Exoplanets and Life in the Universe

Part I

To find planets orbiting other stars we use the Doppler Effect and look for the very tiny shifts in the location of spectral lines caused as the planet orbits its parent star. As an example of the size of the effect we are looking for, consider that Jupiter causes the Sun to wobble by some 12 km/s. Compare this to the radial velocities we measure for stars of hundreds of km/s. The technology to detect such small Doppler Shifts has only existed for about ten years and the first results are now being announced. Of the two-dozen planets detected so far, all are very massive planets (at least the mass of Jupiter). These giants naturally perturb their parent star more than low mass planets and are, therefore, easier to detect. Likewise, none of these planets is directly detected, but their existence is inferred from the radial velocity curves.

Two major surprises have turned up in these first discoveries:

• Most of the planets fall very close to their parent stars
• Many of the planets are in fairly eccentric orbits

According to the formation mechanism I outlined earlier in the course, we do not expect Jovian planets close to the star. Likewise, the planets of our system are in nearly circular orbits.

The number of detection to date suggests that planet formation is common in our galaxy.

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If other stars have planet systems, do some of those planets harbor life? To answer this question we begin by looking at the requirements for life to exist.

• Base Atom - Earth life has chosen C, which is rather abundant in the cosmos. Carbon can make long, complex atom that form the basis of life on Earth.
• Solvent - acts to transport atoms and molecules close to one another as well as acting as a catalyst. Earth life uses water for this role and we have seen that water is very abundant in the universe.
• Master molecule - complex molecule which can replicate itself. Earth life uses DNA.

Even having the requirements, how likely is it that life forms. The famous Urey-Miller experiment put gases thought to be in the primitive Earth atmosphere. A spark simulates lightning and the mixture forms amino acids. Although complex molecules, amino acids are not life, but do represent a significant step toward life. Other energy sources have produced the same results. Perhaps life forms very readily.

Part II

The Drake study tried to put probabilities to the factors that favor life. His commission considered the following:

• Number of suitable stars
• no multiple stars (planetary orbits are unstable)
• long-lived stars (F,G,K, and M types)
• planet within the ecosphere
• Number of solar-type stars that form with planets
• Search for exoplanets is encouraging
• Number of Earth-type planets that form
• About half of the solar system planets are terrestrial
• Chances that life develops on a suitable planet
• Urey-Miller experiment gives encouraging result
• Chances of developing intelligent life
• Measure of lifetime of intelligence as the time since long-distance communication has been possible
• Intelligence Types (Societal and Technological) may not all be reachable
• Technological intelligence may be self-destructive
• Can we communicate?
• Passive - ease-dropping on radio emissions (SETI)
• Active - Pioneer and Voyager spacecraft, signal to M13
• How long does an intelligent society live?
• Roughly gives the number of civilizations

The barrier to detection is the vast distances to the stars. Humans live very short life spans compared to the time required to travel to the stars. Once our technology has been proven (probably within a century) then we need the societal will to commit to a multi-generational space vehicle to the stars.