Sky & Telescope - August 2024 - 28

PLANETARY CHRONICLES by Christopher M. Graney
The Challenging History
of New Worlds
How a revolutionary idea was born and gained acceptance
despite a lack of solid scientific support.
W
hen you're out sharing the night sky with friends
and family, how often are you asked about Earthlike
worlds and the possibility of life on them?
Probably a lot. The questions might center around news
stories about the thousands of exoplanets that have been
discovered, or UFO sightings, or maybe about whether the
interstellar visitor 'Oumuamua was a product of extraterrestrial
technology. And references to pop culture arising from
Star Trek or the cinematic universes of Star Wars and Marvel
might get mixed in, too.
Most people think of the idea of life on other planets as
being based on science. But history and current research
challenge that thinking.
Trillions Upon Trillions
The modern idea of other planets originates with Nicolaus
Copernicus (1473-1543) and Giordano Bruno (1548-1600).
Copernicus envisioned Earth circling the Sun, along with
Mercury, Venus, Mars, Jupiter, and Saturn - the planets then
known. Bruno, a Copernican, considered stars to be faraway
suns, circled by other inhabited earths.
Prior to Copernicus, planets were merely celestial lights
- stars that wandered from one zodiacal constellation to
another (the word planet comes from the Greek for " wanderer " ).
Under Copernicus, however, the idea of the planets
orbiting the Sun meant they had something in common with
Earth. And if planets had one thing in common with Earth,
why couldn't they have others? Why couldn't life be one of
those things? As Bernard Fontenelle (1657-1757 - yes, those
dates are correct) put it in his 1686 book Conversations on the
Plurality of Worlds,
. . . we find that all the planets are of the same
nature, all obscure bodies, which receive no light
but from the sun, and . . . their motions are
the same . . . they are alike; and yet, if we
are to believe that these vast bodies are not
inhabited, I think they were made to little
purpose: why should nature be so partial, as
to except only the earth? . . . I must believe
the planets are peopled as well as the earth.
28 AUGUST 2024 * SKY & TELESCOPE
It was a logical idea, for more reasons than just the obvious
similarities between the other planets and Earth. People
had long thought that life emerged naturally from matter.
Mice were believed to spontaneously form from dirt.
Ancient Jewish Rabbis even discussed whether the dirt
that generated a mouse would be unclean, since the
Bible lists mice as being among the various " creeping
things " that are unclean.
Antonie van Leeuwenhoek, the pioneering Dutch
microscopist of the 17th century, complained of
" respectable and learned men " who told him that
eels were spontaneously generated from dew, " in
confirmation of which they add, that if no dew has
fallen, there will be no eels found. " Edward Topsell's
1658 book The History of Four-Footed Beasts and
Serpents, included a little poetry, inspired by reports
of frogs seen forming from mud:
Durt hath his seed ingendring Frogs full green,
Yet so as feetlesse and without legs on earth they lie,
So as a wonder unto passers-by is seen,
One part hath life, the other earth full dead is nye.
If matter naturally generated life, all planets should
have life on them. Fontenelle said that even were " the
moon but one continued rock " and nothing more, he would
presume it to be populated with rock-eating creatures. And
with the stars being, as he said, " so many suns " each having
" planets, to which they give light, " humankind was looking
at a universe positively teeming with life.
Over the decades, various writers could be found echoing
Fontenelle, discussing how our universe must be full
of worlds like ours, inhabited by lifeforms like ourselves.
By the 1830s, the British science popularizer Thomas
SPAWN OF MUD Edward Topsell's discussion of frogs
in his 1658 History of Four-Footed Beasts and Serpents
reflected the widespread belief, enduring from ancient times
through the end of the 19th century, that some life was
spontaneously generated from lifeless matter. If life was a
natural product of matter, then life on other planets (or even
the Sun!) was inevitable.
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Sky & Telescope - August 2024

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