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Lecture 21: Habitable Zones
around Stars and the Search for
Extraterrestrial Life
Meteo 466
Talk Outline
• Part 1—Basic requirements for life
• Part 2—Definition and boundaries of the
habitable zone
• Part 3—Important recent developments
• Part 4—The galactic habitable zone
• Part 1—What are the requirements for life?
First requirement for life: a liquid or
solid surface
• It is difficult, or impossible,
to imagine how life could
get started on a gas giant
planet
– Need a liquid or solid
surface to provide a stable
P/T environment
• This requirement is
arguably universal
Second requirement for life:
carbon
• Carbon is unique among the
elements in forming long,
complex chains
• Something like 95% of
known chemical compounds
are composed of organic
carbon
• Silicon, which is located right
beneath carbon in the
Periodic Table, forms strong
bonds with oxygen, creating
rocks, not life
Proteins
Third requirement for life (as we
know it) : Liquid water
• Life on Earth requires liquid
water during at least part of
its life cycle
• So, our first choice is to look
for other planets like Earth
• Subsurface water is not
relevant for remote life
detection because it is
unlikely that a subsurface
biota could modify a
planetary atmosphere in a
way that could be observed
(at modest spectral
resolution)
• Part 2—Definition and boundaries of the
habitable zone
Definitions
(from Michael Hart, Icarus, 1978)
• Habitable zone (HZ) -the region around a star
in which an Earth-like
planet could maintain
liquid water on its
surface at some instant
in time
• Continuously habitable
zone (CHZ) -- the
region in which a planet
could remain habitable
for some specified
period of time (e.g., 4.6
billion years)
Michael Hart’s calculations
(Icarus, 1978, 1979)
• 4.6-Gyr CHZ around our own Sun is
quite narrow
– 0.95 AU: Runaway greenhouse
– 1.01 AU: Runaway glaciation
• CHZ’s around other spectral types are
even narrower
– Corollary: Earth may be the only habitable
planet in our galaxy
Problems with Hart’s model
• He concluded, incorrectly, that a fully
glaciated planet could never deglaciate by
building up CO2
• There is strong evidence that this actually
happened on Earth several times in the
aftermath of Snowball Earth glaciations
• Furthermore, there is a well-known negative
feedback that causes CO2 to accumulate on
frozen planets…
The carbonate-silicate cycle
• The habitable zone is relatively wide because of negative feedbacks
in the carbonate-silicate cycle: Atmospheric CO2 should build up as
the planet cools
• Higher CO2 is also at least part of the problem to the faint young Sun
problem on Earth
The carbonate-silicate cycle
• Caveat: For planets with low volcanic outgassing rates and with low
stellar insolation, it may not be possible to maintain warm climates
all the time. Instead, one may get limit cycling behavior, in which the
climate alternates between warm and globally glaciated states (see,
e.g., Kadoya and Tajika, 2014; Menou, 2015; Haqq-Misra et al., 2016)
The ZAMS habitable zone
• With this in mind, one gets a habitable zone that is fairly wide compared
to the mean planetary spacing
• Figure applies to zero-age-main-sequence stars; the HZ moves outward
with time because all main sequence stars brighten as they age
http://www.dlr.de/en/desktopdefault.aspx/tabid-5170/8702_read-15322/8702_page-2/
Outer edge of the HZ
• There is a limit, though, at which the CO2 starts to
condense. This reduces the greenhouse effect by
lowering the tropospheric lapse rate
• Rayleigh scattering by CO2 raises the planet’s albedo
and therefore also competes against the greenhouse
effect
• This leads to something termed the ‘maximum
greenhouse limit’, which one can calculate using a 1-D
climate model (~1.67 AU for a Sun-like star)
Inner edge of the HZ
• Near the inner edge of the HZ, the problem is just the
opposite: a planet can develop either a runaway or
moist greenhouse
• Runaway greenhouse: The oceans evaporate entirely
• Moist greenhouse: The ocean remains liquid, but the
stratosphere becomes wet, leading to rapid
photodissociation of water and escape of hydrogen to
space
– Old limit (Kasting et al., 1993) was 0.95 AU
– New limit (Kopparapu et al., 2014) is ~1.0 AU
– These are for fully saturated, 1-D climate models
3-D modeling of habitable
zone boundaries
• Fortunately, new studies
using 3-D climate models
predict that the runaway
greenhouse threshold is
increased by ~10% because
the tropical Hadley cells act
like radiator fins
– This was pointed out 20 years
ago by Ray Pierrehumbert
(JAS, 1995) in a paper dealing
with Earth’s tropics
• We have adjusted our (1-D)
HZ inner edge back inward
to 0.95 AU to account for this
behavior
Outgoing IR radiation
Leconte et al., Nature (2013)
Empirical HZ limits
• Some researchers don’t
trust theorists
• We can also define
empirical limits on the
HZ
– From looking at its
surface, we can discern
that Venus has not had
liquid water for at least
the last 1 b.y.
