NASA's Webb Lifts Veil on Common but Mysterious Type of Exoplanet

Hot Sub-Neptune Exoplanet Illustration
Credits/Illustration: NASA, ESA, CSA, Dani Player (STScI)

Hot Sub-Neptune Spectrum
Credits/Illustration: NASA, ESA, CSA, Joseph Olmsted (STScI)

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Though they don’t orbit around our Sun, sub-Neptunes are the most common type of exoplanet, or planet outside our solar system, that have been observed in our galaxy. These small, gassy planets are shrouded in mystery…and often, a lot of haze. Now, by observing exoplanet TOI-421 b, NASA’s James Webb Space Telescope is helping scientists understand sub-Neptunes in a way that was not possible prior to the telescope’s launch.

“I had been waiting my entire career for Webb so that we could meaningfully characterize the atmospheres of these smaller planets,” said principal investigator Eliza Kempton of the University of Maryland, College Park. “By studying their atmospheres, we’re getting a better understanding of how sub-Neptunes formed and evolved, and part of that is understanding why they don't exist in our solar system.”

Small, Cool, Shrouded in Haze

The existence of sub-Neptunes was unexpected before they were discovered by NASA’s retired Kepler space telescope in the last decade. Now, astronomers are trying to understand where these planets came from and why are they so common.

Before Webb, scientists had very little information on them. While sub-Neptunes are a few times larger than Earth, they are still much smaller than gas-giant planets and typically cooler than hot Jupiters, making them much more challenging to observe than their gas-giant counterparts.

A key finding prior to Webb was that most sub-Neptune atmospheres had flat or featureless transmission spectra. This means that when scientists observed the spectrum of the planet as it passed in front of its host star, instead of seeing spectral features – the chemical fingerprints that would reveal the composition of the atmosphere – they saw only a flat-line spectrum. Astronomers concluded from all of those flat-line spectra that at least certain sub-Neptunes were probably very highly obscured by either clouds or hazes.

A Different Kind of Sub-Neptune?

“Why did we observe this planet, TOI-421 b? It's because we thought that maybe it wouldn't have hazes,” said Kempton. “And the reason is that there were some previous data that implied that maybe planets over a certain temperature range were less enshrouded by haze or clouds than others.”

That temperature threshold is about 1,070 degrees Fahrenheit. Below that, scientists hypothesized that a complex set of photochemical reactions would occur between sunlight and methane gas, and that would trigger the haze. But hotter planets shouldn't have methane and therefore perhaps shouldn't have haze.

The temperature of TOI-421 b is about 1,340 degrees Fahrenheit, well above the presumed threshold. Without haze or clouds, researchers expected to see a clear atmosphere – and they did!

A Surprising Finding

“We saw spectral features that we attribute to various gases, and that allowed us to determine the composition of the atmosphere,” said the University of Maryland’s Brian Davenport, a third-year Ph.D. student who conducted the primary data analysis. “Whereas with many of the other sub-Neptunes that had been previously observed, we know their atmospheres are made of something, but they're being blocked by haze.”

The team found water vapor in the planet’s atmosphere, as well as tentative signatures of carbon monoxide and sulfur dioxide. Then there are molecules they didn’t detect, such as methane and carbon dioxide. From the data, they can also infer that a large amount of hydrogen is in TOI-421 b’s atmosphere.

The lightweight hydrogen atmosphere was the big surprise to the researchers. “We had recently wrapped our mind around the idea that those first few sub-Neptunes observed by Webb had heavy-molecule atmospheres, so that had become our expectation, and then we found the opposite,” said Kempton. This suggests TOI-421 b may have formed and evolved differently from the cooler sub-Neptunes observed previously.

Is TOI-421 b Unique?

The hydrogen-dominated atmosphere is also interesting because it mimics the composition of TOI-421 b's host star. “If you just took the same gas that made the host star, plopped it on top of a planet's atmosphere, and put it at the much cooler temperature of this planet, you would get the same combination of gases. That process is more in line with the giant planets in our solar system, and it is different from other sub-Neptunes that have been observed with Webb so far,” said Kempton.

Aside from being hotter than other sub-Neptunes previously observed with Webb, TOI-421 b orbits a Sun-like star. Most of the other sub-Neptunes that have been observed so far orbit smaller, cooler stars called red dwarfs.

Is TOI-421b emblematic of hot sub-Neptunes orbiting Sun-like stars, or is it just that exoplanets are very diverse? To find out, the researchers would like to observe more hot sub-Neptunes to determine if this is a unique case or a broader trend. They hope to gain insights into the formation and evolution of these common exoplanets.

“We've unlocked a new way to look at these sub-Neptunes,” said Davenport. “These high-temperature planets are amenable to characterization. So by looking at sub-Neptunes of this temperature, we're perhaps more likely to accelerate our ability to learn about these planets.”

The team’s findings appear May 5 in The Astrophysical Journal Letters.

