A Search for Habitable Planets
JOHANNES KEPLER
A guy who thought a lot about planets
( By permission Sternwarte Kremsmünster)
A Search for Habitable Planets
Kepler MissionA SEARCH FOR HABITABLE PLANETS
Nancy Schaff
Cornell University Dept. of Astronomy
Center for Radiophysics & Space Research
Adapted from a presentation by David Koch
NASA Ames Research Center
A Search for Habitable Planets
OVERVIEW
• What makes for a Habitable Planet
• Different Methods for Finding Planets
• Planets Discovered to Date
• Transit Photometry
• Kepler Mission
A Search for Habitable Planets
A Fundamental NASA Mission Goal:
•Does life in any form however simple or complex, carbon-based or other, exist elsewhere than on Earth?
•Are there Earth-like planets beyond our solar system?
– To place our Solar System in context with other planetary systems
– To provide data on possible platforms for astrobiology beyond our Solar System
These goals imply study of terrestrial planets in the habitable zones of solar-type (or smaller) stars…
A Search for Habitable Planets
WHAT DOES HABITABLE MEAN TO YOU?
• Right temperature
• Air
• Liquid water
• Light to keep you warm and to see
• Radiation shield
• Meteor (asteroid/comet) protection
A Search for Habitable Planets
THINGS THAT AFFECT TEMPERATURE
Looking for a temperature that makes liquid water on the surface of the planet possible…
1. Temperature of star
2. Distance from the star
3. Shape of planet’s orbit: how elliptical
4. Planet’s atmosphere: greenhouse gases
Factors that define the Habitable Zone (HZ) for a star:
A Search for Habitable Planets
aka “Goldilocks zone”
PLANET TEMPERATURE AFFECTS HABITABILITY
A Search for Habitable Planets
THE HABITABLE ZONE
FOR VARIOUS STELLAR TYPES
The Habitable Zone (HZ) in green is the distance from a star where liquid water is expected to exist on the planets surface.
A Search for Habitable Planets
WHAT IS IMPORTANT ABOUT AN ATMOSPHERE?
• Composition (Earth)free oxygen (about 21%)mostly inert (about 78% nitrogen)very little toxic gases
• Composition affects temperatureMinimize day-night extremes Greenhouse gases (water, CO2) hold in the heat
• Acts as an invisible protective shieldCosmic rays (high energy gamma-rays, protons, electrons)Solar wind and solar flares (charged particles)UV - ultravioletMicrometeoroids (e.g., puts holes in Space Shuttle window)
• Transports waterRain
A Search for Habitable Planets
PLANET SIZE AFFECTS HABITABILITY
• Planets form by accretion from a disk of gas and dust
• Too small (about <0.5 ME):
Can’t hold onto a life sustaining atmosphere (Mercury, Mars)
• Too big (about >10 ME):
Can hold onto the very abundant light gases (H2 and He) and turn into a gas giant (Jupiter, Saturn, Uranus, Neptune)
Copyright Lynnette Cook
A Search for Habitable Planets
KEY POINTS THAT GUIDE THE SEARCH
1. Stellar type => Which stars to search
2. HZ => Orbital periods and how long to look
3. Planet sizes => Sensitivity or precision needed
A Search for Habitable Planets
DETECTING EXTRA-SOLAR PLANETS(aka Exoplanets)
That is,Planets orbiting other stars
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TECHNIQUES FOR FINDING EXTRASOLAR PLANETS
Method Mass Limit Status
Pulsar Timing Lunar Successful (4)
Radial Velocity super-Earth Successful (300+)
AstrometryGround sub-Jupiter In developmentSpace super-Earth Under study
Transit Photometry Successful (20+)Ground sub-Jupiter numerous groupsSpace sub-Jupiter HST, CoRoTSpace Earth Kepler
Reflection PhotometrySpace sub-Jupiter Kepler
MicrolensingGround super-Earth OGLE (4)
Direct ImagingSpace Earth Under study
(Source: J. Lissauer)t=period, a=semi-major axis, mp=planet mass, Ap=planet area, I=orbit inclination, e=eccentricity, Ds=distance to star
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EXTRA-SOLAR PLANET DETECTIONS
Note: Masses are only lower limits except for transit cases and typically about 2x greater than shown
N.B. 430 exoplanets as of April 15, 2010
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DISCOVERY OF EXTRASOLAR PLANETS
The “wobble” method gets the orbital period, semi-major axis, and a lower limit on the mass of the planet. This has detected down to 7 Earth-mass planets very close in, (but favors gas giant planets).
