Habitable Zones By Scott DeNoble and Mike Gallivan

Definition The habitable zone is defined as the orbital region around a star in which an Earth-like planet can possess liquid water on its surface and possibly support life

Physical Limits of the Habitable Zone The Habitable zone has both an inner limit and an outer limit Being within the limits of the habitable zone means the planet can contain liquid water on its surface

Inner Limit The inner limit is defined as the point where water would escape from the planet due to a runaway greenhouse effect Any planet which is closer to the star than the inner limit will be too hot and any water will evaporate and be turned to water vapor

Outer Limit The outer limit is defined as the point where the planet is too cold for water to be in liquid form anywhere on the planet Anywhere past this point and the planet will not obtain enough heat from its star to have liquid water

Luminosity

Size of the Star There is a proportional relationship between the size of a star and the size of its habitable zone This is related to the luminosity of the star because the luminosity is dependent on size for main sequence stars If a star is too small then it will not have a habitable zone because it will not have the right elements in its solar system

Other Factors for Habitability Size of the planet Eccentricity of its orbit Company of a large moon Geochemistry Microenvironments

Size of the Planet The size of a planet has a lot to do with whether or not it can be considered habitable It must have enough mass to have an atmosphere The moon is an example of a body within the habitable zone but too small to hold an atmosphere It also can’t be so big that its gravity attracts large metals

Eccentricity If the eccentricity of the planet’s orbit is too large, the planet can travel in and out of the habitable zone The varying temperature can cause planets to not be habitable

Company of a Large Moon A large moon is essential for the development of complex life The gravity of a large moon keeps the planets axis tilt constant If the tilt continually changes, the climate would change drastically making life sustainability nearly impossible

Geochemistry Having the right geochemistry is necessary for a planet to be habitable Earth is 96% carbon, oxygen hydrogen and nitrogen If a planet has not enough of these elements or too many heavier metals, then it is deemed to be uninhabitable

Microenvironments Microenvironments are necessary to define the habitability of some planets If a planet is mostly uninhabitable but some parts of it can fit the conditions to be habitable, then the planet is considered habitable For example, if a planet is almost cool enough on the surface for liquid water but has a few cooler caves where water could exist, then it is considered habitable

Galactic Habitable Zone There are limits to the galactic habitable zone The inner limit is close to the center of the galaxy where there is a high concentration of metals The outer limit is far from the center of the galaxy where there is a low concentration of metals The best chance for a habitable planet is where the galactic habitable zone and the habitable zone of a star overlap

Milky Way Habitable Zone

New Studies Some new studies have indicated that the inner limit of the habitable zone is actually around 0.99 AU This would mean that due to our elliptic orbit we would enter and exit the habitable zone throughout the year which disputes what was formally researched

The Habitable Zone In Our Solar System The only true example of a habitable planet is Earth, as it is the only source of known life, although we have discovered many planets in the habitable zone of other stars Planets such as Mercury and the Gas giants are far outside the habitable zone of our sun whereas Venus and Mars are just outside

Earth Our solar system’s habitable zone ranges from 0.95 AU to 1.5 AU based on current estimations The Earth fits into that range at 1.0 AU Earth also has a large moon which keeps the axis tilt constant over a long period of time

Mercury & Venus Mercury and Venus are the only examples of planets short of our suns Habitable zone Mercury is so close to the sun and so small that almost all its atmosphere escaped On Venus most of water evaporated along with sulfuric acid and thus exhibits a very strong Greenhouse effect Above is an image of Mercury’s atmosphere below is a graph of Venus’s

Mars Mars lacks a moon to stabilize its orbit an thus suffers from climate variations from axial tilt, orbital eccentricity and axial precession due to the lack of a large moon In addition Mars is far enough away that water is almost exclusively ice, although evidence suggests that it once had liquid water on its surface The size of Mars, significantly smaller than Earth or Venus, makes it difficult to hold an atmosphere and thus creates insufficient greenhouse effect for liquid water to exist

The Gas Giants The four gas giants in our solar system; Jupiter, Saturn, Uranus, and Neptune; are all far outside our sun’s habitable zone an thus exhibit many properties that deem life impossible That far outside the habitable zone all water will be ice due to the lack of radiation from the sun Since the gas giants formed with the solar system, they lack the heavier metals that solidify the planets found near the center of the solar system

Research Into Other Systems The discovery of other “exoplanets”, planets in solar systems beyond ours, is difficult due to the low levels of radiation planets emit compared to stars One common method of detecting exoplanets is to determine a change in brightness of a star as they perceive the planet pass in front of it These methods usually take a long time to make measurements and often return false positives making the discovery of these potential habitable planets a tedious process

Results of Research Once the exoplanet is found we can then determine their approximate distance to their given star and determine whether or not they are in the habitable zone or not Many of the found planets are very far away, hundreds of light years away, leaving us little knowledge about them other than their sizes This also makes it virtually impossible to determine if these planets have habitable moons as well

Comparisons to our Solar System Of the exoplanets found within their habitable zones many are much larger than earth and are thus most likely gas giants that are closer to their stars than ours o Many of these are considered either warm Jupiters or warm Neptunes since they will be far closer to their stars than Jupiter or Neptune are to ours o Some of the discovered gas planet are considered Hot Jupiters because they are far within the habitable zones of their stars o Another commonly found exoplanet is a “super-earth” a rocky planet many times larger than earth

Habitable Moons Many of these discovered exoplanets in habitable zones could potentially have moons of the appropriate size In our solar system the only moon that falls within the habitable is our moon which is far too small to support an atmosphere Some of Jupiter and Saturns moons (such as Titan or Ganymede would be large enough to support an atmosphere but they are far outside our solar systems habitable zone Sizes of the larger moons within our solar system

Limitations on Habitable Stars Certain star types form in different ways M Dwarves, one of the most locally common types of stars, form in a manner which makes it highly unlikely for water to exist on its planets Main sequence stars are the best candidates to support life due to their consistent irradiance Post-main sequence stars have pulsating irradiance Other objects such as larger planets that could emit enough radiation to have a habitable zone themselves also lack a consistent luminosity

Thank you for listening!

Sources PLANETS FORMED IN HABITABLE ZONES OF M DWARF STARS PROBABLY ARE DEFICIENT IN VOLATILES. Jack J. Lissauer, Space Science and Astrobiology Division, 245-3, NASA Ames Research Center, Moffett Field, CA Received 2006 September 12; accepted 2007 March 15; published 2007 April 11. Habitable Planets: What Are We Learning from Kepler and Ground- Based Searches? James F. Kasting. ASTROBIOLOGY Volume 11, Number 4, 2011, Mary Ann Liebert, Inc. DOI: /ast The Galactic Habitable Zone an Age Distribution of Complex Life in the Milky Way. By Charles H. Lineweaver, Yeshe Fenner, and Brad K. Gibson Solar Variability and Climate Impact on Terrestrial Planets. By J.-L. Bertaux Service d’Aeronomie du CNRS/IPSL, BP3, Verri`eres- le-Buisson, France

Sources Continued Exotic Earths: Forming Habitable Worlds with Giant Planet Migration by Sean N. Raymond, Avi M. Mandell, and Steinn Sigurdsson. Habitable Climates: The Influence of Eccentricity o Britannica Academic Edition o Wikipedia o o