Sky & Telescope - March 2025 - 64

Neptune's Clouds
more sunlight, explaining why Neptune looks brighter when
the Sun is most active.
One thing that still puzzles us is how the feeble sunlight at
Neptune's extreme distance could catalyze this change, since
Neptune is 30 times farther from the Sun than Earth. This
difference in distance would reduce what is blazing sunlight
at Earth down to the light you might experience from
an overhead office fixture. Perhaps all of the giant planets
respond to the Sun this way, but the effect is swamped by
the bright and dynamic cloud activity on Jupiter and Saturn
and masked by the extreme seasonal variability of Uranus
(S&T: July 2023, p. 14). Maybe only distant, cold Neptune is
stable enough for this faint effect to be discernible.
This new study is probably not the last chapter in the
pFESTOONS White clouds scuttle across Neptune in this image from
Voyager 2, processed to enhance the visibility of various features. The
Great Dark Spot bleeds off the image's left edge.
planet's orbit around the Sun: Neptune's brightness steadily
increased as it neared its solstice in 2005. But there also
seemed to be a secondary signature imprinted on the data,
and surprisingly it seemed to be correlated with the Sun's
11-year activity cycle.
Researchers suggested various solutions to this conundrum.
The Sun emits significantly more ultraviolet radiation
at the peak of its cycle than it does during solar minimum.
This uptick in ultraviolet emission impacts the planets'
atmospheres in various ways. Early on, some speculated that
the variation in the Sun's output - even at distant Neptune
- was somehow influencing the planet's upper tropospheric
cloud cover.
Others speculated that high-energy particles from the surrounding
galaxy might be a more important driver. The Sun's
magnetic field usually shields the solar system from these
particles, but that shielding effect weakens along with the
field itself during the solar cycle's minimum. A fluctuating
number of particles hitting Neptune's atmosphere could produce
complex changes there. Yet another team of researchers
suggested that solar maximum could lead to higher atmospheric
temperatures, driving changes in a different way.
More recently, a new team reanalyzed all of Hubble's
data on Neptune, starting with mine in the early 1990s and
continuing through to the present day. They found a strong
correlation between the number of methane-ice clouds and
the solar cycle: As the Sun neared solar maximum, Neptune
had more bright clouds.
The team hypothesized that the Sun's increased ultraviolet
output triggered the formation of more photochemical products,
which could in turn seed cloud formation. More clouds
then increase the overall planetary brightness by reflecting
64 MARCH 2025 * SKY & TELESCOPE
story of Neptune's long-term variability. But it highlights the
challenge of studying a planet whose seasons last for many
decades: In my 39 years of studying Neptune, the planet still
hasn't completed one full season. The first observations from
Lowell Observatory were made when I was still in kindergarten,
and some of the researchers doing the newest studies
weren't even born yet when I was making the first Hubble
observations of Neptune! The study of Neptune's atmosphere
is by definition multi-generational.
Neptune in the Future
The many mysteries of Neptune's atmosphere continue to
captivate me and my fellow planetary scientists. Why are its
giant storms so short-lived? How can the feeble sunlight at
Neptune's vast distance be driving atmospheric change? As
described above, our current telescopes are chipping away at
the planet's secrets, and soon JWST observations will usher in
a new era in our understanding.
Neptune's secrets are crucial to today's burgeoning field
of planetary science. With more than 5,000 exoplanets now
known around nearby stars, we planetary scientists have a
rich palette of planets to ponder. But to our surprise, the
most populous class of planets seems to be one that is completely
missing from our own solar system: worlds with sizes
between those of Neptune and Earth (S&T: Feb. 2022, p. 20).
To truly understand planets and planet formation, we need to
know what these planets and their atmospheres are like.
With no planets within this size range to study in the local
neighborhood, our local " ground truth " is limited to extrapolation
upward from Earth or downward from Neptune.
Looking ahead, I anticipate with keen interest the development
of NASA's proposed Habitable Worlds Observatory. This
telescope will be a " super-Hubble " that will open a brand-new
chapter in Neptune exploration. It holds the promise of being
able to deliver Voyager-quality images of Neptune's atmosphere
any time it looks at this blue world!
Similarly, the new class of giant ground-based telescopes
in development, which have mirrors measured in dozens of
meters, may also reveal breathtaking details in Neptune's
atmosphere.
Someday, we will send a modern mission to the NeptuNASA
/ JPL
Sky & Telescope - March 2025

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