A Search for Habitable Planets
WE NEED A DIFFERENT APPROACH
• Earth-like planets are about 300 times less massive and about 100 times smaller in area than Jupiter
• Radial velocity (Doppler spectroscopy) method unable to detect Earth-size planets
• Need a different approach that can detect smaller planets
• No method exists for detecting habitable planets from ground-based observatories
• The Kepler Mission uses photometry to detect transits and can detect Earth-size planets from space
• The Kepler Mission is optimized to detect habitable planets in the habitable zone of solar-like stars
Exoplanet encyclopedia http://exoplanet.eu
A Search for Habitable Planets
Detecting Planets by Photometry
HD 209458
A Search for Habitable Planets
USING PHOTOMETRY TO DETECT EARTH-SIZE PLANETS
• The relative change in brightness (L/L) is equal to the relative areas (Aplanet/Astar)
• To measure 0.01% must get above the Earth’s atmosphere
• Method is robust but you must be patient:Require at least 3 transits preferably 4 with same brightness change, duration and temporal separation
Jupiter: 1% area of the Sun (1/100)
Earth or Venus0.01% area of the Sun (1/10,000)
MercuryTransit2006
A Search for Habitable Planets
Kepler MISSION CONCEPT
• Kepler Mission is optimized for finding
habitable planets ( 0.5 to 10 MÅ )
in the HZ ( near 1 AU ) of solar-like stars
• Continuously and simultaneously
monitor 100,000 main-sequence stars
• Use a one-meter Schmidt telescope:
FOV >100 deg2 with an array of 42 CCD
• Photometric precision:
Noise < 20 ppm in 6.5 hours V = 12 solar-like star
=> 4s detection for Earth-size transit
• Mission:
Heliocentric orbit for continuous viewing
> 3.5 year duration
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Kepler PHOTOMETER
Photometer = CCD sensors+ TelescopeKepler will be 9th largest Schmidt ever built
and the largest telescope launched beyond earth-orbit
The Kepler photometer is a simple single purpose instrument• Schmidt telescope design with a 0.95-meter aperture and about 12 degree diameter field-of-view• Pointed at and records data from a single group of stars for the four year duration of the mission
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Kepler Focal Plane
The photometer is composed of just one "instrument," which is, an array of 42 CCDs (charge coupled devices). Each 50x25 mm CCD has 2200x1024 pixels. The CCDs are not used to take pictures. The images are intentionally defocused to improve the photometric precision.
A Search for Habitable Planets
Kepler SPACECRAFT
Schmidt Corrector 0.95 m dia.
Spider with Focal Planeand Local Detector Electronics
Focal Plane95 Mega pixels, 42 CCDs
Primary Mirror1.4 m dia., 85% lt. wt.
Sunshade
Upper Telescope Housing
Lower Telescope Housing
Spacecraft bus integration
Fully assembled Kepler photometerMounted on the spacecraft
A Search for Habitable Planets
FIELD OF VIEW IN CYGNUS
The Kepler star field is a part of the extended solar neighborhood in the Cygnus-Lyra regions along the Orion arm.
It is located on one side of the summer triangle (Deneb-Vega-Altair)
A Search for Habitable Planets
SEARCHING THE EXTENDED SOLAR NEIGHBORHOOD
The stars sampled are similar to the immediate solar neighborhood. The stars actually come from all over the Galaxy near our radius, since they wander after being born. Young stellar clusters and their ionized nebular regions highlight the arms of the Galaxy.
