Our hunt for other worlds will find them—or exclude their existence—only
after continued growth in our hunting skills, bigger telescopes, better
precision spectrography, more exact study of wobbly images, and maybe new
ideas. But it is worthwhile recounting the steps we could and did take
to arrive here.
How do we know there are other suns among the stars?
The sun is our star, and the old understanding that life depends upon the
sun has to be the bottom rung in the ladder of search.
We all see only one sun, the hot disc of every clear daytime. It was not
until the seventeenth century that there was good reason to believe that the
stars in the night sky are really suns, although they look so different.
(The Buddhist scholars always thought so, for they used simple analogy with
real talent.) But how can you reconcile the sun's hot and incomparable glare
to the cool flicker of starlight?
Naked-eye observers held that bright stars were bigger across than the faint
ones are. They do look that way. But from the first telescopic view we saw
that all the magnified stars are still just bright points, if some show
glary haloes to the eye. Even Kepler did not believe that the stars were
suns, for by eye they were much too big. Galileo's telescope made sense of
the facts. Take the sun, figure out how far to move it to make it look as
faint as a bright star, and its size in the sky will dwindle to a luminous
bright point. It is the optics of the eye that surround a bright point with
the dazzling artifact of those glaring rays.
Let me speak of my friend Carl Sagan, outstanding public figure among all
astronomers, who died in December 1996 at the untimely age of 62. It was he who
arranged—and it took some doing—that the distant camera of a Voyager space
probe would point back toward the sun from beyond Neptune to take a picture of
Earth. Out there you do not get a blue disc with a smear of details; you see
only the color, a pale blue glowing dot. The whole Earth is seen as a
planet among planets, just as a sun from afar is only a star among stars.
Distance alone makes planet and sun dwindle.
The more we knew of stars the more we understood that the sun is only a star
along the "main sequence" of stars, a common long-lasting type labeled G2
V. Indeed the Galaxy has a couple of hundred million stars of that very
class!
OK, plenty of suns, but are there any other planetary systems?
When I was a graduate student, it was fashionable to believe that our solar
system might be unique in the Galaxy. I believed that then, too, because the
picture of planet formation was one of near-colliding stars. The theory was
that one star orbited fast and close past the sun to escape to space, but it
pulled out a lot of gas from its collision partner. That gas over a long time
condensed
into the planets, comets, etc. But you could calculate that such a star
collision was very rare, maybe very few over all the age of all the stars.
So we were likely to be alone.
I vividly recall reading the paper of 1939 by Lyman Spitzer that disproved
the conclusion. He showed that a star collision could not make planets;
solar gas was so hot that it would dissipate long before it would condense.
Planets had to be made some other way; maybe they were as plentiful as suns!
How do planets begin?
The root idea is as old as Washington, DC. A star and its
planets—if any—are made at one time by the slow gravitational collapse of a dilute extended
cloud of interstellar gas, a nebula. This proposal is called the nebular
theory (Pierre de Laplace). It was only some fifteen or twenty years ago that
we saw by infrared light the first such "nebular disc" of gas and dust
around a young star. (Even bright Vega has one.) That was the first
confirmation beyond the many plausible fits of the nebular theory to the
single solar system we know, the one and only family of the sun. There are
plenty of nebular discs, even if no sure planets are yet seen in the
discs. Planets take time to form, maybe 1 or 2 hundred million years;
perhaps some will form around Vega too.
Are there signs of life far beyond earth?
Only hints, but plenty of those! Gas clouds in space often show specific
radio emission lines that mean the presence of molecules that would be at
home here in the bio lab, like formaldehyde or ether, not to mention much
more abundant ones like molecular hydrogen, carbon monoxide and carbon dioxide.
This proves that carbon compounds (and others) thought basic to life can form
under the natural processes of deep space. But there is no life in those very
dilute, expanded gas clouds. They span light years and are as dilute as the
very best lab vacuum. Their formation is very slow, and nothing like a cell
could grow, only widely spaced molecules. There is a huge gap between gas
molecules or dust grains and life, even though the molecules are essentially
those of life. A test tube full of blackish stuff is not life; see your
neighborhood snowbank.
The chemistry in and near the stars is the same as it is here. But life is
very special, very dense, very complex, very full of change. Molecules are
implied by life, but atoms and molecules do not of themselves imply life.
They are necessary but not sufficient, as the logician would say. Life is
far more than its components.
Sun-like stars do not imply life, nor do atoms, nor planets, nor molecules—not even those molecules essential for the life we know, like water, carbon
dioxide, and nitrogen compounds. They are only the precursors of life; they
might abound, and life would still be absent.
What would disclose life for sure?
To find life for sure, we need a signal from living things. Jump to the top
of the time scale, and seek radio signals made "artificially" by creatures
as accomplished as we are, or more. Long ago my partner Giuseppe Cocconi and I
put forward the notion that we might look for special narrow-band radio
emission from the stars. If we found anything like the signals we ourselves
could make, we would probably have found life. Complex details of the signal
might make the case irrefutable. So we have been trying this route—a jump to
the very recent features of life, its technical skills—for some 35 years,
though never systematically. We are now beginning a systematic radio search of
the Galaxy that will take many decades.
If this gamble should win, it pays off with disclosing our counterparts.
That is unlike finding organic molecules, or even microorganisms. They are
complex, but they are not what we are, they are more
our ancestors than our partners. The radio search, dubbed SETI, seeks a
grander answer, and is correspondingly less likely to succeed, but it is the
cheapest search.
What is the present state of the "hunt for aliens"?
