The Search for Earth-sized Planets Around Other Stars The Kepler Mission (2009)

2 PHOTOMETRY CAN DETECT EARTH-SIZED PLANETS The relative change in brightness is equal to the area ratio: A planet /A star 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 Venus 0.01% area of the Sun (1/10,000) Mercury Transit 2006 simulated observed, 2004 (2012) Chance of alignment: R star /R orbit

3 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 deg 2 with 42 CCDs Photometric precision of < 20 ppm in 6.5 hours on V mag =12 sunlike star  4  detection of 1 Earth-sized transit

Kepler Parts Schmidt Corrector Lens Primary Mirror Focal Plane detectors

The Completed Spacecraft

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

C ONTINUOUSLY V IEWABLE H IGH D ENSITY S TAR F IELD 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. 7

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 Problem 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.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. Using the “wobble” method, a stellar companion produces a MUCH larger signal.

The False-Positives Problem Nature 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 good ability 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 data is within the line thickness. Kepler precision is 100 times better than that from ground- based observations. Radiation from the planet itself is evident in the bottom panel. The depth of the occultation is similar to that expected from an 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 Planet Detections Orbital Semi-major Axis (AU)

Kepler Planetary Candidates: Orbits Velocity “Wobble” Re 2-6 Re6-15 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 times longer than solar type stars. Kepler The most common type of star…

Finding “earths” around different stars Capability of Kepler to Detect “Earth” in the Habitable Zone R mag A2 A7 F2 F7 G2 G7 K2 K7 M Total Total 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 Mission Goals Find the frequency of terrestrial planets in the GalaxyFind the frequency of terrestrial planets in the Galaxy Characterize the properties of inner planetary systems.Characterize the properties of inner planetary systems. Determine the properties of starsDetermine 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 (frequency of stars with terrestrial planets is less than 1%)