A Search for Habitable Planets
Results announced January 4, 2010 from first six weeks of data collection after science operations commenced on May 12, 2009
• Known as "hot Jupiters" because of their high masses and extreme temperatures• Range in size from similar to Neptune to larger than Jupiter• Orbits ranging from 3.3 to 4.9 days• Estimated temperatures range from 2,200 to 3,000 degrees F, hotter than molten lava and much too hot for life as we know it • All five of the exoplanets orbit stars hotter and larger than Earth's sun.
EARLY RESULTS
A Search for Habitable Planets
SUMMARY
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The Kepler Mission will:
Observe more than 100,000 dwarf stars
continuously for 3.5 to 6+ years
with a precision capable of detecting Earth’s in the HZ
The Kepler Mission can discover:
Planet sizes from that of Mars to greater than Jupiter
Orbital periods from days up to two years
About 600 terrestrial planetary systems if most have 1 AU orbits
About 1000 inner-orbit giant planets based on
already known frequency
A NULL result would also be very significant ! ! !
Results on giants expected 9 months after launch (March 2009)
and will continue for 3.5 to 6+ years
http://kepler.nasa.gov
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Human Orrery
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Introductory questions for
Detecting Extrasolar Planets
from Space Science Sequence Grades 6-8 Unit 4
Great Explorations in Math and Science (GEMS)
http://www.lhsgems.org
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Do you think there are planets orbiting other stars?
... How many?
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Are all stars the same size and temperature as the Sun?
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How might the type of star affect whether it has habitable planets?
A Search for Habitable Planets
If there is a certain zone around a star where life might exist, what must be critical qualities of that
zone?
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What things would you want to know about a newly discovered
planet?
Back to slide 14
A Search for Habitable Planets
Why would exoplanets be hard to detect?
What methods do astronomers use to find
planets?
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WHAT’S THIS?
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What happens when a planet transits a star?
Make a model that you can use to demonstrate a
planet transit.
A Search for Habitable PlanetsFor each team of 4–6 students:
• 1 snake book light
• 1 prepared Ping-Pong ball (see Getting Ready)
• several round, opaque plastic beads (ranging in size from 8 mm to 16 mm in diameter)
• 2 or more pipe cleaners
• 1 or 2 chopsticks or thin wooden dowels
• black thread
• 4" x 6” index cards
• tape
• paper or plastic bag to hold the materials
Create a model “star” by setting up a light bulb and socket in the middle of the classroom for teams to demonstrate their transits.
TRANSIT MODEL DESIGN ACTIVITY
A Search for Habitable Planets
What kind of orbit would allow us to see a transit?
Ball-on-stick demo
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What’s wrong with this statement:
“If a star has an orbiting planet, astronomers can usually detect it by
transit observations”
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Kepler is designed to monitor
brightness of 100,000 stars
simultaneously for over 3 years.
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Scale of the model?
Return to questions about planets (slide 6)
• Are there more questions?• Do transit observations help us
answer the questions?
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DEMO LEGO MODEL
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Transit Tracks
trial investigation from Full Option Science System (FOSS)
Planetary Science course
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Imagine you have a light sensor aimed at the star.
What would the transit of a book look like if you made a graph of
light intensity vs time?
TIME
BR
IGH
TN
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What would the transit of a planet look like if you made a graph of
light intensity vs time?
TIME
BR
IGH
TN
ES
S
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It can lead to finding:
- The SIZE of the planet (based on brightness change and size of star)
- ORBITAL PERIOD of the planet – time between transits
- ORBITAL SIZE – distance from star(from mass of star and orbital period, using Kepler’s 3rd Law)
- TEMPERATURE of planet (from planet’s distance and temperature of star)
Why would those characteristics be important?
The graph is a “light curve”
A Search for Habitable Planets
How does a planet’s size and orbit affect the transit?
A Search for Habitable Planets
Is there a relationship between the planet’s period and how far it is
from the star?
Kepler’s Third Law:the square of the period (in years) equals the cube of the semi-major axis of orbit (in AUs)
P2 = a3
A Search for Habitable Planets
A Search for Habitable Planets
A Search for Habitable Planets