Detecting Terrestrial Planets by Transits:

The Kepler Mission (2009)

A Fundamental NASA Mission Goal:

–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 place our Solar System in context with other planetary systems

–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, semi-major 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

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TECHNIQUES FOR FINDING EXTRASOLAR

PLANETS

Method Yield Mass Limit Status

Pulsar Timing m/M sin i; aLunar Successful (3)

Radial Velocity m/M sini ; aNeptuneSuccessful (~220)

Astrometry m/M ; a; all distant companionsGround: Telescope Neptune

OngoingGround: Interferometer <Jupiter In developmentSpace: Interferometer Uranus Being studied

Transit Photometry R ; asini=1Ground Neptune Successful (7)Space Super-Earth Launched: COROTSpace Venus Planned: Kepler

Reflection or Eclipse : albedo/R Photometry from Space Saturn Successful (2)

Microlensing: f(m,M,r,Ds,DL )Ground Super-Earth Successful (5)

Direct Imaging albedo/R; a ; all companionsGround Saturn Being studiedSpace Earth Being studied

Why is Water Essential for Life (as we know it)?

• It is one of the most common moleculesIt is one of the most common molecules• It is liquid in the right temperature range for organic chemistryIt is liquid in the right temperature range for organic chemistry• It is a polar molecule, which allows interesting surface chemistry It is a polar molecule, which allows interesting surface chemistry (hydrophobic and hydrophilic molecules)(hydrophobic and hydrophilic molecules)• It is a weak solvent for many simple organic chemicals (and conductive)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 structures like proteins to survive and fold (silicon bonds are too rigid)• It allows a lot of hydrogen bond chemistry to occurIt allows a lot of hydrogen bond chemistry to occur• It has “local structure” (hydrogen bonding makes it almost crystalline; allowing It has “local structure” (hydrogen bonding makes it almost crystalline; allowing

capillary action) but is globally liquidcapillary action) but is globally liquid• Its frozen state is less dense than its liquid state Its frozen state is less dense than its liquid state (so ice doesn’t collect at the bottom of bodies of water)(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 dissolves salts well, and allows a range of acidity (proton donors)

• It is observed to be an essential ingredient of life on Earth!It is observed to be an essential ingredient of life on Earth!

Habitable Zones (liquid surface water)

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.

KeplerThe most commontype of star…

Many other

conditions may

be “habitab

le”

Life here could have started at the bottom of the ocean at volcanic vents.

Europa

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 MissionQuestions 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

centralM

aP

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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.

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.

Example: if Jupiter is at 5 AU, how long is its orbital period?

2.11125;125532 PP

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

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PHOTOMETRY CAN DETECT EARTH-SIZED PLANETS

• The relative change in brightness is equal to the relative areas (Aplanet/Astar)

• 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)

Jupiter: 1% area of the Sun (1/100)

Earth or Venus0.01% area of the Sun (1/10,000)

MercuryTransit2006

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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 4 detection of 1 Earth-sized transit

Focus mechanisms

42 CCDs read every 3 seconds

Focal plane electronics 15 minute integrations

Sunshade

0.95 m diameter Schmidt corrector

Radiator and heat pipe for cooling focal plane

105 sq deg FOV Focal plane assembly:

CCDs, field flattening lenses fine guidance sensors

Graphite cyanate structure

1.4 m diameter primary mirror

Kepler Comes Together

CCDs have been delivered from E2V and are being mounted into focal plane packages with filters and sapphire correcting lenses

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

Kepler Parts Exist!

Schmidt Corrector Lens

Primary Mirror

Launch Vehicle

and Orbit

Delta rocket (well-tested)

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.

Ecliptic plane

Equatorial plane

Minimum solar avoidance

Minimum solar avoidance

Region of maximum star density

De

c

Right Ascension

Gal

actic

pla

ne

Galactic plane

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Kepler CCDs on the Sky

• Full Moon

Kepler Fields and Images

Images are de-focussed to FWHM ~6” to improve precision

Each of the 21 CCDs (2048x2048) samples 5 square degrees

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 ProblemsThere are several common sources of false positives. They produce the right

signal for the wrong reasons but some are easy to deal with:1. Grazing eclipses of one star by another

2. Cool dwarf stars transiting giants and supergiants

3. 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. Full eclipses in a faint background binary whose light is combined with a foreground bright

star

2. 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…

10000

1000

100

10

1 0.2 10 1.40.4 1.20.80.6 1.6

Potential for Planetary Detections

Expected # of planets found, assuming one planet of a given size & semi-major axis per star and random orientation of orbital planes.

# of PlanetDetections

Orbital Semi-major Axis (AU)

The Importance of Small Cool Stars

The Immediate Solar NeighborhoodThe 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 in Favor of finding M-star Habitable PlanetsFactors 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.

Factors Against Finding M-star Habitable PlanetsFactors Against Finding M-star Habitable Planets

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%)

• 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”!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

other planets like ours in the universe?”Drawing by H. Martin; © 1991 The New Yorker Magazine, Inc.

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THE HABITABLE ZONE BY 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 (Kasting, Whitmire, and Reynolds 1993).

B0

Stellar Radii and Planetary Orbital Semi-Major Axis (A.U.)1001010.10.010.001

••••

•

•

A0

F0G0

K0M0S

tella

r M

ass

(Sun

=1) 10

1

0.1

Solar System

Habitable Zone

2 Msun

1 Msun

0.5 Msun