Transcript Lecture PowerPoint

Detecting Terrestrial Planets by Transits:
The Kepler Mission (2009)
A Fundamental NASA Mission Goal:
– To place our Solar System in
context with other planetary
systems
–To place our Sun in context
with other solar-like stars
Q:2 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 provide data on possible
platforms for astrobiology
beyond our Solar System
These imply study of terrestrial
planets in the habitable zones
of solar-type stars…
Discovery of Extrasolar planets
The “wobble” method gets
the orbital period, semimajor axis, and a lower
limit on the mass of the
planet. This can detect
down to Neptune-mass
planets relatively close in,
(but could see our Jupiter if
you look long enough).
A Big Surprise : Close-in Jupiters
It is easiest to find a massive planet that is close to the star (it repeats
quickly and has a large velocity amplitude). The first discovery, 51
Peg, had a 4 day orbit (0.05 AU!) and the mass of Jupiter. Many are
now known, but that doesn’t mean they are most common, just
easiest to find and reasonably common (~10% of stars).
Properties of the systems found
TECHNIQUES FOR FINDING EXTRASOLAR
PLANETS
Method
Yield
Mass Limit
Pulsar Timing
m/M sin i; a
Radial Velocity
m/M sini ; a Neptune
Lunar
Status
Successful (3)
Successful (~220)
Astrometry
m/M ; a; all distant companions
Ground: Telescope
Neptune
Ongoing
Ground: Interferometer
<Jupiter
In development
Space: Interferometer
Uranus
Being studied
Transit Photometry
Ground
Space
Space
R ; a; sini=1
Neptune
Successful (7)
Super-Earth Launched: COROT
Venus
Planned: Kepler
Reflection or Eclipse :
albedo/R
Photometry from Space
Saturn
Successful (2)
Microlensing:
Ground
f(m,M,r,Ds,DL )
Super-Earth Successful (5)
Direct Imaging
Ground
Space
albedo/R; a ; all companions
Saturn
Being studied
Earth
Being studied
6
Why is Water Essential for Life (as we know it)?
•
•
•
•
•
•
•
•
•
It is one of the most common molecules
It is liquid in the right temperature range for organic chemistry
It is a polar molecule, which allows interesting surface chemistry
(hydrophobic and hydrophilic molecules)
It is a weak solvent for many simple organic chemicals (and conductive)
It allows structures like proteins to survive and fold (silicon bonds are too rigid)
It allows a lot of hydrogen bond chemistry to occur
It has “local structure” (hydrogen bonding makes it almost crystalline; allowing
capillary action) but is globally liquid
Its frozen state is less dense than its liquid state
(so ice doesn’t collect at the bottom of bodies of water)
It dissolves salts well, and allows a range of acidity (proton donors)
• It is observed to be an essential ingredient of life on Earth!
Habitable Zones (liquid surface water)
The most common
type of star…
Kepler
Because most stars
keeps getting brighter,
the continuously
habitable zone is
smaller than the
habitable zone at a
given time. But that is
not true for low-mass
stars, which also live
10-100 times longer
than solar type stars.
Europa
Life here could have started at
the bottom of the ocean at
volcanic vents.
Many other
conditions
may be
“habitable”
Planetary Transits
A transit is like an eclipse, only
smaller… This has been seen for a
few cases (confirming the radial
velocity detections).
HST measurement of HD209458
Purpose of the Kepler Mission
Questions Kepler Asks
•
•
•
•
•
Are terrestrial planets common or rare?
How many are in the habitable zone?
What are their sizes & distances?
Can we learn anything about their atmospheres?
Are there dependences on stellar properties?
Answers Kepler (hopefully) Will Provide
•
•
•
•
Discovers thousands of planets, both terrestrial and giant
Characterizes the planetary population within 1.5 AU
Associations between stellar types and terrestrial planets
Finds reflected light from inner Jovian planets which
provide density and phase functions
• Finds true Earth analogs
Kepler’s Third Law of Planetary Motion
3) The orbital period of a planet is proportional to its semi-major
axis, in the relation P2 ~ a3
The more general form of this law (crucial for determining all
masses in Astronomy) is
a3
P2 
M central
For the planets (with the Sun as the central mass), you can take
the units to be AU for a (semi-major axis) and years for P (with M
in solar masses). Then all the numbers are “1” for the Earth.
Example: if Jupiter is at 5 AU, how long is its orbital period?
P 2  53  125; P  125  11.2
Kepler didn’t understand the physical basis of these laws (though he
suspected they arose because the Sun attracted the planets, perhaps through
magnetism he speculated.
Information from Transits
Kepler’s Third Law: The
orbital period of a planet is
proportional to its
semi-major axis,
in the relation P2 ~ a3
PHOTOMETRY CAN DETECT EARTH-SIZED PLANETS
•
The relative change in brightness 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
•
This is also needed for getting a high duty cycle
•
Method is robust but you must be patient:
Require at least 3 transits, preferably 4 with same brightness change,
duration and temporal separation
(the first two establish a possible period, the third confirms it)
14
Kepler Mission Design
• Kepler is optimized for finding habitable/terrestrial planets
( 0.5 to 10 M ) in the HZ ( out to 1 AU ) of cool stars (F-M)
• Continuously and simultaneously monitor >100,000 dwarf stars
using a 1-meter Schmidt telescope: FOV >100 deg2 with 42 CCDs
• Photometric precision of < 20 ppm in 6.5 hours on
Vmag=12 sunlike star  4s detection of 1 Earth-sized transit
Focal plane electronics
15 minute integrations
Sunshade
42 CCDs
read every
3 seconds
1.4 m diameter
primary mirror
0.95 m diameter
Schmidt corrector
Focus
mechanisms
105 sq deg FOV
Focal plane assembly:
CCDs, field flattening lenses
fine guidance sensors
Radiator and heat pipe
for cooling focal plane
Graphite cyanate
structure
15
Kepler Comes Together
Construction of the spacecraft is
underway at Ball Aerospace
Corp. in Boulder, Colorado.
