Download Phys 214. Planets and Life

Phys 214. Planets and Life
Dr. Cristina Buzea
Department of Physics
Room 259
E-mail: [email protected]
(Please use PHYS214 in e-mail subject)
Lecture 30. Habitability in star systems.
Extrasolar planets.
March 28th, 2008
Contents
Textbook pages 329-332, 338-353, 360-392
The evolution of surface habitability in a star system
Stars and their classification
Which stars do make good stars
Detection of extrasolar planets
The nature & evolution of habitability
A star’s habitable zone = the range of distances from the star where liquid water can be stable on
the surface of a suitable planet.
Being in a star habitable zone is NOT enough to make a world habitable. The size of the planet
is very important!
E.g. The Moon is in the habitable zone of the Sun but is not habitable (too small to retain an atmosphere
necessary for liquid water to be stable).
E.g. Europa is located outside the Sun’s habitable zone and yet may be habitable (because is tidally heated,
allowing liquid water to exist beneath its icy surface)
The nature & evolution of habitability
Habitable zones evolve with time.
Over time the Sun’s habitable zone
has widened and moved farther
from the Sun.
When the Sun was younger its
habitable zone was narrower and
closer to the Sun.
In the future, the Sun’s habitable
zone will be wider and farther
from the Sun.
The end of habitability of Earth:
conservative estimates ~ 1 billion
years from now.
optimistic estimates ~ 3-4 billion
years from now.
The nature & evolution of habitability
The Sun brightens gradually with time because as H is converted into He in
the core, the number of H nuclei decreases, decreasing the fusion rate.
To maintain the balance with the gravity of the outer layers, the core
compensates by shrinking and heating up.
The outer layer surrounding the core (that still contains unburned H)
becomes very hot that ignites nuclear fusion.
The total rate of fusion is so high that the star increases in size and emits
more light.
The Sun will expand into a red giant when will run out of nuclear fuel. The
Earth surface will heat to 700oC, oceans will evaporate, runaway
greenhouse effect followed by the total loss of its atmosphere. Life not
able to survive even beneath the surface.
When the Sun ejects its
outer layer into space to
become a planetary nebula,
most likely the Earth will
be destroyed.
Surface habitability factors
Critical factors that determine the habitability of a
planet:
1) Size
E.g. Mars currently lacks surface habitability mostly because
of its small size
Large size enough to allows plate tectonics to exist.
Venus (similar size to Earth) but lacks plate tectonics because
of the runaway greenhouse effect - too close to the Sun.
2) distance from parent star
(the planet must be not too close or too far from its star. Must
lie within the habitable zone)
3) presence of an atmosphere
Without enough atmospheric pressure, liquid water
cannot be stable & protection against harmful
solar radiation.
To have a long lasted atmosphere, the planet must
have enough gasses trapped in interior to be
outgassed, a magnetic field, and at least a
moderate rotation.
Global warming
The recent gradual rise in the Earth’s average surface
temperature is commonly referred to as global warming.
The detailed causes of the recent warming remain an
active field of research, but the scientific consensus is
that the increase in atmospheric greenhouse gases due
to human activity caused most of the warming
observed since the start of the industrial era.
Earth - Global warming
Measurements of CO2 concentrations over the past 400,000 years show a direct correlation
with global surface temperatures.
The atmospheric CO2 concentration is higher than it has been at any time during the last
400,000 years.
While there is no doubt that global temperatures are increasing, it is now becoming clear
that human activity is indeed causing global warming.
Earth - Global warming
The green stripe shows the variation between different models that take into account
both:
-natural factors (changes in sun’s brightness, effects of volcanic eruptions) &
- emission of greenhouse gases by humans.
Global warming
These changes are expressed in
terms of radiative forcing = a
measure of the influence that a
factor has in altering the
energy balance of the climate
system.
Positive forcing tends to warm the
surface while negative forcing
tends to cool it.
(check
http://www.physics.queensu.
ca/~phys214/Links.htm
Climate change 2007
Changes in the atmospheric abundance of greenhouse gases and
aerosols, in solar radiation and in land surface properties.
Aerosols
Natural and anthropogenic aerosols.
The amount of man-made aerosols is
considerable and can induce weather
cycles over populated regions, where they
can affect cloud formation.
NASA research: anthropogenic aerosols
induce clear weekly cycles over New
York City.
The aerosols were thickest midweek and
lightest on weekends, affecting clouds
formation (J. of Geophysical Res. 110, 2005)
STRATOSPHERE
TROPOSPHERE
St. Helen, 1080
40 km plume
Solar irradiance
Some researchers estimated that the Sun
may have contributed about 25–50% of
the increase in the average global
surface temperature during 1900–2000.
