Is there life "out there"? If so, is there intelligent life out there?

One way we can address the issue is to make a reasoned guess, based upon everything we know about astronomy, physics, chemistry, biology, and a host of other disciplines. Many years ago the radio astronomer Frank Drake did just this, combining all the "knowledge" in the form of a mathematical equation now named for him: THE DRAKE EQUATION. In its simplest form, it looks like this:

where N_ic is the number of intelligent civilizations (say, in the Milky Way galaxy), R_ic is the rate of formation of such civilizations, and L_ic is their lifetimes. (Note the units, R_ic is number/year, L_ic is years, so N_ic is number).

R_ic is itself a number that depends on many things, so let us break it down into parts that are easier to deal with:

R_*=Rate of star formation (Astronomy)

P_p=Probability of a star having planets (Astronomy)

P_e=Probability that Habitable Zone lasts long enough for life to arise - Continuously Habitable Zone (CHZ) or "ecoshell" (Astronomy, Planetary Evolution, Chemical and Biological Evolution)

N_e=Number of planets in the CHZ (Astronomy, Planetary Evolution, Chemical and Biological Evolution)

P_L=Probability that life will arise in the CHZ (Chemical and Biological Evolution)

P_I=Probability that the life will develop an "intelligent" civilization (Chemical and Biological Evolution)

L_ic=Lifetime of intelligent civilization (Speculative Sociology)

Now comes the fun part - estimating the values of each term. This will be the main focus for this chapter.

By looking at star-forming regions, the rates of stellar evolution, and other lines of evidence, it seems as if the current rate of star formation in the Galaxy as about 10 stars per year. So R_*=10. This will be the easiest term to determine!

The nebular model (now supported by numerous observations) suggests that in order to conserve angular momentum, stars will be born with either a companion star, planets, or possibly both. We know that 1/2 of all stars in the sky are binary stars, so planets are not "needed". Of the other half, planets may be required. Observations with the IRAS satellite, which surveyed the sky at infrared wavelengths, suggest that at least 1/4 of all main sequence stars possess the sort of debris around them that is associated with the planet-formation process. IRAS, however, could not detect the oldest, sparsest systems, however (Our Solar System would have escaped detection, if viewed by IRAS from a distant location), so that the number is probably higher. Let's set P_p=1/2.

The probability that a planet will be located in a "habitable zone" will depend on the size of the habitable zone. Within the zone it is "just right". Outside, it is too hot (too close to the star) or too cold (too far from the star). This is the "Goldiocks" condition.

What is the temperture of a planet going to be? We can use the Stefan-Bolotzmann Law and the Inverse-Square Law of light to calculate it.

A planet absorbs

Using the fact that the luminosity of a star is its surface area (4p R², where R is the star's radius) times its Stefan-Boltzmann factor (s T⁴), the fact that a spherical planet presents a circular cross-section (p R², where here R is the radius of the planet), we get

The planet emits

(I say roughly, as this does not include effects due to planetary rotation, etc.)

In equilibrium, these must be equal! So equate the two and solve for T:

Example: Let T_*=5800, R_*=7x10¹⁰ cm (values for the Sun)

The freezing point of water is 273 K, and the boiling point is 373 K, under 1 Atm pressure. Venus is currently too hot for liquid water. Mars is too cold. The Earth is "just right".

We can do the same calculation for other types of stars as well.

However, this is a very simplistic, naïve calculation, in that it ignores the evolution of the star (its L changes with time), and the evolution of the planet, which affects its climate. In effect, what we really need to know is the size of the Continuously Habitable Zone, which we will define as the zone where water is liquid over 800My-1By, long enough for biological evolution to yield something viable. This zone may be quite narrow!

How big are CHZs ("ecoshells")?

Michael Hart was the first person to try to find out using realistic computer simulations that included a large number of important parameters. First he lookeed at the evolution of the Earth's atmosphere, and included the following processes:

• Rate of outgassing of volatiles (H, C, N, O) from the interior
• Condesation of H₂O vapor into oceans
• Solution of atmospheric gases into oceans
• Photodissociation of H₂O in the upper atmosphere
• Escape of H from the uppermost atmosphere (exosphere)
• Chemical reactions in atmospheric gases
• Presence of life and variations in biomass
• Photosynthesis and burial of organic sediments
• Urey reaction (CaSiO₃+CO₂=>CaCO₃+SiO₂)
• Oxidation of surface minerals (2FeO+O=>Fe₂O₃)
• Variations in the luminosity of the Sun
• Variations in the albedo of the Earth
• Greenhouse effect

The criteria he assumed for life to arise were

• Liquid water with T<42C for 0.8by
• Concurrent presence of C and N in atmosphere and oceans
• Absence of free O in atmosphere

His starting conditions:

• No atmosphere
• Albedo=0.15 (rock)
• 4.5 by ago

His process:

Using time steps of 2.5my, vary the composition of juvenile volatiles until the best fit to present conditions is reached.

