Finally, given all of the required conditions, how many of these planets actually have life on them? Are the odds for life in the Universe as measly as one instance in hundreds of billion billions? What little we know suggests life may not only common, but that life may well be a natural consequence of stellar and planetary formation. In casting this possibility into perspective, NASA's Origins and Astrobiology programs refer to life as being a "cosmic imperative".

Planets - rare or common?

In 1995, a massive object was found to be orbiting the star 51 Pegasi. Since that discovery, 24 large bodies, some many times the size of Jupiter, have been discovered orbiting other stars. In 4 short years we've discovered more extrasolar planetary bodies than exist in our own solar system! This apparent predominance of large Jupiters is an artifact of the current discovery process wherein the easy to detect planets are found first.

If you only consider extrasolar bodies which are less than 13 Jupiter masses in size - and assume that they are part of a planetary system - that's still 12 potential planetary systems discovered in the last few years. Although the planetary systems we've found thus far are on the unusual side, their existence suggests that planet formation is common. This view is further bolstered by Hubble Space Telescope images of the Orion nebula that show many proplyds e.g. disc-shaped dust clouds swirling around young stars. The presence of these planetary nurseries, themselves located within stellar nurseries, certainly suggests rampant planet formation throughout the universe.

The right stuff for life

We now have ample evidence of other planetary systems, and a suspicion that among the menagerie of systems with orbiting giant planets, there would be many with some modest planets of earth-like dimensions. But, would these earth-sized planets have the "right stuff" for life? In order for life, to use the phrase "as we know it" to exist, certain biogenic i.e. life-forming compounds are required. These compounds not only need to be formed, they need to be delivered to the proper locations where they can mix together and set the stage for life's origin.

Initially, only hydrogen and helium formed in our universe. Heavier elements, among which are those considered to be the building blocks of biogenic compounds (i.e. carbon, nitrogen, phosphorus, oxygen, and sulfur) were generated in repeated cycles of star formation and death. This process would repeat itself over the course of millions to billions of years with each cycle blowing newly fused heavier elements out into the universe. Only after several generations of star birth and death was there the possibility for planets to be formed with any hope of harboring the exciting carbon chemistry we call life.

Surveys of our galaxy, and of the universe, show that the elements for life are evenly distributed, to the first order. Furthermore, known interstellar chemistry is encouraging in its ubiquity and complexity of organic chemistry. At the two-atom level, known compounds are diverse and range from molecular hydrogen to aluminum chloride. As the number of atoms in molecular species increase, compounds of carbon, hydrogen, oxygen, and nitrogen predominate, to the exclusion of all other elements in detectable compounds of seven atoms or more. The plethora of organic (i.e. carbon-containing) compounds range from amino acids such as glycine, to complex refractory compounds such as polycyclic aromatic hydrocarbons (PAHs).

These days you need to have an organic chemistry book next to your telescope as you explore the universe: the elements and compounds for life seem to be present everywhere and are woven into the very fabric of the cosmos.

Right stuff Vs Left Stuff: Life's chemistry is very specific

One of the core mysteries of life is its preference, no its requirement, for "handedness" in its chemistry. In fact, the "stereo-specific" (preference for one possible shape of a molecule over another) use and production of organic molecules has been a hallmark of life on Earth. Left-handed amino acids (building blocks of proteins) and right-handed sugars are the basis of everything we call living. The origin of this characteristic of life is hotly debated. Did life start with single-handed chemistry or did such a preference evolve after life had begun? The answer seems to lie out among the stars.

For years, meteorites have been shown to be a rich source of a wide variety of organic compounds that could have been critical to the origin of life on Earth. Occasional reports of a left-handed bias in amino acids were contentious, until last year. John Cronin and Sandra Pizzarello examined the ratio of left- and right-handed amino acids in Murchison meteorite. They found an excess of left-handed amino acids. Normally this would be attributed to terrestrial contamination but these were extraterrestrial amino acids - compounds that aren't used by life on this planet.

What are the implications? Our left handedness may have started with a bias in the organic compounds that arrived in the vicinity of Earth as it formed. In addition, some of Earth's organic compound inventory was acquired from materials originating outside of, and from a time prior to the formation of our Solar System.

In other words, our biochemistry could be could be left-handed because we started with left-handed molecules originating in a molecular cloud, one formed of debris strewn from the corpses of generations of dying stars. This idea is reinforced with the recent finding of circular polarization in a star forming region of Orion OMC-1. These findings seem to demonstrate a mechanism that could induce a preferred handedness in the organic compounds within of a molecular cloud, compounds that could eventually be delivered to a planetary body such as Earth.

Is life easy?

On Earth, life arose pretty darn quick. The heavy bombardment which followed Earth's formation ended around 3.9 billion years ago. Only when this pummeling began to ebb could Earth witness its first opportunity to form an ocean - and have it last. 400 million years later, we have clear evidence that microbial communities existed - communities of organisms. That's fast! Furthermore, chemists have suggested, looking at the stability of complex biogenic compounds, that the origination of life must be "easy" happening on the order of tens of millions of years.

The message? As soon as Earth had an ocean, it could form life, and it did. The obvious question which follows? What were the ingredients needed for life to arise here - and where else can we expect to find similar conditions in our solar system where life could also have arisen?

In our Solar System, there are four other planetary-sized bodies that have or have had liquid water at some point in their life: Venus, Mars, and Jupiter's moons Europa, and possibly Callisto*. Of the nine planets in our solar system, there's definitely life on one, and the probability for life's prerequisite conditions and the possibility for life (extant or extinct) on four other planetary-sized bodies.

What are the odds?

When you look at the night sky, most of the stars you see are within 80 light years. Most of these stars burn too bright and too fast to be able to sustain a planet with liquid water on the surface - certainly not long enough for life to emerge and evolve. Exclude all the stars which fall into the "live hard die fast" camp and there are still about 1,000 suitable stars within that same distance. 1,000 within our galactic neighborhood. If the odds of life appearing on a world roughly similar to our own are just better than 1 in 1,000, then we could expect to have neighbors. Maybe not close enough to visit, but close enough to see!

Since we are still figuring out how to crawl through our own solar system, we won't be travelling to meet our neighbors any time soon. However we will be able to look in on them. Maybe, in about twenty years we'll know. The search begins when NASA builds the Terrestrial Planet Finder, a large space-based interferometric telescope, and looks for another pale blue dot. Simple examinations of the light from these worlds will tell us whether their atmospheres have been perturbed by the presence of life.

We are limited by our level of exploration, not by the possibilities. If our Solar System is at all typical, then planetary systems with habitable planets must be common.

If we don't look, we won't find anything.