NSCI 314

LIFE IN THE COSMOS

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

SUITABLE STARS AND PLANETS

Dr. Karen KolehmainenDepartment of Physics, CSUSB

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

SEARCHING FOR LIFE IN OTHER SOLAR SYSTEMSWE WILL TAKE A CONSERVATIVE APPROACH:

-WE WILL CONSIDER ONLY LIFE THAT IS BASED ON THE ELECTROMAGNETIC INTERACTION, i.e., 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)

2. NOT NEAR A SITE OF COSMIC VIOLENCE3. ROCKY PLANET - SOLID SURFACE4. MASSIVE ENOUGH TO RETAIN A

REASONABLY DENSE ATMOSPHERE 5. CORRECT TEMPERATURE RANGE FOR LIQUID

WATER (SOLVENT)6. STABLE ENVIRONMENT FOR SEVERAL

BILLION YEARS (TIME NEEDED ON EARTH FOR “ADVANCED” LIFE FORMS TO EVOLVE)

ABUNDANCES OF ELEMENTS HYDROGEN AND HELIUM (BUT NO HEAVIER

ELEMENTS) ARE PRODUCED SHORTLY AFTER THE BIG BANG (DURING THE FIRST FEW MINUTES OF THE UNIVERSE).

HEAVIER ELEMENTS ARE PRODUCED VIA FUSION IN STARS (AND IN SUPERNOVA EXPLOSIONS ), AND RETURNED TO THE INTERSTELLAR MEDIUM VIA PLANETARY NEBULAE AND SUPERNOVAE.

HEAVIER ELEMENTS ARE INCORPORATED IN NEW STARS (AND THEIR PLANETARY SYSTEMS) FORMED FROM THE ENRICHED INTERSTELLAR MEDIUM.

ABUNDANCES OF ELEMENTS THEREFORE, PLANETARY SYSTEMS AROUND

YOUNGER STARS CONTAIN LARGER ABUNDANCES OF HEAVY ELEMENTS (ANYTHING HEAVIER THAN H AND He) THAN PLANETARY SYSTEMS OF OLDER STARS.

THUS PLANETS THAT CONTAIN ELEMENTS NECESSARY FOR LIFE ARE MORE LIKELY TO BE FOUND ORBITING YOUNGER STARS THAN OLDER STARS.

PLANETS MAY NOT EVEN BE ABLE TO FORM AROUND VERY OLD STARS WITH VERY LOW ABUNDANCES OF HEAVY ELEMENTS.

ABUNDANCES OF ELEMENTS POPULATION II STARS: OLD STARS, LOW

ABUNDANCES OF HEAVY ELEMENTS (0.1 – 0.5%)

POPULATION I STARS: YOUNGER STARS, HIGHER ABUNDANCES OF HEAVY ELEMENTS (1 - 2%)

PLANETS ORBITING POPULATON I STARS ARE MORE SUITABLE FOR LIFE – MORE HEAVY ELEMENTS

MOST STARS EXISTING TODAY (INCLUDING THE SUN) ARE POPULATION I STARS.

WHERE ARE POP I AND POP II STARS?

POPULATION I (PLANETS MAY BE SUITABLE CANDIDATES FOR LIFE):– DISKS OF SPIRAL GALAXIES, ESPECIALLY

IN SPIRAL ARMS– MOST IRREGULAR GALAXIES

POPULATION II (PLANETS ARE PROBABLY NOT SUITABLE CANDIDATES FOR LIFE):– NUCLEUS AND OUTER HALO OF SPIRAL

GALAXIES, PLUS GLOBULAR CLUSTERS– ELLIPTICAL GALAXIES

COSMIC VIOLENCE SOME ASTRONOMICAL ENVIRONMENTS WOULD

PROBABLY BE FATAL TO LIFE:– HIGH INCIDENCE OF GAMMA RAYS OR X-RAYS (HIGH

ENERGY PHOTONS) – FREQUENT STELLAR COLLISIONS OR NEAR-

COLLISIONS LOTS OF COMETARY IMPACTS LOCATIONS WHERE THIS IS A PROBLEM:

– NEAR GALACTIC CENTER (RADIATION FROM VICINITY OF CENTRAL BLACK HOLE, PLUS FREQUENT STELLAR COLLISIONS)

– NEAR SUPERNOVAE – NEAR ANY OTHER X-RAY AND GAMMA RAY SOURCE

ROCKY PLANETS

PROBABLY OCCUR MOST OF THE TIME AS LONG AS:

– THE ABUNDANCE OF HEAVY ELEMENTS IS HIGH ENOUGH TO MAKE ROCK, i.e., IN A SOLAR SYSTEM AROUND A POPULATION I STAR.

– THE PLANET IS IN THE INNER PART OF ITS SOLAR SYSTEM. (THEREFORE LIGHT GAS MOLECULES LIKE HYDROGEN AND HELIUM ARE MOVING FAST ENOUGH TO ESCAPE FROM ITS ATMOSPHERE, PREVENTING IT FROM BECOMING A GAS GIANT.)

ATMOSPHERE NEEDED TO KEEP WATER (OR OTHER LIQUID

SOLVENT) FROM BOILING AWAY INTO SPACE HELPS TO SPREAD HEAT AROUND, AVOIDING

TEMPERATURE EXTREMES MAY ALSO SUPPLY GASES FOR ORGANISMS TO

BREATHE MAY ALSO SUPPLY GASES NEEDED FOR CHEMICAL

REACTIONS INVOLVED WITH THE ORIGIN OF LIFE PLANET MUST BE MASSIVE ENOUGH TO RETAIN AN

ATMOSPHERE, i.e., ITS GRAVITY MUST BE SUFFICIENT TO PREVENT MOST GAS MOLECULES FROM ESCAPING. – RECALL THAT IN OUR SOLAR SYSTEM, MARS IS

TOO LIGHT.

RIGHT TEMPERATURE RANGE MAINTAINED VIA A NEARLY CIRCULAR ORBIT

AT THE RIGHT DISTANCE FROM ITS STAR FOR A TEMPERATURE ALLOWING LIQUID WATER (i.e., IN THE “HABITABLE ZONE”)

OR

PROVIDED BY A SOURCE OF INTERNAL HEAT (E.G., RADIOACTIVE DECAY, OR TIDAL FORCES AS WITH JUPITER’S MOONS IO AND EUROPA). THIS CAN HAPPEN EVEN IF THE PLANET OR MOON IS LOCATED FARTHER FROM ITS STAR.

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 (e.g., 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 LuminositySpectral

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 LuminositySpectral

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.)

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 CO2 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

– 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.