NSCI 314

LIFE IN THE COSMOS

13 - WHERE TO SEARCH FOR LIFE OUTSIDE OUR SOLAR SYSTEM:

SUITABLE STARS AND PLANETSAND EXTRASOLAR PLANETS

Dr. Karen KolehmainenDepartment of Physics, CSUSB

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

SEARCHING FOR LIFE IN OTHER SOLAR SYSTEMS

WE WILL TAKE A CONSERVATIVE APPROACH: -WE WILL CONSIDER ONLY LIFE THAT IS BASED ON THE

ELECTROMAGNETIC INTERACTION(IS MADE OF ATOMS AND MOLECULES).

- WE WILL ONLY CONSIDER LIFE THAT USES CARBON-BASED CHEMISTRY.

- WE WILL ONLY CONSIDER LIFE THAT USES WATER AS ITS LIQUID SOLVENT.

- IGNORE POSSIBILITY OF “EXOTIC LIFE,” SUCH AS LIFE THAT USES A LIQUID SOLVENT OTHER THAN WATER, SILICON-BASED LIFE, OR LIFE BASED ON THE STRONG FORCE.

– THEREFORE CONSIDER ONLY EARTH-LIKE PLANETS (OR LARGE MOONS) WITH LIQUID WATER.

– IF EXOTIC LIFE CAN EXIST, THEN LIFE MAY EXIST IN A LARGER RANGE OF LOCATIONS AND BE MORE COMMON THAN WHAT WE WILL ESTIMATE.

PROPERTIES OF A PLANET THAT IS SUITABLE FOR LIFE

1. - RELATIVELY LARGE ABUNDANCES OF CARBON, NITROGEN, AND OXYGEN (PLUS TRACE AMOUNTS OF HEAVIER ELEMENTS)

• IN ORDER TO HAVE SUFFICIENT QUANTITIES OF ELEMENTS HEAVIER THAN HYDROGEN AND HELIUM, THE PLANET MUST ORBIT A FAIRLY YOUNG (POPULATION I) STAR

2. - NOT NEAR A SITE OF COSMIC VIOLENCE• IN CERTAIN LOCATIONS, SUCH AS NEAR THE

CENTER OF A GALAXY, A PLANET MIGHT BE BOMBARDED BY FREQUENT COMETARY IMPACTS OR HAVE HIGH LEVELS OF DAMAGING RADIATION

PROPERTIES OF A PLANET THAT IS SUITABLE FOR LIFE

1. - ROCKY PLANET - SOLID SURFACE• PROBABLY THIS WILL USUALLY BE THE CASE IF

THE PLANET IS LOCATED IN THE INNER PART OF THE SOLAR SYSTEM ORBITING A RELATIVELY YOUNG (POPULATION I) STAR

2. - MASSIVE ENOUGH TO RETAIN A REASONABLY DENSE ATMOSPHERE

• IF THE ATMOSPHERE ESCAPES INTO SPACE, THERE CAN BE NO LIQUID WATER (OR ANY OTHER LIQUID) ON THE SURFACE

• REMEMBER THAT THIS IS THE PROBLEM WITH MARS IN OUR SOLAR SYSTEM

PROPERTIES OF A PLANET THAT IS SUITABLE FOR LIFE

1. - CORRECT TEMPERATURE RANGE FOR LIQUID WATER (SOLVENT)

• THIS REQUIRES A PLANET TO HAVE A NEARLY CIRCULAR ORBIT AROUND ITS STAR WITHIN THE “HABITABLE ZONE,” OR TO HAVE A SOURCE OF INTERNAL HEAT (SUCH AS TIDAL HEATING)

2. - STABLE ENVIRONMENT FOR SEVERAL BILLION YEARS (TIME NEEDED ON EARTH FOR “ADVANCED” LIFE FORMS TO EVOLVE)

HABITABLE ZONE SPHERICAL SHELL SURROUNDING STAR IN

WHICH ANY ORBITING PLANETS WILL HAVE THE RIGHT TEMPERATURE FOR LIQUID WATER

INNER EDGE OF ZONE IS WHERE AVERAGE TEMPERATURE = 100oC

OUTER EDGE OF ZONE IS WHERE AVERAGE TEMPERATURE = 0oC

WHERE IS THE HABITABLE ZONE FOR FOR OUR SUN?