The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).

To learn more about Webb, visit:  https://science.nasa.gov/webb

Source: NASA's James Webb Space Telescope/News

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Space Telescope Science Institute, Baltimore

Hannah Braun
Space Telescope Science Institute, Baltimore

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WIYN 3.5-meter Telescope at Kitt Peak Discovers Extremely Strange Orbit of Rare Exoplanet

PR Image noirlab2418a
Artist’s Impression of a Hot Jupiter Progenitor Orbiting a Star

PR Image noirlab2418b
TIC 241249530 b Orbital Comparison Illustration

PR Image noirlab2418c
NEID on the WIYN 3.5-meter Telescope

PR Image noirlab2418d
WIYN 3.5-meter Telescope

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Videos


PR Video noirlab2418a
Cosmoview Episode 84: WIYN 3.5-meter Telescope at Kitt Peak Discovers Extremely Strange Orbit of Rare Exoplanet


PR Video noirlab2418b
Cosmoview Episodio 84: Astrónomos descubren inusual exoplaneta gigante con una órbita extremadamente rara


PR Video noirlab2418c
TIC 241249530 b Orbital Comparison Animation

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An exoplanet’s elongated, backwards orbit holds clues to the formation history and future trajectories of high-mass gas giants

Using the WIYN 3.5-meter telescope at the U.S. National Science Foundation Kitt Peak National Observatory, a Program of NSF NOIRLab, astronomers have discovered the extreme orbit of an exoplanet that’s on its way to becoming a hot Jupiter. This exoplanet not only follows one of the most drastically stretched-out orbits of all known transiting exoplanets but is also orbiting its star backwards, lending insight into the mystery of how hot Jupiters evolve.

At present there are over 5600 confirmed exoplanets in just over 4000 star systems. Within this population, about 300–500 exoplanets fall into the curious class known as hot Jupiters — large, Jupiter-like exoplanets that orbit very close to their star, some even as close as Mercury is to our Sun. How hot Jupiters end up in such close orbits is a mystery, but astronomers postulate that they begin in orbits far from their star and then migrate inward over time. The early stages of this process have rarely been observed, but with this new analysis of an exoplanet with an unusual orbit, astronomers are one step closer to unraveling the hot Jupiter mystery.

The discovery of this exoplanet, named TIC 241249530 b, originated with the detection by NASA’s Transiting Exoplanet Survey Satellite (TESS) in January 2020 of a dip in a star’s brightness consistent with a single Jupiter-sized planet passing in front of, or transiting, it. To confirm the nature of these fluctuations and eliminate other possible causes, a team of astronomers used two instruments on the WIYN 3.5-meter Telescope at the U.S. National Science Foundation Kitt Peak National Observatory (KPNO), a Program of NSF NOIRLab.

The team first utilized the NASA-funded NN-EXPLORE Exoplanet and Stellar Speckle Imager (NESSI) in a technique that helps to ‘freeze out’ atmospheric twinkling and eliminate any extraneous sources that might confuse the signal’s source. Then, using the NASA-funded NEID spectrograph, the team measured the radial velocity of TIC 241249530 b by carefully observing how its host star’s spectrum, or wavelengths of its emitted light, shifted as a result of the exoplanet orbiting it.

Arvind Gupta, NOIRLab postdoctoral researcher and lead author of the paper published in Nature, praised NESSI and NEID as being critical to the team’s efforts to characterize and confirm the exoplanet’s signal. “NESSI gave us a sharper view of the star than would have been possible otherwise, and NEID precisely measured the star’s spectrum to detect shifts in response to the orbiting exoplanet,” explained Gupta. Gupta particularly noted the unique flexibility of NEID’s observation-scheduling framework as it allows for swift adaptation of the team’s observing plan in response to new data.

“The WIYN telescope is playing a crucial role in helping us understand why the planets found in other solar systems can be so different from system to system,” said NSF's Chris Davis, program director for NSF NOIRLab. “The collaboration between NSF and NASA on the NN-EXPLORE program continues to yield impressive results in exoplanet research.”

Detailed analysis of the spectrum confirmed that the exoplanet is approximately five times more massive than Jupiter. The spectrum also revealed that the exoplanet is orbiting along an extremely eccentric, or stretched-out, path. The eccentricity of a planet’s orbit is measured on a scale from 0 to 1, with 0 being a perfectly circular orbit and 1 being highly elliptical. This exoplanet has an orbital eccentricity of 0.94, making it more eccentric than the orbit of any other exoplanet ever found via the transiting method [1]. For comparison, Pluto’s highly elliptical orbit around the Sun has an eccentricity of 0.25; Earth’s eccentricity is 0.02.

If this planet was part of our Solar System its orbit would stretch from its closest approach ten times closer to the Sun than Mercury all the way out to its most distant extent at Earth’s distance. This extreme orbit would cause temperatures on the planet to vary between that of a summer’s day to hot enough to melt titanium.

