Download The Search for Habitable Worlds

The Search for
Habitable Worlds
How Would We Know One If We Saw One?
Dr. Victoria Meadows
NASA Astrobiology Institute
Jet Propulsion Laboratory/California Institute of Technology
What Is Astrobiology?
• Astrobiology is the scientific study of life in the
universe, its past, present and future.
• Astrobiology seeks to answer three questions:
– How does life begin and develop?
– Does life exist elsewhere in the universe?
– What is life’s future on Earth and beyond?
• Astrobiology is an interdisciplinary science
– combines biology, chemistry, geology, astronomy,
planetary science, paleontology, oceanography,
physics, and mathematics to answer these questions.
The Search for Planets Around Other Stars
109
106
Where would we start the search
for life outside our Solar System?
First, find a habitable world
The Search for Planets Around Other Stars
There are many
challenges to
observing extrasolar
planets
– in the visible, they
don’t give off their
own light
– they are VERY far
away, which
makes them very
faint
– They are lost in
the glare of their
star
Indirect Detection of
Extrasolar Planets
These techniques use changes in the
position or brightness of a star to infer
the existence of a planet
The Doppler Technique
http://planetquest.jpl.nasa.gov
Astrometry
Transit
Microlensing

Suitable Parent Stars
• To be a suitable “parent” a star must
– live long enough
• stars 1.5M (O,B,A) age too quickly
– Be bright enough so that the planet
doesn’t have to be too close
• stars 0.5M (M) will tidally lock
– Be “stable”
– Favor stars with high “metallicity”
– Special constraints on a binary system
• Therefore we search for planets
around F, G and K stars (yellow to
orange)
A Multitude of Worlds
116
• 107 Planets
• 93 Planetary
Systems
• 12 Multiple Star
Systems
Not bad for not being
able to see anything!
But there’s one
problem...
Too Big!
• These planets are “giant planets”
– smallest found so far is about the size of
Neptune (0.1 MJ)
– 12% of stars surveyed have giant planets
• Small, rocky, Earth-like terrestrial planets
around “friendly” stars still elude us
The Kepler Mission
Measures stellar brightness
changes lasting for 2-16 hours
caused by transiting terrestrial
planets.
Monitoring 100,000 stars for 4
years!
Launch 2007
Kepler
Transit Telescope
T. Brown and D. Charbonneau
• Transit gives
planet size and
orbital period
The Space Interferometry Mission
• Launch in 2009
• Optical
interferometry
• Astrometry 100x
more accurate (1-2µ
arcseconds)
• Search for planets >
1 M around the few
nearest stars, and
5-10 M planets
around stars within
10pc.
• Technology
demonstration for
spacebourne
interferometry.
Direct Detection
of Extrasolar Planets
Infrared Nulling Interferometer
• Uses multiple mirrors to
simulate the angular
resolution of a much larger
telescope.
• Two architectures
– “free-flyer” in precision
formation
– fixed structure (“TPF on a
stick”).
• Uses destructive interference
to place the star in a “null”,
reducing its light by a factor
of a million
Visible Light Coronagraph
•
•
•
•
A coronagraph blocks the
light from a bright object so
that fainter nearby things can
be seen.
Implemented on large optical
telescope.
The coronagraph must
minimize both the direct light
from the star, and minimize
the telescope diffraction
pattern to maximize angular
resolution.
Current designs use “masks”
to simulate a telescope of a
different configuration to
preferentially scatter light in a
restricted area on the focal
plane.
Terrestrial Planet Finders
Terrestrial Planet Finder
NASA
Direct detection of planets
Launch 2011-2015
Darwin ESA
What Is a Habitable World?
A world that can maintain liquid water
on its surface
What Makes a Habitable World?
• Location, Location,
Location
• Planet Mass:
Atmospheric mass and
plate tectonics
• Atmospheric
Composition: reflectivity
and climate balance
• Circular(ish) Orbits
The Family of Earths
Modern
Proterozoic
Archean
1. Modern day Earth is only one of the “habitable Earths”
2. A habitable world does not require high levels of
atmospheric oxygen.
