Download not predisposed to a Goldilocks view of habitability

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
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
Models 2 and 4 (Kroupa IMF)
•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
Results: fraction of stars with a habitable planet
integrated over all epochs
1.2% of stars host a habitable planet
0.9% host tidally-locked planets
0.3% host non-locked planets
Total
Tidally-locked
Non-locked
Results: Present day number of habitable planets per pc
Too little time
Too metal poor
Results: Fraction of habitable planets integrated over
all epochs
Comparison with Other Work
50% of the planets in my model are found within the annular region above
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