The Search for Earth-sized PlanetsAround Other Stars

The Kepler Mission (2009)

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PHOTOMETRY CAN DETECT EARTH-SIZED PLANETS

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

• To measure 0.01% must get above the Earth’s atmosphere

• This is also meets the need for 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

simulatedobserved, 2004 (2012)

Chance of alignment:R

star/R

orbit

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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 >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 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 Parts

Schmidt Corrector Lens

Primary Mirror

Focal Plane detectors

The Completed Spacecraft

Away She Goes! 10:47pm EDT Mar. 6, 2009

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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 Field of View

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 False-Positives ProblemThere are several common sources of false positives. They produce the right There are several common sources of false positives. They produce the right signal for the wrong reasons, but some are easy to deal with:signal for the wrong reasons, but some are easy to deal with:

1.1. Grazing eclipses of one star by anotherGrazing eclipses of one star by another

2.2. Cool dwarf stars transiting giants and supergiantsCool dwarf stars transiting giants and supergiants

3.3. White dwarfs transiting solar-type starsWhite 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.Using the “wobble” method, a stellar companion produces a MUCH larger signal.

The False-Positives ProblemNature can generate the right signal for the

wrong reasons and these 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…

Kepler has a goodability to detect shifts in the location of the light during transit(this helps a lot!)

The Certification Program

Eclipsing Binary

There are a number of other tests that “dips” have to pass. They have to be consistent, and when modeled yield planetary radii. There should be no secondary eclipse (unless planetary). A “blender” analysis should rule out combinations of stellar eclipses. Radial velocity tests of increasing precision must be passed.

Candidates must pass a “rain plot” test with Kepler data, and a high resolution image search for faint background stars.

KEPLER DETECTS LIGHT FROM A PLANET ITSELF

Scatter of the data points in the Kepler datais within the line thickness.

Kepler precision is 100 times better than that from ground-based observations.

Radiation from the planet itselfis evident in the bottom panel.

The depth of the occultation issimilar to that expected froman Earth-size planet orbiting

a solar-like star.

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

10000

1000

100

10

1 0.2 10 1.40.4 1.20.80.6 1.6Orbital Semi-major Axis (AU)

Kepler Planetary Candidates:Orbits

Velocity “Wobble”

1.25-2 Re

2-6 Re 6-15 Re

Kepler Planetary Candidates:Frequency

Multi-planet Systems

There have been a number of multiple-transit systems found. They have up to five planets. The first instances of transit timing variations have also been detected; these show the gravitational interactions between the planets. This method can also detect non-transiting planets.

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…

Finding “earths” around different stars

Capability of Kepler to Detect “Earth” in the Habitable Zone

Rmag A2 A7 F2 F7 G2 G7 K2 K7 M2 9 39 38 40 52 28 10 8 2 0 10 98 110 102 186 136 45 44 8 0 11 167 200 318 618 478 197 126 24 8 12 197 423 770 2048 1704 681 465 136 33 13 119 488 1402 6311 5966 2294 1694 414 163 14 37 244 1574 16488 18627 7383 5803 1724 564 15 0 50 935 34883 49386 21978 18791 5946 1998 16 0 2 356 60984 96058 51866 52172 19638 7497 Total 657 1555 5497 121570 172383 84454 79103 27892 10263 Total 0 0 142 856 2346 3227 8140 2308 10263

Most stars are small...

The nearest stars...

Kepler's ability to find “earths” depends on the size and brightness of a star, and on the orbital period of the “temperate zone”. It also depends on how many of each type of star there are. Small cool stars are the most common: can they harbor “habitable” planets?

Summary of Kepler Summary of Kepler Mission GoalsMission Goals

• Find the frequency of terrestrial planets in the Find the frequency of terrestrial planets in the Galaxy Galaxy

• Characterize the properties of inner planetary Characterize the properties of inner planetary systems. systems.

• Determine the properties of stars Determine the properties of stars

(single & multiple) hosting planets. (single & multiple) hosting planets. • Discover terrestrial planets in habitable zonesDiscover terrestrial planets in habitable zones• (or show that they are rare).(or show that they are rare).• Detect true Earth analogs (?)Detect true Earth analogs (?)

A NULL result would also be very significant A NULL result would also be very significant

(frequency of stars with terrestrial planets is less than (frequency of stars with terrestrial planets is less than 1%)1%)