The Instantaneous Habitable Zone
“The region around a star in which an Earth-like planet could maintain
liquid water at some instant in time” (0.93-1.37AU for our Solar System)
H2O
Image courtesy of J.F.Kasting.
CO2
After Kasting, Whitmire and Reynolds, 1993.
The Continuously Habitable Zone
• The region in which a
planet could remain
habitable for some
specified period of time
• Our Solar System has
had a CHZ spanning
0.95-1.15AU in the past
4.6 Gy.
• The Sun may become
10% brighter in the
next 1.1 Gy, so earth
may be too hot in
another 500-900My!
Image courtesy of J.F.Kasting.
Learning About Distant Worlds
Radio
Infrared
Visible
UltraViolet X-Ray
How Can We TelljjIf A
Planet is Habitable?
“Environmental”
Characteristics
parent star, placement in solar system,
other planets
“Photometric” Characteristics
brightness, color, how it varies over time
Gauging the Greenhouse
Planetary Energy balance is given by:
σTe4 = S(1-A)/4
The effective radiating temperature Te denotes the
average temperature of the emitting layer
Teffective
Venus
Earth
Mars
Tsurface
-43C
-17C
-55C
470C
15C
-50C
Δ 37 C
Δ 520 C
Greenhouse
513C
32C
5C
A planet’s greenhouse effect is at least as important in determining
that planet’s surface temperature as is its distance from the star!
crisp
Remote-Sensing
O3
In the visible, sunlight is
reflected and scattered back to
the observer, and is absorbed by
materials on the planet’s surface
and in its atmosphere.
Net
60
Stratopause
50
The planet is warm and
gives off its own infrared
radiation. As this radiation
escapes to space, materials
in the atmosphere absorb it
and produce spectral
features.
Emission
40
Ozone
Absorption
30
20
Tropopause
10
Absorption
Water Vapor
0
200
250
300
The Earth From Space in the Infrared
H2O
N2O
CH4
O3
CO2
The Earth From Space In The Visible
Crisp, Meadows
Viewing Angle Differences
Phase and Seasonal Variations
How can we tell if a planet is
inhabited?
Hi!
Without direct contact with an alien civilization, or travelling to the nearest
solar system, our best chance for finding life in the Universe is to look for
global changes in the atmosphere and surface of a terrestrial planet.
The Signs of Life
CH4
O3
Life Changes a Planet’s Atmosphere
Life Changes a Planet’s Surface
Life Changes a Planet’s Appearance Over Time
Gas or surface signatures that change with day-night, or seasons
What a planet looks like from space depends on many things…..
Task 1: Spectra
Task 2: The Climate Model
(SMARTMOD)
Synthetic
Spectra
Radiative
Transfer
Model
Atmospheric
Thermal Structure
and Composition
Radiative
Fluxes
and Heating
Rates
Atmospheric
and surface
optical
properties
Stellar
Spectra
Climate
Model
Atmospheric
Composition
UV Flux and
Atmospheric
Temperature
Atmospheric
Chemistry
Model
Atmospheric Escape,
Meteorites, Volcanism,
Weathering products
Exogenic
Model
Biological
Effluents
Atmospheric
Thermal
Structure and
Composition
Geological
Model
Atmospheric
Thermal Structure
and Composition
Biology Model
Virtual Planetary Laboratory
Task 3: The Coupled Climate-Chemistry Model
Task 4: The Abiotic Planet Model
The Virtual
Planetary
Laboratory
Task 5: The Inhabited Planet Model
Observer
VPL TEAM MEMBERS
NAME
INSTITUTION
CONTRIBUTION
Dr. Victoria Meadows*
Dr. Mark Allen*
Dr. Linda Brown*
Dr. Rebecca Butler
Dr. David Crisp*
Dr. Chris Parkinson
Dr. Giovanna Tinetti
Dr. Thangasamy Velusamy*
Dr. Mark Richardson*
Dr. Ian McKewan
Prof. Yuk Yung*
Dr. Wesley Huntress, Jr*
Prof. James Kasting *
Ms. Kara Krelove
Mr. Pushker Karecha
Dr. Antigona Segura
Ms Irene Schneider
Mr Shawn Goldman
Prof. Norm Sleep*
Dr. Martin Cohen*
Dr. Robert Rye*
Dr. David DesMarais*
Dr. Kevin Zahnle*
Dr. Francis Nimmo