OUR SUN’S HABITABLE ZONE INNER EDGE: ABOUT 0.85 - 0.95 AU FROM SUN OUTER EDGE: ROUGHLY 1.4 - 1.7 AU FROM SUN

– EXACT VALUES DEPENDS ON DETAILS OF GREENHOUSE EFFECT

HABITABLE ZONE MOVES OUTWARD WITH TIME, AS SUN BRIGHTENS SLIGHTLY– WHEN SOLAR SYSTEM FIRST FORMED, INNER EDGE

AT 0.65 – 0.80 AU AND OUTER EDGE AT 1.1 – 1.5 AU CONTINUOUSLY HABITABLE ZONE (FOR FIRST

FEW BILLION YEARS AFTER SOLAR SYSTEM FORMED)– INNER EDGE AT 0.85 TO 0.95 AU– OUTER EDGE AT 1.1 TO 1.5 AU

OUR SUN’S HABITABLE ZONE WHERE ARE PLANETS RELATIVE TO SUN’S

HABITABLE ZONE? EARTH IS AT 1 AU – WITHIN CONTINUOUSLY

HABITABLE ZONE (OBVIOUSLY!) VENUS IS AT 0.72 AU – TOO CLOSE NOW, BUT

MAY HAVE BEEN BARELY HABITABLE VERY EARLY IN ITS HISTORY.

MARS IS AT 1.52 AU – PROBABLY WITHIN HABITABLE ZONE NOW, BUT POSSIBLY NOT EARLIER IN ITS HISTORY.– BUT THE REAL PROBLEM IS THAT MARS IS TOO

LIGHT, SO IT LOST MOST OF ITS ATMOSPHERE.

PROPERTIES OF MAIN SEQUENCE STARS

SPECT. TYPE

BRIGHTNESS (SUN=1)

NUMBER OF STARS

(IN MW)PERCENT OF TOTAL

O

B

A

F

G

K

M

100,000

500

10

2

0.9

0.2

0.005

80,000

360 MILLION

2.4 BILLION

12 BILLION

28 BILLION

60 BILLION

290 BILLION

0.00002%

0.09%

0.6%

3%

7%

15%

73%

COLOR

BLUE

BLUE

WHITE

YELLOW

YELLOW

ORANGE

RED

HABITABLE ZONES AROUND OTHER STARS

FOR BRIGHTER STARS:– HABITABLE ZONE IS FARTHER FROM STAR AND

LARGER IN EXTENT (E.G., 5 TO 20 AU FOR AN A-TYPE MAIN SEQUENCE STAR)

FOR FAINTER STARS:– HABITABLE ZONE IS CLOSER TO STAR AND SMALLER

IN EXTENT (E.G., 0.02 TO 0.06 AU FOR AN M-TYPE MAIN SEQUENCE STAR)

– HABITABLE ZONE MAY BE SO SMALL THAT IT IS UNLIKELY THAT ANY PLANETS ARE FOUND WITHIN IT

– IF PLANET IS TOO CLOSE TO STAR, OTHER POSSIBLE PROBLEMS INCLUDE:

• SOLAR FLARES • PLANET’S ROTATION MAY BE TIDALLY LOCKED (MIGHT BE

OK IF ATMOSPHERE CAN SPREAD HEAT AROUND ENOUGH)

STABLE ENVIRONMENT

STAR MUST NOT CHANGE TOO MUCH IN TEMPERATURE OR BRIGHTNESS FOR SEVERAL BILLION YEARS

THIS REQUIRES A MAIN SEQUENCE STAR THAT IS COOLER/REDDER/FAINTER THAN MID-F SPECTRAL TYPE – MAIN SEQUENCE LIFETIME IS TOO SHORT FOR

HOTTER/BLUER/BRIGHTER STARS– THIS REQUIREMENT ELIMINATES ONLY A FEW

PERCENT OF ALL MAIN SEQUENCE STARS

PROPERTIES OF MAIN SEQUENCE STARS

SPECT. TYPE

BRIGHTNESS (SUN=1)

LIFETIME (YEARS)

# OF STARS (IN MW)

PERCENT OF TOTAL

O

B

A

F

G

K

M

100,000

500

10

2

0.9

0.2

0.005

5 MILLION

10 MILLION

500 MILLION

1 BILLION

10 BILLION

100 BILLION

1 TRILLION

80,000

360 MILLION

2.4 BILLION

12 BILLION

28 BILLION

60 BILLION

290 BILLION

0.00002%

0.09%

0.6%

3%

7%

15%

73%

COLOR

BLUE

BLUE

WHITE

YELLOW

YELLOW

ORANGE

RED

SUITABLE STARSDEFINED 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 (CARBON-BASED, USES WATER AS LIQUID SOLVENT)

WHAT PROPERTIES MUST A STAR POSSESS IN ORDER TO BE A SUITABLE STAR?

