NSCI 314 LIFE IN THE COSMOS 13 - WHERE TO SEARCH FOR LIFE OUTSIDE OUR SOLAR SYSTEM: SUITABLE STARS AND PLANETS AND EXTRASOLAR PLANETS Dr. Karen Kolehmainen Department of Physics, CSUSB

SUITABLE STARS DEFINED AS STARS AROUND WHICH PLANETS THAT ARE SUITABLE FOR LIFE MIGHT BE ORBITING. REMEMBER THAT WE ARE RESTRICTING OUR ATTENTION TO LIFE SIMILAR TO THAT ON THE EARTH (e.g., CARBON- BASED, USES WATER AS LIQUID SOLVENT) WHAT PROPERTIES MUST A STAR POSSESS IN ORDER TO BE A SUITABLE STAR?

SUITABLE STARS A SUITABLE STAR MUST BE: 1.MAIN SEQUENCE 2.SPECTRAL TYPES K, G, AND SOME F 3.POPULATION I (ENOUGH HEAVY ELEMENTS) 4.NOT TOO NEAR GALACTIC CENTER 5.SINGLE STARS (NOT BINARY OR MULTIPLE)? IN OUR MILKY WAY GALAXY (OR A SIMILAR SPIRAL GALAXY), THIS LIMITS US TO ABOUT 5 TO 10% OF STARS (MORE IF WE ALLOW SOME BINARIES).

NUMBER OF SUITABLE STARS LIMITATIONS ON SEARCH TIMES (WE'LL TALK ABOUT HOW TO SEARCH LATER) OUT TO 100 LY:OUT TO 1,000 LY: 20,000 SUITABLE STARS 20 MILLION SUITABLE STARS 200,000 STARS 200 MILLION STARS SEARCH RATE: 1 PER DAY 1 PER HOUR 1 PER MINUTE 1 PER SECOND OUT TO 100 LY: 55 YEARS 2.3 YEARS 14 DAYS 6 HOURS OUT TO 1000 LY: 55,000 YEARS 2,300 YEARS 38 YEARS 230 DAYS

PROPERTIES OF A SUITABLE PLANET A SUITABLE PLANET IS A PLANET ON WHICH LIFE COULD BE FOUND PROPERTIES: –MUST ORBIT A SUITABLE STAR –TEMPERATURE MUST BE IN THE CORRECT RANGE FOR LIQUID WATER VIA EITHER: MUST ORBIT ITS STAR WITHIN THE HABITABLE ZONE, PLUS ORBIT MUST BE NEARLY CIRCULAR SO THAT IT IS ENTIRELY WITHIN THE HABITABLE ZONE, OR MUST HAVE A SIGNIFICANT SOURCE OF INTERNAL HEAT (PERHAPS ON A LARGE MOON EXPERIENCING SIGNIFICANT TIDAL FORCES FROM THE PLANET IT ORBITS) –MUST BE ROCKY, HAVE A SOLID SURFACE –MUST BE MASSIVE ENOUGH TO RETAIN A REASONABLY THICK ATMOSPHERE

HELPFUL (BUT MAYBE NOT CRUCIAL) FEATURES FOR A SUITABLE PLANET PRESENCE OF A LARGE MOON –PRODUCES TIDES (TIDEPOOLS ARE A POSSIBLE PLACE FOR THE ORIGIN OF LIFE) –STABILIZES ROTATION AXIS (PREVENTS SEASONAL CHANGES FROM VARYING DRAMATICALLY WITH TIME) PLATE TECTONICS –PROVIDES A VARIETY OF ENVIRONMENTS –PLAYS A ROLE IN CO 2 CYCLE ON EARTH, WHICH HELPS REGULATE CLIMATE –MAY BE INEVITABLE FOR A SUFFICIENTLY MASSIVE ROCKY PLANET (STILL HOT INSIDE)

