13 - EXTRASOLAR PLANETS

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1 NSCI 314 LIFE IN THE COSMOS 13 - EXTRASOLAR PLANETS Dr. Karen Kolehmainen Department of Physics, CSUSB

2 EXTRASOLAR PLANETS? DO PLANETS ORBIT AROUND OTHER STARS? WE WOULD EXPECT SO, BASED ON OUR KNOWLEDGE OF THE FORMATION OF OUR SOLAR SYSTEM. LUMPS OF MATERIAL IN THE SOLAR NEBULA FORMED PLANETS. THE FORMATION OF PLANETS SEEMS LIKE A NATURAL CONSEQUENCE OF STAR FORMATION. WHAT DO OBSERVATIONS TELL US? NOTE: WE DO NOT HAVE THE ABILITY TO SEND SPACECRAFT OVER INTERSTELLAR DISTANCES (i.e., TO OTHER SOLAR SYSTEMS) TO LOOK FOR PLANETS.

3 PROTOPLANETARY DISKS WE HAVE OBSERVED FLAT DISKS OF GAS AND DUST ORBITING MANY YOUNG STARS. RECENT OBSERVATIONS HAVE FOUND THAT MOST YOUNG SUN-TYPE STARS HAVE THESE. MASS OF DISK IS A FEW PERCENT OF THE MASS OF THE STAR. (IN OUR SOLAR SYSTEM, MASS OF ALL PLANETS COMBINED IS 0.2% OF SUN'S MASS.) THESE APPEAR TO BE SOLAR SYSTEMS IN PROCESS OF FORMATION. FLAT SHAPE EXPECTED MASS IS SUFFICIENT

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

7 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 MOST CURRENT GENERATION TELESCOPES, 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.

8 DIRECT OBSERVATION CURRENT TECHNOLOGY IS AT THE BORDERLINE OF BEING ABLE TO DETECT EXTRASOLAR PLANETS VIA DIRECT IMAGING. EASIEST TO DETECT IF: PLANET IS LARGER PLANET IS FARTHER FROM STAR A FEW LARGE PLANETS DISCOVERED THIS WAY SO FAR, BUT MAY BE MORE IMPORTANT IN THE FUTURE. EARTH-SIZED PLANETS MAY BECOME VISIBLE DURING NEXT FEW DECADES.

9 DIRECT OBSERVATION 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

10 TRANSITS A PLANET PASSES IN BETWEEN US AND THE STAR IT ORBITS. IN OUR OWN SOLAR SYSTEM, WE CAN OBSERVE MERCURY AND VENUS DO THIS (APPEARS AS LITTLE BLACK DOT AGAINST DISK OF SUN). HOWEVER, OTHER STARS ARE TOO FAR AWAY TO SEE DISK, SEE ONLY POINT OF LIGHT. 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.

11 TRANSITS THIS WORKS ONLY IF ORBIT IS SEEN EDGE-ON (SMALL FRACTION OF SOLAR SYSTEMS). EASIEST TO DETECT IF PLANET IS LARGER FEW 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.

12 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. A FEW EXTRASOLAR PLANETS HAVE BEEN DISCOVERED THIS WAY, BUT THIS TECHNIQUE MAY WORK BETTER 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.

13 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 CM B

14 ORBITS IF OBJECT A IS HEAVIER THAN OBJECT B, THEN CM IS CLOSER TO OBJECT A: A X CM B 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.

15 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 EASIER TO DETECT IF: PLANET IS MORE MASSIVE PLANET IS FARTHER FROM STAR ONLY A FEW EXTRASOLAR PLANETS DISCOVERED THIS WAY SO FAR, BUT IT MAY BE MORE SUCCESSFUL IN THE FUTURE

16 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. THE FASTER THE RELATIVE MOTION, THE MORE THE WAVELENGTH CHANGES. SEE DEMONSTRATION (JAVA APPLET) AT:

17 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

18 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.

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

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

21 DOPPLER EFFECT DETECTION OF PLANETS PLANET AND STAR ORBIT AROUND THEIR COMMON CENTER OF MASS SINCE THE STAR IS MUCH HEAVIER, IT MOVES IN A SMALLER CIRCLE (OR ELLIPSE) PLANET IS UNSEEN, BUT LIGHT FROM STAR IS ALTERNATELY BLUESHIFTED AND REDSHIFTED DUE TO WIGGLE OF STAR CYCLE REPEATS OVER AND OVER AGAIN

22 DOPPLER EFFECT DETECTION OF PLANETS WORKS ONLY IF ORBIT IS SEEN NEARLY EDGE-ON EASIEST TO DETECT IF PLANET IS MORE MASSIVE PLANET CLOSER TO STAR OVER 100 PLANETS DISCOVERED SINCE 1995 VIA THIS TECHNIQUE OVER 90% OF EXTRASOLAR PLANETS DISCOVERED THIS WAY

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

24 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

25 RESULTS OVER 200 EXTRASOLAR PLANETS HAVE BEEN DISCOVERED SINCE 1995, MOST USING THE DOPPLER EFFECT TECHNIQUE. SEVERAL STARS HAVE BEEN FOUND TO HAVE TWO OR MORE PLANETS. MOST PLANET MASSES ARE IN JUPITER RANGE. (MANY ARE EVEN HEAVIER.) THE LIGHTEST PLANET FOUND SO FAR IS 5.5 EARTH MASSES. 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).

26 DISTRIBUTION OF PLANETS MERCURY VENUS 0.5 A.U. EARTH 1.0 A.U. 2.3 A.U. 2.5 A.U. 2.5 A.U. MARS 1.0 A.U. 3.3 A.U. 2.0 A.U.

27 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

28 EXPLANATION?? OBSERVED MASSIVE PLANETS WERE FORMED FARTHER OUT FROM STAR (>5 AU), WHERE GAS GIANTS ARE EXPECTED TO FORM AFTER FORMATION, THE PLANETS MIGRATED TO NEW ORBITS DUE TO GRAVITATIONAL INTERACTIONS WITH OTHER PLANETS MATERIAL IN THE SOLAR DISK (NEAR THE END OF SOLAR SYSTEM FORMATION) OTHER STARS PASSING NEARBY

29 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

30 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 OR TYPICAL. OUR CURRENT TECHNOLOGY CANNOT DETECT EARTH-LIKE PLANETS.

31 WE ARE JUST BEGINNING TO BE ABLE TO DETECT JUPITER-LIKE PLANETS (AT JUPITER'S DISTANCE FROM THE STAR). THERE ARE 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. MAYBE THE OTHER 90% OF STARS (OR MANY OF THEM, AT LEAST) MAY HAVE PLANETARY SYSTEMS MORE LIKE OURS, WHICH WE CANNOT YET DETECT. IMPROVED TECHNOLOGY WILL ANSWER THIS, PROBABLY WITHIN THE NEXT DECADE. NASA IS PLANNING A TERRESTRIAL PLANET FINDER.

32 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 OR < 13 EARTH MASSES (?) (1 EARTH MASS ~ JUPITER MASSES)