Water Is Destroyed, Then Reborn in Ultrahot Jupiters

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. Credit: NASA/JPL-Caltech/Vivien Parmentier/Aix-Marseille University (AMU).  › Full image and caption

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.

News Media Contact

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

Written by Adam Hadhazy

Source: JPL-Caltech/News

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NASA's Hubble Makes One Millionth Science Observation

Transiting Exoplanet HAT-P-7b
Credit: NASA, ESA, and G. Bacon (STScI)

All-Sky illustration of all Hubble observations as of June 27, 2011

Credit: NASA, ESA, and R. Thompson (CSC/STScI)

NASA's Hubble Space Telescope crossed another milestone in its space odyssey of exploration and discovery. On Monday, July 4, the Earth-orbiting observatory logged its one millionth science observation during a search for water in an exoplanet's atmosphere 1,000 light-years away.

"For 21 years Hubble has been the premier space science observatory, astounding us with deeply beautiful imagery and enabling ground-breaking science across a wide spectrum of astronomical disciplines," said NASA Administrator Charles Bolden. He piloted the space shuttle mission that carried Hubble to orbit. "The fact that Hubble met this milestone while studying a faraway planet is a remarkable reminder of its strength and legacy."

Although Hubble is best known for its stunning imagery of the cosmos, the millionth observation is a spectrocopic measurement, where light is divided its component colors. These color patterns can reveal the chemical composition of cosmic sources.

Hubble's millionth exposure is of the planet HAT-P-7b, a gas giant planet larger than Jupiter orbiting a star hotter than our Sun. HAT-P-7b, also known as Kepler 2b, has been studied by NASA's planet-hunting Kepler observatory after it was discovered by ground-based observations. Hubble now is being used to analyze the chemical composition of the planet's atmosphere.

"We are looking for the spectral signature of water vapor. This is an extremely precise observation and it will take months of analysis before we have an answer," said Drake Deming of the University of Maryland and NASA's Goddard Space Flight Center in Greenbelt, Md. "Hubble demonstrated it is ideally suited for characterizing the atmospheres of exoplanets, and we are excited to see what this latest targeted world will reveal."

"This exciting set of observations in search of water vapor on a planet orbiting another star is laying the foundation for how we plan to use the James Webb Space Telescope's more powerful vision to look for liquid water on nearby planets, the necessary precursor to extraterrestrial life," said Matt Mountain, STScI director.

Hubble was launched April 24, 1990, aboard space shuttle Discovery's STS-31 mission. Its discoveries revolutionized nearly all areas of astronomical research from planetary science to cosmology. The observatory has collected more than 50 terabytes of data to-date. The archive of that data is available to scientists and the public at: http://hla.stsci.edu/

Hubble's odometer reading includes every observation of astronomical targets since its launch and observations used to calibrate its suite of instruments. Hubble made the millionth observation using its Wide Field Camera 3, a visible- and infrared-light imager with an on-board spectrometer. It was installed by astronauts during Hubble Servicing Mission 4 in May 2009.

"The Hubble keeps amazing us with groundbreaking science," said Sen. Barbara Mikulski, the chairwoman of the Senate Commerce, Justice, Science and Related Agencies Appropriations Subcommittee that funds NASA. "I championed the mission to repair and renew Hubble not just to get one million science observations, but also to inspire millions of children across the planet to become our next generation of stargazers, scientists, astronauts and engineers."

CONTACT

Trent Perrotto
NASA Headquarters, Washington, D.C.
trent.j.perrotto@nasa.gov
202-358-0321

Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4514
villard@stsci.edu

Discovery of a Retrograde or Highly Tilted Extrasolar Planet

The panels show two possibilities for the bizarre orbit of HAT-P-7b. The top panel shows a "polar" orbit in which the planet goes over the north and south poles of the star. The bottom panel shows a "retrograde" orbit in which the planet revolves in the opposite direction as the star's rotation. Astronomers cannot distinguish these two possibilities because the exact orientation of the star's rotation axis is not yet known. Illustrations: Simon Albrecht/MIT

Figure 1

An illustration of the concept of the Rossiter-McLaughlin effect. Each star generally rotates by itself and has an approaching part and a receding part. During a planetary transit, we can see the Rossiter-McLaughlin effect, which is an apparent anomaly of the stellar radial velocity, the star appears to be receding if the transiting planet hides an approaching part and vice versa. We can observe this effect by precise radial velocity measurements. Note that if the planet orbits in a prograde manner, the planet first hides an approaching side and subsequently hides a receding side. Inversely, if the planet orbits in a retrograde manner, the effect occurs in reverse.

Figure 2　

The observational result of the Rossiter-McLaughlin effect on UT May 30, 2008 taken with the Subaru HDS (Narita et al. 2009). This figure shows that the HAT-P-7b first hides a receding part of the HAT-P-7 and subsequently hides an approaching side.