HELPFUL (BUT MAYBE NOT CRUCIAL) FEATURES FOR A SUITABLE PLANET IMPACTS NOT TOO FREQUENT –A SUFFICIENTLY LARGE IMPACT COULD WIPE OUT ALL LIFE ON THE PLANET –FREQUENCY OF IMPACTS MAY DEPEND ON POSITIONS OF LARGE PLANETS WITHIN SOLAR SYSTEM THEIR GRAVITATIONAL EFFECTS EXPEL COMETS TO OORT CLOUD JUPITER MAY PROTECT EARTH IN THIS RESPECT BUT OCCASIONAL IMPACTS MAY BE HELPFUL FOR EVOLUTION OF ADVANCED LIFE FORMS –IMPACTS CAUSE MASS EXTINCTIONS (E.G., DINOSAURS) –MASS EXTINCTIONS OPEN UP ECOLOGICAL NICHES FOR NEW SPECIES

HELPFUL (BUT MAYBE NOT CRUCIAL) FEATURES FOR A SUITABLE PLANET OCCASIONAL MAJOR CLIMATE CHANGES MAY BE HELPFUL FOR EVOLUTION OF ADVANCED LIFE FORMS - THESE CAN “STIMULATE” EVOLUTION BY OPENING UP NEW ECOLOGICAL NICHES FOR A VARIETY OF SPECIES. - THE END OF THE “SNOWBALL EARTH” STAGE IN THE EARTH'S HISTORY (A VERY SEVERE GLOBAL ICE AGE) HAPPENED ABOUT THE SAME TIME AS THE “CAMBRIAN EXPLOSION,” A MAJOR INCREASE IN THE DIVERSITY OF LIFE FORMS ON EARTH, AND THE ORIGIN OF “ADVANCED” ORGANISMS. - SIMILAR, BUT LESS SEVERE, CLIMATE CHANGES MAY HAVE PROMPTED THE EVOLUTION OF HUMANS.

WE HAVE ALREADY EXAMINED THE SUITABILITY OF PLANETS IN OUR SOLAR SYSTEM (ORBITING OUR SUN). HOW COMMON DO WE THINK SUITABLE PLANETS ARE IN OTHER SOLAR SYSTEMS? LET’S EXAMINE: –METHODS FOR DETECTING EXTRASOLAR PLANETS (PLANETS OUTSIDE OUR SOLAR SYSTEM, i.e., ORBITING OTHER STARS) –WHAT WE HAVE DISCOVERED ABOUT EXTRASOLAR PLANETS –NOTE: WE DO NOT HAVE THE ABILITY TO SEND SPACECRAFT OVER INTERSTELLAR DISTANCES (i.e., TO OTHER SOLAR SYSTEMS) TO LOOK FOR EXTRASOLAR PLANETS.

METHODS FOR DETECTING EXTRASOLAR PLANETS DIRECT OBSERVATION TRANSITS GRAVITATIONAL LENSING ASTROMETRY DOPPLER EFFECT (MOST SUCCESSFUL)

DIRECT OBSERVATION (USING EITHER VISIBLE LIGHT OR INFRARED RADIATION) PROBLEMS: PLANET IS MUCH FAINTER THAN THE STAR IT ORBITS EXAMPLE: USING VISIBLE LIGHT, THE SUN IS 1 BILLION TIMES BRIGHTER THAN JUPITER, SEEN FROM THE SAME DISTANCE. USING INFRARED, THE SUN IS “ONLY” 100,000 TIMES BRIGHTER THAN JUPITER. RESOLUTION - ABILITY TO SEE SEPARATELY TWO OBJECTS THAT ARE CLOSE TOGETHER WITH CURRENT GENERATION TELESCOPES (EVEN THE HUBBLE SPACE TELESCOPE), THE IMAGE OF A PLANET WOULD APPEAR BLENDED TOGETHER WITH THE IMAGE OF THE STAR IT ORBITS. FAINTNESS AND RESOLUTION COMBINED MAKE THE PROBLEM EVEN WORSE.

