The Habitability of the Milky Way Galaxy Mike Gowanlock University of Hawaii NASA Astrobiology Institute

Habitability Habitability is the study of the environments that can support life on Earth and beyond – The solar system (planets, satellites, other small bodies) – Extrasolar planets – Galaxies – Etc. Motivated by the numerous conditions that life is found on the Earth, and the increasing number of extrasolar planet detections

Circumstellar Habitable Zone of our Solar System

The Galactic Habitable Zone (GHZ) The region(s) of the Milky Way galaxy that have the highest carrying capacity for complex life – Focus on land based animal (complex) life that can be extinguished by astrophysical events

Previous Work Early work suggested that the inner region of the GHZ is defined by hazards to a planet’s biosphere (e.g. supernovae) The outer region of the GHZ is defined by insufficient metals to produce planets Conducive to a “Goldilocks” zone view of habitability

Some Literature on the GHZ Galactic chemical evolution considered (Gonzalez et al., 2001) The GHZ lies between 7-9 kpc from the Galactic centre that widens with time (Lineweaver et al., 2004) The entire disk of the Galaxy may be suitable for complex life, but the processes are too challenging to quantify (Prantzos, 2008) The entire disk is found to contain habitable planets, but there is a higher density towards the Galactic centre (Gowanlock et al., 2011) GHZ of Elliptical Galaxies (Suthar & McKay, 2012) GHZ of the Andromeda galaxy (Carigi et al., 2013) GHZ of MW and Andromeda (Spitoni et al., 2014)

Limits to Habitability & the GHZ: Some Open Questions How does planet formation vary as a function of location and time in the Milky Way? Are Jupiter-mass planets good or bad for terrestrial planets existing in the habitable zones of their host stars? Is the Earth a good model for understanding other terrestrial planets in the context of the habitability of the Milky Way? How often do transient radiation events inhibit planets? – Supernovae, gamma ray bursts, etc. How often do close encounters with flyby stars influence the habitability of planetary systems? At what frequency can planets tolerate these events and recover?

Modelling the Galactic Habitable Zone We model stars and planets on an individual basis using a Monte Carlo simulation – In contrast to other work, we do not use a probabilistic model, which implicitly assumes that the Galaxy has 2 spatial dimensions Utilize 3 dimensional models of the stellar number density distribution Assign each star various properties that reproduce the major observable properties of the Milky Way Select supernovae from within this pre-existing stellar distribution – Self-consistent model of star formation and supernova sterilizations Result: not predisposed to a Goldilocks view of habitability

What part of the Galaxy is Modelled? Bulge: high stellar density, possibly too many supernovae. Elliptical orbits, need information regarding dynamics. Halo: not enough metallicity for the formation of planets The disk: noticeable metallicity gradient, favourable stellar density in particular regions, contains the majority of the stars in the Galaxy

Constructing A Model of the Milky Way Galaxy Assign stars properties using: – Star Formation Rate – Star Formation History – Stellar Mass – Metallicity

Star Formation Rate A measure of how often stars are born Stars have a higher formation rate in the centre of the galaxy A high star formation rate in a region indicates that there are more potential homes for life over time

A Model of the Star Formation History at the Solar Neighbourhood R=8 kpc Naab & Ostriker (2006)

Star Formation History We use an inside-out formation history of the Milky Way – Mean age of stars: Oldest are found towards the inner Galaxy Youngest are found in the outer Galaxy

Stellar Mass/The Initial Mass Function (IMF) We assign masses from the Salpeter and Kroupa IMF (two models) Gives the main sequence lifetime of the stars

Metallicity A measure of how much metal there is in a given region of the Galaxy

Metallicity With too little metallicity, Earth-mass planets are unable to form Metallicity declines with radial distance from the galactic centre Metallicity increases with time through stellar nucleosynthesis

Metallicity

Putting it Together: A Model of the Habitability of the Galaxy Stellar Density, Star Formation Rate Metallicity and Planet Formation Supernovae Rate Time required for Complex Life to Evolve

Metallicity is correlated with Planet Formation Generations of births and deaths of stars are required before protoplanetary disks form

