Sky & Telescope - February 2022 - 21

NASA / JPL-CALTECH / EXOPLANET EXPLORATION PROGRAM
The
Planets
That
Aren't
What
They
Seem
Super-Earths and miniNeptunes
abound in the
galaxy, but astronomers are
still working out what these
worlds actually look like.
W
hen astronomers started scouring space for signs of
planets around other stars, they thought they'd be
looking in the mirror. Everyone expected to see ourselves
reflected back at us - after all, there's supposed to be
nothing special about us or our place in the cosmos. It should
have been a tick-box exercise to find copy after copy of home.
Yet the more worlds we found, the more we realized that,
when it comes to exoplanets, the universe is more like a
deceptive hall of mirrors - it's hard to distinguish between
truth and illusion.
We've found entire classes of planets that simply don't
exist in our own backyard. Hot Jupiters that buzz around
their stars in days, worlds where iron rain whips about on
ferocious winds. Yet perhaps the most jarring revelation is
that there are entire solar systems without any planets that
look like ours. The Sun's planets can be neatly pigeonholed as
either small, rocky worlds or large gas and ice giants. There's
nothing in between to muddy the waters. Yet time and again
we're finding stars that only have planets with sizes betwixt
those of Earth and Neptune.
" According to our traditional models of planet formation,
they shouldn't even exist, " says Björn Benneke (University of
Montreal).
Yet of the 4,438 confirmed exoplanets, 2,422 have radii
between Earth's and Neptune's (1 to about 4 Earth radii).
The fact that they appear to dominate the exoplanet population
has left astronomers more than a little red-faced. " We
should really be able to explain the most common type of
planet, " Benneke says. " Instead, they are the least-understood
type of planet right now. "
Like biologists trying to classify a newly discovered species,
astronomers searched for a simple way to categorize
these unfamiliar worlds. Size initially seemed a good choice.
Observers found the vast majority of these mid-size planets
by using the transit method, which looks for starlight
dimmed by an orbiting planet periodically getting in the way.
The amount of dimming, coupled with the size of the star,
allows us to estimate the planet's radius.
There are plenty of planets between 1 and 1.5 Earth radii
and even more between 2 and 4 Earth radii. But there's a
significant drop in between, known as the " Fulton gap " after
Benjamin Fulton (now at Caltech), who headed up the 2017
study presenting the observational evidence for the deficit.
Astronomers used it to divide mid-size exoplanets into two
groups: Worlds below the gap became rocky " super-Earths, "
and planets above it were gaseous " mini-Neptunes. "
Yet a run of recent research suggests the division may
not be that straightforward. " There's a gray zone between
the two, " says Angelos Tsiaras (University College London).
Super-Earths could be masquerading as mini-Neptunes, or
mini-Neptunes might become super-Earths over time. MiniIN
BETWEEN The most bountiful planets in the galaxy - the ones
with sizes and masses between those of Earth and Neptune, shown here
as artist's concepts - have no counterpart in our solar system. Their appearance
and evolution remain a puzzle.
sk yand tele scope .o r g * FEBRUARY 2022 21
Sky & Telescope - February 2022

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