Imagine
a place where the weather forecast is always the same: scorching temperatures,
relentlessly sunny, and with absolutely zero chance of rain. This hellish
scenario exists on the permanent daysides of a type of planet found outside our
solar system dubbed an "ultrahot Jupiter." These worlds orbit
extremely close to their stars, with one side of the planet permanently facing
the star.
What
has puzzled scientists is why water vapor appears to be missing from the toasty
worlds' atmospheres, when it is abundant in similar but slightly cooler
planets. Observations of ultrahot Jupiters by NASA's Spitzer and Hubble space
telescopes, combined with computer simulations, have served as a springboard
for a new theoretical study that may have solved this mystery.
According
to the new study, ultrahot Jupiters do in fact possess the ingredients for
water (hydrogen and oxygen atoms). But due to strong irradiation on the
planet's daysides, temperatures there get so intense that water molecules are
completely torn apart.
"The
daysides of these worlds are furnaces that look more like a stellar atmosphere
than a planetary atmosphere," said Vivien Parmentier, an astrophysicist at
Aix Marseille University in France and lead author of the new study. "In
this way, ultrahot Jupiters stretch out what we think planets should look
like."
While
telescopes like Spitzer and Hubble can gather some information about the
daysides of ultrahot Jupiters, the nightsides are difficult for current
instruments to probe. The new paper proposes a model for what might be
happening on both the illuminated and dark sides of these planets, based largely
on observations and analysis of the ultrahot Jupiter known as WASP-121b, and
from three recently published studies, coauthored by Parmentier, that focus on
the ultrahot Jupiters WASP-103b,
WASP-18b and HAT-P-7b, respectively. The new study suggests
that fierce winds may blow the sundered water molecules into the planets'
nightside hemispheres. On the cooler, dark side of the planet, the atoms can
recombine into molecules and condense into clouds, all before drifting back
into the dayside to be splintered again.
Water
is not the only molecule that may undergo a cycle of chemical reincarnation on
these planets, according to the new study. Previous detections of clouds by
Hubble at the boundary between day and night, where temperatures mercifully
fall, have shown that titanium oxide (popular as a sunscreen) and aluminum
oxide (the basis for ruby, the gemstone) could also be molecularly reborn on
the ultrahot Jupiters' nightsides. These materials might even form clouds and
rain down as liquid metals and fluidic rubies.
Among
the growing catalog of planets outside our solar system -- known as exoplanets
-- ultrahot Jupiters have stood out as a distinct class for about a decade.
Found in orbits far closer to their host stars than Mercury is to our Sun, the
giant planets are tidally locked, meaning the same hemisphere always faces the
star, just as the Moon always presents the same side to Earth. As a result,
ultrahot Jupiters' daysides broil in a perpetual high noon. Meanwhile, their
opposite hemispheres are gripped by endless nights. Dayside temperatures reach
between 3,600 and 5,400 degrees Fahrenheit (2,000 and 3,000 degrees Celsius),
ranking ultrahot Jupiters among the hottest exoplanets on record. Nightside
temperatures are around 1,800 degrees Fahrenheit cooler (1,000 degrees
Celsius), cold enough for water to re-form and, along with other molecules,
coalesce into clouds.
Hot
Jupiters, cousins to ultrahot Jupiters with dayside temperatures below 3,600
degrees Fahrenheit (2,000 Celsius), were the first widely discovered type of
exoplanet, starting back in the mid-1990s. Water has turned out to be common in
their atmospheres. One hypothesis for why it appeared absent in ultrahot
Jupiters has been that these planets must have formed with very high levels of
carbon instead of oxygen. Yet the authors of the new study say this idea could
not explain the traces of water also sometimes detected at the
dayside-nightside boundary.
To
break the logjam, Parmentier and colleagues took a cue from well-established
physical models of the atmospheres of stars, as well as "failed
stars," known as brown dwarfs, whose properties overlap somewhat with hot
and ultrahot Jupiters. Parmentier adapted a brown dwarf model developed by Mark
Marley, one of the paper's coauthors and a research scientist at NASA's Ames
Research Center in Silicon Valley, California, to the case of ultrahot
Jupiters. Treating the atmospheres of ultrahot Jupiters more like blazing stars
than conventionally colder planets offered a way to make sense of the Spitzer
and Hubble observations.
"With
these studies, we are bringing some of the century-old knowledge gained from
studying the astrophysics of stars, to the new field of investigating
exoplanetary atmospheres," said Parmentier.
Spitzer's
observations in infrared light zeroed in on carbon monoxide in the ultrahot
Jupiters' atmospheres. The atoms in carbon monoxide form an extremely strong
bond that can uniquely withstand the thermal and radiational assault on the
daysides of these planets. The brightness of the hardy carbon monoxide revealed
that the planets' atmospheres burn hotter higher up than deeper down.
Parmentier said verifying this temperature difference was key for vetting
Hubble's no-water result, because a uniform atmosphere can also mask the
signatures of water molecules.
"These
results are just the most recent example of Spitzer being used for exoplanet
science -- something that was not part of its original science manifest,"
said Michael Werner, project scientist for Spitzer at NASA's Jet Propulsion
Laboratory in Pasadena, California. "In addition, it's always heartening
to see what we can discover when scientists combine the power of Hubble and
Spitzer, two of NASA's Great Observatories."
Although
the new model adequately described many ultrahot Jupiters on the books, some
outliers do remain, suggesting that additional aspects of these worlds'
atmospheres still need to be understood. Those exoplanets not fitting the mold
could have exotic chemical compositions or unanticipated heat and circulation
patterns. Prior studies have argued that there is a more significant amount of
water in the dayside atmosphere of WASP-121b than what is apparent from
observations, because most of the signal from the water is obscured. The new
paper provides an alternative explanation for the smaller-than-expected water
signal, but more studies will be required to better understand the nature of
these ultrahot atmospheres.
Resolving
this dilemma could be a task for NASA's next-generation James Webb Space
Telescope, slated for a 2021 launch. Parmentier and colleagues expect it will
be powerful enough to glean new details about the daysides, as well as confirm
that the missing dayside water and other molecules of interest have gone to the
planets' nightsides.
"We
now know that ultrahot Jupiters exhibit chemical behavior that is different and
more complex than their cooler cousins, the hot Jupiters," said
Parmentier. "The studies of exoplanet atmospheres is still really in its
infancy and we have so much to learn."
The new study is forthcoming in the journal Astronomy and Astrophysics.
NASA's
Jet Propulsion Laboratory, Pasadena, California, manages the Spitzer Space
Telescope mission for NASA's Science Mission Directorate, Washington. Science
operations are conducted at the Spitzer Science Center at Caltech in Pasadena.
Spacecraft operations are based at Lockheed Martin Space, Littleton, Colorado.
Data are archived at the Infrared Science Archive housed at IPAC at Caltech.
Caltech manages JPL for NASA.
Hubble
is a project of international cooperation between NASA and ESA. NASA's Goddard
Space Flight Center in Greenbelt, Maryland, manages Hubble. The Space Telescope
Science Institute (STScI)
in Baltimore conducts Hubble science operations.
News Media Contact
Calla Cofield
Jet Propulsion Laboratory, Pasadena, Calif.
818-393-1821
Calla.e.cofield@jpl.nasa.gov
Written by Adam Hadhazy
