Kepler - A Search for Extraterrestrial Planets

Nick GautierJet Propulsion LaboratoryCalifornia Institute of Technology January 30, 2009

Overview

• Extrasolar planets and NASA’s goals

• Star types, planet habitability

• How to find extrasolar planets, particularly Earth-sized ones

• The Kepler mission

• Expected results from Kepler

Mercury

Mars

Jupiter

Saturn

Uranus

Neptune

Pluto

Doppler 3m/sfor G2V star EarthVenus

Rough range of habitable planets

336 Extrasolar planets as of 29 January 2009(0 extrasolar planets as of January 1989)

286 Planetary Systems

A Fundamental NASA Mission Goal:

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 goals imply study of terrestrial planets in the habitable zones of solar-type (or smaller) stars…

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Stellar Sizes and Masses

The mass (in solar masses) and radius (in AU) of dwarf stars, also known as main-sequence stars or luminosity class V, are shown in black.

The Sun has a radius of 0.00467 AU and a mass of 1 solar mass.

Giant stars, luminosity class III, of the same spectral type are shown in red.

What do we mean by “Habitable”?

• Right size to have an atmosphere but not too much atmosphere– Too small, less than ~0.5 Mearth, surface gravity <0.8g

• Can’t hold onto atmosphere like Mercury and Mars

– Too big, more than ~10 Mearth, surface gravity >2.2g

• Holds on to very abundant light gases hydrogen and helium and turns into a gas giant like Jupiter, Saturn, Uranus and Neptune

• Surface, if any, is deep in atmosphere and very hot

• Right temperature, roughly, to allow liquid water on the surface– Too close to parent star

• Surface is too hot for liquid water

– Too far from parent star• Surface is so cold that water is permanently ice and other parts of the atmosphere,

like CO2, may freeze as well

• This does exclude habitability by exotic life forms that might be imagined.– We are concentrating on planets that might support life that we know can exist.

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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).

What Are We Looking For?

• Planets with masses between 0.5 MEarth and 10 MEarth

(between 1.6x10-3 MJupiter and 3.2x10-2 MJupiter)

• Planets in the habitable zone of their parent star– Periods of up to 16 years for dwarf A stars and as little as a

few days for very cool dwarf M stars

(up to 3 years for F5 dwarfs and as little as 1 month for the hottest M dwarfs)

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Techniques for Finding Extrasolar Planets

Method Yield Mass Limit Status

Pulsar Timing m/M ; Lunar Successful (7)

Radial Velocity m sini ; Uranus Successful (~300)

Astrometry m ; Ds ; aGround: Telescope Jupiter SuccessfulGround: Interferometer <Jupiter In developmentSpace: Interferometer Uranus Being studied

Transit Photometry A ; sini=1Ground <Jupiter Successful (55)Space Venus Corot (a few), Kepler

(soon)

Reflection Photometry: albedo*A ; Space Saturn Kepler (soon)

Microlensing: f(m,M,r,Ds,DL )Ground sub-Uranus Successful (8)

Direct Imaging albedo*A ; Ds ; a ; MGround Saturn Some success (11)Space Earth Being studied

Detection of Extrasolar Planets by Radial Velocity

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 strongly favors gas giant planets).

Mercury

Mars

Jupiter

Saturn

UranusNeptune

Pluto

3 m/s, G2V starEarth

Habitable zone for several stellar types0.5 m/s

G2VK0VG2VK0V

Venus

Doppler methods with commonly available velocity precision cannot search deep into the habitable zones of solar-like stars.Even the highest precision velocity measurements cannot detect Earth-mass planets around G stars like the Sun.

We need a different approach to find Earth-sized planets

A transit is like an eclipse, only smaller… Not only does the orbital plane have to be lined up, but you have to be looking at the right time (a few hours every 365 days for a true-Earth analog). A number of transiting gas giants have already been seen by the dips in the light of their stars that they cause.

HST measurement of HD209458

We only see the dip, not an image as shown here.

Planetary Transits

Planetary TransitsThe likelihood of a transit is governed by simple geometry. For a true-Earth analog it is about 0.5%, but it rises to almost 10% for very close-in planets. These also occur much more frequently (short orbital periods), and so are favored.