To add to the unusual nature of the exoplanet’s orbit, the team also found that it’s orbiting backwards, meaning in a direction opposite to the rotation of its host star. This is not something that astronomers see in most other exoplanets, nor in our own Solar System, and it helps inform the team’s interpretation of the exoplanet’s formation history.

The exoplanet’s unique orbital characteristics also hint at its future trajectory. It’s expected that its initial highly eccentric orbit and extremely close approach to its host star will ‘circularize’ the planet’s orbit, since tidal forces on the planet sap energy from the orbit and cause it to gradually shrink and circularize. Discovering this exoplanet before this migration has taken place is valuable as it lends crucial insight into how hot Jupiters form, stabilize, and evolve over time.

“While we can’t exactly press rewind and watch the process of planetary migration in real time, this exoplanet serves as a sort of snapshot of the migration process,” Gupta said. “Planets like this are incredibly rare and hard to find, and we hope it can help us unravel the hot Jupiter formation story.”

"We’re especially interested in what we can learn about the dynamics of this planet's atmosphere after it makes one of its scorchingly close passages to its star," said Jason Wright, Penn State professor of astronomy and astrophysics who supervised the project while Gupta was a doctoral student at the university. "Telescopes like NASA's James Webb Space Telescope have the sensitivity to probe the changes in the atmosphere of the newly discovered exoplanet as it undergoes rapid heating, so there is still much more for the team to learn about the exoplanet."

TIC 241249530 b is only the second exoplanet ever discovered to demonstrate the hot Jupiter pre-migration phase. Together, these two examples observationally affirm the idea that higher-mass gas giants evolve to become hot Jupiters as they migrate from highly eccentric orbits toward tighter, more circular orbits.

“Astronomers have been searching for exoplanets that are likely precursors to hot Jupiters, or that are intermediate products of the migration process, for more than two decades, so I was very surprised — and excited — to find one,” Gupta said. “It’s exactly what I was hoping to find.”

Source: NSF’s National Optical-Infrared Astronomy Research Laboratory (NOIRLab)/News

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Notes

[1] One exoplanet has been found with a higher eccentricity. HD 20782 b has an eccentricity of 0.956 but is not transiting, thus the orientation of its orbit compared to its host star cannot be determined. This emphasizes the importance of the discovery of TIC 241249530 b whose orbital characteristics could be determined thanks to its transiting its star.

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More information

This research was presented in a paper entitled “A hot Jupiter progenitor on a super-eccentric, retrograde orbit” to appear in Nature. DOI: 10.1038/s41586-024-07688-3

NSF NOIRLab (U.S. National Science Foundation National Optical-Infrared Astronomy Research Laboratory), the U.S. center for ground-based optical-infrared astronomy, operates the International Gemini Observatory (a facility of NSF, NRC–Canada, ANID–Chile, MCTIC–Brazil, MINCyT–Argentina, and KASI–Republic of Korea), Kitt Peak National Observatory (KPNO), Cerro Tololo Inter-American Observatory (CTIO), the Community Science and Data Center (CSDC), and Vera C. Rubin Observatory (operated in cooperation with the Department of Energy’s SLAC National Accelerator Laboratory). It is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with NSF and is headquartered in Tucson, Arizona. The astronomical community is honored to have the opportunity to conduct astronomical research on I’oligam Du’ag (Kitt Peak) in Arizona, on Maunakea in Hawai‘i, and on Cerro Tololo and Cerro Pachón in Chile. We recognize and acknowledge the very significant cultural role and reverence that these sites have to the Tohono O’odham Nation, to the Native Hawaiian community, and to the local communities in Chile, respectively.

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Links

• Research paper: A hot Jupiter progenitor on a super-eccentric, retrograde orbit
• Penn State University press release
• Massachusetts Institute of Technology press release
• Simons Foundation Flatiron Institute press release
• Instituto de Astrofísica de Canarias (IAC) press release
• Photos of WIYN 3.5-meter Telescope
• Videos of the WIYN 3.5-meter Telescope
• Check out other NOIRLab Science Releases

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Contacts:

Arvind Gupta
Postdoctoral Fellow
NSF NOIRLab
Email: arvind.gupta@noirlab.edu

Josie Fenske
Jr. Public Information Officer
NSF NOIRLab
Email: josie.fenske@noirlab.edu

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NASA's Webb Investigates Eternal Sunrises, Sunsets on Distant World

Hot Gas Giant Exoplanet WASP-39 b (Artist’s Concept)
Credits: Illustration: NASA, ESA, CSA, Ralf Crawford (STScI)

Hot Gas Giant Exoplanet WASP-39 b Transit Light Curve (NIRSpec)
Credits: Illustration: NASA, ESA, CSA, Ralf Crawford (STScI)

Hot Gas Giant Exoplanet WASP-39 b Transmission Spectrum (NIRSpec)
Credits: Illustration: NASA, ESA, CSA, Ralf Crawford (STScI)

Images

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Researchers using NASA’s James Webb Space Telescope have finally confirmed what models have previously predicted: An exoplanet has differences between its eternal morning and eternal evening atmosphere. WASP-39 b, a giant planet with a diameter 1.3 times greater than Jupiter, but similar mass to Saturn orbits a star about 700 light-years away from Earth, is tidally locked to its parent star. This means it has a constant dayside and a constant nightside—one side of the planet is always exposed to its star, while the other is always shrouded in darkness.

