August 05, 2004
Earth-Like Planets May Be Rare Around Other Stars
So far most stars with planets oribiting around them have gas giants with highly elliptical orbits that prevent Earth-like planets from developing in less elliptical orbits.
Researchers from the UK believe that our Solar System could have formed differently from many other star systems, making places like our home much more rare in the Universe. After examining the 100 or so known extrasolar planetary systems, they found that they probably formed in a manner different from our own Solar System - in a way that's hostile to the formation of life. Planets could form in several different ways, and how the Earth formed is actually quite rare. It will still be 5 more years or so before astronomers have equipment with the resolution to confirm this.
Martin Beer thinks our solar system's planets may have formed in a way that is rare as compared to how planets form around most stars.
Martin Beer of the University of Leicester, UK, and co-workers argue that our Solar System may be highly unusual, compared with the planetary systems of other stars. In a preprint published on Arxiv1, they point out that the alien planets we have seen so far could have been formed by a completely different process from the one that formed ours. If that is so, says Beer, "there won't necessarily be lots of other Earths up there".
But current ways of detecting extrasolar (not from our solar system) planets are biased toward planets that are the sort being found so far.
It will take a few years to resolve this debate. The vast majority of extrasolar planets have been detected by measuring the way a star wobbles as a result of the gravity of an orbiting planet. This technique is inherently sensitive to heavy planets with short orbital periods, so those are the ones we are finding.
NASA's planned Kepler mission, to be launched tentatively in 2007, will be able to find more Earth-like planets.
Kepler also is being designed to detect planets in an orbit like the Earth at the same distance from their star as the Earth is from our Sun. With a measure of the orbit of the planet and with information about the planet's star, scientists can determine if the planet might have liquid water on its surface and, perhaps, sustain life.
This is a familiar story in one sense: More advanced scientific and technological capabilities are accelerating the rate at which scientific discoveries can be made. Any speculation about the odds on the existence of intelligent species around other planets is fairly uninformed at this point. We will have far more data about the frequency of extrasolar planets with promising conditions within several years and our ability to listen for signs of intelligent life will grow by leaps and bounds in the coming years as well.
Also see my recent post Will Intelligent Alien Life Be Discovered Within 20 Years?
I've been thinking we may actually be better off with worlds quite un-Earthlike in most respects. If we only have dead worlds like the Moon and Mars to work with, or the larger moons of gas giants, we'll have free reign to terriform or create artificial habitats. If we find an earthlike world we'll probably never be able to live there without first sterilizing it - which we probably won't be willing to do (and shouldn't do).
Alternatively, we would have to alter ourselves to an extreme degree to adapt ourselves to a new, independantly evolved biosphere. What would result from that would probably no longer be human.
If Earth-like planets (garden worlds) are rare, it just means that the future of the human race is in artificial environments (either terriformed planets or space habitats). It means lots and lots of real estate for man, because there are no aliens to compete with.
Since life existed in only single-celled format for 85% of Earth's history, its reasonable to assume that any "Earth-like" worlds we encounter will have only single-celled life.
Also, these findings could turn out to be wrong. The doppler effect can only detect gas giants near the parent star. However, when compared with the metallicy of the star, there is a positive corelation between iron content of the star and the presence of a planet. These large planets have been found around 20% of the stars with greater amounts of iron than the sun and 10% of the stars with iron comparible to the sun. None of these planets have been found around stars with significantly less iron than the sun. The total number of stars surveyed is around 1,000.
This seems to comfirm the theory of steller evolution that stars of high "metal" content tend to produce planets. Since only 10% of the stars with an iron content like the sun have been found to have these "hot jupiters" around it, there are 90% of these star left that may have terrestrial planets in the goldilocks orbit. The Kepler probe should be able to find if this is true or not.
It's obvious that a solar system like our own is not detectable with current methods, whereas the presence of detectable planets rules out any Earths. The question we should be asking right now isn't "how many G-type stars have planets we can see right now", but "how many do not." It is around the latter that other earths will be found.
You said above:
" It means lots and lots of real estate for man, because there are no aliens to compete with."
And I assume that you think this way because the humans are very different from other forms
of life due to the originality of the solar system.
But the other alien life forms, can still be hostile to humans even if they are DIFFERENT from us.
If you have seen the movie "Alien", the space creatures were able to survive in atmospheres
that were hostile to humans, but they were STILL using humans for their nutritional requirements!
This is an important nuance!!!!!
That question should be answered by the Keplar probe that will go up in 2007 or so. Also, I just read that a group of universities are planning to build a new observatory in the Andies that should be operational by 2015. This obervatory will have 70 times the observational capability of the Hubble-telescope. Although its viewing range will be limited to mostly the southern hemisphere, it should be able to sample enough G-type stars to give a statistical analysis. It will also be able to look at Alpha Centuri as well.
Until then, we are left with the 10% of the G-type stars with sun-like metallicy that have "hot jupitors".