multiple-star systemsDifficulty forming stable orbits and climates 26.

Even if a planet is in the right spot, it must have plate tectonics 27, must avoid runaway greenhouse effects or freezing 28, and must satisfy other conditions, in addition to factors of chance 29. For these reasons, the rare Earth hypothesis has been put forward, which posits that life more complex than the most simple prokaryotic cells is rare enough that humanity is unlikely to ever detect it 30.

In contrast to this, recent research suggests that cold planets can maintain subsurface oceans, heated by radioactivity or other forces. This may hold even in rogue planets that are in interstellar space and not in orbit around any star 31. Such rogue planets may outnumber planets that orbit stars 32. Hycean planets, or large ocean worlds, appear to be common and may be another promising home for life 33.

Several ongoing and upcoming missions are intended to search for biosignatures, indicators of the presence of life, on exoplanets. One of them is the James Webb Space Telescope, launched in December 2021. It will look for spectral lines from exoplanets suggesting the presence of gases, such as high levels of oxygen, that would only exist in the presence of life 34.

Alternative Biochemistry

There is no consensus on the definition of life. NASA considers life to be "a self-sustaining chemical system capable of Darwinian evolution" 35. Nothing in NASA's formulation precludes life based on radically different biochemistries than what is familiar on Earth.

Habitability discussions often revolve around conditions conducive to life as we know it on Earth, but there are hypothetical biochemistries that might allow other forms of life to thrive elsewhere. Some proposed alternatives are as follows.

DescriptionDifference from Earth LifePossible HabitatsChallenges
SiliconSilicon instead of carbon as a basis for organic chemistry.Hot environments 36Compared to carbon, silicon is less common and can form fewer compounds 37.
Metal oxideOxidized metals, such as tungesten, instead of carbon 38.Hot environments such as Mercury-
Hydrogen SulfideHydrogen sulfide (H2S) as a solvent instead of water 39Interstellar planets-
AmmoniaAmmonia (NH3) as a solvent 39Interstellar planets with ammonia lakes-
Hydrogen PeroxideHydrogen peroxide (H2O2) as a solventMars 40-
Liquid NitrogenLiquid nitrogen as a solventCold environments, such as Triton, a moon of Neptune 41.-
Supercritical hydrogenHydrogen as a solventGas giants 41Requires high pressure; areas where this is possible may be limited.
Sulfuric AcidSulfuric acid as a solventClouds of Venus 41-
Supercritical carbon dioxideSupercritical CO2 as a solventLarge planets with dense, high-pressure atmospheres 42.-
Silicon DioxideSilicon dioxide (SiO2) as a solventVery hot environments 43.-
MethaneMethane (CH4) as a solventTitan, a moon of Saturn 44.-
Dust and PlasmaBehaviors of life shown by dust particles suspented in plasma 45.Interstellar medium-
Cosmic necklaceMagenetic monopoles connected by cosmic strings 46.Interior of stars-

Intelligent Life

The existence of extraterrestrial life is speculative, and of extraterrestrial civilization, even more speculative. The Drake equation 47, developed by the astronomer Frank Drake, is a rough attempt at a framework for thinking about this question.

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Source: Wikipedia.

The parameters, and some rough estimates of them, are as follows.

ParameterDescriptionEstimate
NNumber of civilizations in the Milky Way with which we can communicate.Anywhere from 1 (humans are the only civilization in the galaxy) to millions 48
R*Average rate of star formation in the Milky Way1.5-3 per year 49, 50
fpFraction of stars that have planetsClose to 1 51
neFor each star with planets, the average number that can support lifeVery uncertain. 0.1 52, maybe less 30
flFraction of planets that can support life that actually do harbor lifeClose to 1 53, but very little hard evidence
flFraction of planets with life that develop intelligent life0.0002 54 or near 1 53
fcFraction of civilizations that release signals into spaceNear 1 53
LAverage length of time that a civilization emits signals420 years 55 or billions of years 56

The Fermi paradox, attributed to the physicist Enrico Fermi, asks if there are possibly many intelligent civilizations in the Milky Way, why do we not see clear evidence of them 57? Many answers have been proposed, though none has achieved consensus. It may be, as per the Rare Earth Hypothesis 30, that life is too rare for other civilizations to have been detected, or perhaps life is common but intelligent life is rare.

Several efforts to detect alien intelligent life have been attempted and so far turned up no convincing discoveries.

PhenomenonPossible Intelligence ExplanationObservational Evidence
1977 Wow! SignalAnomolous radio pulses might indicate a civilization's communication.Unknown, possibly from a comet 58.
'OumuamuaBracewell probe using solar sail technologyMost likely a natural comet 59
KIC 8462852, also known as Tabby's Star or Boyajian's StarDimming might be caused by construction of a Dyson swarmProbably dust 60
Galaxy-scale infrared lightA Kardeshev Type III civilization might build sufficiently many Dyson spheres to be detected at intergalactic distancesA survey of 100,000 galaxies found no evidence of excess infrared light 61.
Unidentified Flying Objects (UFOs)Alien spacecraft using speculative technologyMany mundane explanations are offered for UFO sightings.



Fine Tuning and the Anthropic Principle

The fine-tuned universe is the notion that only a small portion of the full space of cosmic physical parameters is conducive for life, and we happen to live in a universe that shows this property. There are many formulations of the fine tuning principle, one of which is as follows.

ConstantObserved ValueConsequences if Different
Ratio of the electromagnetic force to the gravitational force between a pair of protons~1036If much smaller, a long-lasting universe would be impossible.
Percentage of mass converted to energy when four nucleons fuse into Helium-40.007If much less, only hydrogen would exist. If much more, no hydrogen would exist.
Ratio of the mass density of the universe to "critical density"Close to 1If too big, universe would collapse quickly. If too small, stars could not form.
Cosmological constant, ratio of the density of dark energy to the critical energy density of the universe~10-122If much larger, space would have expanded too fast for star formation.
Ratio of the gravitational energy required to pull a large galaxy apart to the energy equivalent of its mass~10-5If too small, stars could not form. If too large, stars could not survive.
Number of spatial dimensions3, excluding possible compactified dimensions posited by string theoryLife could not exist in 2 or 4 dimensions.

Source: Rees 62.

Responses to apparent fine tuning often invoke the anthropic principle 63. The weak anthropic principle is a form of survivorship bias, that the universe must be fine tuned for life because if it wasn't, no one could notice that fact. The strong anthropic principle holds that fine tuning is evidence that the universe was created for conscious life, either by God or by an advanced intelligence.

Proposed SolutionRationaleAnthropic Principle
Multiverse 64There must be many universes, only some of which are conducive to lifeWeak anthropic principle
Top-down cosmology 65Initial universe consisted of a superposition of many conditionsWeak anthropic principle
Carbon chauvinism 66Conscious life might be radically different from humans and could thrive with different cosmic constantsNo anthropic principle
Independent Parameters 67The many seemingly independent variables constituting fine-tuning may indicate deeper, still-unknown physics.No anthropic principle
Alien Design 68Our universe was designed by another intelligence, leading to a sort of natural selectionStrong anthropic principle
Intelligent Design 69God designed the universe to be hospitable to humansStrong anthropic principle

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