Using Webb’s NIRSpec (Near-Infrared Spectrograph), astronomers confirmed a temperature difference between the eternal morning and eternal evening on WASP-39 b, with the evening appearing hotter by roughly 300 Fahrenheit degrees (about 200 Celsius degrees). They also found evidence for different cloud cover, with the forever morning portion of the planet being likely cloudier than the evening.

Astronomers analyzed the 2- to 5-micron transmission spectrum of WASP-39 b, a technique that studies the exoplanet’s terminator, the boundary that separates the planet’s dayside and nightside. A transmission spectrum is made by comparing starlight filtered through a planet’s atmosphere as it moves in front of the star, to the unfiltered starlight detected when the planet is beside the star. When making that comparison, researchers can get information about the temperature, composition, and other properties of the planet’s atmosphere.

“WASP-39 b has become a sort of benchmark planet in studying the atmosphere of exoplanets with Webb,” said Néstor Espinoza, an exoplanet researcher at the Space Telescope Science Institute and lead author on the study. “It has an inflated, puffy atmosphere, so the signal coming from starlight filtered through the planet’s atmosphere is quite strong.”

Previously published Webb spectra of WASP-39b’s atmosphere, which revealed the presence of carbon dioxide, sulfur dioxide, water vapor, and sodium, represent the entire day/night boundary – there was no detailed attempt to differentiate between one side and the other.

Now, the new analysis builds two different spectra from the terminator region, essentially splitting the day/night boundary into two semicircles, one from the evening, and the other from the morning. Data reveals the evening as significantly hotter, a searing 1,450 degrees Fahrenheit (800 degrees Celsius), and the morning a relatively cooler 1,150 degrees Fahrenheit (600 degrees Celsius).

“It’s really stunning that we are able to parse this small difference out, and it’s only possible due Webb’s sensitivity across near-infrared wavelengths and its extremely stable photometric sensors,” said Espinoza. “Any tiny movement in the instrument or with the observatory while collecting data would have severely limited our ability to make this detection. It must be extraordinarily precise, and Webb is just that.”

Extensive modeling of the data obtained also allows researchers to investigate the structure of WASP-39 b’s atmosphere, the cloud cover, and why the evening is hotter. While future work by the team will study how the cloud cover may affect temperature, and vice versa, astronomers confirmed gas circulation around the planet as the main culprit of the temperature difference on WASP-39 b.

On a highly irradiated exoplanet like WASP-39 b that orbits relatively close to its star, researchers generally expect the gas to be moving as the planet rotates around its star: Hotter gas from the dayside should move through the evening to the nightside via a powerful equatorial jet stream. Since the temperature difference is so extreme, the air pressure difference would also be significant, which in turn would cause high wind speeds.

Using General Circulation Models, 3-dimensional models similar to the ones used to predict weather patterns on Earth, researchers found that on WASP-39 b the prevailing winds are likely moving from the night side across the morning terminator, around the dayside, across the evening terminator and then around the nightside. As a result, the morning side of the terminator is cooler than the evening side. In other words, the morning side gets slammed with winds of air that have been cooled on the nightside, while the evening is hit by winds of air heated on the dayside. Research suggests the wind speeds on WASP-39 b can reach thousands of miles an hour!

“This analysis is also particularly interesting because you’re getting 3D information on the planet that you weren’t getting before,” added Espinoza. “Because we can tell that the evening edge is hotter, that means it’s a little puffier. So, theoretically, there is a small swell at the terminator approaching the nightside of the planet.”

The team’s results have been published in Nature.

The researchers will now look to use the same method of analysis to study atmospheric differences of other tidally locked hot Jupiters, as part of Webb Cycle 2 General Observers Program 3969.

WASP-39 b was among the first targets analyzed by Webb as it began regular science operations in 2022. The data in this study was collected under Early Release Science program 1366, designed to help scientists quickly learn how to use the telescope’s instruments and realize its full science potential.

The James Webb Space Telescope is the world's premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).

Source: NASA's James Webb Space Telescope/News

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Space Telescope Science Institute, Baltimore, Maryland

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Related Links and Documents

• Webb's Impact on Exoplanet Research

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To Inspiral or Not to Inspiral

Illustration of the exoplanet WASP-12 b
Credit:NASA/JPL-Caltech

 

Title: Doomed Worlds I: No New Evidence for Orbital Decay in a Long-Term Survey of 43 Ultra-Hot Jupiters
Authors: Elisabeth R. Adams et al.
First Author’s Institution: Planetary Science Institute
Status: Accepted to PSJ

Ultra-hot Jupiters: Fleeting Beauties?

