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G. Chin (GSFC) 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?

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the search for habitable worlds

G. Chin (GSFC)

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
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 stars5
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
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
The Doppler Technique

http://planetquest.jpl.nasa.gov

suitable parent stars
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
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
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

R. Hasler

the kepler mission

Kepler

Launch 2007

T. Brown and D. Charbonneau

The Kepler Mission

Measures stellar brightness changes lasting for 2-16 hours caused by transiting terrestrial planets.

Monitoring 100,000 stars for 4 years!

  • Transit gives planet size and orbital period

Transit Telescope

the space interferometry mission
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.
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

Infrared Nulling Interferometer
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.

Visible Light Coronagraph
terrestrial planet finders
Terrestrial Planet Finders

Terrestrial Planet Finder

NASA

Direct detection of planets

Launch 2011-2015

Darwin ESA

what is a habitable world
What Is a Habitable World?

A world that can maintain liquid water on its surface

what makes a habitable world
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

The Family of Earths

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 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

CO2

Image courtesy of J.F.Kasting.

After Kasting, Whitmire and Reynolds, 1993.

the continuously habitable zone
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
Learning About Distant Worlds

Radio

Infrared

Visible

Ultra-

Violet

X-Ray

slide27

How Can We Tell If A Planet is Habitable?

jj
  • “Environmental” Characteristics
    • parent star, placement in solar system, other planets
  • “Photometric” Characteristics
    • brightness, color, how it varies over time
gauging the greenhouse
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

Δ 37 C Δ 520 C

A planet’s greenhouse effect is at least as important in determining

that planet’s surface temperature as is its distance from the star!

After Table 9.1, Bennet, Shostak, Jakosky, 2003

remote sensing

Net

60

Stratopause

50

Emission

40

Ozone

Absorption

30

20

Tropopause

10

Absorption

Water Vapor

0

200

250

300

Remote-Sensing

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.

O3

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.

slide33

Viewing Angle Differences

Phase and Seasonal Variations

how can we tell if a planet is inhabited

Hi!

How can we tell if a planet is inhabited?

DEAFENING

SILENCE!

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
The Signs of Life

CH4

O3

Life Changes a Planet’s Atmosphere

slide37

Life Changes a Planet’s Appearance Over Time

Gas or surface signatures that change with day-night, or seasons

the virtual planetary laboratory

Observer

Synthetic

Spectra

Atmospheric and surface optical properties

Radiative

Transfer

Model

Task 1: Spectra

Task 2: The Climate Model

(SMARTMOD)

Stellar

Spectra

Radiative Fluxes

and Heating Rates

Atmospheric

Thermal Structure and Composition

Task 3: The Coupled Climate-Chemistry Model

Climate

Model

UV Flux and

Atmospheric

Temperature

Atmospheric

Composition

Task 4: The Abiotic Planet Model

Atmospheric

Chemistry

Model

Atmospheric

Thermal Structure and Composition

Atmospheric Escape,

Meteorites, Volcanism,

Weathering products

Task 5: The Inhabited Planet Model

Virtual Planetary Laboratory

Exogenic

Model

Geological

Model

Atmospheric

Thermal Structure

and Composition

Biological

Effluents

Biology Model

The Virtual Planetary Laboratory
vpl team members
VPL TEAM MEMBERS

NAME INSTITUTION CONTRIBUTION

Dr. Victoria Meadows* JPL /SSC PI: radiative transfer/astronomical observing

Dr. Mark Allen* JPL/Caltech chemical models

Dr. Linda Brown* JPL laboratory spectroscopy

Dr. Rebecca Butler JPL spectroscopic database

Dr. David Crisp* JPL radiative transfer modeling

Dr. Chris Parkinson JPL/Caltech upper atmosphere modeling

Dr. Giovanna Tinetti JPL/USC/NRC planetary models, effect of orbit

Dr. Thangasamy Velusamy* JPL astronomical instrumentation models

Dr. Mark Richardson* Caltech global models, upper atmosphere boundary

Dr. Ian McKewan Caltech parallelization algorithms, model interfacing

Prof. Yuk Yung* Caltech chemical models

Dr. Wesley Huntress, Jr* CIW geophysical laboratory data

Prof. James Kasting * Penn. State climate modeling, escape processes

Ms. Kara Krelove Penn. State->Arizona climate modeling

Mr. Pushker Karecha Penn. State Archean ecosystems

Dr. Antigona Segura Penn. State astrophysics, climate modeling

Ms Irene Schneider Penn. State geosciences

Mr Shawn Goldman Penn. State radiation and biology

Prof. Norm Sleep* Stanford geology, geochemical cycles

Dr. Martin Cohen* UC, Berkeley stellar spectra

Dr. Robert Rye* USC microbiology, parameterization of life

Dr. David DesMarais* NASA Ames microbiology

Dr. Kevin Zahnle* NASA Ames impact processes, chemical models

Dr. Francis Nimmo The Royal Society plate tectonics, geochemical cycles

Dr. Monika Kress U. Washington solar system architectures, volatile delivery

Prof. Janet Seifert Rice University biochemistry, ancient metabolisms

Dr. Nancy Kiang GISS biometeorology, leaf structure

Dr. John Armstrong Weber University climate studies, earth systems

Dr. Cherilynn Morrow* Space Science Institute education and public outreach

Dr. Jamie Harold Space Science Institute education and public outreach

Dr. Ray Wolstencroft Royal Observatory Edinburgh polarization, chlorophyll signatures

Dr. Jeremy Bailey Australian Centre for Astrobiology terrestrial planet observations

Ms. Sarah Chamberlain Australian Centre for Astrobiology terrestrial planet observations

SURF Students 2003: Will Fong, Sam Hsiung, Robert Li (Caltech).

the family of earths44
The Family of Earths

Modern

  • The oxygen content of the Earth’s atmosphere has significantly changed over 4.6 billion years.

Proterozoic

Archean

slide45

Modern Earth

355ppm CO2

slide46

Proterozoic

0.1PAL O2 100ppm CH4

15% decrease in ozone column depth

Segura, Krelove, Kasting, Sommerlatt,Meadows,Crisp,Cohen

slide47

Archean

N2 99.8%

2000ppm CO2

1000ppm CH4

100ppm H2

Karecha, Kasting, Segura, Meadows, Crisp, Cohen

earths around other stars

F2V

G2V

Earths Around Other Stars
  • 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

O3

O2

CO2

Krelove,Kasting,Cohen,Crisp,Meadows

terrestrial planet finders49
Terrestrial Planet Finders

Terrestrial Planet Finder

NASA

Direct detection of planets

Launch 2011-2015

Darwin ESA

the terrestrial planet finder mission
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
NASA’s Life Finder
  • chemical signatures of life at R~1000
summary and conclusions

G. Chin (GSFC)

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.
summary
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 Earth-sized planets around nearby stars.