– Mars, on the other hand,
looks as if it was
habitable at about 3.8 Ga
Updated habitable zone
(Kopparapu et al., 2013, 2014)
• Note the change in the x-axis from distance units to stellar flux
units. This makes it easier to compare where different objects lie
Credit: Sonny Harman
Updated habitable zone
(Kopparapu et al., 2013, 2014)
Conservative HZ
• We should define the HZ conservatively when designing our space
telescopes, so that we don’t underestimate the problem
Credit: Sonny Harman
Updated habitable zone
(Kopparapu et al., 2013, 2014)
Optimistic HZ
• Once we’ve got these telescopes built and launched, we can afford
to be more optimistic, so that we don’t overlook interesting planets
Credit: Sonny Harman
• What are the potential problems for
planets orbiting stars different from the
Sun?
– Let’s do the early (more massive and
bluer) stars first
– What are two obvious potential problems?
Planets around early-type stars
•
Planets orbiting stars earlier than
about F0 have at least two problems
1. Short main sequence lifetimes (~2 b.y. for
an F0 star)
Stellar
lifetimes
• A star’s main sequence
lifetime decreases rapidly
with its mass
M
K G
F
A
B
O
http://cherrymountainobservatory.com/basics_stellar_systems.html
Planets around early-type stars
•
Planets orbiting stars earlier than
about F0 have at least two problems
1. Short main sequence lifetime (~2 b.y. for
an F0 star)
2. High levels of UV radiation (about 20
times that of the Earth)
– But this may not be a problem once an O2rich atmosphere develops 
Chapman mechanism for ozone
production
Wavelength
( < 200 nm)
1)
2)
3)
4)
O2 + h  O + O
O + O2 + M  O3 + M
O3 + h  O2 + O
( < 800 nm)
O + O3  2 O2
•
Around M stars the far-UV flux is enhanced
relative to the near-UV and visible fluxes 
Earth-like planets around F, G,
and K stars
Complete spectra
UV spectra
• The “Earth” is assumed to be at a distance equivalent to 1 AU in the
extrasolar planet system. First, scale the orbital radius by L, then
move the planet inward or outward until its calculated surface
temperature is 288 K.
Segura et al., Astrobiology (2003)
Earth-like planets around F, G,
and K stars
Ozone number density
Temperature
• The planet around the F star develops a “super” ozone layer because of
the abundance of short-wavelength UV radiation ( < 200 nm) that can
dissociate O2  UV is not a problem once O2 levels become high
Segura et al., Astrobiology (2003)
Earth-like planets around F, G,
and K stars
• This figure shows the
incident and surface UV
fluxes for the F, G, and
K-star planets
• The F-star planet has
the highest incident UV
flux, but the lowest UV
flux at the ground
• The G-star (Sun) and Kstar surface UV fluxes
are close to the same
Segura et al., Astrobiology (2003)
• What are the potential problems for
planets orbiting later (dim, red) stars?
– There are probably more than two…
ZAMS habitable zones
• Older diagram: Not quite as pretty as the other one, but it illustrates
the tidal locking problem for planets around late K and M stars
Kasting et al., Icarus (1993)
Possible problems for planets
around M stars
• Tidal locking
– But this can probably be
overcome by atmospheric
and oceanic heat transport
• Lack of magnetic field 
atmospheres may be
removed by flares and
sputtering (H. Lammer et al.,
Astrobiol., 2007)
• Initial deficiency in volatiles
due to smaller orbits, hotter
accretion environment, more
energetic collisions (J.
