Sky & Telescope - March 2022 - 38

Altitude
Brightness (W/m2 per micron)
SPECTRA: C. MORLEY ET AL. / ASTROPHYSICAL JOURNAL 2012; CLOUDS DIAGRAM: LEAH
TISCIONE / S&T, SOURCE: PETER GAO / UNIV. OF CALIFORNIA, BERKELEY
Planet or Failed Star?
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
1
No clouds
absorption feature from silicate cloud particles, darkening
the spectra between 9 and 10 microns. Detecting this feature
allows us to determine that there are specific minerals present
in the atmosphere, like enstatite and quartz. In exoplanets,
we see a similar trend based on a water absorption feature
in hot Jupiter spectra: The water feature is mostly obscured -
which means the clouds peak in thickness - at temperatures
where silicate clouds should be plentiful.
One intriguing insight is that clouds appear to be thicker
Clouds
2
3
4
Wavelength (microns)
CLOUD EFFECT A brown dwarf's spectrum (colored lines) looks
dramatically different depending on its clouds. Shown here are two possibilities
for a theoretical, 1300K object: no clouds (yellow), with clear
features around 1 micron, and thin iron and silicate clouds (red), which
blot out those features.
can grow into larger organic molecules that condense, like
the smog enveloping Los Angeles.
When clouds or hazes form, the appearance of a planet
changes dramatically, like the transformation from a sunny
morning to an afternoon thunderstorm. Instead of us being
able to easily see molecular features, the clouds can obscure
our view, absorbing light at all wavelengths. Some exoplanets
have been found to be so cloudy or hazy that we cannot see
any molecules through the clouds at all, thwarting our ability
to measure the atmospheric composition. More commonly,
clouds only somewhat obscure our view, decreasing the sizes
of molecular features we detect and making it harder to
extract information about how much of a given element or
compound is there. To measure the compositions of planetary
atmospheres precisely, we must first understand their clouds.
We've learned in recent years that the clouds on brown
dwarfs and giant exoplanets appear to be very similar. Several
brown dwarfs have infrared spectra that actually reveal an
5
6
on lower-mass objects. Perhaps the clouds are more easily
lofted to high altitudes when the force of gravity tugging
them down is smaller. This physics means it's more challenging
to measure the compositions of lower-mass objects than
high-mass objects, because clouds obscure their spectra more.
Sometimes, we can even tell that storm systems on exoplanets
and brown dwarfs vary over time, as we watch an
object brighten and fade as it rotates. One such young world,
VHS J1256−1257b, has thick silicate clouds that blanket much
of the planet. Observations with the Hubble Space Telescope
show that it varies up to 25% in brightness with each planetary
rotation, getting fainter as darker cloudy regions come
into view and brighter as clearer regions do. Understanding
these variations allows us to trace weather on other worlds
- not involving our own familiar water clouds, but instead
planet-wide convection and circulation patterns that create
storm clouds of dust.
Despite the excitement of studying these exotic worlds,
we are perhaps most excited about observations of planets
a little more familiar to us. The coldest free-floating planet
known is the nearby gas giant WISE J0855−0714 (WISE 0855
for short), where the temperature averages about 250K, or
−10°F: a moderately cold day in Minnesota. The world is cold
enough for water ice to condense in its atmosphere, just like
it does in Earth's upper atmosphere. WISE 0855 also appears
to vary in brightness, changing by a few percent during its
approximately 12-hour day. No direct proof yet exists of the
ice clouds themselves, but the evidence so far is promising.
Hazes
Salts
Silicates
Aluminum oxide
Titanium dioxide
700K
900K
1400K
Temperature
FORECAST Clouds' altitudes and compositions depend in part on how hot their world's atmosphere is. Shown here are the condensates that
scientists predict should form for a range of temperatures common on gas giants that orbit very close to their parent stars. Some brown dwarfs have
these temperatures, too. Colors are ballpark examples of how the clouds might appear to the human eye.
38 MARCH 2022 * SKY & TELESCOPE
1900K
2200K

Sky & Telescope - March 2022

Table of Contents for the Digital Edition of Sky & Telescope - March 2022