Sky & Telescope - January 2023 - 12

COSMIC RELIEF by David Grinspoon
Worlds Apart
IN COMPARATIVE PLANETOLOGY we
can't run controlled experiments, altering
factors such as initial composition
or distance from a star and then seeing
what happens over billions of years. We
can only study the random experiments
nature has chosen to run and try to
piece together the overall story.
The closest we may ever get to a controlled
experiment for Earth's evolution
is a comparison with Venus. Our sister
world seems to have started with similar
ingredients and conditions as our
planet - though about a third closer to
the Sun - yet it evolved into such an
utterly different planet. We're still trying
to work out the timing and mechanisms
for this divergence, and to find
evidence for or against past oceans on
our neighboring world. These questions
are a major motivation for the fleet of
Venus missions we'll launch in the next
decade (S&T: May 2022, p. 12).
They are tough problems to solve
because there are so many variables.
Did Earth and Venus actually form out
of different stuff? How do rotation rates
and magnetic fields affect planetary
evolution? How did the massive collision
that formed the Moon change our
planet? Also, how could we possibly
answer these questions by studying only
two Earth-size planets?
12 JANUARY 2023 * SKY & TELESCOPE
Can a newly found exoplanet help explain why Earth and Venus diverged so radically?
Fortunately, help is arriving in patterns
of photons from distant stars that
reveal previously hidden planetary companions.
Exoplanets can rescue comparative
planetology from the curse of too
few examples. It goes both ways: Solar
system planets give us ground truth for
filling in the vague clues we can gather
from a growing harvest of exoplanets,
and in turn this multitude will provide
context for our home system, teaching
us which characteristics are widespread
and which are just local quirks.
An exciting new discovery illustrates
the complementary nature of these
efforts. An international team led by
Laetitia Delrez (University of Liège, Belgium)
was using a telescope in Chile to
verify a planet found by the TESS mission
around LP 890-9, a red dwarf star
100 light-years distant, when it found
another planet orbiting the same star.
This lucky find, dubbed SPECULOOS-2c,
is slightly larger than Earth
and located just at the inner edge of
that system's habitable zone. Orbiting
its star at only a tenth of Mercury's
distance from the Sun, SPECULOOS-2c
receives about 90% of the starlight that
Earth receives from the Sun. This means
that - depending on evolutionary
details that are still poorly understood
- the planet might be something like
Like the seven-planet Trappist-1 system,
pictured here in an artist's concept, the red
dwarf LP 890-9 system may host at least one
planet bearing liquid water on its surface.
an exo-Earth, or it may instead resemble
an exo-Venus. How can we tell?
Its atmosphere would give us a clue.
The James Webb Space Telescope could
observe starlight passing through its
atmosphere, and the mix of gases seen
would be very different for a watery
Earth than for a parched Venus.
Of all stars known to have planetary
systems, LP 890-9 is now the second
coolest, in both meanings of the word.
Such relatively dim red dwarf stars can
host planets that orbit very close by but
nonetheless may be habitable, in the
sense of having stable liquid water on
their surfaces. And having nearly Earthsize
planets in this zone, which as far as
we know might have inhabited oceans
- well, that's just cool.
What makes a planet become like
Earth, with stable oceans and a surface
biosphere, as opposed to a scorching,
desiccated world like Venus? The next
decade of exploration with spacecraft,
aided by continuing exoplanet discoveries,
should reveal the answers.
¢ DAVID GRINSPOON is a senior scientist
at the Planetary Science Institute.
MARK GARLICK / SCIENCE PHOTO LIBRARY
Sky & Telescope - January 2023

Table of Contents for the Digital Edition of Sky & Telescope - January 2023