Lissauer, Ap. J., 2007)
GRIN: Great Images in NASA
http://www.fanboy.com/science/
Possible problems for planets
around M stars
• Long, bright, pre-main
sequence evolution
– As pointed out by Luger and
Barnes (Astrobiology, 2015), M
stars can take tens to hundreds
of millions of years to collapse
(as compared to a few million
years for solar-type stars
– During this time, these stars
can be up to 10 times their
eventual main sequence
brightness
– If planets form during this time
period, there is a chance that
they will be devolatilized by a
runaway greenhouse during
their early histories
Ramirez and Kaltenegger, ApJ (2014)
Part 3: Important recent developments
Kepler Mission
• This space-based telescope
will point at a patch of the
Milky Way and monitor the
brightness of ~160,000 stars,
looking for transits of Earthsized (and other) planets
• 105 precision photometry
• 0.95-m aperture  capable
of detecting Earths
• Launched: March 5, 2009
• Died (mostly): April, 2013
http://www.nmm.ac.uk/uploads/jpg/kepler.jpg
Kepler target field: The stars in this field range from a few
hundred to a few thousand light years in distance
1. Big planets are not rocky
• One of the most important
results from Kepler is that we
have learned when a planet
is likely to be rocky
– Transit depth gives us the
planet’s radius
– Some Kepler target stars
are bright enough so that
we can make radial velocity
(Doppler) measurements of
the planets, giving the mass
– Measuring mass and radius
gives us density
– Planetary density peaks at
about 1.5 Earth radii,
suggesting that this is the
upper limit for a rocky planet
Weiss and Marcy, ApJ (2014)
2. Proxima Centauri b
• There is a planet in the
habitable zone around
Proxima Centauri!
– This planet was found
from the ground
(HARPS) using the radial
velocity method
• Star: Red dwarf, 0.12
MSun, 4 light years
distant
• Planet: M sini =
1.270.2 M, stellar flux
= 0.65 S
3. TRAPPIST-1 System
• These planets were found
by the transit method
with an automated 60-cm
telescope in Chile
• Stellar characteristics
• Very late M star
• Distance: 12.1 pc (~40
light years)
• Mass: ~0.08 MSun
• Te: ~2560 K
• Luminosity: 0.00052 LSun
• Radius: 0.12 Rsun
• Metallicity: [Fe/H] =
+0.040.08
Gillon et al., Nature (2017)
Updated habitable zone
• Three of the seven Trappist-1 planets fall within the conservative
HZ, as does Proxima Centauri b
• Part 4: The galactic habitable zone
The galactic habitable zone
• There may also be a
preferred time and location
within the galaxy for
habitable planets to exist
• Stars that are too close to
the center of the galaxy are
subject to frequent nearcollisions and more
supernovae and gamma ray
bursts
• Stars that are too far out in
the galaxy (or that evolved
too early in its history) may
be too metal-poor
• This figure is concerned with
the time necessary to evolve
complex life (assumed to be
41 billion years)
Ref.: Lineweaver et al., Science (2004)
The galactic habitable zone
• Same as the previous figure,
but removing the
requirement for 4 b.y. of
continuous evolution
• This is the region of the
galaxy in which life in
general might be present
• Fortunately, the GHZ is large
compared to the local solar
neighborhood; hence, this
should not pose any
significant problems in
looking for life
Ref.: Lineweaver et al., Science (2004)
Conclusions
• Detectable life requires, at a minimum, a planet with a solid (or
liquid) surface, sufficient availability of carbon, and surface liquid
water
• Habitable zones should be defined conservatively if they are
being used to generate design parameters for future spacebased telescopes
– 3-D climate models are needed to further refine the boundaries of the HZ
• Bright blue stars are not good candidates for harboring habitable
planets because of short main sequence lifetimes and high solar
UV fluxes
– The latter problem goes away for planets with O2-rich atmospheres
• Dim red stars have multiple problems for habitability: 1) Tidal
locking, 2) solar wind stripping, 3) deficiencies in volatiles,
magnified by long, bright, pre-main-sequence evolution
• A galactic habitable zone probably exists, but it is large
compared to the area of space that we can search in the near
future