Sky & Telescope - March 2022 - 4

SPECTRUM by Peter Tyson
Cloudy Nights
CLOUDS ARE THE BANE of astronomers. They can shut down a
night of stargazing - or a week. Even scattered ones can ruin your
observing plan. In the most excruciating instance, they can dash
hopes of witnessing totality during a solar eclipse you traveled half
the world to see.
The irony is thick: With a clear night sky, you can see for light-years, yet
cumulus just overhead can trip you up at the very start of such a visual marathon.
Clouds can do the same at the other end. As Tom Dobbins remarks in his
article about Venus on page 52, even though our twin is the brightest planet in
the sky, it's a " bitterly disappointing " sight for visual observers because of its
all-encompassing and largely featureless clouds. When looking at the second
rock from the Sun, your sight of the finish line is unobstructed for tens of millions
of kilometers, only to be blocked at the tape.
The same thing happens with far more distant exoplanets and brown dwarfs,
the subject of Caroline Morley's article on page 34. As she notes, the clouds
and hazes that often enshroud these two kinds of
extrasolar worlds can absorb light at all wavelengths,
denying us clues about the molecular composition of
their atmospheres.
More commonly, the aerosol sheaths of these
pJupiter's clouds make the
view, as seen strikingly in
this Juno spacecraft image.
But in many other instances
in astronomy, clouds just get
in the way.
far-off orbs only partially obscure our view, so we
can still get some idea of what their air contains. And
the clouds themselves can tell stories of a world's
weather over time, just as they can on Jupiter and
other gas giants in our solar system.
All the same, clouds can frustrate exoplanet
researchers just as much as they can backyard
astronomers. That's why professionals are beside
themselves with anticipation for the James Webb
Space Telescope, set to launch only weeks from now
as I write this. JWST's extreme sensitivity at infrared wavelengths will help
astronomers cut through the veils of many exoplanets and brown dwarfs and
measure the molecules they've been hiding.
Doing so will help us answer the question at the heart of Morley's piece:
What exactly are brown dwarfs? Are they failed stars or giant planets? JWST
will address much more than this question, of course, including whether any
exoplanet atmospheres offer conditions that might support life.
One thing JWST will not do, though, is look at Venus
- the telescope's protective sunshield will block the inner
solar system from its view. So our sister planet's clouds will
continue to hinder both our sight and our insight.
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Sky & Telescope - March 2022

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