astrobiology: life on other planets

AstrobiologySearching for life on other planets

Friday, February 13, 2009

The Drake equation can be used to organize our thoughts


Fermi paradox


Fermi paradox

• Time for an intelligent species to colonize galaxy: 106 years


Fermi paradox


• Age of the Galaxy: 1010 years


Fermi paradox


• Age of the Galaxy: 1010 years

• where are they?


How many planets are there?


We now have discovered many planets around other stars, but no Earth analogs (yet)


We now have discovered many planets around other stars, but no Earth analogs (yet)

mic

role

nsin

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pulsarhabitable?


Future missions approved by NASA will probe ‘habitable zone’


Kepler Transit search(scheduled launch 2/2009)


In a transit search, we see when a planet passes in front of its star


Transit searches are a cheap way to hunt for planets


USING PHOTOMETRY TO DETECT EARTH-SIZE PLANETS

• The relative change in brightness (ΔL/L) is equal to the relative areas (Aplanet/Astar)

Small planets need ultra-precise photometry. Must be done with wide-field CCD imager in space.

Jupiter: 1% area of the Sun (1/100)

Earth or Venus0.01% area of the Sun (1/10,000)


What have we learned from planet searches so far?


Only high metallicity stars have planets

more than 100metal-poor stars (Mayor et al. 2003). In addition, aDoppler survey of!150 low-metallicity stars has been underwayat Keck for the past two years (Sozzetti et al. 2004). No planetshave been announced from either of these surveys, suggestingthat the rate of occurrence of Jovian-mass planets with orbitalperiods less than 3 yr does not exceed (and is likely lower than)a few percent around metal-poor stars.

A single substellar object, HD 114762b, with M sin i !11MJ has been found orbiting a metal-poor ("Fe/H# $ %0:655)field star (Latham et al. 1989). Interestingly, we measure a lowv sin i (1.7 km s%1) for this F-type star. Fewer than 5% of the starswith comparable spectral type have v sin i < 2:0 km s%1, sug-gesting that this particular star may be viewed close to pole-on.Assuming that the stellar rotation axis is aligned with the orbitalrotation axis, it is possible that the companion to HD 114762may have a substantially higher mass, conceivably even a stellarmass, a suggestion first made by Cochran et al. (1991).

It has been suggested that the paucity of spectral lines inmetal-poor stars results in poorer detectability that impedes the detec-tion of Jovian-mass planets. To address this issue, we calculatedthe mean radial velocity error for stars in each 0.25 dex metal-licity bin. For [Fe/H] between%0.75 and 0.5, the mean Dopplerprecision is 4 m s%1. The lowest metallicity bin only suffers amodest degradation in velocity precision to !6 m s%1. Thus,there is no significant detectability bias against the detection ofplanets in the parameter space that we have defined to have uni-form detectability. If gas giant planets orbit metal-poor stars as

often as they orbit solar-metallicity stars, it seems very likely thatthey would have been detected by now.

3.2. The Volume-limited Sample

Avolume-limited sample is often desirable as an unbiased sam-ple, and virtually all spectroscopic investigations of the planet-metallicity correlation have referenced such a sample as a control.We contend that a volume-limited sample is not the best com-parison sample for this investigation because it does not nec-essarily represent the stars on Doppler surveys. To investigatethis, we defined a volume-limited subset of 230 FGK-type starsanalyzed with SME. Figure 6 shows the density of the entire(1040 star) planet search sample as a function of distance forspecified ranges of absolute visual magnitude. The points oneach curve mark the distance where the sample size incrementsby about 40 stars. Intrinsically faint stars dominate the 20 pcsample, and the sample composition gradually shifts to earliertype, intrinsically bright stars at larger distances. We define thevolume-limited sample to have a radius of 18 pc, inside of whichthe number of FGK-type stars per unit volume on the planetsearch programs is nearly constant as a function of distance. Be-yond this distance the number density of intrinsically faint starsbegins to decline rapidly.

TABLE 3

Stars with Uniform Planet Detectability

Star ID Planet /Star

HD 142 ............................... P

HD 2039 ............................. P

HD 4203 ............................. P

HD 8574 ............................. P

HD 10697 ........................... P

Note.—Table 3 is published in its entiretyin the electronic edition of the AstrophysicalJournal. A portion is shown here for guidanceregarding its form and content.

Fig. 4.—Percentage of stars with detected planets rises with iron abundance.In all, a subset of 850 stars were grouped according to metallicity. This subset ofstars had at least 10 Doppler measurements over 4 yr, providing uniform de-tectability for the presence of planets with velocity amplitudes greater than30 m s%1 and orbital periods less than 4 yr. The numbers above each bar on thehistogram indicate the ratio of planets to stars in each bin. Thirteen stars had"Fe /H#< %1:0, and no planets have been discovered around these stars.