Planets of normal stars went unknown or only hinted at until the fall of
1995. Out of a few hundred stars examined with the best new Doppler
techniques or by direct orbit-wobble viewing, we now have evidence that eight
normal stars have planetary mass companions. That astonishing new sample
suggests that between ten and a hundred million solar systems are to be found
in the Galaxy around sun-like stars.
If that sample poll holds up, we will come to believe what Carl Sagan
conjectured in his television series Cosmos in 1980. He foresaw an
Encyclopedia Galactica listing of all the planetary systems and their details
in a huge many-volumed data base.
Our one and only solar system does have many counterparts, but we do not yet
know how varied they may be. Do they all have big Jupiters—the only class
of planet we can now find? Do they have earths too? Or moons like Europa that
might hold life under ice crusts? We do not know, and we will be decades
finding out.
Long shots might pay off sooner, if we try from the Earth by staring with
computer at millions of stars to see dark planets causing a rare drop in
the starlight, or maybe millimeter radio waves that show cool planets.
Otherwise we will need big space-borne instruments to see planet
atmospheres.
What about the chances of life similar to our own in its awareness of
the world around?
Our true counterparts are akin in awareness of the universe, no matter how
the creatures may or may not differ in looks or limb count or size or
speech. What they are like is still mostly imaginative fiction. We simply do
not know enough to say. Some day we will. We need both to hunt and to think.
For me the most striking lesson of NOVA's Hunt for Alien Worlds was the
wide recognition that merely wishing it true doesn't get at truth.
There is a big difference between an earth wobble and the wobble of a star
out there, even if they look the same at first. Science requires that we
find what is the case, not just what we expect, hope for, or even seek
devotedly and long. That is its purest charm, and its most profound risk.
There is a sharp but subtle distinction between tested fact and fiction, a
distinction by no means obvious, simple, or even final.
Let the search go on. Hope for new means, new ideas, new people, but with
the old resolution not to fool ourselves. We can progress, but we cannot in
advance be certain of progress. We can try.
This is a golden age for sky viewers, one not matched since the time of
Galileo, who around 1610 was professor at Padua near Venice. He first showed
stars unseen by any eye, and showed that the moon was a rocky and
mountainous place. Maybe, maybe some day—with lots of luck, by 2100!--we
will find life, even fellow astronomers, well beyond earth. I envy those who
witness it!
Dr. Philip Morrison, Institute Professor and Professor of Physics at the
Massachusetts Institute of Technology, is a distinguished theoretical
astrophysicist and a pioneer in the search for extraterrestrial intelligence
through radio communication. In one of his many roles as a science educator,
Dr. Morrison serves on NOVA's Board of Advisors.
By Philip Morrison
We all see only one sun, the hot disc of every clear daytime. It was not
until the seventeenth century that there was good reason to believe that the
stars in the night sky are really suns, although they look so different.
(The Buddhist scholars always thought so, for they used simple analogy with
real talent.) But how can you reconcile the sun's hot and incomparable glare
to the cool flicker of starlight? 
Let me speak of my friend Carl Sagan, outstanding public figure among all
astronomers, who died in December 1996 at the untimely age of 62. It was he who
arranged—and it took some doing—that the distant camera of a Voyager space
probe would point back toward the sun from beyond Neptune to take a picture of
Earth. Out there you do not get a blue disc with a smear of details; you see
only the color, a pale blue glowing dot. The whole Earth is seen as a
planet among planets, just as a sun from afar is only a star among stars.
Distance alone makes planet and sun dwindle.
The root idea is as old as Washington, DC. A star and its
planets—if any—are made at one time by the slow gravitational collapse of a dilute extended
cloud of interstellar gas, a nebula. This proposal is called the nebular
theory (Pierre de Laplace). It was only some fifteen or twenty years ago that
we saw by infrared light the first such "nebular disc" of gas and dust
around a young star. (Even bright Vega has one.) That was the first
confirmation beyond the many plausible fits of the nebular theory to the
single solar system we know, the one and only family of the sun. There are
plenty of nebular discs, even if no sure planets are yet seen in the
discs. Planets take time to form, maybe 1 or 2 hundred million years;
perhaps some will form around Vega too.
more abundant ones like molecular hydrogen, carbon monoxide and carbon dioxide.
This proves that carbon compounds (and others) thought basic to life can form
under the natural processes of deep space. But there is no life in those very
dilute, expanded gas clouds. They span light years and are as dilute as the
very best lab vacuum. Their formation is very slow, and nothing like a cell
could grow, only widely spaced molecules. There is a huge gap between gas
molecules or dust grains and life, even though the molecules are essentially
those of life. A test tube full of blackish stuff is not life; see your
neighborhood snowbank.
If this gamble should win, it pays off with disclosing our counterparts.
That is unlike finding organic molecules, or even microorganisms. They are
complex, but they are not what we are, they are more
our ancestors than our partners. The radio search, dubbed SETI, seeks a
grander answer, and is correspondingly less likely to succeed, but it is the
cheapest search. 
Our one and only solar system does have many counterparts, but we do not yet
know how varied they may be. Do they all have big Jupiters—the only class
of planet we can now find? Do they have earths too? Or moons like Europa that
might hold life under ice crusts? We do not know, and we will be decades
finding out. 
There is a big difference between an earth wobble and the wobble of a star
out there, even if they look the same at first. Science requires that we
find what is the case, not just what we expect, hope for, or even seek
devotedly and long. That is its purest charm, and its most profound risk.
There is a sharp but subtle distinction between tested fact and fiction, a
distinction by no means obvious, simple, or even final.