DIRECT OBSERVATION CURRENT TECHNOLOGY CANNOT DETECT EXTRASOLAR PLANETS VIA DIRECT IMAGING. FUTURE IMAGING TECHNOLOGY (WITHIN THE NEXT FEW YEARS TO DECADES): -TELESCOPES IN EARTH ORBIT -WILL USE INTERFEROMETRY (COMBINATION OF IMAGES FROM SEVERAL TELESCOPES TO IMPROVE RESOLUTION) -MAY BE ABLE TO DETECT CHANGES IN BRIGHTNESS DUE TO CLOUD COVER OR SEASONAL CHANGES -SPECTROSCOPIC ANALYSIS WILL BE ABLE TO DETECT COMPOSITION OF ATMOSPHERE

TRANSITS A PLANET PASSES IN BETWEEN US AND THE STAR IT ORBITS. THE PLANET PARTIALLY BLOCKS LIGHT FROM THE STAR, CAUSING A TEMPORARY DECREASE IN THE STAR’S BRIGHTNESS. BRIGHTNESS DIPS REPEATEDLY, ONCE PER ORBIT OF THE PLANET. THIS WORKS ONLY IF ORBIT IS SEEN EDGE-ON (SMALL FRACTION OF SOLAR SYSTEMS). NO EXTRASOLAR PLANETS HAVE BEEN DISCOVERED THIS WAY, BUT IT WAS USED TO VERIFY THE EXISTENCE OF SEVERAL PLANETS THAT HAD BEEN ALREADY DISCOVERED VIA THE DOPPLER EFFECT.

GRAVITATIONAL LENSING LIGHT FROM A DISTANT OBJECT PASSES BY SOME NEARER OBJECT (AN EXTRASOLAR PLANET IN OUR CASE) ON ITS WAY TO US. GRAVITATIONAL EFFECTS OF NEARER OBJECT BEND THE PATH OF THE LIGHT. AS A RESULT, THE DISTANT OBJECT APPEARS SHIFTED IN POSITION OR IN MULTIPLE IMAGES. PROBLEM: PLANETS AREN’T MASSIVE ENOUGH TO CAUSE SIGNIFICANT BENDING. NO EXTRASOLAR PLANETS HAVE BEEN DISCOVERED THIS WAY, BUT THIS TECHNIQUE MAY WORK IN THE FUTURE. THIS WORKS BETTER WHEN A MORE MASSIVE OBJECT (E.G., A STAR OR GALAXY) IS BENDING THE LIGHT. THIS HAS BEEN OBSERVED.

ORBITS AN UNDERSTANDING OF THIS IS NEEDED TO DISCUSS THE TWO REMAINING TECHNIQUES – ASTROMETRY AND THE DOPPLER EFFECT. OBJECT A AND OBJECT B (COULD BE TWO STARS, OR A STAR AND A PLANET) ORBIT AROUND THEIR COMMON CENTER OF MASS (CM). IF OBJECT A AND OBJECT B HAVE THE SAME MASS, THEN CM IS HALFWAY IN BETWEEN: A X B CM

ORBITS IF OBJECT A IS HEAVIER THAN OBJECT B, THEN CM IS CLOSER TO OBJECT A: A X B CM IF A IS MUCH HEAVIER THAN B, THEN OBJECT A “WIGGLES” A LITTLE AS OBJECT B ORBITS IT. THIS IS THE CASE IF OBJECT A IS A STAR AND OBJECT B IS A PLANET.

