Illustration: NASA, ESA, and G. Bacon (STScI)
Credits: Science: NASA, ESA, and T. Evans (University of Exeter)
Scientists have discovered the strongest evidence to date for a
stratosphere on a planet outside our solar system, or exoplanet. A
stratosphere is a layer of atmosphere in which temperature increases
with higher altitudes.
"This result is exciting because it shows that a common trait of most
of the atmospheres in our solar system — a warm stratosphere — also can
be found in exoplanet atmospheres," said Mark Marley, study co-author
based at NASA's Ames Research Center in California's Silicon Valley. "We
can now compare processes in exoplanet atmospheres with the same
processes that happen under different sets of conditions in our own
solar system."
Reporting in the journal Nature, scientists used data from NASA's
Hubble Space Telescope to study WASP-121b, a type of exoplanet called a
"hot Jupiter." Its mass is 1.2 times that of Jupiter, and its radius is
about 1.9 times Jupiter's — making it puffier. But while Jupiter
revolves around our sun once every 12 years, WASP-121b has an orbital
period of just 1.3 days. This exoplanet is so close to its star that if
it got any closer, the star's gravity would start ripping it apart. It
also means that the top of the planet's atmosphere is heated to a
blazing 4,600 degrees Fahrenheit (2,500 degrees Celsius), hot enough to
boil some metals. The WASP-121 system is estimated to be about 900
light-years from Earth — a long way, but close by galactic standards.
Previous research found possible signs of a stratosphere on the
exoplanet WASP-33b as well as some other hot Jupiters. The new study
presents the best evidence yet because of the signature of hot water
molecules that researchers observed for the first time.
"Theoretical models have suggested stratospheres may define a
distinct class of ultra-hot planets, with important implications for
their atmospheric physics and chemistry," said Tom Evans, lead author
and research fellow at the University of Exeter, United Kingdom. "Our
observations support this picture."
To study the stratosphere of WASP-121b, scientists analyzed how
different 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
response to certain wavelengths of light, depending on the temperature
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 wavelengths 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 temperature is increasing
with height," said Tiffany Kataria, study co-author based at NASA's Jet
Propulsion Laboratory, Pasadena, California. "We’re excited to explore
at what longitudes this behavior persists with upcoming Hubble
observations."
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.
Depending on the material, these electrons will emit light at specific
wavelengths as they lose energy: sodium produces orange-yellow and
strontium produces red in this process, for example. The water molecules
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
heating 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 Fahrenheit (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 candidates, as they
are commonly seen in brown dwarfs, "failed stars" that have some
commonalities with exoplanets. Such compounds are expected to be present
only on the hottest of hot Jupiters, as high temperatures are needed to
keep them in a gaseous state.
"This super-hot exoplanet is going to be a benchmark for our
atmospheric models, and it will be a great observational target moving
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.
The Hubble Space Telescope is a project of international cooperation
between NASA and ESA (European Space Agency). NASA's Goddard Space
Flight Center in Greenbelt, Maryland, manages the telescope. The Space
Telescope Science Institute (STScI) in Baltimore, Maryland, conducts
Hubble science operations. STScI is operated for NASA by the Association
of Universities for Research in Astronomy, Inc., in Washington, D.C.
The California Institute of Technology (Caltech) manages the Jet
Propulsion Laboratory (JPL) for NASA.
Links
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- The science paper by T. Evans et al.
- NASA's Hubble Portal
- Journal Nature
- NASA JPL's Feature Story
- NASA's Exoplanets Portal
- University of Exeter's Release
Contacts
Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, California
818-354-6425
elizabeth.landau@jpl.nasa.gov
Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4514
villard@stsci.edu
