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JOHANNES KEPLER
A Search for Habitable Planets
( By permission Sternwarte Kremsmünster)
A guy who thought a lot about planets
A Search for Habitable Planets
Kepler Mission
A 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
OVERVIEW
A Search for Habitable Planets
•
What makes for a Habitable Planet
•
Different Methods for Finding Planets
•
Planets Discovered to Date
•
Transit Photometry
•
Kepler Mission
A Fundamental NASA Mission Goal:
A Search for Habitable Planets
– To place our Solar System in context
with other planetary systems
– To provide data on possible platforms
for astrobiology beyond our Solar
System
•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?
These goals imply study of
terrestrial planets in the habitable
zones of solar-type (or smaller)
stars…
WHAT DOES HABITABLE MEAN TO YOU?
A Search for Habitable Planets
•
Right temperature
•
Air
•
Liquid water
•
Light to keep you warm and to
see
•
Radiation shield
•
Meteor (asteroid/comet)
protection
THINGS THAT AFFECT TEMPERATURE
A Search for Habitable Planets
Looking for a temperature that makes
liquid water on the surface of the planet
possible…
Factors that define the
Habitable Zone (HZ) for a star:
1.
Temperature of star
2.
Distance from the star
3.
Shape of planet’s orbit: how elliptical
4.
Planet’s atmosphere: greenhouse
gases
PLANET TEMPERATURE AFFECTS HABITABILITY
A Search for Habitable Planets
aka “Goldilocks zone”
THE HABITABLE ZONE
FOR VARIOUS STELLAR TYPES
A Search for Habitable Planets
The Habitable Zone (HZ) in green is the distance from a star where liquid
water is expected to exist on the planets surface.
WHAT IS IMPORTANT ABOUT AN ATMOSPHERE?
A Search for Habitable Planets
•
Composition (Earth)
free oxygen (about 21%)
mostly inert (about 78% nitrogen)
very little toxic gases
•
Composition affects temperature
Minimize day-night extremes
Greenhouse gases (water, CO2) hold in the heat
•
Acts as an invisible protective shield
Cosmic rays (high energy gamma-rays, protons, electrons)
Solar wind and solar flares (charged particles)
UV - ultraviolet
Micrometeoroids (e.g., puts holes in Space Shuttle window)
•
Transports water
Rain
PLANET SIZE AFFECTS HABITABILITY
A Search for Habitable Planets
•
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
KEY POINTS THAT GUIDE THE SEARCH
A Search for Habitable Planets
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
TECHNIQUES FOR FINDING EXTRASOLAR PLANETS
A Search for Habitable Planets
Method
Mass Limit
Status
Pulsar Timing
Lunar
Successful (4)
Radial Velocity
super-Earth
Successful (300+)
sub-Jupiter
super-Earth
In development
Under study
sub-Jupiter
sub-Jupiter
Earth
Successful (20+)
numerous groups
HST, CoRoT
Kepler
Astrometry
Ground
Space
Transit Photometry
Ground
Space
Space
Reflection Photometry
Space
sub-Jupiter
Kepler
super-Earth
OGLE (4)
Earth
Under study
Microlensing
Ground
Direct Imaging
Space
t=period, a=semi-major axis, mp=planet mass, Ap=planet area,
I=orbit inclination, e=eccentricity, Ds=distance to star
(Source: J. Lissauer)
EXTRA-SOLAR PLANET DETECTIONS
A Search for Habitable Planets
N.B. 430 exoplanets
as of April 15, 2010
Note: Masses are only lower limits except for transit cases
and typically about 2x greater than shown
DISCOVERY OF EXTRASOLAR PLANETS
A Search for Habitable 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).
WE NEED A DIFFERENT APPROACH
A Search for Habitable Planets
• 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 solarlike stars
Exoplanet encyclopedia http://exoplanet.eu
Detecting Planets by Photometry
A Search for Habitable Planets
HD 209458
USING PHOTOMETRY TO DETECT
EARTH-SIZE PLANETS
•
A Search for Habitable Planets
The relative change in brightness (DL/L) is equal to the relative
areas (Aplanet/Astar)
Mercury
Transit
2006
Jupiter:
1% area of the Sun (1/100)
Earth or Venus
0.01% area of the Sun (1/10,000)
•
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
Kepler MISSION CONCEPT
A Search for Habitable Planets
• 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
Kepler PHOTOMETER
A Search for Habitable Planets
Photometer = CCD sensors+ Telescope
Kepler 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 0.95meter aperture and
about 12 degree
diameter field-ofview
• Pointed at and
records data from a
single group of
stars for the four
year duration of the
mission
Kepler Focal Plane
A Search for Habitable Planets
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.
Kepler SPACECRAFT
A Search for Habitable Planets
Schmidt Corrector 0.95 m dia.
Sunshade
Spider with Focal Plane
and Local Detector Electronics
Upper Telescope Housing
Focal Plane
95 Mega pixels, 42 CCDs
Lower Telescope Housing
Fully assembled Kepler photometer
Mounted on the spacecraft
Primary Mirror
1.4 m dia., 85% lt. wt.
Spacecraft bus integration
FIELD OF VIEW IN CYGNUS
A Search for Habitable Planets
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)
SEARCHING THE EXTENDED SOLAR NEIGHBORHOOD
A Search for Habitable Planets
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.
EARLY RESULTS
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.
SUMMARY
A Search for Habitable Planets
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
29
Human Orrery
A Search for Habitable Planets
A Search for Habitable Planets
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
A Search for Habitable Planets
Do you think there are planets orbiting
other stars?
... How many?
A Search for Habitable Planets
Are all stars the same size and
temperature as the Sun?
A Search for Habitable Planets
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?
A Search for Habitable Planets
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?
WHAT’S THIS?
A Search for Habitable Planets
A Search for Habitable Planets
What happens when a planet transits a
star?
Make a model that you can
use to demonstrate a planet
transit.
TRANSIT MODEL DESIGN ACTIVITY
For 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
A Search for Habitable Planets
Create a model “star” by
setting up a light bulb and
socket in the middle of the
classroom for teams to
demonstrate their transits.
A Search for Habitable Planets
What kind of orbit would allow us to see
a transit?
Ball-on-stick demo
A Search for Habitable Planets
What’s wrong with this statement:
“If a star has an orbiting planet,
astronomers can usually detect it by
transit observations”
A Search for Habitable Planets
Kepler is designed to
monitor
brightness of
100,000 stars
simultaneously
for over 3 years.
A Search for Habitable Planets
Scale of the model?
Return to questions about planets (slide 6)
• Are there more questions?
• Do transit observations help us
answer the questions?
DEMO LEGO MODEL
A Search for Habitable Planets
A Search for Habitable Planets
Transit Tracks
trial investigation from
Full Option Science System (FOSS)
Planetary Science course
A Search for Habitable Planets
BRIGHTNESS
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
A Search for Habitable Planets
BRIGHTNESS
What would the transit of a planet look
like if you made a graph of light
intensity vs time?
TIME
The graph is a “light curve”
A Search for Habitable Planets
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?
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