ASTROMETRY LOOK FOR WIGGLES IN A STAR’S PROPER MOTION DUE TO ITS ORBITAL MOTION AROUND CENTER OF MASS OF STAR-PLANET SYSTEM PROPER MOTION: PATH OF STAR ACROSS SKY (RELATIVE TO OTHER STARS) DUE TO ACTUAL MOTION THROUGH SPACE (MUST OBSERVE FOR MANY YEARS TO SEE ANY SUCH MOTION) WORKS ONLY IF ORBIT SEEN NEARLY FACE-ON ONLY ONE EXTRASOLAR PLANET DISCOVERED THIS WAY SO FAR, BUT THE TECHNIQUE MAY BE MORE SUCCESSFUL IN THE FUTURE

DOPPLER EFFECT A SHIFT IN THE WAVELENGTH OF A WAVE DUE TO RELATIVE MOTION OF THE SOURCE AND THE OBSERVER IF THE SOURCE AND OBSERVER ARE MOVING TOWARDS EACH OTHER, THE WAVELENGTH IS SHORTENED. IF THE SOURCE AND OBSERVER ARE MOVING AWAY FROM EACH OTHER, THE WAVELENGTH IS LENGTHENED. SEE DEMONSTRATION (JAVA APPLET) AT:

DOPPLER EFFECT FOR SOUND WAVES, A CHANGE IN WAVELENGTH IS A CHANGE IN PITCH. –THE SOUND IS HIGHER PITCHED IF THE SOURCE AND OBSERVER ARE MOVING TOWARDS EACH OTHER. –THE SOUND IS LOWER PITCHED IF THE SOURCE AND OBSERVER ARE MOVING AWAY FROM EACH OTHER. EXAMPLE: SIREN ON A MOVING CAR

DOPPLER EFFECT FOR LIGHT WAVES, A CHANGE IN WAVELENGTH IS A CHANGE IN COLOR. –THE LIGHT IS BLUER IF THE SOURCE AND OBSERVER ARE MOVING TOWARDS EACH OTHER (BLUESHIFT). –THE LIGHT IS REDDER IF THE SOURCE AND OBSERVER ARE MOVING AWAY FROM EACH OTHER (REDSHIFT). EXAMPLE: LIGHT COMING FROM DISTANT GALAXIES IS REDSHIFTED DUE TO THE EXPANSION OF THE UNIVERSE.

STELLAR DOPPLER SHIFT DETECTION Star Moves Toward Observer LIGHT FROM STAR IS BLUE SHIFTED Unseen Planet Moves Away From Observer

STELLAR DOPPLER SHIFT DETECTION Star Moves Away From Observer LIGHT FROM STAR IS RED SHIFTED Unseen Planet Moves Towards Observer

DOPPLER EFFECT DETECTION OF PLANETS PLANET AND STAR ORBIT AROUND THEIR COMMON CENTER OF MASS SINCE THE STAR IS MUCH HEAVIER, IT JUST WIGGLES BACK AND FORTH A LITTLE PLANET IS UNSEEN, BUT LIGHT FROM STAR IS ALTERNATELY BLUESHIFTED AND REDSHIFTED DUE TO WIGGLE OF STA. CYCLE REPEATS OVER AND OVER AGAIN WORKS ONLY IF ORBIT IS SEEN NEARLY EDGE-ON OVER 100 PLANETS DISCOVERED SINCE 1995 VIA THIS TECHNIQUE

WHAT CAN WE DETERMINE? ORBITAL PERIOD (TIME NEEDED FOR ONE ORBIT) AVERAGE DISTANCE OF PLANET FROM STAR ECCENTRICITY (SHAPE) OF ORBIT LOWER LIMIT ON PLANET’S MASS

RESULTS MOST PLANET MASSES ARE IN JUPITER RANGE (MANY ARE EVEN HEAVIER) MOST PLANETS ARE VERY CLOSE TO STAR –HALF OF ALL DISCOVERED PLANETS ARE CLOSER IN THAN 0.5 AU –MANY ARE CLOSER TO THEIR STARS THAN MERCURY IS TO OUR SUN MOST ORBITS ARE VERY ECCENTRIC (HIGHLY ELLIPTICAL - FAR FROM CIRCULAR) SEVERAL STARS HAVE BEEN FOUND TO HAVE TWO OR MORE PLANETS

DISTRIBUTION OF PLANETS MERCURYVENUSEARTH 0.5 A.U.1.0 A.U. MARS 1.0 A.U.2.0 A.U. 2.3 A.U. 2.5 A.U. 3.3 A.U.

