We only have one example of a planet with life: Earth. But within the next generation, it should become possible to detect signs of life on planets orbiting distant stars. If we find alien life, new questions will arise. For example, did that life arise spontaneously? Or could it have spread from elsewhere? If life crossed the vast gulf of interstellar space long ago, how would we tell?
New research by Harvard astrophysicists shows that if life can travel
between the stars (a process called panspermia), it would spread in a
characteristic pattern that we could potentially identify.
"In our theory clusters of life form, grow, and overlap like bubbles
in a pot of boiling water," says lead author Henry Lin of the
Harvard-Smithsonian Center for Astrophysics (CfA).
There are two basic ways for life to spread beyond its host star. The
first would be via natural processes such as gravitational
slingshotting of asteroids or comets. The second would be for
intelligent life to deliberately travel outward. The paper does not deal
with how panspermia occurs. It simply asks: if it does occur, could we
detect it? In principle, the answer is yes.
The model assumes that seeds from one living planet spread outward in
all directions. If a seed reaches a habitable planet orbiting a
neighboring star, it can take root. Over time, the result of this
process would be a series of life-bearing oases dotting the galactic
landscape.
"Life could spread from host star to host star in a pattern similar
to the outbreak of an epidemic. In a sense, the Milky Way galaxy would
become infected with pockets of life," explains CfA co-author Avi Loeb.
If we detect signs of life in the atmospheres of alien worlds, the
next step will be to look for a pattern. For example, in an ideal case
where the Earth is on the edge of a "bubble" of life, all the nearby
life-hosting worlds we find will be in one half of the sky, while the
other half will be barren.
Lin and Loeb caution that a pattern will only be discernible if life
spreads somewhat rapidly. Since stars in the Milky Way drift relative to
each other, stars that are neighbors now won't be neighbors in a few
million years. In other words, stellar drift would smear out the
bubbles.
This research has been accepted for publication in The Astrophysical Journal Letters.
Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.
For more information, contact:
Christine Pulliam
Media Relations Manager
Harvard-Smithsonian Center for Astrophysics
617-495-7463
cpulliam@cfa.harvard.edu