[Geophysical Research Letters 33 (5)]
Movie: Solar flares
Some researchers suggests that climate
models overestimate the relative effect
of greenhouse gases compared to solar
forcing and underestimate the cooling
effects of volcanic dust.
However, even with an enhanced climate
sensitivity to solar forcing, most of
the warming since the mid-20th
century is likely attributable to the
increases in greenhouse gases!!!
Stars
Finding a planet on which life might arise is
selecting a sun that can provide sufficient
light and heat to support habitable
worlds.
Stars spend 90% of their lives fusing hydrogen
into helium in their cores and slowly
brightening.
A star that exhausts its hydrogen core begins
to grow larger and brighter, becoming a
giant or supergiant star.
Habitable planets are around stars that are
in the long-lasting H-fusing stage of
their life.
When stars finished the fusion fuel they die.
Relatively low-mass stars like our sun eject
their outer layers into space as planetary
nebulae, leaving behind a type of dead star
called white dwarf.
Higher mass stars die in titanic explosions –
called supernovae, in which their cores
collapse in either neutron stars or black
holes.
Stars
One of the fundamental principles of stellar evolution is that the
more massive a star is the faster it evolves.
A star less massive than our Sun will have a longer lifetime.
A star more massive than our Sun will have a shorter lifetime.
How do we classify stars?
Because all stars are born with basically the same composition (98% H & He), the physics
that determines the star characteristics is straightforward.
During the H burning phase, the star’s surface temperature (or spectral type) and total
luminosity is determined almost entirely by one thing – star mass!
There are seven major spectral type of stars: O, B, A, F, G, K, M.
The spectral sequence
OBAFGKM runs from
hot to cool in terms of
surface temperature of
stars.
Mnemonic Oh,
Be
A
Fine
Girl,
Kiss
Me
Which stars do make good suns?
O-type stars (most luminous) have lifetimes too short for planet formation.
O-type stars in our galaxy are very rare (less than 1%)
A- and F-type stars (more luminous than the
Sun) have lifetimes long enough for planets to
form and for simple life to appear, but not long
enough for advanced life to develop.
B-type stars (much more luminous than our
Sun) have lifetimes long enough for planets
to form but not for life to appear.
Stars like our Sun have
lifetimes long enough
for advance life to
evolve.
K- and M-type stars (less luminous
than the Sun) have lifetimes long
enough for advanced life to evolve.
However, they might not have many
habitable planets around them
because their habitable zones are
very narrow.
Which stars do make good suns?
K- and M-type stars have lifetimes long enough for advanced life to evolve. However,
they might not have many habitable planets around them because their habitable zones
are very narrow.
Objections to their habitability:
Closer planets might be locked in a synchronous rotation, with one side facing the star.
Smaller stars have frequent and energetic flares that might negatively affect life on closer
planets.
Which stars do make good suns?
•
Depending on the stellar type (mass and luminosity), habitable planets will be at
different distances from the parent star
Which stars do make good suns?
Many brown dwarfs in constellation Orion. Infrared image of a Jupiter-size planet orbiting a brown dwarf.
Brown dwarfs are substellar objects with insufficient mass to sustain nuclear fusion in
their cores. They have higher surface temperatures than planets and masses between 10to 80 times that of Jupiter. Brown dwarfs have no habitable zones because they are so
dim.
However, recent infrared observations suggest that planets are forming around them, and
one brown-dwarf has a Jupiter-size planet orbiting it.
Habitability of planets orbiting a multiple star system
Multiple star systems in our galaxy are fairly common, making up
around 30% of the total stars.
Stable planetary orbits:
- A large orbit around both stars in a close binary system
- An orbit close to one of the stars in a wide binary system
Detection of extrasolar planets
Two ways to search for extrasolar planets:
1) Directly – pictures or spectra of planets
2) Indirectly – by measuring stellar properties
(position, brightness, spectra)
• astrometric technique
• Doppler technique
• transit technique
• Gravitational lensing
The few extrasolar planets
that have been detected
directly to date are very
large and at great
distances from their
parent star.
Detection of extrasolar planets - Astrometry
Astrometry uses regular changes in the
positions of the parent stars with respect to
more distant stars as they move across the
sky to detect extrasolar planets around
other star systems.
Most extrasolar planets have been detected by
observing the gravitational tug they exert
on the stars they orbit.
All the objects in a star system, including the
star, orbit the system’s center of mass.
Orbital path of the Sun around the center of mass of the
Solar System as it would appear from a distance of 30
light-years away. 1960-2025
The center of mass of the solar system is close to
center but not exactly at the center of the Sun.
