NSCI 314 LIFE IN THE COSMOS 12 - WHERE TO SEARCH FOR LIFE OUTSIDE OUR SOLAR SYSTEM: SUITABLE STARS AND PLANETS Dr. Karen Kolehmainen Department of Physics, CSUSB

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, 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 VIOLENCE 3. ROCKY PLANET - SOLID SURFACE 4. 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 = 100 o C OUTER EDGE OF ZONE IS WHERE AVERAGE TEMPERATURE = 0 o C WHERE IS THE HABITABLE ZONE FOR FOR OUR SUN?

OUR SUN’S HABITABLE ZONE INNER EDGE: ABOUT AU FROM SUN OUTER EDGE: ROUGHLY 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 OBAFGKMOBAFGKM 100, , MILLION 2.4 BILLION 12 BILLION 28 BILLION 60 BILLION 290 BILLION % 0.09% 0.6% 3% 7% 15% 73% COLOR BLUE WHITE 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 OBAFGKMOBAFGKM 100, MILLION 10 MILLION 500 MILLION 1 BILLION 10 BILLION 100 BILLION 1 TRILLION 80, MILLION 2.4 BILLION 12 BILLION 28 BILLION 60 BILLION 290 BILLION % 0.09% 0.6% 3% 7% 15% 73% COLOR BLUE WHITE YELLOW ORANGE RED

SUITABLE STARS DEFINED 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 STARS SOME 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 OR THE 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 STARS 2.SPECTRAL TYPES K, G, AND SOME F 3.POPULATION I (ENOUGH HEAVY ELEMENTS) 4.NOT TOO NEAR GALACTIC CENTER 5.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) Name Luminosity (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 A Luyten B Ross G2 M5 G2 K1 M5 M6 M2 A1 A2 (WD) M6 M4

Name Luminosity Spectral Type Ross 248 Epsilon Eridani CD-36°15693 Ross 128 Luyten 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 M6 K2 M2 M5 K5 K7 F5 F5 (WD) M4 M2 M4

Name Luminosity Spectral Type G51-15 Epsilon Indi Luyten Luyten Tau Ceti BD Kapteyn’s Star CD Kruger 60 A Kruger 60 B Ross 614 A Ross 614 B CD A M7 K4 M5 M6 G8 M4 M1 M0 M4 M

Name Luminosity Spectral Type BD CD 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 CO 2 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.