Ultra-hot Jupiters are gas giants orbiting close to their stars, with orbital periods less than roughly three days. Because these planets are large and close to their stars, they produce large signals, making them promising targets for detection and characterization. But, you know what they say: all good things must come to an end. These planets are expected to experience large tidal effects from their stars, resulting in a loss of angular momentum, orbital decay, and, eventually, the star engulfing the planet.

Several lines of evidence support the picture that ultra-hot Jupiters are subject to orbital decay over long timescales. For instance, stars hosting hot Jupiters tend to be younger than the average exoplanet host star, and ultra-hot Jupiters are rarer around older host stars. A recent research article even reports a direct detection of a planetary engulfment event from the sudden, short-lived increase in brightness of a faint star. While this evidence paints a compelling picture, it is difficult to estimate how quickly we expect ultra-hot Jupiters to experience orbital decay given theoretical uncertainty in stellar tidal effects.

Keeping Time: Working Hard or Hardly Working?

Because we expect orbital decay to occur and we know of thousands of transiting exoplanets, some of which have been observed for decades, several teams have searched for orbital decay and found two promising detections: WASP-12 b and Kepler-1658 b. Searching for orbital decay relies on the detection of transit-timing variations. This is when a planet passes in front of its star along our line of sight earlier or later than expected. There are many sources of transit-timing variations in addition to orbital decay, including precession, perturbations from companion planets or stars, or acceleration of the host star toward Earth.

Let’s say we observe the transit of a planet at time t = 0 and know its period, P. We expect to observe transits at time P, 2P, 3P, etc. In the case of orbital decay, the period of the planet is getting shorter as time goes on, meaning we need to factor in an additional quadratic term encoding the rate at which the period shrinks. Then, to detect a statistically significant signal of orbital decay, we need to show that the quadratic model fits the data better than the constant-period linear model. The authors of today’s article attempt to do exactly this but with an impressive level of care and attention to detail.

This science depends upon accurate and precise measurements of transit times for each planet in the authors’ sample, most of which have been observed by several teams with various instruments and methodologies over years or decades. Moreover, each transit time must be reported in one unified timing system (click here for more info on one of the most common timing systems). Not every transit observation properly identifies its timing system or accurately converts between timing systems, meaning any historical inaccuracies complicate such studies.

Statistical Methods: Comparing Models

The authors of today’s article compile transit times for 43 ultra-hot Jupiters and take new transit data for six of those planets to extend the temporal baseline of observations. To assess whether the linear (constant period) or quadratic (changing period) model fits the data better, the authors use the Bayesian information criterion (BIC), a model selection criterion that awards a good fit but penalizes additional parameters to avoid overfitting. The authors calculate the difference in the BIC (ΔBIC) between the linear and quadratic models, with a larger ΔBIC suggesting the quadratic model is preferred.

The authors additionally perform a variety of steps to ensure the quality of the data. They perform omit-one tests, where individual transit times are removed from the analysis and flagged if they alter the ΔBIC result by more than 25%. This step is essential since one transit time recorded inaccurately or in the wrong system could result in a spurious detection of orbital decay. The authors additionally perform a “rescaling test,” where the error bars are scaled up to account for unrealistically small error bars in reported transit times.

Results

As shown in Figure 1, four planets out of the sample of 43 had a ΔBIC above the detection threshold, including WASP-12 b, which had been found previously to show orbital decay. The authors measure WASP-12 b’s period to be shrinking by 30 milliseconds per year, matching previously reported values. The planets WASP-121 b and WASP-46 b show tentative period increases, but these results are highly dependent on one or a few data points, warranting further observations. The planet TrES-1 b has prior tentative claims of its period decreasing, and the authors find a tentative period decrease of 18 milliseconds per year. However, this rate of period shrinkage suggests stellar tidal effects that would differ greatly from theoretical predictions, perhaps suggesting a cause of period decrease other than orbital decay.

Figure 1: The value of each planet’s ΔBIC shown relative to a threshold of ΔBIC = 30 (top), zoomed in results (middle), and rescaled results (bottom) with scaled up uncertainties, indicating only WASP-12 b definitively shows signs of orbital decay. Credit: Adams et al. 2024

Only one planet, WASP-12 b, was found to have a clear period decrease after rescaling error bars, as shown in the bottom panel of Figure 1. The authors predict that if the orbits of the other planets in the sample were decaying as rapidly as the orbit of WASP-12 b, they could have found significant detections of period decrease in roughly half the sample. There is thus no evidence that orbital decay is common among ultra-hot Jupiters, which is possibly confounding considering the other lines of evidence that suggest ultra-hot Jupiters are subject to decay. Though patience is required, as time goes on, it will be possible to search for orbital decay around more planets at higher precision, helping us ascertain the ultimate fate of close-in planets.

