Too young for life?

In 1960 radio astronomer Frank Drake and his team pointed a radio telescope at several nearby stars (including Epsilon Eridani) as part of Project Ozma and listened for signals from extraterrestrial intelligence. None were heard. What they did not know at the time is that Epsilon Eridani, while indeed similar to our sun, is probably much too young to have planets with life - much less intelligent life - upon them.

The first solid evidence of life on Earth has been found to date from 3.85 billion years ago. Our Sun is estimated to be 4.5 billion years old. As such, there was a time interval approaching 1 billion years before life managed to gain a foothold on Earth. Intelligent life took another 3.85 billion years to appear. Estimates of the age of Epsilon Eridani range from 500 million to a billion years. As such, were it to be assumed that events are occurring at the same pace and along the same path as they did in our solar system, conditions on such putative habitable planets would just now be ripe for life's initial appearance.

The dust cloud surrounding Epsilon Eridani is suspected of having as much as 1,000 times more dust than the inner regions of our own solar system. JAC researchers suggest that this may mean that "it has about 1,000 times more comets". As solar systems form, it is expected that rather intense bombardment of all planets occurs - and that this continues for quite some time.

In our solar system this period of heavy bombardment lasted for its first 600 million years. It was thought that during this time conditions on Earth (and elsewhere) would have been so chaotic that life would have had little chance to arise or (if it did) to thrive before being wiped out. Indeed, many think that life may have arisen more than once on Earth before finally taking hold. As a tribute to life's tenacity it would seem that the moment that life could arise on Earth - it did. The window between the end of the bombardment and the oldest known fossils (at least those that have survived) is only 50 - 100 million years.

Large planets - life's friend or foe?

The presence of large planets can have beneficial and detrimental effect upon the chances for habitable planets to form and for conditions amenable to life to be maintained for long periods of time. In a SpaceRef story earlier this year, "Hot Jupiters and Rare Earths: Planets are common. Are we?", the effect of the formation (and location) of large jovian class planets and the formation of habitable Earth-like worlds was examined.

According to one model, giant gas planets may push each other around. If, for example, you have 3 large gas planets forming in a solar system out in regions where ices are abundant, they will excite each other's orbital eccentricities. Over time this will cause orbits to shift around. Eventually planets will actually swap their relative order with respect to one another and their parent star. As this happens, the inner most planet moves outward, the outermost planet moves inward, and the planet in the middle is ejected from the solar system altogether While this model tends to explain why many of the hot Jupiters discovered thus far have high orbital eccentricities, this is not the leading explanation.

The other, more accepted model involves interactions between forming planets and with the dust disk from which they are forming. An analog to this process has been observed in Saturn's giant ring system. Small saturnian moons orbiting close to the rings tend to disrupt the organization of the rings. In so doing, spiral density waves are produced. These perturbations result in movement of the moons themselves via gravitational interactions. On the larger scale of a solar nebula, so the theory goes, spiral density waves clear out a space within the developing dust disk and locks the planet into a position in the cleared space. Later, as nebula moves inward and material falls into the sun, the planet moves inward, hence closer to the local star.

As is always the case with the universe one thing always affects another. The closer a large Jupiter-sized planet is to its local star during planetary formation, the smaller the chance that Earth-like planets will form. Evidence of this effect can be seen in our own solar system in the asteroid belt between Mars and Jupiter where Jupiter's gravitational influence has either hindered planetary formation or caused objects to collide and fragment before they achieved planetary size. As such, if the object is to find more potential Earths, then the emphasis should be to search within solar systems wherein large gas giants form further out than those currently known.