From NASA Ames and JPL: “Water Is Destroyed, Then Reborn in Ultrahot Jupiters”

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From NASA Ames

Aug. 9, 2018

Calla Cofield
Jet Propulsion Laboratory, Pasadena, Calif.
818-393-1821
Calla.e.cofield@jpl.nasa.gov

Written by Adam Hadhazy

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These simulated views of the ultrahot Jupiter WASP-121b show what the planet might look like to the human eye from five different vantage points, illuminated to different degrees by its parent star. The images were created using a computer simulation being used to help scientists understand the atmospheres of these ultra-hot planets. Ultrahot Jupiters reflect almost no light, rather like charcoal. However, the daysides of ultrahot Jupiters have temperatures of between 3600°F and 5400°F (2000°C and 3000°C), so the planets produce their own glow, like a hot ember. The orange color in this simulated image is thus from the planet’s own heat. The computer model was based on observations of WASP-121b conducted using NASA’s Spitzer and Hubble space telescopes. Credits: NASA/JPL-Caltech/Vivien Parmentier/Aix-Marseille University (AMU)

NASA/Spitzer Infrared Telescope

NASA/ESA Hubble Telescope

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.

Star-planet hybrids

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.

Further information from
mwatzke@cfa.harvard.edu

The Kreidberg et al. paper reports observations of the ultra-hot Jupiter WASP-103b by NASA’s Hubble Space Telescope (HST) and Spitzer Space Telescope. The researchers estimate the dayside temperature of the planet is 4800 degrees Fahrenheit (2700 degrees Celsius) making it one of the hottest exoplanets known. The nightside temperature of the planet is much cooler, at 2900 degrees Fahrenheit (1600 degrees Celsius).

Unlike the case for cooler hot Jupiters, Kreidberg and collaborators do not detect any sign of water vapor in WASP-103b using Hubble data. As explained in papers led by Vivien Parmentier, Jacob Arcangeli and Megan Mansfield, and the JPL press release, the water is being torn apart by radiation from the planet’s host star. However, Kreidberg and collaborators do detect evidence for carbon monoxide on the dayside of WASP-103b using Spitzer data. This is a much hardier molecule than water.

They also detect evidence for a temperature inversion, formed by the same mechanism as temperature inversions on the Earth. In both planets this effect is caused by ultraviolet radiation being absorbed in the upper atmosphere, causing it to become hotter. In the case of Earth, ozone is the molecule most responsible for the absorption, while in the case of WASP-103b sodium might be responsible or perhaps exotic molecules like titanium or vanadium oxides.

A crucial observational advance by Kreidberg and her team was that they observed the planet for an entire orbit, enabling them to map the climate at every longitude and derive detailed information about the temperatures on the planet’s dayside and nightside. This is only the second time that such a complete exoplanet observation has been performed with HST.

Kreidberg and collaborators also use modeling of the WASP-103b data to estimate the magnetic field of the planet. They estimate that the magnetic field is about twice that of the Earth and half that of Jupiter.

“WASP-103b is unlike anything in our Solar System”, said Kreidberg. “It orbits right next to its parent star — less than 2 million miles, 20 times closer than Mercury is to the Sun. The planet completes a full orbit (its “year”) every 22 hours, and the strong pull of its star’s gravity distorts it into an egg-like shape. It’s heated up on the permanent hot dayside to 4800 degrees F, which is hotter than many stars. You definitely wouldn’t want to live there! And yet in some ways, the planet isn’t so exotic — we found that it has a thermal inversion in its atmosphere that formed in a very similar way as the stratosphere on Earth. It serves as a reminder that even on the most unwelcoming of extrasolar worlds, they’re still subject to the same laws of physics and chemistry that we abide by here on Earth.”

The Kreidberg et al. paper is available online at https://xxx.lanl.gov/abs/1805.00029 and has been published in The Astronomical Journal.

Other papers included in the JPL press release are:

• Mansfield et al: https://arxiv.org/abs/1805.00424 and published in The Astronomical Journal: http://iopscience.iop.org/article/10.3847/1538-3881/aac497/pdf
• Parmentier et al: https://arxiv.org/abs/1805.00096 and published in Astronomy & Astrophysics
• Arcangeli et al: https://arxiv.org/abs/1801.02489 and published in The Astrophysical Journal Letters: http://iopscience.iop.org/article/10.3847/2041-8213/aab272/pdf

Contact information for Dr Laura Kreidberg:
laura.kreidberg@cfa.harvard.edu
775-233-7497

Contact information for Professor Avi Loeb:
aloeb@cfa.harvard.edu
617-913-5598

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