Title:  WHY WE MIGHT BE ALONE:
        The Case Against Earth-Like Planets
             

Jeffrey F. Bell
Hawaii Institute of Geophysics & Planetology


Abstract:

     Since 1960 the idea that our Galaxy is filled with life-bearing 
planets has grown from an idle speculation by a few astronomers 
into a major official justification for the US space exploration 
program (and many basic science projects as well). However, over the
last 40 years numerous discoveries have called this doctrine
into question:
  
-- The "Drake equation" was predicated on an ageless steady-state 
universe, whereas the appearance of free oxygen and animal life on 
Earth took at least 1/3 the now-accepted age of the universe.
-- The accumulation of adequate amounts of heavy elements in the 
interstellar medium to form Earth-mass planets probably did not 
occur until shortly before the Earth actually formed; this implies 
that most such planets are too young to have free oxygen or ozone. 
-- Rising luminosity of H-burning stars implies that the habitable
zone around a solar-type star moves significantly outward over 
geologic time, so that rare combinations of geological and biological
evolution are necessary to avoid Venus-like desertification or 
Europa-like global freezing.
-- Global extinctions caused by impacts or astrophysical catastrophes
may be much more common in other system and/or other regions of
the Galaxy.
-- Earth's large moon was created by a low-probability event; if the
Moon played a key role in evolution of Earth's climate and biosphere
the histories of typical Earth-sized planets may be very different.
-- The so-called Jovian planets detected to date around other stars 
are very different from Jupiter; many are in highly elliptical 
"Earth-Killer" orbits which suggest that they are are actually small 
brown-dwarf stars which are not useful flags for "normal" planetary 
systems. The current practice of calling any object smaller than
15 Jupiter masses a "planet" (even if not orbiting a star!) is 

With a few notable exceptions, the astronomy community ignores 
these issues and continues to promote the Old-Time Religion that the
discovery of abundant Earth-like planets is inevitable. We should 
also prepare ourselves and the public for the possible alternate 
result that both exo-Jupiters and exo-Earths will be very rare. 
(Projects already underway could prove this definitively within the 
next 5-7 years.) Given the current inflated expectations of the 
general public, such a result could have a catastrophic effect on the
credibility of space scientists. A similar backlash after the negative
results from the Viking soil microbe experiments of 1976 destroyed 
the Mars program for 20 years.