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Detecting Terrestrial Planets by Transits: The Kepler Mission (2009) A Fundamental NASA Mission Goal: – To place our Solar System in context with other planetary systems –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 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, semimajor 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 TECHNIQUES FOR FINDING EXTRASOLAR PLANETS Method Yield Mass Limit Pulsar Timing m/M sin i; a Radial Velocity m/M sini ; a Neptune Lunar Status Successful (3) Successful (~220) Astrometry m/M ; a; all distant companions Ground: Telescope Neptune Ongoing Ground: Interferometer <Jupiter In development Space: Interferometer Uranus Being studied Transit Photometry Ground Space Space R ; a; sini=1 Neptune Successful (7) Super-Earth Launched: COROT Venus Planned: Kepler Reflection or Eclipse : albedo/R Photometry from Space Saturn Successful (2) Microlensing: Ground f(m,M,r,Ds,DL ) Super-Earth Successful (5) Direct Imaging Ground Space albedo/R; a ; all companions Saturn Being studied Earth Being studied 6 Why is Water Essential for Life (as we know it)? • • • • • • • • • It is one of the most common molecules It is liquid in the right temperature range for organic chemistry It is a polar molecule, which allows interesting surface chemistry (hydrophobic and hydrophilic molecules) 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 a lot of hydrogen bond chemistry to occur It has “local structure” (hydrogen bonding makes it almost crystalline; allowing capillary action) but is globally liquid Its frozen state is less dense than its liquid state (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 is observed to be an essential ingredient of life on Earth! Habitable Zones (liquid surface water) The most common type of star… Kepler 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. Europa Life here could have started at the bottom of the ocean at volcanic vents. Many other conditions may be “habitable” 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 Mission Questions 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 a3 P2 M central 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. Example: if Jupiter is at 5 AU, how long is its orbital period? P 2 53 125; P 125 11.2 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. 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 PHOTOMETRY CAN DETECT EARTH-SIZED PLANETS • The relative change in brightness is equal to the relative areas (Aplanet/Astar) Mercury Transit 2006 Jupiter: 1% area of the Sun (1/100) Earth or Venus 0.01% area of the Sun (1/10,000) • 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) 14 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 4s detection of 1 Earth-sized transit Focal plane electronics 15 minute integrations Sunshade 42 CCDs read every 3 seconds 1.4 m diameter primary mirror 0.95 m diameter Schmidt corrector Focus mechanisms 105 sq deg FOV Focal plane assembly: CCDs, field flattening lenses fine guidance sensors Radiator and heat pipe for cooling focal plane Graphite cyanate structure 15 Kepler Comes Together 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 CCDs have been delivered from E2V and are being mounted into focal plane packages with filters and sapphire correcting lenses Kepler Parts Exist! Primary Mirror Schmidt Corrector Lens Delta rocket (well-tested) Launch Vehicle and Orbit 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. 19 Kepler CCDs on the Sky • Full Moon Kepler Fields and Images Each of the 21 CCDs (2048x2048) samples 5 square degrees Images are de-focussed to FWHM ~6” to improve precision 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 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. 2. 3. Grazing eclipses of one star by another Cool dwarf stars transiting giants and supergiants 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. 2. Full eclipses in a faint background binary whose light is combined with a foreground bright star 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… + = Potential for Planetary Detections 10000 1000 # of Planet Detections 100 10 1 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Orbital Semi-major Axis (AU) Expected # of planets found, assuming one planet of a given size & semi-major axis per star and random orientation of orbital planes. The Importance of Small Cool Stars The Immediate Solar Neighborhood The 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 Against Finding M-star Habitable Planets Factors 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. 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%) 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 Drawing by H. Martin; © 1991 other planets like ours in the universe?” The New Yorker Magazine, Inc. 28 THE HABITABLE ZONE BY STELLAR TYPES 2 Msun 1 Msun 0.5 Msun 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). 29