Original astrobite edited by Ivey Davis.

Source: American Astronomical Society (AAS-Nova)

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About the author, Kylee Carden:

I am a first-year PhD student at The Ohio State University, where I am an observer of planets outside the solar system. I’m involved with the transiting exoplanet survey of the upcoming Roman Space Telescope and working with high-resolution spectroscopic observations of exoplanet atmospheres. I am a huge fan of my cat Piccadilly, cycling, and visiting underappreciated tourist sites.

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Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

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NASA's Hubble Observes Exoplanet Atmosphere Changing Over 3 Years

WASP-121 b (Artist's Concept)
Credits: Illusration: NASA, ESA, Quentin Changeat (ESA/STScI), Mahdi Zamani (ESA/Hubble)

Temperature Forecast for Exoplanet WASP-121 b (Tylos)
Credits: Visualization: NASA, ESA, Quentin Changeat (ESA/STScI), Mahdi Zamani (ESA/Hubble)

Exoplanet WASP-121 b (Tylos) Simulated Weather Patterns
Credits: Visualization: NASA, ESA, Quentin Changeat (ESA/STScI), Mahdi Zamani (ESA/Hubble)

Release Images | Release Videos

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By combining several years of observations from NASA's Hubble Space Telescope along with conducting computer modelling, astronomers have found evidence for massive cyclones and other dynamic weather activity swirling on a hot, Jupiter-sized planet 880 light-years away.

The planet, called WASP-121 b, is not habitable. But this result is an important early step in studying weather patterns on distant worlds, and perhaps eventually finding potentially habitable exoplanets with stable, long-term climates.

For the past few decades, detailed telescopic and spacecraft observations of neighboring planets in our solar system show that their turbulent atmospheres are not static but constantly changing, just like weather on Earth. This variability should also apply to planets around other stars, too. But it takes lots of detailed observing and computational modelling to actually measure such changes.

To make the discovery, an international team of astronomers assembled and reprocessed Hubble observations of WASP-121 b taken in 2016, 2018 and 2019.

They found that the planet has a dynamic atmosphere, changing over time. The team used sophisticated modelling techniques to demonstrate that these dramatic temporal variations could be explained by weather patterns in the exoplanet's atmosphere.

The team found that WASP-121 b's atmosphere shows notable differences between observations. Most dramatically, there could be massive weather fronts, storms, and massive cyclones that are repeatedly created and destroyed due to the large temperature difference between the star-facing side and dark side of the exoplanet. They also detected an apparent offset between the exoplanet's hottest region and the point on the planet closest to the star, as well as variability in the chemical composition of the exoplanet's atmosphere (as measured via spectroscopy).

The team reached these conclusions by using computational models to help explain observed changes in the exoplanet's atmosphere. "The remarkable details of our exoplanet atmosphere simulations allows us to accurately model the weather on ultra-hot planets like WASP-121 b," explained Jack Skinner, a postdoctoral fellow at the California Institute of Technology in Pasadena, California, and co-leader of this study. "Here we make a significant step forward by combining observational constraints with atmosphere simulations to understand the time-varying weather on these planets."

"This is a hugely exciting result as we move forward for observing weather patterns on exoplanets," said one of the principal investigators of the team, Quentin Changeat, a European Space Agency Research Fellow at the Space Telescope Science Institute in Baltimore, Maryland. "Studying exoplanets' weather is vital to understanding the complexity of exoplanet atmospheres on other worlds, especially in the search for exoplanets with habitable conditions."

WASP-121 b is so close to its parent star that the orbital period is only 1.27 days. This close proximity means that the planet is tidally locked so that the same hemisphere always faces the star, in the same way that our Moon always has the same side pointed at Earth. Daytime temperatures approach 3,450 degrees Fahrenheit (2,150 degrees Kelvin) on the star-facing side of the planet.

The team used four sets of Hubble archival observations of WASP-121 b. The complete data-set included observations of WASP-121 b transiting in front of its star (taken in June 2016); WASP-121 b passing behind its star, also known as a secondary eclipse (taken in November 2016); and the brightness of WASP-121 b as a function of its phase angle to the star (the varying amount of light received at Earth from an exoplanet as it orbits its parent star, similar to our Moon's phase-cycle). These data were taken in March 2018 and February 2019, respectively.

"The assembled data-set represents a significant amount of observing time for a single planet and is currently the only consistent set of such repeated observations," said Changeat. The information that we extracted from those observations was used to infer the chemistry, temperature, and clouds of the atmosphere of WASP-121 b at different times. This provided us with an exquisite picture of the planet changing over time."

Hubble's unique capabilities also are evident in the broad expanse of science programs it will enable through its Cycle 31 observations, which began on December 1. About two-thirds of Hubble's time will be devoted to imaging studies, while the remainder is allotted to spectroscopy studies, like those used for WASP-121 b. More details about Cycle 31 science are in a recent announcement.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble and Webb science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.