Need to look at thousands of stars to get good statistical sample of planets.

4) Transit durations vary from a couple of hours to 20-30 hours for planets in the habitable zone depending on the type of parent star, the orbit of the planet and where the planet crosses the stellar disk.

Information from TransitsKepler’s Third Law: The orbital period of a planet is proportional to its semi-major axis, in the relation P2 ~ a3

With additional measurement of reflected light you can also get an idea of the planet’s atmosphere (reflectivity, albedo). With additional measurement of the “wobble” you get the planet’s mass, and combined with the size you get the density (composition).

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Transit Photometry Can Detect Earth-sized Planets

• The relative change in brightness is equal to the area ratio: Aplanet/Astar

• Can see ~Jupiter-size planets from the ground. To see Earth-size planets with 0.01% depths must get above the Earth’s atmosphere

• Need high duty cycle observation since transits only last a few hours

• 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

simulated observed, 2004 (2012)

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 (type F-M)

• Continuously and simultaneously monitor >150,000 stars using a 1-meter Schmidt telescope with a field-of-view of >100 deg2 with 42 CCDs

• Photometric precision of < 20 ppm in 6.5 hours on Vmag= 12 solar-like star

4 detection of 1 Earth-sized transitFocusmechanisms42 CCDs read every 3 seconds

Focal plane electronics15 minute integrationsSunshade0.95 m diameterSchmidt correctorRadiator and heat pipe for cooling focal plane105 sq deg FOV Focal plane assembly:

CCDs, field flattening lenses fine guidance sensors

Graphite cyanate structure1.4 m diameterprimary mirror

• Heliocentric orbit for continuous visibility of target field

• 3.5 year lifetime with capability of 6 years

Bandpass: 420-830 nm

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Earth-trailing Heliocentric Orbit

Need 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. 18

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Field of View in Cygnus

Size of full moon

Between the constellations of Cygnus and Lyra near the plane of the Milky Way, the array of squares shows the actual field of view of each Kepler CCD.

Searches 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.

Potential for Detections

• Kepler covers a wide range of planet sizes, orbital distances, and stellar types.

• Hundreds of terrestrial-size planets will be found if such planets are common.

• Kepler should provide a robust answer to the frequency of terrestrial and larger planets in and near the habitable zone.

• The null result - no Earth-sized planets found - is significant. This would mean that very few other planetary systems have Earth-like planets

• Kepler can find true Earth analogs

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Detectable Planets for a V=12 Star

The detectable planet size is shown for a nearly central transit as a function of the stellar size and orbital size. For a solar-like star (G2V) at 1 AU a 0.8 Earth area (A) planet can be detected. Detections are based on a total SNR >8 and >3 transits in 4 years.

Potential for 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)

10000

1000

100

10

1 0.2 10 1.40.4 1.20.80.6 1.6

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Expected Results from KeplerHypothesis: all dwarf stars have planets and monitor 100,000 dwarf

stars for 4 years

Transits of terrestrial planets:

About 50 planets if most have R~1.0 R (M~1.0 M)

About 185 planets if most have R~1.3 R (M~2.2 M )

About 640 planets if most have R ~2.2 R (M~10 M )

About 70 cases (12%) of 2 or more planets per system

Transits of thousands of terrestrial planets:If most have orbits much less than 1 AU

Modulation of reflected light of giant inner planets:About 870 planets with periods ≤1 week, 35 with transitsAlbedos for 100 giants planets also seen in transit

Transits of giant planets:About 135 inner-orbit planet detections Densities for about 35 giants planets from radial velocity dataAbout 30 outer-orbit planet detections

Results expected will most likely be a mix of the above

Can We Tell Composition from Radius?Find Water Worlds?

Valencia, Sasselov, O’Connell ApJ 2007 25

Models of Super-earths suggest that the core-mantle structure is much less important than the water content in determining size. Radius differences are quite

measureable.

With a velocity precision of 10cm/s, one can hope to measurethe masses of a few close-interrestrial transiting planets, andget their mass and density.