PROPERTIES OF SUITABLE STARS

MAIN SEQUENCE - MOST NON-MAIN SEQUENCE STARS (RED

GIANTS & WHITE DWARFS) AREN’T STABLE ENOUGH IN BRIGHTNESS AND TEMPERATURE OVER A LONG ENOUGH TIME

- 90% OF STARS ARE MAIN SEQUENCE

SUFFICIENTLY LONG MAIN SEQUENCE LIFETIME

- AT LEAST SEVERAL BILLION YEARS

- SPECTRAL TYPES M, K, G, AND SOME F - 97% OF MAIN SEQUENCE STARS ARE OF THESE

SPECTRAL TYPES

PROPERTIES OF SUITABLE STARS

SUFFICIENTLY LARGE LUMINOSITY - REASONABLY LARGE HABITABLE ZONE THAT ISN’T TOO CLOSE TO THE STAR

- SPECTRAL TYPE M TOO FAINT, HABITABLE ZONE TOO SMALL AND TOO CLOSE TO STAR

LOCATION IN SPIRAL ARMS OR DISK OF A SPIRAL GALAXY, OR IN AN IRREGULAR GALAXY- STARS HERE ARE YOUNGER (POPULATION I) AND THUS HAVE SUFFICIENT ABUNDANCE OF HEAVY ELEMENTS

- NOT TOO NEAR BLACK HOLE IN GALACTIC CENTER (AVOIDS COSMIC VIOLENCE)

SUITABLE STARSSOME STARS IN BINARY OR MULTIPLE STAR

SYSTEMS ARE EXCLUDED - 50% OF STARS ARE BINARY OR MULTIPLE- SOME PLANETS IN BINARY SYSTEMS WILL

NOT HAVE STABLE ORBITS- PLANETARY ORBITS IN DOUBLE OR MULTIPLE

STAR SYSTEMS CAN BE STABLE IF:THE STARS ARE FAR APART, AND THE PLANET IS MUCH CLOSER TO ONE STAR (THE ONE IT ORBITS) THAN TO THE OTHER STAR

ORTHE TWO STARS ARE CLOSE TOGETHER, AND THE PLANET ORBITS BOTH STARS AT A DISTANCE THAT IS LARGE COMPARED TO THEIR SEPARATION

SUITABLE STARS DEFINED AS STARS AROUND WHICH A PLANET (OR

PLANETS) SUITABLE FOR LIFE COULD BE ORBITING.

THIS RESTRICTS US TO:1. MAIN SEQUENCE STARS2. SPECTRAL TYPES K, G, AND SOME F3. POPULATION I (ENOUGH HEAVY ELEMENTS)4. NOT TOO NEAR GALACTIC CENTER5. 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).

LET’S LOOK AT NEARBY SUITABLE STARS.

THE NEAREST 40 STARS (WITHIN 14 LY)

NameLuminosity (Sun = 1)

Spectral Type

Sun

Proxima Centauri

Alpha Centauri A

Alpha Centauri B

Barnard’s Star

Wolf 359

BD+36°2147

Sirius A

Sirius B

Luyten 726-8 A

Luyten 726-8 B

Ross 154

1

0.00005

1.58

0.44

0.0003

0.00002

0.006

23.0

0.003

0.00006

0.00004

0.0005

G2

M5

G2

K1

M5

M6

M2

A1

A2 (WD)

M6

M6

M4

Name Luminosity Spectral Type

Ross 248

Epsilon Eridani

CD-36°15693

Ross 128

Luyten 796-6

61 Cygani A

61 Cygni B

Procyon A

Procyon B

BD+59°1915 A

BD+59°1915 B

BD+43°44 A

BD+43°44 B

0.0001

0.30

0.012

0.0003

0.0003

0.08

0.04

7.6

0.0005

0.002

0.0015

0.006

0.0004

M6

K2

M2

M5

M5

K5

K7

F5

F5 (WD)

