A habitable planet is one where liquid water can exist on the surface. The term “habitable” is derived from the Latin word habitare, meaning “to dwell”. A habitable planet must have a stable climate, sufficient atmospheric pressure and an appropriate temperature range to allow for the existence of liquid water. It must also have a rocky surface with enough gravity to maintain an atmosphere.
The first criteria for habitability is that the planet must be in the so-called “habitable zone” around its star, where it receives enough stellar radiation to maintain liquid water on its surface. The inner edge of the habitable zone is defined by the runaway greenhouse effect, in which a planet’s atmosphere traps too much heat and causes the planet to become uninhabitably hot. The outer edge of the habitable zone is defined by the frost line, beyond which any water present on a planet would freeze solid.
The second criterion for habitability is that the planet must have a stable climate. Climate instability can be caused by changes in the output of the star (such as during a star’s main sequence lifetime), or by changes in orbital parameters (such as eccentricity). If a planet’s orbit becomes too eccentric, it will experience extreme swings in temperature that could make it uninhabitable. Additionally, if a star undergoes large changes in luminosity (brightness), this will also cause significant changes in climate on any orbiting planets.
Thirdly, a habitable world must have an atmospheric pressure within certain limits. Too little atmospheric pressure results in an extremely thin atmosphere that does not protect against UV radiation or retain heat well; both of these conditions make liquid water impossible to sustain on a planetary surface. On Earth, our air pressure at sea level is about 1013 hPa (hectopascals), but pressures only need to be within about an order of magnitude of this value for habitability (between 100-1000 hPa) . For example, Mars has an atmospheric pressure of 600 hPa, which is just barely within this range. However, even if all other conditions are met, without sufficient atmosphericpressure no liquid water can exist on a world’s surface regardlessof temperature—this was one reason why Venus was once thoughtto be potentially habitable despite being incredibly hot due totrapping too much solar radiation within its dense carbon dioxideatmosphere (the average temperature on Venus’ surfaceis 462 °C! ).
Fourthly, habitability requires that planets have surfaces composedof rocks and metals rather than gas and ice—withouta solid or molten rocky exterior there can be no plate tectonicsand thus no oceans or continents (and likely no life aswe know it). Additionally,”life” itself generallyrequires some kind of substrate uponwhich to gain purchase and multiply—be it soilor ocean sediment —soa gaseous orblike Jupiterwould not usually meet this criterion either(although there are potential exceptions ). Finally(and relatedly),a body needs enough mass sothat its own gravitycan hold onto an atmosphereduring billionsof years: less massiveworlds like Pluto rapidlylose their atmospheres over time due tobulletin escape velocity . In short then:for long-term habitabilitymass matters! Thisis one reason whyVenus mayhave been inhabitable inthe past but isnow decidedlyunliveable :its lower massmeans it lostmuch morehydrogenand heliumover time thando larger worlds likeEarth . Andthis brings us neatlyto our next topic…
GRAVITY & ATMOSPHERIC RETENTION:THE IMPORTANCE OF MASS IN HABITABILITY As we’ve just seen , mass matters whenit comes to maintainingan atmosphere over long periods of time . But how importantis gravity really? After all , smaller moons like our own Moonor Saturn’s Enceladusseem able do just fine with relatively weak gravitationalpulls comparedto giant worlds likethe gas giantsJupiter and Saturnthemselves . So what gives ? Well , partofthe answerliesin another important factorwhenit comes toplanetary habitability :surface temperature ! You see ,objects with very low densities tendto have very high surface temperaturesbecause they absorb more sunlight per unit area than moremassive bodies do — thinkabout howhot Mercuryis comparedtocold Ceresor even colderPluto ! In fact , most knownexoplanets discoveredby astronomersso far tendto fall intothis category because they’re smalland close to their stars ; many are what we call”super-Earths”with radii 1..2 timesthat of Earthbut masses severaltimesour world’s … meaningthey’re mostly made up offrozen volatileslike waterice , methaneiceor ammoniaice insteadof rocks& metals . Super-Earths arecommonplace throughoutour Milky Way galaxybut unfortunatelymost seemunlikely candidatesfor hosting lifeas we know it simplybecause they lacksturdy surfacesuponwhich living thingscould take root& multiply ! Thereare however someimportant exceptions worth mentioning … MOONS & PLANETARY SATELLITES:POTENTIAL HABITABLE WORLDS IN ORBIT? One typeof object inthe solar systemthat doespossess arocky surfaceto stand uponis amoon — natural satellitesorbiting largerplanets & dwarf planesthat formed alongsidethem during formation processessome 4..5 billion years ago