Mercury

Mars

Jupiter

Saturn

Uranus

Neptune

Pluto

3 m/s, G2V starEarth

Habitable zone for several stellar types

0.5 m/s, 3

G2VK0VM0V

G2VK0VK5VM0V

Venus

Limits of measurability with best expected radial velocity accuracy (~10 cm/s) for stellar types G2-M0. The mass Earth-sized planet could be accurately measured if close in to a K5-M dwarf.

Other Science with Kepler

•THE UNEXPECTED…. Kepler will have unprecedented

precision andtime coverage.

• Magnetic Activity: short-term behavior, cycles for huge sample; highly relevant for “Sun-Earth connection”

• Fundamental Stellar Parameters: many new eclipsing binaries, asteroseismology (great spectral type coverage)

• Stellar (differential?) rotation for a huge sample; star spots• Variable Stars: stellar pulsation; behavior of giants and

supergiants; dust formation events• Protoplanetary systems – disks, accretion, magnetic activity• Interacting binaries, accretion disks and streams, cataclysmic

variables and novae• Quasar and Seyfert galaxy (AGN) variability

Summary

The Kepler Mission will:

Observe more than 100,000 dwarf starscontinuously for 4 to 6+ yearswith a precision capable of detecting Earth’s in the HZ

The Kepler Mission can discover:

Planet sizes from that of Mars to greater than JupiterOrbital periods from days up to two yearsAbout 600 terrestrial planetary systems if most have 1

AU orbitsAbout 1000 inner-orbit giant planets based on

already known frequency

A NULL result would also be very significant (frequency of terrestrial planets is less than 5%)

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Kepler Exists!

Schmidt Corrector Lens

Primary Mirror

Upper TelescopeBarrel

Kepler Has Come Together

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

Construction of the spacecraft is complete at Ball Aerospace Corp. in Boulder, Colorado.

The Science Operations Center has opened at Ames Research Labs in Sunnyvale, CA

The Spacecraft is Now at Cape Canaveral

Kepler launches on March 5in 33 days!

New Yorker Cartoon

“Well, this mission answers at least one big question: Are thereother planets like ours in the universe?”Drawing by H. Martin; © 1991

The New Yorker Magazine, Inc.

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Backup Slides

Potential Problems

• Are stars quiet enough?– Will the natural variability in solar-like stars, even

though quite small, allow detection of the shallow transits expected from Earth-like planets?

• Rejection of false positives.– Several types of astrophysical phenomena can

masquerade as planetary transits.– These can’t be sorted out by Kepler data alone.

The Visible Sun has

Dark and Bright Spots

SOHO/MDIVisibleContinuum19-July-2001

Behavior of Solar Microvariability

Active Sun

The total irradiance and visible output of the Sun vary on all timescales because of the magnetic activity and possibly convection.

Inactive Sun

Precision Stellar Light Curves Show Variability

Actual COROT data: starspots and a giant exoplanet transit

We Believe Most Stars Are Quiet Enough

– Variability noise declines with rotation as stars age• Magnetic activity declines• Spot passage period increases

– Solar-type stars slowed enough by 2-3 Gyr • Rotation-activity relationship well-known• Stellar spin-down timescales well-known

– 70% of solar-type stars appear slow and quiet enough• Galaxy >10 Gyr old and star formation ~constant• Detailed galactic population models confirm • Actual observations of stellar activity confirm

– However• Preliminary data from the COROT satellite indicate stars may be

less quiet than we think; only 50% may be quiet enough.• This may somewhat reduce the effectiveness of Kepler but will

not be a crippling effect.

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 another2. Cool dwarf stars transiting giants and supergiants3. White dwarfs transiting solar-type stars

Look at transit shape: 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 colors.Gravitational focussing makes a white dwarf transit into a bump instead of a dip!Stellar companions are easily detected from their large radial velocity variation.

The other types generate the right signal for thewrong reasons and are harder to remove:1. Full eclipses in a faint background binary whose light is combined with a foreground bright star2. Triple star systems with a bright primary and a faint eclipsing secondary pair

+ =

For these false positives, extensive ground-based observation will be required to confirm detections before they are announced.

> Kepler will have good ability to measure shifts in the location of the light during transit to detect background eclipsing binaries, but some will slip through.> Ground based high resolution imaging will find other background eclipsing binaries

The Hard False-Positive Problems

> Eclipses in multiple systems will be the hardest to sort out and require lots of ground based observation