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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)