Sky & Telescope - February 2022 - 24

Mini-Neptunes
3,050
3,000
2,950
2,900
2,850
2,800
2,750
0.4 0.5 0.6
0.8
1.0
1.5
2.0 2.5 3.0 4.0 5.0
Wavelength (microns)
pWATER IN THE AIR The transmission spectrum of K2-18b shown
here is derived by examining the planet's apparent size at different
wavelengths as it transits its star. A feature at 1.4 microns indicates the
presence of water vapor.
argues that the absorption came from methane, not water,
which would make K2-18b distinctly more Neptunian.) If
Benneke and Tsiaras are right, the water vapor might lead to
a familiar phenomenon. " The water vapor could condense
out to form falling water droplets, " says Benneke. In other
words, there's a chance that it's raining water on this alien
world - again, not something we'd normally associate with
a Neptune-like planet (although it can rain other substances
on gas and ice giants).
It's unlikely K2-18b is a one-off. " Mini-Neptunes could
be water worlds, " says Martin Turbet (Geneva Observatory).
Astronomers have previously assumed that a thick layer of
hydrogen is needed to explain both the large radius and fairly
low density of planets above the Fulton gap. " We discovered
that it is possible to explain [it] without the need for hydrogen, "
Turbet says. " We can do it only with water. "
In research published in June 2020, Turbet drew inspiration
from our growing understanding of climate change here
on Earth. We know that water vapor is a particularly potent
0.1
1,000
100
10
1
greenhouse gas, trapping heat from the Sun and preventing
its escape back into space. A water world close to its star
would receive a lot of energy, which would evaporate significant
quantities of water into its atmosphere. The water vapor
would then trap more stellar energy, raising the ambient temperature
and creating yet more vapor in a runaway process.
" This dilates the atmosphere and puffs up the planet, " Turbet
says. Over time, the exoplanet could even reach the same
radius as those previously considered to have Neptune-like
hydrogen atmospheres.
Water vapor may even be the culprit behind the flat spectra
initially seen by Benneke and others. Absorption bands
will be some 10 times weaker in a water-dominated atmosphere
compared with a hydrogen-dominated one, Turbet
explains, resulting in a flat-looking spectrum that's indistinguishable
from that of a cloudy planet. " Existing telescopes
don't have the precision to probe the differences between
these two scenarios, " Turbet says. Fortunately, the James
Webb Space Telescope, due to launch in December 2021 as
this article goes to press, should be able to help answer this
question (S&T: Nov. 2021, p. 20). Webb has the capability
of detecting distinct absorption features at infrared wavelengths,
where water vapor absorbs photons quite efficiently.
Super-Earth Origin Stories
Taken together, these recent findings actually make it harder
to understand where all the observed mid-size planets come
from. One idea is that super-Earths and mini-Neptunes
are two stages of the same planet. That's what Travis Berger
(University of Hawaiʻi, Mānoa) thinks. In work published
in August 2020, he found evidence that mini-Neptunes can
shrink over time, the loss of their atmosphere revealing a
super-Earth underneath. The Fulton gap would be populated
by exoplanets in transition.
Recent work by Trevor David (Flatiron Institute) and others
supports this argument. When sorted by their host star's
age, the exoplanets in the Fulton gap change in size. Based
Why Were mini-Neptunes and Super-Earths Such a Surprise?
We thought we knew how giant
planets were made. Once a
massive-enough core has come
together, it has sufficient gravity to
begin gathering up gas from the
disk in which it was born. The rate
of this accretion increases exponentially
once the mass of accumulated
gas matches the mass of the core,
growing quickly into a giant planet,
says Hongping Deng (University of
Cambridge, UK).
Conventional wisdom says that
it's impossible to stop this juggernaut
partway through to produce
the huge numbers of planets we see
between 1 and 4 Earth radii. Yet in
recent work, Deng and others argue
that such a feat could be possible
if we account for an oft-neglected
force: magnetism.
The team's simulations show that
the magnetic field in a planet-forming
disk triggers fragmentation, cre24
FEBRUARY 2022 * SKY & TELESCOPE
ating nascent planets much smaller
than expected. The protoplanets stir
up gas around them, building up a
magnetic shield that inhibits further
inflow of gas. Only the biggest initial
cores have sufficient gravity to outdo
magnetism and amass enough gas
to match the giant planets we see
in our solar system. If Deng is right,
this could explain why less than
19% of known exoplanets have radii
greater than Neptune's.
Transit depth (parts per million)
Pressure (millibars)
BENNEKE ET AL. / ASTROPHYSICAL JOURNAL 2019
Sky & Telescope - February 2022

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