NSCI 314

LIFE IN THE COSMOS

13 - EXTRASOLAR PLANETS

Dr. Karen KolehmainenDepartment of Physics, CSUSB

http://physics.csusb.edu/~karen/

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.

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

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 TOGETHERWITH 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.

DIRECT OBSERVATIONCURRENT 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.

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

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.

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.

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.

ORBITSAN 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

ORBITSIF 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 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

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: http://lectureonline.cl.msu.edu/~mmp/applist/doppler/d.htm

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

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

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

METHODS FOR DETECTING EXTRASOLAR PLANETS(PLANETS ORBITING OTHER STARS)

DIRECT OBSERVATION

TRANSITS

GRAVITATIONAL LENSING

ASTROMETRY

DOPPLER EFFECT (MOST SUCCESSFUL)

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 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).

DISTRIBUTION OF PLANETS

MERCURY VENUS EARTH

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.

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), 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

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 OR “TYPICAL.”

– 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 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.”

STELLAR/PLANETARY HIERARCHY

STARS 0.08 TO 20 SOLAR MASSES

BROWN DWARFS0.013 TO 0.08 SOLAR MASSES13 - 80 JUPITER MASSESMASSES 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 ~ 0.003 JUPITER MASSES)