M4

M4

M2

M4

Name Luminosity Spectral Type

G51-15

Epsilon Indi

Luyten 372-58

Luyten 725-32

Tau Ceti

BD+5 1668

Kapteyn’s Star

CD-39 14192

Kruger 60 A

Kruger 60 B

Ross 614 A

Ross 614 B

CD-25 10553A

0.000016

0.14

0.0003

0.0001

0.47

0.0005

0.004

0.03

0.0002

0.0003

0.0003

0.00005

0.00005

M7

K4

M5

M6

G8

M4

M1

M0

M4

M4

M4

M

M

NameLuminosity Spectral

Type

BD-12 4523

CD-37 15492

0.0004

0.0002

M4

M4

OF THE 40 CLOSEST STARS (THOUGHT TO BE TYPICAL), 4 STARS (10% OF TOTAL), ARE “SUITABLE STARS” – SUN, EPSILON ERIDANI, EPSILON INDI, AND TAU CETI. (THESE ARE MARKED WITH ARROWS ABOVE.)

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 CO2 CYCLE ON EARTH, WHICH

HELPS REGULATE CLIMATE– A SUFFICIENTLY MASSIVE ROCKY PLANET (STILL

HOT INSIDE) MAY AUTOMATICALLY MEET THIS REQUIREMENT

HELPFUL (BUT MAYBE NOT CRUCIAL) FEATURES FOR A SUITABLE PLANET IMPACTS NOT TOO FREQUENT

– A SUFFICIENTLY LARGE IMPACT COULD WIPE OUT ALL LIFE

– THIS 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 THE 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.

EXTRASOLAR PLANETS? SOMETIMES CALLED EXOPLANETS 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 (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 HAVE BEEN DISCOVERED THIS WAY SO FAR, BUT THIS METHOD WILL BE MORE IMPORTANT IN THE FUTURE.

EARTH-SIZED PLANETS MAY BECOME VISIBLE DURING THE 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 (PLANET APPEARS AS A LITTLE BLACK DOT AGAINST DISK OF SUN).

HOWEVER, OTHER STARS ARE TOO FAR AWAY TO SEE A DISK, SEE ONLY A POINT OF LIGHT.

THE PLANET PARTIALLY BLOCKS LIGHT FROM THE STAR, CAUSING A TEMPORARY DECREASE IN THE STAR’S BRIGHTNESS.

THE 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

A 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 (A STAR WITH AN EXTRASOLAR PLANET IN OUR CASE) ON ITS WAY TO US.

THE GRAVITATIONAL EFFECTS OF THE NEARER OBJECT (STAR WITH PLANET) BEND THE PATH OF THE LIGHT.

AS A RESULT, THE DISTANT OBJECT APPEARS SHIFTED IN POSITION OR IN MULTIPLE IMAGES.

THE STAR CAUSES MOST OF THE BENDING, BUT ITS PLANET CAUSES A LITTLE BIT OF BENDING.

PROBLEM: PLANETS AREN’T MASSIVE ENOUGH TO CAUSE SIGNIFICANT BENDING OF LIGHT. THIS WORKS BETTER WHEN A MORE MASSIVE OBJECT IS BENDING THE LIGHT.

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

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)

THE PLANET IS UNSEEN, BUT LIGHT FROM STAR IS ALTERNATELY BLUESHIFTED AND REDSHIFTED DUE TO THE MOTION 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

CLOSE TO 300 PLANETS DISCOVERED SINCE 1995 VIA THIS TECHNIQUE

OVER 90% OF EXTRASOLAR PLANETS DISCOVERED THIS WAY

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 300 EXTRASOLAR PLANETS HAVE BEEN

DISCOVERED SINCE 1995, MOST USING THE DOPPLER EFFECT TECHNIQUE.

AT LEAST 20 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 4 EARTH MASSES.

MANY 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 MANY 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). A FEW SUCH PLANETS HAVE BEEN FOUND. SOLAR SYSTEMS THAT CONTAIN JUPITER-LIKE PLANETS AT JUPITER-LIKE DISTANCES FROM THE STAR 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 HIERARCHYSTARS 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)