Detection of extrasolar planets - Doppler technique
Detection of Doppler shifts in the spectra of the parent stars has been the MOST successful in
detecting extrasolar planets around other star systems.
Stars exhibit Doppler shift only if they are moving toward or away from us along the line of sight.
The wavelengths of radiation from a star that is moving toward us are shorter.
The wavelengths of radiation from a star that is moving away from us are longer.
Detection of extrasolar planets - Doppler technique
The radial velocity curve of a star with an extrasolar planet is a plot of radial velocity against
time.
If a star has an extrasolar planet
- the amplitude of its radial velocity curve is related to the planet’s mass.
- the wavelength of its radial velocity curve is related to the planet’s orbital period.
- the symmetry of its radial velocity curve
If a star has a high mass planet
is related to the planet’s orbital shape.
at a small distance form it, its
(The uneven nature of the change in velocity
radial velocity curve would
indicates that the planet is in a highly elliptical orbit.)
have a large amplitude, short
wavelength.
Massive planets closed to their
parent star -easiest to detect
using the Doppler shift.
A low mass planet far from its
parent star would be the most
difficult to detect using the
Doppler shift method.
Detection of extrasolar planets - Doppler technique
If we view a planetary orbit face-on, we will not detect any
Doppler shift at all.
We can detect Doppler shift only if the planet and star have
a part of their orbital velocities directed toward or away
from us.
When we measure the mass of a planet using the Doppler shift method, we know that it is
mass could well be larger. The Doppler shift method of detecting extrasolar planets only
give us the minimum mass of a planet because we don’t necessarily know the angle the
planet’s orbit makes with our line of sight.
Most of the extrasolar planets detected to date are found very close to the parent stars.
Detection of extrasolar planets - Transit technique
The transit method for detecting extrasolar
planets is based on detection of brightness
changes in a star as a planet passes in front of
it. These changes depend on the planet size.
When Mercury or Venus passes in front of the
disk of the Sun, we call this a transit.
For the transit of an extrasolar planet to be
observed, the orbital plane of the planet has
to be aligned along our line of sight.
This method detects planets orbiting closed to
their star.
Detection of extrasolar planets - Transit technique
•Hubble Space Telescope field of view in the Sagittarius Window Eclipsing Extrasolar Planet Search
(SWEEPS).
•Half of these stars are bright enough for Hubble to monitor for any small, brief and periodic dips in
brightness caused by the passage of an exoplanet passing in front of the star.
•The green circles identify 9 stars that are orbited by planets with periods of a few days. Planets so
close to their stars with such short orbital periods are called "hot Jupiters."
Detection of extrasolar planets - Gravitational lensing
Gravitational lensing is the process by which a massive object magnifies and distorts
the light from an object behind it. Mass distorts space -> light rays bend slightly
when passing near objects with large mass.
Gravitational lensing detection has discovered the lowest-mass planet to date – only 5
times the mass of earth planets in large orbits.
Properties of extrasolar planet discovered so far
Hot-jovian describes the most common type of extrasolar planet discovered to date.
The orbits of most extrasolar planets detected to date are highly elliptical.
Most of the extrasolar planetary systems discovered to date are very different than our
own solar system having Jovian-sized planets close to their parent stars.
Properties of extrasolar planet discovered so far
The existence of giant planets in sub-Mercurian orbits and in excentric orbits has come as a
surprise and is forcing theorists to revise their understanding of how young planetary
systems evolve.
According to our current theory of planet formation, Jupiter-like planet cannot form close to
its parent star because it would be too hot for gases to condense.
However, they can form farther out and then migrate inward.
The inward migration of a Jovian-like planet in an extrasolar planetary system will alter
the probability of life appearing on inner terrestrial planets - it would greatly decrease
the chance because the orbits of the inner, terrestrial-like planets would be disrupted
How to detect life on extrasolar planets
Spectra from different Earth-like planets
The simplest way we might detect life on an extrasolar planet is
to analyze the spectrum of reflected light from the planet.
Galactic constraints
Green – habitable zone
Both the amount of heavy elements AND the amount of radiation from the center determine
the position of a galaxy’s Galactic Habitable Zone.
It is difficult for planets with life to form around Sun-like stars in the inner parts of the disk
of our galaxy because there would be too much harmful radiation
It is difficult for Earth-like planets to form around Sun-like stars in the outer parts of the
disk of our galaxy because there would be insufficient amounts of heavy elements.
Impact rates and Jupiter
If our solar system hadn’t formed with a Jupiter-sized planet, the rate of impacts on the
Earth may have been much higher, possibly preventing the appearance of life.
Next lecture
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The search for extraterrestrial intelligence
The Drake equation
Interstellar travel
Conclusions
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The final review lecture to be posted online later
today!