Source: HubbleSite/News

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Ray Villard
Space Telescope Science Institute, Baltimore, Maryland

Bethany Downer
ESA/Hubble

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Quentin Changeat
ESA/STScI

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Related Links and Documents

• Science Paper: The science paper by Quentin Changeat et al. (arXiv.org), PDF (23.45 MB)
• NASA's Hubble Portal
• ESA/Hubble's Release

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NASA’s Webb Detects Tiny Quartz Crystals in Clouds of Hot Gas Giant

Exoplanet WASP-17 b (Artist's Concept)
Credits: Artwork: NASA, ESA, CSA, Ralf Crawford (STScI)

Exoplanet WASP-17 b (MIRI Transmission Spectrum)
Credits: Illustration: NASA, ESA, CSA, Ralf Crawford (STScI)
Science: David Grant (University of Bristol), Hannah R. Wakeford (University of Bristol), Nikole Lewis (Cornell University)

Release Images

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Researchers using NASA’s James Webb Space Telescope have detected evidence for quartz nanocrystals in the high-altitude clouds of WASP-17 b, a hot Jupiter exoplanet 1,300 light-years from Earth. The detection, which was uniquely possible with MIRI (Webb’s Mid-Infrared Instrument), marks the first time that silica (SiO2) particles have been spotted in an exoplanet atmosphere.

“We were thrilled!” said David Grant, a researcher at the University of Bristol in the UK and first author on a paper being published today in the Astrophysical Journal Letters . “We knew from Hubble observations that there must be aerosols – tiny particles making up clouds or haze – in WASP-17 b’s atmosphere, but we didn’t expect them to be made of quartz.”

Silicates (minerals rich in silicon and oxygen) make up the bulk of Earth and the Moon as well as other rocky objects in our solar system, and are extremely common across the galaxy. But the silicate grains previously detected in the atmospheres of exoplanets and brown dwarfs appear to be made of magnesium-rich silicates like olivine and pyroxene, not quartz alone – which is pure SiO2.

The result from this team, which also includes researchers from NASA’s Ames Research Center and NASA’s Goddard Space Flight Center, puts a new spin on our understanding of how exoplanet clouds form and evolve. “We fully expected to see magnesium silicates,” said co-author Hannah Wakeford, also from the University of Bristol. “But what we’re seeing instead are likely the building blocks of those, the tiny ‘seed’ particles needed to form the larger silicate grains we detect in cooler exoplanets and brown dwarfs.”

Detecting Subtle Variations

With a volume more than seven times that of Jupiter and a mass less than one-half of Jupiter, WASP-17 b is one of the largest and puffiest known exoplanets. This, along with its short orbital period of just 3.7 Earth-days, makes the planet ideal for transmission spectroscopy: a technique that involves measuring the filtering and scattering effects of a planet’s atmosphere on starlight.

Webb observed the WASP-17 system for nearly 10 hours, collecting more than 1,275 brightness measurements of 5- to 12-micron mid-infrared light as the planet crossed its star. By subtracting the brightness of individual wavelengths of light that reached the telescope when the planet was in front of the star from those of the star on its own, the team was able to calculate the amount of each wavelength blocked by the planet’s atmosphere.

What emerged was an unexpected “bump” at 8.6 microns, a feature that would not be expected if the clouds were made of magnesium silicates or other possible high-temperature aerosols like aluminum oxide, but which makes perfect sense if they are made of quartz.

Crystals, Clouds, and Winds

While these crystals are probably similar in shape to the pointy hexagonal prisms found in geodes and gem shops on Earth, each one is only about 10 nanometers across – one-millionth of one centimeter.

“Hubble data actually played a key role in constraining the size of these particles,” explained co-author Nikole Lewis of Cornell University, who leads the Webb Guaranteed Time Observation (GTO) program designed to help build a three-dimensional view of a hot Jupiter atmosphere. “We know there is silica from Webb’s MIRI data alone, but we needed the visible and near-infrared observations from Hubble for context, to figure out how large the crystals are.”

Unlike mineral particles found in clouds on Earth, the quartz crystals detected in the clouds of WASP-17 b are not swept up from a rocky surface. Instead, they originate in the atmosphere itself. “WASP-17 b is extremely hot – around 2,700 degrees Fahrenheit (1,500 degrees Celsius) – and the pressure where the quartz crystals form high in the atmosphere is only about one-thousandth of what we experience on Earth’s surface,” explained Grant. “In these conditions, solid crystals can form directly from gas, without going through a liquid phase first.”