Figure 3

The observational result of the Rossiter-McLaughlin effect on UT July 1, 2009 taken with the Subaru HDS (Winn et al. 2009). This result also indicates a retrograde orbit of HAT-P-7b as well as the previous figure.

Two teams of astronomers have found that extrasolar planet HAT-P-7b, discovered in 2008, has a retrograde or highly tilted orbit. On UT May 30, 2008, a Japanese collaboration led by Norio Narita (National Astronomical Observatory of Japan) used the Subaru Telescope’s High Dispersion Spectrograph (HDS) to observe the HAT-P-7 planetary system, which is about 1000 light years distant from Earth, and found the first evidence of a retrograde orbit of the extrasolar planet HAT-P-7b. On UT July 1, 2009, a US collaboration led by Joshua N. Winn (Massachusetts Institute of Technology) also used the Subaru Telescope’s HDS to independently observe the HAT-P-7 system and likewise concluded that extrasolar planet HAT-P-7b has a retrograde or polar orbit. Both observational results were independently submitted and accepted to scientific journals in August 2009, and were published in October 2009.

The HAT-P-7b is the first planet that indicates a retrograde orbit at the time of publications in scientific journals. Such retrograde or spin-orbit misaligned planets are important for understanding the diversity of planetary systems, and they provide important evidence for assessing current planetary migration models. It is now well known that extrasolar planets have diverse orbits, and recent planetary migration models have predicted the existence of such retrograde or highly tilted extrasolar planets. The Subaru findings provide an important milestone for understanding the orbital evolution of planetary systems.

Extrasolar planets are planets beyond our Solar System. With the advent of large ground-based telescopes and innovative instruments to enhance the flexibility of observations since the 1990s, over 400 extrasolar planets have been discovered since the first one was identified in 1995. The discoveries taught us that orbits of extrasolar planets are very different from those of the planets in the Solar System. For instance, dozens of extrasolar Jovian planets orbit their host stars with a period of a few days ("hot Jupiters"), and many of the extrasolar planets have significant eccentricities ("eccentric planets"). In order to understand the diversity of planetary orbits, many theoretical models for planetary migration have been developed.

Widely held beliefs about planetary system formation have maintained that planetary systems form in rotating protoplanetary disks surrounding protostars. Thus the planetary orbital axis and the stellar spin axis are generally considered to be well aligned, as is true for the planets in the Solar System. However, recent theories have not followed suit. For example, planetary migration models considering gravitational interactions between multiple giant planets ("planet-planet scattering models") or considering Kozai cycles due to a distant companion star ("Kozai migration") predict that a significant fraction of migrated planets have tilted, or even retrograde orbits to the stellar spin axis. Retrograde orbits are those in which the planetary orbit is tilted by over 90 degrees to the stellar spin axis.

A collaboration led by Norio Narita at National Astronomical Observatory of Japan has used the Subaru Telescope to make observations that test such planetary migration models. The team focused on the Rossiter-McLaughlin effect, which is an apparent irregularity in the star’s radial velocity; the star appears to be receding if the transiting planet hides an approaching part and vice versa. By measuring the Rossiter-McLaughlin effect, one can estimate the sky-projected angle between the stellar spin axis and the planetary orbital axis. The Subaru telescope succeeded in detecting the effect in the TrES-1 transiting planetary system in 2007 (a press release at the Subaru website on August 23, 2007), and since then the Subaru Telescope has made observations of several transiting planetary systems. During the observation of the planetary system on UT May 30, 2008, the Japanese team found the first evidence of a retrograde or highly tilted orbit of the HAT-P-7b based on the Rossiter-McLaughlin effect. The result indicates that the planet first hides the receding part of the stellar surface and then the approaching part. The independent observations on UT July 1, 2009 of the US collaboration led by Joshua N. Winn at MIT confirmed these findings (a press release at the MIT website on November 12, 2009).

At this point, however, the migration model for HAT-P-7b has not yet been firmly discriminated. Thus further observations of this system to search for outer massive planets or a binary companion would be interesting. In addition, since the HAT-P-7 system is within the field of view of the Kepler mission, further characterization of this interesting planet HAT-P-7b will be undertaken in the near future.

These studies were published in the Publications of Astronomical Society of Japan Letters (issue published on October 25, 2009) and the Astrophysical Journal Letters (issue publised on October 1, 2009).

REFERENCES:

Norio Narita, Bun'ei Sato, Teruyuki Hirano, Motohide Tamura, 2009 "First Evidence of a Retrograde Orbit of a Transiting Exoplanet HAT-P-7b" Publ. Astron. Soc. Japan, Vol. 61, No. 5, L35-L40.

Joshua N. Winn, John Asher Johnson, Simon Albrecht, Andrew W. Howard, Geoffrey W. Marcy, Ian J. Crossfield, Matthew J. Holman, 2009 "HAT-P-7: A Retrograde or Polar Orbit, and a Third Body" The Astrophysical Journal Letters, Volume 703, Issue 2, pp. L99-L103.