MAIN RESULTS:

• His "best" initial gas composition was 84% H₂O, 14%CO₂, 1%CH₄, 0.2%N₂
• Most of the H₂O vapor condensed promptly into oceans
• Early atmosphere was dominated by CO₂
• CO₂was later removed by the Urey Reaction
• Oxygen released by photolysis of H₂O vapor and later by photosynthesis. This O destroys the CH₄ (the O being consumed in the process, of course). By 2 by ago, most of the CH₄ was gone, leaving N₂ as the dominant gas by default (it is the only thing left!).

Since then, there has been a slow buildup of O₂. By 420my ago, enough O₂ and O₃ had built up to provide protection from solar UV, making life on land tolerable.

It is possible, by assigning plausible values to those parameters included in the models, to simulate the evolution of the Earth's atmosphere in a manner that is consistent with the basic ideas of atmospheric studies and stellar evolution. It is consistent with the temperature necessary for life to arise, the mass of the oceans, the volatiles in sedementary rocks, and the composition of the present atmosphere.

Furthermore, it does not involve any extraordinary events, but is based entirely on processes already discussed in the scientific literature.

OTHER IMPORTANT RESULTS AND LIMITS

In these models, once CH₄ was gone and the luminosity of the Sun reached its current value, if T(surface)<278K, Runaway Glaciation occurs, and in none of the simulations is it ever reversed. This occurs 2by ago if the Earth were located 1.01 AU from the Sun, a mere 1% further away!

If the earth were at 0.9 AU from the Sun, a Runaway Greenhouse Effect occurs 4 by ago, and in none of the simulations is it ever reversed!

These results, which include runaway effects, provide only a very narrow (0.06 AU) CHZ for the Earth.

Here, the Mass-Luminosity relation of main sequence stars, and their evolution, must be included. Higher-mass stars have larger and thicker regions around them where liquid water can exist. However, as we have already seen, they evolve faster than low-mass stars, so that this zone moves outward with time. The CHZ is that region that is within the liquid-water zone throughout the 0.8by needed for life to take hold, from beginning to end.

RESULTS

Due to the mutual and simultaneous evolution of the star and the atmosphere of the planet, the CHZs are NARROW. In fact, for a variety of reasons, their thickness goes to ZERO for masses less than 0.8 solar masses, and for masses greater than 1.2 solar masses.

r< < r_inner

Planet always too hot for oceans to condense

r<r_inner

Oceans exist in early stages. Buildup in atmospheric gases and increase in stellar luminosity lead to a Runaway Greenhouse Effect after 1by. It is L_*(t> 1by) that determines r_inner.

r> > r_outer

Runaway Glaciation occurs as soon as most of the CH₄ (etc.) is gone - usually occurring at t=2.5by.

r> r_outer

Runaway Glaciation does not occur until after 3.5by. It is L_*(t> 3.5by) that sets the value of r_outer.

OVERALL PICTURE

The evolution of other terrestrial planets will be similar to that of the Earth if inside the CHZ

CHZs are widest around G0 main sequence stars, and shrink to zero at F7 at the hot end, and K1 at the cool end.

In all cases D r< 0.1 AU, suggesting that P_e~0.01 Whew! Finally!

"It appears therefore, that there are probably fewer planets in our galaxy suitable for evolution of advanced life than had been previously thought." M. Hart (1979).

1. The most recent studies of the composition of the earliest gases suggest that they may have been richer on CH₄ and NH₃ than in Hart's models.
2. CH₄ and NH₃ are rapidly destroyed by sunlight, leading to no CH₄-dominated phase. In the long run, this seems to have dominated over the above "problem". This diminishes the importance of the original Miller-Urey experiment (remember?) in that the availability of CH₄ is lower that the M-U experiment used.
3. Plate tectonics helps to replenish the CO₂ that is removed by the Urey reaction. It gets recycled. This effect was not included in the Hart models, and is critical for determining the long-term climate of the planet.

The Cool Sun Problem.

One other "problem" has bothered some investigators: the fact that the Sun was once only 0.7 times its current luminosity. A cooler Sun requires a larger Greenhouse Effect to keep the surface temperature at a suitable value. While some models allow for an increased GE early on, this is a point of concern, especially if it runs counter to the analysis of gases trapped in ancient rocks.

However, we know that T Tauri stars have stellar winds. If they carry away enough mass into the main sequence phase, then the initial mass of the very early Sun was larger than today, and so therefore was its luminosity. But it may require a 10% loss of mass.

In my opinion, I do not believe that we can feel confident that this potential problem is behind us.

For years, dynamicists have been running computer simulations of solar system formation, to see what kind of planets form where out of all the debris. Then it’s a matter of finding how many end up in the CHZ. Our own solar system has 1 (Earth). Models suggest that this may be typical, but it may be less. It depend a lot on the initial conditions. For now, we will use N_e~1.

Here we have only one example, our own planet, and it is dangerous to make vast conclusions from a sample of only one. The pre-biotic chemical evolution seems easy, but it does not necessarily mean life. Nevertheless, we also no of no reason why the basic chemical pathways utilized here on Earth would be prevented elsewhere. P_L~1?!