Fig. 5.—Same results as Fig. 4, but divided into 0.1 dex metallicity bins. Theincreasing trend in the fraction of stars with planets as a function of metallicity iswell fitted with a power law, yielding the probability that an FGK-type star has agas giant planet: P(planet) $ 0:03"(NFe=NH)=(NFe=NH)&#

2:0.

Fig. 6.—Stellar density for a range of absolute visual magnitudes calculatedin distance bins, each with 41–43 stars. Intrinsically faint stars dominate thenearby solar neighborhood but are rapidly lost beyond 20 pc. Intrinsically brightstars become the dominant constituent of the planet search samples at distancesgreater than about 40 pc.

FISCHER & VALENTI1110 Vol. 622

3 × Solar0.3 × Solar


Stars with planets are young. The Sun may be one of the oldest stars with planets– 37 –

Fig. 7.— The age-metallicity relation for the local Galactic disk: The top-left panel plotsdata for the VF05 stars from Sample A; the lower-left panel shows data for the VF05 stars

from Sample B; the top-right panel plots results for the Edvardsson et al (1993) dataset;and the lower-right panel plots the AMR defined by the Nordstrom et al (2004) dataset.

In each case, the solid pentagon marks the location of the Sun. The two upper panels alsoshow the best-fit linear and second-order relations for the VF05 data; the large crosses inthe upper-right panel plot the AMR derived by Rocha-Pinto et al (2000); and the errorbars

in the lower-left panel provide an indication of the range of uncertainties associated with theVF05 age estimates.

– 40 –

Fig. 10.— The age distribution of local disk stars: The upper two panels show the agedistributions for the volume-complete samples considered in the present study: the shaded

histogram plots the summed probability distribution for stars in the VF05 dataset, and thedotted histogram is based on the median ages for those stars; the solid histogram includes

non-VF05 stars, whose ages are estimated using the linear AMR plotted in Figure 7. Inboth cases, the vertical bars mark the median ages for the full sample (solid line) and forthe VF05 stars alone (dotted line). The lower left panel plots the age distribution of stars

known to have planetary companions: the shaded histogram shows data for 107 VF05 starswith isochrone-based ages; a further 23 stars have age estimates that are based on the linear

AMR. The vertical bar (dotted) marks the median age for the VF05 host stars. Finally,the lower right panel compares the cumulative age distributions of the 107 VF05 exoplanet

hosts and the 239 VF05 stars from Sample B; a Kolmogorov-Smirnov test indicates that theprobability is less than 5% that the two samples are drawn from the same parent population.

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What planets support life?


What is life?


Life as we know it


What planets support life?

What kinds of planets can support life?What fraction of planets that can support life do support life?


The Habitable Zone is the range of distances from a star which allow a planet to support life


What are the minimal conditions for life?

• How hot?

• How cold?

• How dry?

• How acid?

• How salty?

• How radioactive?

• How poisonous?

• How much pressure?

• How barren?

Organisms that push these limits are calledextremophiles


Thermophiles thrive at 90ºC (190ºF)


Endolithic life eat and breath rock two miles undeground


3 inches annual precipitation

-68ºC

Dry 200 mph winds evaporate all moisture

Dry valleys of antarctica


Cryptoendolithic ecosystem inside rocks

White lichen

Black Lichen

Algae


Bacteria living at bottoms of perpetually frozen lakes.


Extreme life in Permafrost


2 mm decadal precipitation

recent discoveries of life below 4 inches

Atacama Desertdryest place on earth


Lifeless desert in Oceans

• Centers of oceans have very little life.

• Plenty of liquid water

• Plenty of sunlight energy

• Missing some key chemicals– Phosphorus


Lessons from terrestrial life

• Life can exist with only a bare minimum of ingredients:– Liquid Water– Some energy source

• Sunlight, Rocks, Geothermal energy– Basic chemical ingredients

• Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus


Old Habitable Zone Theory

• If planet is too close to star, fries through runaway greenhouse effect

• If planet is too far from star, freezes, can’t support life.


New Habitable Zone Theory

• Many other factors besides distance from the Sun help determine planetary climate– Greenhouse effect– Plate Tectonics– Impacts (Early Earth atmosphere stripped)– Tidal Heating (Io, Europa)

• Liquid water can be found in a variety of unlikely environments– Europa, Callisto, Ganymede– Early Mars very wet, present Mars dry?


How many habitable planets will actually be alive?

• Basic ingredients to make life are common throughout the cosmos

• Look at History of Life on Earth• Life began shortly after Earth cooled• Suggests that Life is easy to make.


