Sky & Telescope - March 2022 - 40

Planet or Failed Star?
there is evidence that most brown dwarfs have compositions
similar to one another, with patterns in their spectra largely
explained by their temperatures. Our picture fits together:
Brown dwarfs likely form the same way stars do.
For the lowest-mass brown dwarfs - the free-floating
planets - the picture that's emerging may be more complex.
Many of the hot, young, low-mass brown dwarfs are cloudier
than their high-mass counterparts, making their compositions
even more challenging to determine. We also have not yet
measured precise chemical makeups for the older, colder, freefloating
planets, though with JWST we soon will, providing the
most reliable comparisons to the solar system's objects to date.
And for the exoplanets themselves, we have much work to
do. Exoplanet compositions are just barely in reach of current
telescopes. Evidence so far suggests that exoplanets may have
a wide range of oxygen abundances compared to stars, but
other elements have remained largely out of reach. When
JWST begins science operations, its enhanced sensitivity and
wavelength range will enable us to measure molecules in a
variety of exoplanet atmospheres and compare the results
NEIGHBORS
The third- and fourth-closest " star " systems to
the Sun solely contain brown dwarfs: the binary
WISE J1049-5319 (a.k.a. Luhman 16 AB) at 6.5
light-years, and WISE J0855-0714 at 7.3 lightyears.
But with an estimated mass of only 3 to
10 Jupiters, WISE 0855 would qualify as a freefloating
planet.
with both stars and brown dwarfs. Perhaps with this information,
we will have clarity in our categories, with exoplanets
appearing radically different from their stellar compatriots.
Or, perhaps more likely, we will find new unanticipated complexities
in the atmospheres of these faraway worlds.
¢ CAROLINE MORLEY is an assistant professor of astronomy
at the University of Texas, Austin. Her research focuses on understanding
the atmospheres of exoplanets and brown dwarfs.
BY THE NUMBERS By number of atoms,
Jupiter and a typical brown dwarf
are fairly similar in composition. Carbon
exists in gaseous methane on both
objects. Oxygen is tough to measure
on Jupiter, because it's condensed into
water clouds below the higher nitrogencloud
layer. Nitrogen, meanwhile, is
complicated on brown dwarfs, because
it's only measured in the form of gaseous
ammonia - molecular nitrogen is
likely also abundant, but undetectable.
Periodic Table of the Elements
Top Five Elements in the
Atmosphere of Jupiter
92.5%
Top Five Elements in the
Atmosphere of a Brown Dwarf
91.1%
7.3%
0.1%
0.03%
0.02%
8.9%
0.03%
0.03%
0.002%
1
2
H
HYDROGEN
He
HELIUM
C
CARBON
6
N
NITROGEN
7
O
OXYGEN
8
H
HYDROGEN
1
2
He
HELIUM
C
CARBON
6
O
OXYGEN
8
N
NITROGEN
7
40 MARCH 2022 * SKY & TELESCOPE
TERRI DUBÉ / S&T, SOURCES: LUNAR AND PLANETARY INSTITUTE, M. R. LINE ET AL.
/ ASTROPHYSICAL JOURNAL 2017; JUPITER: SEAN WALKER / S&T; BROWN DWARF
ILLUSTRATION: NASA / JPL-CALTECH
Sky & Telescope - March 2022

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