How can a planet be "hotter than hot?" The answer is when heavy
metals are detected escaping from the planet's atmosphere, instead of
condensing into clouds.
Observations by NASA's Hubble Space
Telescope reveal magnesium and iron gas streaming from the strange world
outside our solar system known as WASP-121b. The observations represent
the first time that so-called "heavy metals"—elements heavier than
hydrogen and helium—have been spotted escaping from a hot Jupiter, a
large, gaseous exoplanet very close to its star.
Normally, hot
Jupiter-sized planets are still cool enough inside to condense heavier
elements such as magnesium and iron into clouds.
But that's not
the case with WASP-121b, which is orbiting so dangerously close to its
star that its upper atmosphere reaches a blazing 4,600 degrees
Fahrenheit. The temperature in WASP-121b's upper atmosphere is about 10
times greater than that of any known planetary atmosphere. The WASP-121
system resides about 900 light-years from Earth.
"Heavy metals
have been seen in other hot Jupiters before, but only in the lower
atmosphere," explained lead researcher David Sing of the Johns Hopkins
University in Baltimore, Maryland. "So you don't know if they are
escaping or not. With WASP-121b, we see magnesium and iron gas so far
away from the planet that they're not gravitationally bound."
Ultraviolet
light from the host star, which is brighter and hotter than the Sun,
heats the upper atmosphere and helps lead to its escape. In addition,
the escaping magnesium and iron gas may contribute to the temperature
spike, Sing said. "These metals will make the atmosphere more opaque in
the ultraviolet, which could be contributing to the heating of the upper
atmosphere," he explained.
The sizzling planet is so close to its star that it is on the cusp of being ripped apart by the star's gravity. This hugging distance means that the planet is football shaped due to gravitational tidal forces.
"We picked this planet because
it is so extreme," Sing said. "We thought we had a chance of seeing
heavier elements escaping. It's so hot and so favorable to observe, it's
the best shot at finding the presence of heavy metals. We were mainly
looking for magnesium, but there have been hints of iron in the
atmospheres of other exoplanets. It was a surprise, though, to see it so
clearly in the data and at such great altitudes so far away from the
planet. The heavy metals are escaping partly because the planet is so
big and puffy that its gravity is relatively weak. This is a planet
being actively stripped of its atmosphere."
The researchers used
the observatory's Space Telescope Imaging Spectrograph to search in
ultraviolet light for the spectral signatures of magnesium and iron
imprinted on starlight filtering through WASP-121b's atmosphere as the
planet passed in front of, or transited, the face of its home star.
This
exoplanet is also a perfect target for NASA's upcoming James Webb Space
Telescope to search in infrared light for water and carbon dioxide,
which can be detected at longer, redder wavelengths. The combination of
Hubble and Webb observations would give astronomers a more complete
inventory of the chemical elements that make up the planet's atmosphere.
The
WASP-121b study is part of the Panchromatic Comparative Exoplanet
Treasury (PanCET) survey, a Hubble program to look at 20 exoplanets,
ranging in size from super-Earths (several times Earth's mass) to
Jupiters (which are over 100 times Earth's mass), in the first
large-scale ultraviolet, visible, and infrared comparative study of
distant worlds.
The observations of WASP-121b add to the
developing story of how planets lose their primordial atmospheres. When
planets form, they gather an atmosphere containing gas from the disk in
which the planet and star formed. These atmospheres consist mostly of
the primordial, lighter-weight gases hydrogen and helium, the most
plentiful elements in the universe. This atmosphere dissipates as a
planet moves closer to its star.
"The hot Jupiters are mostly
made of hydrogen, and Hubble is very sensitive to hydrogen, so we know
these planets can lose the gas relatively easily," Sing said. "But in
the case of WASP-121b, the hydrogen and helium gas is outflowing, almost
like a river, and is dragging these metals with them. It's a very
efficient mechanism for mass loss."
The results will appear online today in The Astronomical Journal .
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 in Washington, D.C.
Contact:
Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
David Sing
Johns Hopkins University, Baltimore, Maryland
dsing@jhu.edu
Source: Hubble/Site
