Sky & Telescope - August 2024 - 33

ESO / M. KORNMESSER
Meanwhile, in 1659 Christiaan Huygens published measurements
he'd made using a micrometer fitted to a telescope
to correctly determine the relative sizes of the planets. But
despite the huge size difference between Earth and Jupiter,
even Huygens assumed Jupiter would be so much like Earth
that there had to be Jovian sailing ships with pulleys, just
like on Earth! After all, Earth has oceans and ships equipped
with ropes and pulleys - why wouldn't Jupiter? Plus, Jupiter's
retinue of moons would be so very useful for navigation!
Whewell, however, used science to argue in his 1853 book
Of the Plurality of Worlds: An Essay that the planets were
diverse, not just other Earths. They received widely differing
amounts of light and heat from the Sun and had widely differing
densities and surface gravities (something Isaac Newton
had worked out a century earlier). Geological evidence
indicated that Earth had existed for vast periods of time
without human beings, suggesting that intelligent life wasn't
a given even here. Whewell thought that even if the other
planets in the solar system weren't populated by sentient and
intelligent beings, they might still host life. He imagined a
watery Jupiter with " aqueous, gelatinous creatures; too sluggish,
almost, to be deemed alive, floating on their ice-cold
water, shrouded forever by their humid skies. " Not so different
from the " vast living balloons " Carl Sagan suggested
could live on Jupiter in his 1980s television series, Cosmos.
Whewell's astronomer friend John Herschel (William's
son) was among the many who disagreed. Herschel said that
there might yet be intelligent fish in the deeper, warmer Jovian
waters. Such fish might even build underwater structures to
rival London's Crystal Palace! Of course, even now it would
be a monumental discovery if a future space probe found even
one small colony of the most microscopic gelatinous creatures
in the waters under the ice of Jupiter's moon Europa.
Planetary diversity goes far beyond what Whewell envisioned.
He surely would have been surprised to find that Venus
is utterly unlike Jupiter, and yet both worlds are so unlike
Earth as to be uninhabitable. No doubt he would also have
been shocked by the diversity seen in the oddball planets found
in exoplanet systems. The orange dwarf star HD 83443 in the
southern constellation Vela, for example, features one Jupitersize
planet in an orbit that makes Mercury's look large, while
another planet has a comet-like eccentric orbit. The exoplanet
TrES-2b is darker than coal. The planets of the TRAPPIST-1 system
are all in tight, lockstep orbits around a red dwarf. Moreover,
half the exoplanets discovered are of a size not found in
our solar system - worlds bigger than Earth, yet smaller than
Neptune (S&T: Mar. 2017, p. 22). Of course, it might be that
our solar system is the one that's odd. Posting on NASA's exoplanet
exploration site (https://exoplanets.nasa.gov/), science
writer Pat Brennan has proposed that ours is " the weirdest
solar system we've found so far. "
If Whewell would be shocked by recent discoveries, what
of Fontenelle or Bruno or Dick? And what would they think
about the scarcity of life? The idea of spontaneous generation
finally died off in the late 19th century, thanks to scientists
VISITOR FROM BEYOND 'Oumuamua's 2017 passage through the
solar system prompted suggestions that the object was the product of
extraterrestrial intelligence. Such an idea presupposes that extraterrestrial
intelligent life is common enough for us to encounter their machines.
building on the work of Leeuwenhoek, who showed that even
eels beget young. Today science is very far from spontaneous
generation. Current hypotheses propose that life on
Earth originated some 3.8 billion years ago through a process
neither reproducible in a lab nor found reoccurring in nature,
and that more complex life originated in a merger of two
primitive, single-celled organisms - a merger so improbable
that it occurred but once in our planet's history, and only
after primitive life had existed for many hundreds of millions
of years. Thus, palace-building fish under Europa's ice
stretches scientific plausibility, yet a lifeless Europa does not.
The Challenge of History
History shows that science has repeatedly challenged the idea
of other Earths. Science - be it the mistaken theorizing of
Kepler and Cassini, or the work of Whewell that has stood
the test of time - has always pointed toward diversity, not a
universe full of Earths. Our best efforts today show that most
planets differ greatly from our Earth, most stars differ greatly
from our Sun, and most planetary systems differ greatly from
our solar system. Indeed, science even tells us that most of
the matter and energy of the universe is " dark, " differing
greatly from the stuff of ordinary experience.
Despite this, we human beings have consistently embraced
the assumption that the universe is full of worlds like Earth
populated by life that just naturally emerges from matter. That
is an exciting universe. Attributing 'Oumuamua to extraterrestrial
intelligence; reputable media outlets featuring fantastical
accounts of UFOs; NASA panelists being questioned about
what they are " hiding " when they can't produce evidence
supporting alien visitations - all reflect the ongoing embrace
of assumptions that follow the long tradition of Bruno, Fontenelle,
and Dick. And as Kepler, Cassini, and Whewell show
us, that tradition is less based in science than we think.
¢ CHRISTOPHER M. GRANEY is an astronomer and historian
of science with the Vatican Observatory. This article is
adapted from his paper " The challenging history of other
Earths " , published in the International Journal of Astrobiology
22:6 (December, 2023).
sk yand tele scope .o r g * AUGUST 2024 33
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Sky & Telescope - August 2024

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