Dr. Monika Kress
Prof. Janet Seifert
Dr. Nancy Kiang
Dr. John Armstrong
Dr. Cherilynn Morrow*
Dr. Jamie Harold
Dr. Ray Wolstencroft
Dr. Jeremy Bailey
Ms. Sarah Chamberlain
JPL /SSC
JPL/Caltech
JPL
JPL
JPL
JPL/Caltech
JPL/USC/NRC
JPL
Caltech
Caltech
Caltech
CIW
Penn. State
Penn. State->Arizona
Penn. State
Penn. State
Penn. State
Penn. State
Stanford
UC, Berkeley
USC
NASA Ames
NASA Ames
The Royal Society
U. Washington
Rice University
GISS
Weber University
Space Science Institute
Space Science Institute
Royal Observatory Edinburgh
Australian Centre for Astrobiology
Australian Centre for Astrobiology
PI: radiative transfer/astronomical observing
chemical models
laboratory spectroscopy
spectroscopic database
radiative transfer modeling
upper atmosphere modeling
planetary models, effect of orbit
astronomical instrumentation models
global models, upper atmosphere boundary
parallelization algorithms, model interfacing
chemical models
geophysical laboratory data
climate modeling, escape processes
climate modeling
Archean ecosystems
astrophysics, climate modeling
geosciences
radiation and biology
geology, geochemical cycles
stellar spectra
microbiology, parameterization of life
microbiology
impact processes, chemical models
plate tectonics, geochemical cycles
solar system architectures, volatile delivery
biochemistry, ancient metabolisms
biometeorology, leaf structure
climate studies, earth systems
education and public outreach
education and public outreach
polarization, chlorophyll signatures
terrestrial planet observations
terrestrial planet observations
SURF Students 2003: Will Fong, Sam Hsiung, Robert Li (Caltech).
The Family of Earths
Modern
Proterozoic
Archean
• The oxygen content of the Earth’s atmosphere
has significantly changed over 4.6 billion years.
Modern Earth
355ppm CO2
Proterozoic
0.1PAL O2
100ppm CH4
15% decrease in
ozone column depth
Archean
N2 99.8%
2000ppm CO2
1000ppm CH4
100ppm H2
Earths Around Other Stars
F2V
G2V
O3
O2
Krelove,Kasting,Cohen,Crisp,Meadows
• Modeling self-consistent atmospheres for
planets around other stars
• Producing spectra of these cases
– what we would see looking down from space
– what a microbe would see looking up at the sky
O3
CO2
Terrestrial Planet Finders
Terrestrial Planet Finder
NASA
Direct detection of planets
Launch 2011-2015
Darwin ESA
The Terrestrial Planet Finder Mission
• Goal: Direct detection and characterization of
Earth-sized planets in their habitable zones.
– Are there nearby Earth-like planets?
• Search 150 stars up to 45 light years away
– Do they have atmospheres?
– Is there any sign of life?
– How to planets form?
NASA’s Life Finder
• chemical signatures
of life at R~1000
Summary and Conclusions
•In about a decade, we will be able to characterize extrasolar terrestrial planets.
•To understand what we find, we need to understand
•The possible range of habitable planets
•The evolution of habitable worlds (the Earth’s history included)
•Techniques for characterization of extrasolar terrestrial planets
•Observational:
•remote-sensing (photometry, atmospheric thermal structure and
composition, surface types, clouds, aerosols, etc.) and
•Theoretical:
•environmental models, including atmospheric chemistry, climate, carbon
cycle, hydrological cycle, and biospheric models.
http://planetquest.jpl.nasa.gov
Summary
• The search for extrasolar
planets can be done indirectly
or directly
– indirectly: Doppler (radial
velocity), astrometry, transit,
gravitational microlensing
– Directly: nulling interferometry,
coronography
• The direct techniques are
technologically challenging but
will provide the capability to
detect and characterize Earthsized planets around nearby
stars.