Download Pathways to Habitability: from disks to active stars, planets and life

Pathways to Habitability: from disks
to active stars, planets and life
Colin P. Johnstone
PATHWAYS TO HABITABILITY PROJECT
What are the astrophysical conditions required for
the formation of habitable planets?
Department of Astrophysics,
University of Vienna
- circumstellar disks
- planet formation
- water transport
- stars: magnetic activity
- binary star systems
Institut für Weltraumforschung,
Graz
- planetary atmospheres
- planetary magnetospheres
THE CLASSICAL
HABITABILE ZONE
The classical definition of planetary
habitability is based on the planet's
'equilibrium temperature'
STELLAR RADIATION
PLANET THERMAL
RADIATION
Input energy from star:
Planet thermal blackbody radiation:
Equating input and output energies gives:
For the Earth: Teq = 250 K...
...but our surface has a
temperature of ~270 K !
For the Earth: Teq = 250 K...
...but our surface has a
temperature of ~270 K !
GREENHOUSE
EFFECT HEATS
SURFACE ABOVE
EQUILIBRIUM
TEMPERATURE
CLASSICAL HABITABILE ZONE
STELLAR MASS DEPENDENCE
STELLAR AGE DEPENDENCE
For the last ~4.5 Gyr, the Sun has been
getting bigger and more luminous
Solar Mass
Half Solar Mass
NOW: everything fine
In a few Gyr: oceans evaporated
A few Gyr ago: oceans frozen
THE FAINT YOUNG SUN PARADOX
In the past, the Sun was too faint
During the initial disk phase, the first stages of
planet formation take place
Planets formed in the gas disk gain
large atmospheres made of hydrogen
During the initial disk phase, the first stages of
planet formation take place
When disk is gone, a distribution of solid bodies remains
- how do these bodies form planets?
- where is the water?
- how is the water transported?
TIME
During collision, water can be transported
between bodies and lost into space
red = not water
blue = water
Without a gas disk, planets are exposed to
the radiation of their stars
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X-ray and extreme
ultraviolet irradiation of
an atmosphere causes
heating and expansion
STELLAR RADIATIVE EVOLUTION
VISIBLE WAVELENGTHS
X-RAY + EUV
STELLAR RADIATIVE EVOLUTION
VISIBLE WAVELENGTHS
X-RAY + EUV
SECONDARY ATMOSPHERES
As the liquid surfaces of young planets solidify, they
release gases forming secondary atmospheres
For secondary atmospheres, X-ray and EUV heating
casuses little mass loss
WE NEED WINDS
NON-THERMAL MASS LOSS
Interactions with the stellar wind cause erosion
of atmosphere
red = ions
blue = neurals
Early Sun
Early Earth
Stronger XUV
Stronger Wind
Larger atmosphere
Smaller magnetosphere
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