Intelligence seems to be a powerful adaptation. We do not know if it will always occur, but nature seems to fill ecological niches, including this one. P_I~1?

L_ic

Case A: We blow ourselves up (or something equally nasty) almost as soon as we have the technology to do it. Lic~50 years

Case B: We learn to survive everything. Lic~10¹⁰ years. Note that extinctions due to impacts limit L_ic to less than 10⁷ years unless the civilizations are spacefareing (established off of the Earth).

So L_ic=50-10¹⁰ years. WE DON’T REALLY KNOW, and this is the most uncertain part of the entire Drake Equation.

RESULT

which equals 0.05 L_ic. A very useful approximation (particularly if the Kasting et al. models are more realistic than the Hart models, increasing the P_e term to something close to 1 instead of 1/100) is to set the product of the above numerical factors to ONE, so that

CONSEQUENCES

There are huge uncertainties! The Milky Way may have hundreds of intelligent civilizations, or just one (us).

HOW DO WE DECIDE THE USSUE?

Well, we can solve the Drake Equation until the cows come home, and that will not tell us the answer. The only way to decide the issue for sure is to LOOK! We must do the experiment. (This has not, however, been a useless exercise - it has helped us to better-define the problem.)

WHERE DO WE LOOK?

There are a number of possible strategies

• Nearby stars with planets
• Stars with CHZs
• Everywhere

HOW DO WE SEARCH?

Spacecraft

• Fast
• Slow (Cryogenic and/or "Generation")
• Robots

Remote Observations (much cheaper!)

• Radio
• Other

SEARCHES FOR EXTRASOLAR PLANETS

1. At optical wavelengths, planets (terrestrial and jovian) are about 10⁹ times fainter than the stars they orbit.
2. At IR wavelengths, the ratio is "only" 10⁶.

Adaptive optics and interferometry (including space-based interferometers) may be able to do the job.

Astrometric ("reflex motion")

To detect a "Jupiter" around a solar-like star requires measuring the motion of the star to a precision of a bit smaller than 1 mas (10^-3 arcsec).

Current techniques can reach ~1mas, upcoming interferometers maybe 0.1 mas.

To detect partial eclipses of a star by a Jupiter requires accuracies of 1 part in 10⁵. May be possible with current techniques, but we would need to look LOTS OF STARS MANY TIMES in hopes of catching the eclipse events.

The Sun moves ~13m/s due to the gravitational pull of Jupiter. Current surveys can reach 10 m/s and are improving even more!

RESULTS OF THE SEARCHES

Photometric and Astrometric - maybe one each. Maybe.

Imaging - Adaptive optics and coronagraphic observations have detected dusty debris disks surrounding some stars (b Pictoris, BD+31° 643, HR 4796, HD 141569, HD 163296, Vega, Fomalhaut, e Eri). Imaging with the Hubble may have detected a young Jupiter being ejected from a very young star system - TMR-1.

The first detection was of planets orbiing the pulsar PSR 1257+12. Although the technique used was timing of the pulse arrival time instead of true spectroscopy, the principle is the same. 2, possibly 3 planets have been detected, but these are "phoenix" or "zombie" planets of little interest for The Search.

More importantly, since 1995 well over a dozen main sequence stars (51 peg, 47 Uma, 70 Vir, HR 3522, etc.) have show evidence for velocity variations consistent with the presence of at least one jovian-mass planet!

To find the latest on this subject, from one the experts in the field, visit Goeff Marcy's Planet Search site.

However, it looks like the majority of these systems have the detected jovian planet within 1 AU of the star - well within the "snow line"! What's going on? Why all the "Hot Jupiters"?

Good question! Of these systems, only 47 UMa looks like our solar system. Most of the others have jovians much closer to the star than where they cold have formed. Models of the evolution of the protostellar disks indicate that if there is enough stuff in the disk, the planets can migrate inward from where they formed. Jupiters formed beyond the "snow line" can now be at at 0.2 AU. Any terrestrial planets that formed in that region may have migrated right into the star!

Are systems like our own the exception, rather than the rule? Maybe, but the spectroscopic search technique is heavily biased in favor of finding hot jupiters. The closer the planet is to the star, the greater the velocity it imparts to the star, and the easier it is to detect.

OTHER INTERESTING POSSIBLE SEARCHES

The atmosphere of the Earth exhibits spectroscopic features that are very difficult to have without the presence of living organisms. Free oxygen is so reactive that for large amounts to be present, a robust source for resupply is required. This is especially true if the atmosphere has CH₄ in it. In principle, one could detect these signatures in the light of a distant star-planet system if one had enough light. The spectrum of these molecules could be detected in the Sun-Earth system with technology of the not-too-distant future. So maybe you will witness the discovery of another planet with life on it!

Useful References on CHZs (technical):

Hart, M. 1978. The evolution of the atmosphere of the earth,Icarus,33, 23-39.

Hart, M. 1979. Habitable zones about main sequence stars,Icarus,37, 351-357.

Kasting, J.F., Whitmire, D.P., and Reynolds, R.T. 1993. Habitable zones around main sequence stars,Icarus,101, 108-128.