Metallicity is correlated with Planet Formation for Gas Giant Planets Fischer & Valenti (2005)

Planet Formation We use the probability of forming a gas giant planet in combination with a model of solar system formation (Ida & Lin 2005) to determine if a star is assigned a planet in its habitable zone If a hot Jupiter is also populated, then we denote that planetary system as uninhabitable Model 1 Model 4

Supernovae The deaths of stars produce supernovae that release cosmic rays, gamma-rays, and x-rays that are fatal to life on nearby planets In particular, the radiation depletes an atmosphere of ozone leaving a planet exposed to its host star

Supernovae Rate The high stellar density in the inner region of the Galaxy means that there is a greater probability of planetary systems being sterilized by supernovae

Type II Supernova SNII supernova occur at the end of a massive star’s lifetime (>8 solar masses) Every star in this mass range will become a SNII Complex life will not survive if a planet is within a particular distance from the SN, determined by the absolute magnitude of the event

Type Ia Supernova Type Ia supernova can occur at the end of a low mass star’s lifetime in a binary star system Initially one star evolves off of the main sequence forming a white dwarf The white dwarf accretes material from the companion star

Type Ia Supernova When the white dwarf reaches the Chandrasekhar limit (1.4 solar masses), the star collapses and it explodes 1% of all white dwarfs are expected to become SNIa (Pritchett et al., 2008)

Supernovae Stars with M>8 solar masses are SNII – ~1% of stars Every star with M<8 solar masses is a SNIa candidate – Nearly 99% of the stars have M<8 solar masses – Only 1% of these

Sterilization Distances We utilize the absolute magnitudes of SNII and SNIa to assign sterilization distances – We normalize the absolute magnitudes to a typical SN event and associated sterilization distance – SNII have a sterilization distance between 2 and 27 pc – SNIa have a sterilization distance between 13 and 26 pc

Complex Life Sufficient time must be allowed to permit the evolution of land-based complex life This took ~4 billion years on Earth This model assumes that Earth’s time scale is typical of the evolution of life in general Timescales considered: – Formation of the Earth 4.55 Gya – Evidence of Cyanobacteria 2.7 Gya – Formation of the ozone layer 2.3 Gya – Rise of metazoan (animal) life 0.75 Gya This is the largest limitation of our work because we only have a single data point (life on Earth)

Results: fraction of stars not nearby a supernova event Models 1-4 These plots ignore planets and are only concerned with those stars bathed in the flux by a nearby supernova

Results: The total number of habitable planets vs. radius 1.2% of stars host a habitable planet at some point in the history of the Galaxy 50% of the habitable planets lie at R<4.1 kpc The greatest number of habitable planets are located in the inner Galaxy despite the high SNe rate Models 2 and 4 (Kroupa IMF)

Results: fraction of stars with a habitable planet integrated over all epochs Total Non-locked Tidally-locked 1.2% of stars host a habitable planet 0.9% host tidally-locked planets 0.3% host non-locked planets

Results: Present day number of habitable planets per pc Too little time Too metal poor

Results: Fraction of habitable planets integrated over all epochs

50% of the planets in my model are found within the annular region above Comparison with Other Work

Next Steps We find that the greatest number of habitable planets exist in the inner Galaxy (R>2.5 kpc) Suggests we should model R<2.5 kpc (the galactic bulge) Need to consider modelling stellar trajectories to account for the eccentric orbits of the stars in the region Need to update planet formation as a result of new exoplanet detections

Next Steps Questions: – Is there a region and epoch such that planets are always uninhabitable? – Will close encounters cause planets to be ejected or orbits significantly altered? – How often do close encounters between flyby stars and planetary systems dynamically disrupt small bodies and cause a heavy bombardment-like event?

Conclusions We find that the GHZ is not limited to an annular region, or a “Goldilocks” view of habitability We predict that 1.2% of all stars in the Galaxy have a habitable planet with the ability to host complex life Are we located in a position favourable for the emergence of complex life? – Given that the majority of habitable planets are located in the inner Galaxy, I do not believe that we are in a favourable location