Basic ingredients of life



• Organic molecules detected in interstellar space.




• Water (Ice) detected throughout galaxy, solar system.




• Water (Ice) detected throughout galaxy, solar system.

• The basic chemical ingredients of life are common throughout the galaxy.


Organic Molecules in interstellar space

• Amino Acids• Nucleic Acids• Soot


http://www.allthesky.com/nebulae/big/coals-b.jpg

http://www.allthesky.com/nebulae/big/coals-b.jpg

Given the right ingredients, how easy is it to make life?

34


Earliest Life

• Bands of Carbon in ancient rock


Conclusion from oldest life


Conclusion from oldest life• Earth was not habitable until

3.8 billion years ago.– Too many impacts melted

surface.




surface.• 3.8 billion years ago, many

fewer impacts.– Earth became inhabitable.






• Oldest life on Earth 3.8 billion years old.







• Life formed on Earth as soon as Earth could support life.







• Life formed on Earth as soon as Earth could support life.

• Life is easy to form?


Searching for life in the Solar System

38


Canceled


Detection of life around other stars

Indefinitely postponed


http://origins.jpl.nasa.gov/habitable-planets/images/ra6in13-coronagraph.jpg

http://origins.jpl.nasa.gov/habitable-planets/images/ra6in13-coronagraph.jpg

Biomarkers in atmosphere


Biomarkers in atmosphere• Earth’s atmosphere shows

strong signals of two biogenic molecules– Oxygen

• Produced by plants– Methane

• Produced by Cows






• Normally, methane burns in Oxygen– Natural Gas






• Normally, methane burns in Oxygen– Natural Gas

• Two can only exist in combination because both being produced by life.


What planets support intelligent life?


Simple life


http://www.oar.noaa.gov/spotlite/archive/images/loihi_4.jpg

http://www.oar.noaa.gov/spotlite/archive/images/loihi_4.jpg

Complex Life


http://images.google.com/imgres?imgurl=http://jajhs.kana.k12.wv.us/vwv/animal/rep_amph/graytreefrog1.jpg&imgrefurl=http://jajhs.kana.k12.wv.us/vwv/animal/rep_amph/rep_amph.htm&h=496&w=350&sz=39&tbnid=qyJvfyHAZawJ:&tbnh=126&tbnw=89&start=13&prev=/images%253Fq%253Dtree%2526hl%253Den%2526lr%253D%2526ie%253DUTF-8

http://images.google.com/imgres?imgurl=http://jajhs.kana.k12.wv.us/vwv/animal/rep_amph/graytreefrog1.jpg&imgrefurl=http://jajhs.kana.k12.wv.us/vwv/animal/rep_amph/rep_amph.htm&h=496&w=350&sz=39&tbnid=qyJvfyHAZawJ:&tbnh=126&tbnw=89&start=13&prev=/images%253Fq%253Dtree%2526hl%253Den%2526lr%253D%2526ie%253DUTF-8

Lessons from Extremophiles

• Complex life on Earth restricted to narrow range of habitats.– Not in Antarctica, too cold, dry– Not inside rocks, nothing to eat, breath– Not inside geothermal vents, too hot– Not in clouds, too heavy– Not in driest deserts, too dry


Formation of Oxygen Atmosphere

• Life begins to saturate atmosphere with Oxygen

• Oxygen kills off life• Oxygen combines with

rock• Life comes back,

makes more oxygen• Oxygen kills off life• Process continued for

800 million years.


Lessons from formation of life

• Complex multicellular life did not evolve until recently.– Cambrian Explosion 600 Mya.

• Complex life could not have evolved without Oxygen atmosphere.

• Complex life more fragile than simple life.• Complex life difficult to evolve.


Lessons from Mars

• Planetary climates can change

• Complex life (if it ever existed) likely wiped out today.

• Simple life could have survived.


http://www.lpi.usra.edu/expmars/edbrief/updateFS3.gif

http://www.lpi.usra.edu/expmars/edbrief/updateFS3.gif

• Sun may be among the first stars to have planets

• Life may be common

• Complex life may be rare

• Complex life may take a long time to form

• We may be alone?

Answer to Fermi Paradox?


Intelligent life• No information how

common intelligent life is.– Took most of history of

earth to evolve an earthworm.

• Definition of Intelligent life: Ability to operate radio transmitters

• Search for intelligent life by searching for radio transmission

• Search for Extraterrestrial Intelligence: SETI– Cannot be federally funded

by congressional mandate– Now part of NASA’s

Astrobiology Institute– Just another means to

search for life– Privately Funded SETI

institute


The End


astrobiology: life on other planets

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