The Science Operations Center
has opened at Ames Research
Labs in Sunnyvale, CA
CCDs have been delivered from
E2V and are being mounted into
focal plane packages with filters and
sapphire correcting lenses
Kepler Parts Exist!
Primary Mirror
Schmidt Corrector Lens
Delta rocket (well-tested)
Launch Vehicle
and Orbit
Earth-trailing orbit;
slowly falls behind;
telemetry rates fall,
so number of target
stars falls
CONTINUOUSLY VIEWABLE HIGH DENSITY STAR FIELD
One region of high star field density far (>55°) from the ecliptic plane where the
galactic plane is continuously viewable is centered at RA=19h45m Dec=35°.
The 55° ecliptic plane avoidance limit is defined by the sunshade size for a large
aperture wide field of view telescope in space.
19
Kepler CCDs on the Sky
•
Full Moon
Kepler Fields and Images
Each of the 21 CCDs (2048x2048) samples 5 square degrees
Images are de-focussed to FWHM ~6” to improve precision
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.
The Easy False-Positives Problems
There are several common sources of false positives. They produce the right
signal for the wrong reasons but some are easy to deal with:
1.
2.
3.
Grazing eclipses of one star by another
Cool dwarf stars transiting giants and supergiants
White dwarfs transiting solar-type stars
A full eclipse is flat-bottomed, a grazing eclipse is more bowl or “V” shaped.
Giants and supergiants can be known from their spectra and photometric behavior.
Gravitational focussing makes a white dwarf transit into a bump instead of a dip!
The Hard False-Positives Problem
The other types generate the right signal for the
wrong reasons and are harder to remove:
1.
2.
Full eclipses in a faint background binary whose light is combined with a foreground bright
star
Triple star systems with a bright primary and a faint eclipsing secondary pair
For this reason, extensive ground-based
astronomy will be required to confirm
detections before they are announced…
+
=
Potential for Planetary Detections
10000
1000
# of Planet
Detections
100
10
1
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
Orbital Semi-major Axis (AU)
Expected # of planets found, assuming one planet of a given size &
semi-major axis per star and random orientation of orbital planes.
The Importance of Small Cool Stars
The Immediate Solar Neighborhood
The 120 stars closest to the Sun are shown by spectral type.
Hot stars are green, G stars (like the Sun) are yellow, cooler K
stars are orange, and the coolest M stars are red. There are
more than 10 times as many M stars as G stars, and they
constitute ¾ of the total.
This is true in general in this Galaxy and others.
Factors Against Finding M-star Habitable Planets
Factors in Favor of finding M-star Habitable Planets
Many more stars
Habitable zone much longer lived and stable
A half-solar mass star lives about 100 billion years, and
a 0.1 solar mass star lives a few trillion years.
Inner giant planets less common (this is observed,
and expected)
Wet planets may be more likely in the habitable
zone(??)
Habitable planets are easier to find by transits
(detectability)
Because habitable planets will have short-period orbits
Kepler is most sensitive to them (and the stars are smaller,
although fainter). These will be the first habitable planets to
be announced.
Small Habitable zone
Yes, but they are much longer-lasting
Habitable planets are tidally locked to the star
Because the planets must be close to be warm, one
side of the planet always has day, and the other always
night. But if there is an atmosphere thick enough for life,
it will redistribute the heat.
Giant flares occurring frequently,
or strong UV/X-ray fluxes
M stars are often known as flare stars. The duration of
the flaring stage is only about a billion years or 0.1-1% of
the star’s life. Anyway, tidally locked planets keep one
face away from the star. Finally, life which lives under an
ocean or icecap couldn’t care less about flares.
Habitable planets will be hard to study by
imaging (detectability)
True, although M stars will typically be closer since
there are more of them.
Summary of Kepler Mission Goals
• Find the frequency of terrestrial planets in the Galaxy
• Characterize the properties of inner planetary systems.
• Determine the properties of stars
(single & multiple) hosting planets.
• Discover terrestrial planets in habitable zones
•
(or show that they are rare).
• Detect true Earth analogs
A NULL result would also be very significant
(frequency of stars with terrestrial planets is less than 5%)
Kepler is uniquely qualified to detect Earth-sized
extrasolar planets “before this decade is out”!
New Yorker Cartoon
“Well, this mission answers at least one big question: Are there
Drawing by H. Martin; © 1991
other planets like ours in the universe?”
The New Yorker Magazine, Inc.
28
THE HABITABLE ZONE BY STELLAR TYPES
2 Msun
1 Msun
0.5 Msun
The Habitable Zone (HZ) in green is the distance from a star where liquid water is
expected to exist on the planets surface (Kasting, Whitmire, and Reynolds 1993).
29