The Search for Extrasolar Planets Since it appears the conditions for planet formation are common, we’d like to know how many solar systems there are, and what they look like. Indirect Methods: 1) Doppler shift of the star’s orbit this is the main one so far 2) Astrometric wobble of the star’s orbit Semi-direct Methods: 1) Transits (light blocked by the planet) might also see phases 2) Microlensing (planet’s gravity) Direct Methods: 1) Planet imaged directly (perhaps with coronograph) reflected or emitted (IR or radio) light 2) Planet imaged by interferometer

Precision Radial Velocity Searches Shift is 1 part in 100 million

Discovery of Extrasolar planets We get the orbital period, semimajor axis, and a lower limit on the mass of the planet. This can only do giant planets relatively close in (but could see Jupiter).

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 present in some numbers).

Properties of the systems found: 1

Properties of the systems found: 2

Astrometry This works best for large orbits (which take a long time) and stars that are nearby. Interferometry would allow very small motions to be measured.

“Microlensing” : Gravitational lenses In principle, this method could even see Earth-mass planets. You have to have a huge and long-time monitoring program with high time resolution and good photometric precision. The downside is that you will only detect the planet once, and can’t learn anything more about it. One detection has been claimed (but how to confirm it?).

The Problem with Direct Imaging 1)The host star is FAR brighter (10 6 ) than any planet (except very young Jupiters in the infrared). 2)The planet is VERY close in angle (micro- arcsecs) to the star, so any stray light from the star can overwhelm the light from the planet. Reflected light Thermal emission

Interferometric Missions Darwin Terrestrial Planet Finder Perhaps a decade from now we will be able to directly image older extrasolar giant planets.

Nulling Interferometry You can try to keep the star at a destructive null fringe, while the planet will be slightly off the fringe and so still visible. Might be able to reduce the star’s brightness by a million times?

Planetary Transits A transit is like an eclipse, only smaller… This has been seen for a few cases (confirming the radial velocity detections).

13 PHOTOMETRY CAN DETECT EARTH-SIZED PLANETS The relative change in brightness is equal to the relative areas (A planet /A star ) 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 Venus 0.01% area of the Sun (1/10,000)

Information from Transits HST measurement of HD The distance and size of the planet come out directly. If you have radial velocity as well you get the mass, and thus the density. It is unlikely you could ever image the planet or get its spectrum, but you can get the thermal spectrum and something about the atmosphere during eclipses.

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”!

16 Kepler MISSION CONCEPT The Kepler Mission is optimized for finding habitable planets ( 0.5 to 10 M  ) in the HZ ( near 1 AU ) of solar-like stars Continuously and simultaneously monitor 100,000 dwarf stars using a 1-meter Schmidt telescope: FOV >100 deg 2 with 42 CCDs Photometric precision of < 20 ppm in 6.5 hours on V mag = 12 solar-like star  4  detection for one Earth-sized transit

Kepler CCDs on the Sky Full Moon

Transit Detectability The strict periodicity of planetary transits provides an extremely powerful filter against misleading stellar signals. You need 3 transits to be sure you’ve seen it.

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.Grazing eclipses of one star by another 2.M dwarfs 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 + =

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)

22 T HE H ABITABLE Z ONE BY S TELLAR T YPES 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).

Search Methods : what they can find Detections by 2005