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14

SEPTEMBER-OCTOBER 2017

SPACE CHRONICLES

The phenomenon is similar to what

happens with fireworks, which get

their colors from chemicals emitting

light. When metallic substances are

heated and vaporized, their electrons

move into higher energy states. De-

pending on the material, these elec-

trons will emit light at specific

wavelengths as they lose energy:

sodium produces orange-yellow and

strontium produces red in this pro-

cess, for example. The water mo-

lecules in the atmosphere of WASP-

121b similarly give off radiation as

they lose energy, but in the form of

infrared light, which the human eye

is unable to detect.

In Earth’s stratosphere, ozone gas

traps ultraviolet radiation from the

Sun, which raises the temperature of

this layer of atmosphere. Other solar

system bodies have stratospheres,

too; methane is responsible for heat-

ing in the stratospheres of Jupiter and

Saturn’s moon Titan, for example. In

solar system planets, the change in

temperature within a stratosphere is

typically around 100 degrees Fahren-

heit (about 56 degrees Celsius). On

WASP-121b, the temperature in the

stratosphere rises by 1,000 degrees

(560 degrees Celsius). Scientists do

not yet know what chemicals are

causing the temperature increase in

WASP-121b’s atmosphere. Vanadium

oxide and titanium oxide are candi-

dates, as they are commonly seen in

brown dwarfs, “failed stars” that

have some commonalities with exo-

planets. Such compounds are ex-

pected to be present only on the

hottest of hot Jupiters, as high tem-

peratures are needed to keep them in

a gaseous state.

“This super-hot exo-

planet is going to be a benchmark for

our atmospheric models, and it will

be a great observational target mov-

ing into the Webb era,”

said Hannah

Wakeford, study co-author who

worked on this research while at

NASA’s Goddard Space Flight Center,

Greenbelt, Maryland.

T

his diagram presents evidence for the existence of a stratosphere on a

planet orbiting another star. As on Earth, the stratosphere increases in tem-

perature with altitude. The water emissions from the Jupiter-sized planet’s

upper atmosphere show this. The results are in marked contrast to the spectrum

of a failed star, a brown dwarf, which shows water absorption because the at-

mosphere is cooling with altitude increase. [NASA, ESA, and A. Feild (STScI)]

To study the stratosphere of WASP-

121b, scientists analyzed how differ-

ent molecules in the atmosphere

react to particular wavelengths of

light, using Hubble’s capabilities for

spectroscopy. Water vapor in the

planet’s atmosphere, for example,

behaves in predictable ways in re-

sponse to certain wavelengths of

light, depending on the tempera-

ture of the water.

Starlight is able to penetrate deep

into a planet’s atmosphere, where it

raises the temperature of the gas

there. This gas then radiates its heat

into space as infrared light. However,

if there is cooler water vapor at the

top of the atmosphere, the water

molecules will prevent certain wave-

lengths of this light from escaping to

space. But if the water molecules at

the top of the atmosphere have a

higher temperature, they will glow at

the same wavelengths.

“The emission

of light from water means the tem-

perature is increasing with height,”

said Tiffany Kataria, study co-author

based at NASA’s Jet Propulsion Labo-

ratory, Pasadena, California.

“We’re

excited to explore at what longitudes

this behavior persists with upcoming

Hubble observations.”

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