THE PROBLEM IN UNDERSTANDING THIS OUR MODELS OF SOLAR SYSTEM FORMATION PREDICT SMALL ROCKY PLANETS CLOSE TO STAR AND MASSIVE GAS GIANTS FARTHER AWAY (>5 AU), AS IN OUR SOLAR SYSTEM BUT MOST OBSERVED SOLAR SYSTEMS HAVE MASSIVE PLANETS (PROBABLY GAS GIANTS) CLOSE TO STAR

EXPLANATION?? OBSERVED MASSIVE PLANETS WERE FORMED FARTHER OUT FROM STAR (>5 AU) AFTER FORMATION, THE PLANETS MIGRATED TO NEW ORBITS DUE TO GRAVITATIONAL INTERACTIONS WITH –OTHER PLANETS –MATERIAL IN THE PROTOPLANETARY DISK –OTHER STARS PASSING NEARBY

MIGRATING PLANETS COMPUTER MODELING INDICATES –PLANETS ARE MORE LIKELY TO MIGRATE INWARD THAN OUTWARD –NEW ORBIT IS USUALLY HIGHLY ECCENTRIC –WHEN A LARGE PLANET MIGRATES, SMALLER PLANETS ARE PROBABLY THROWN INTO THE STAR OR OUT OF THE SOLAR SYSTEM BY GRAVITY OF MIGRATING MASSIVE PLANET –HENCE THERE ARE PROBABLY NO SUITABLE PLANETS IN THE SYSTEM

ARE MIGRATING PLANETS COMMON? IF THEY ARE THE NORM, PLANETS THAT ARE SUITABLE FOR LIFE MAY BE RARE. BUT KEEP IN MIND THAT… –MASSIVE PLANETS CLOSE TO THEIR STARS ARE EASIEST TO DETECT (LARGEST DOPPLER EFFECT). –THEREFORE “OBSERVATIONAL BIAS” IS PRESENT. OUR SAMPLE OF KNOWN EXTRASOLAR PLANETS IS NOT REPRESENTATIVE. –OUR CURRENT TECHNOLOGY CANNOT DETECT EARTH-LIKE PLANETS.

WE ARE JUST BEGINNING TO BE ABLE TO DETECT JUPITER-LIKE PLANETS (AT JUPITER'S DISTANCE FROM THE STAR). THERE ARE PRELIMINARY REPORTS OF A FEW SUCH PLANETS. SOLAR SYSTEMS CONTAINING JUPITER-LIKE PLANETS FARTHER OUT ARE MORE LIKELY TO HAVE EARTH-TYPE PLANETS CLOSER IN TO THE STAR. WE HAVE FOUND EXTRASOLAR PLANETS ORBITING ABOUT 10% OF STARS EXAMINED. THEREFORE THE OTHER 90% OF STARS MAY HAVE PLANETARY SYSTEMS MORE LIKE OURS, WHICH WE CANNOT YET DETECT. IMPROVED TECHNOLOGY WILL ANSWER THIS, PROBABLY WITHIN THE NEXT DECADE.

STELLAR/PLANETARY HIERARCHY STARS 0.08 TO 20 SOLAR MASSES BROWN DWARFS TO 0.08 SOLAR MASSES JUPITER MASSES MASSES IN BETWEEN THOSE OF PLANETS AND STARS GAS GIANT PLANETS 0.04(?) - 13 JUPITER MASSES ROCKY (TERRESTRIAL) PLANETS <0.04(?) JUPITER MASSES (1 EARTH MASS ~ JUPITER MASSES)