Understanding what the clouds are made of is crucial for understanding the planet as a whole. Hot Jupiters like WASP-17 b are made primarily of hydrogen and helium, with small amounts of other gases like water vapor (H2O) and carbon dioxide (CO2). “If we only consider the oxygen that is in these gases, and neglect to include all of the oxygen locked up in minerals like quartz (SiO2), we will significantly underestimate the total abundance,” explained Wakeford. “These beautiful silica crystals tell us about the inventory of different materials and how they all come together to shape the environment of this planet.”

Exactly how much quartz there is, and how pervasive the clouds are, is hard to determine. “The clouds are likely present along the day/night transition (the terminator), which is the region that our observations probe,” said Grant. Given that the planet is tidally locked with a very hot day side and cooler night side, it is likely that the clouds circulate around the planet, but vaporize when they reach the hotter day side. “The winds could be moving these tiny glassy particles around at thousands of miles per hour.” WASP-17 b is one of three planets targeted by the JWST Telescope Scientist Team ’s Deep Reconnaissance of Exoplanet Atmospheres using Multi-instrument Spectroscopy (DREAMS) investigations, which are designed to gather a comprehensive set of observations of one representative from each key class of exoplanets: a hot Jupiter, a warm Neptune, and a temperate rocky planet. The MIRI observations of hot Jupiter WASP-17 b were made as part of GTO program 1353.

The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and the Canadian Space Agency.

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About This Release

Credits:

Media Contact:

Margaret W. Carruthers
Space Telescope Science Institute, Baltimore, Maryland

Christine Pulliam

Space Telescope Science Institute, Baltimore, Maryland

Science:

David Grant (University of Bristol), Hannah R. Wakeford (University of Bristol), Nikole Lewis (Cornell University)

Permissions: Content Use Policy

Contact Us:

Direct inquiries to the News Team.

Related Links and Documents

• Science Paper: The science paper by D. Grant et al., PDF (1.85 MB)
• JWST Telescope Scientist Team

Source:  NASA's James Webb Space Telescope/News

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Hot Jupiters: Planets Can Be Anti-Aging Formula for Stars

An artist’s illustration shows a gas giant planet (lower right) closely orbiting its host star (left), with another star in the distance (upper right). The two stars are themselves in orbit with each other. Credit: Illustration: NASA/CXC/M.Weiss. X-ray: NASA/CXC/Potsdam Univ./N. Ilic et al.

JPEG (869 kb) - Large JPEG (7.3 MB) - Tiff (37.1 MB) - More Images 

A Tour of Hot Jupiters - More Videos

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An artist’s illustration shows a gas giant planet (lower right) closely orbiting its host star (left), with another star in the distance (upper right). The two stars are themselves in orbit with each other. As explained in our latest press release, a team of scientists used NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton to test whether such exoplanets (known as “hot Jupiters”) affect their host star in comparison to the star that does not have one. The results show that these exoplanets can make their host star act younger than it is by causing the star to spin more quickly than it would without such a planet.

The double-star (or “binary”) system in the illustration is one of dozens that astronomers studied using Chandra and XMM-Newton to look for the effects of hot Jupiters on their host stars. A hot Jupiter can potentially influence its host star by tidal forces, causing the star to spin more quickly than if it did not have such a planet. This more rapid rotation can make the host star more active and produce more X-rays, making it appear younger than it really is.

The stars in binary systems form at the same time. The separation between the stars studied by the team, however, is much too large for them to influence each other or for the hot Jupiter to affect the other star. Studying such systems eliminates the challenge that astronomers face in precisely determining the age of individual stars, allowing them to avoid trying to account for the natural decrease in spin rate and activity that occur as stars age. In this new study, the companion star acts as a control for the star with the hot Jupiter.

The team measured the amounts of X-rays produced by the stars to determine how “young” they are acting by studying almost three dozen systems in X-rays (the final sample contained 10 systems observed by Chandra and 6 by ESA’s XMM-Newton, with several observed by both telescopes). The study revealed that the stars with hot Jupiters tended to be brighter in X-rays and therefore more active than their companion stars without hot Jupiters. In the illustration the more active star with the hot Jupiter shows flaring activity and the distant companion star does not. The illustration also shows some of the exoplanet’s atmosphere being blasted away by radiation from its host star.

Separate graphics show Chandra data for two of the systems where one star is orbited by a hot Jupiter (HD189733 and WASP-77) and two with neither star orbited by a hot Jupiter (HD46375 and HD109749). In the latter two systems one of the stars hosts a planet that is more distant or has a lower mass than a hot Jupiter. The stars with hot Jupiters are clearly brighter than their companion stars, including a non-detection for the companion in WASP-77. The stars without hot Jupiters have comparable brightness to their companions. This dependence of a star's X-ray brightness on the type of planet it hosts shows that hot Jupiters make their host stars act younger than they really are.

Labeled X-ray image of the HD189733 and WASP-77 systems
Credit: NASA/CXC/Potsdam Univ./N. Ilic et al

Labeled X-ray image of the HD46375 and HD109749 systems
Credit: NASA/CXC/Potsdam Univ./N. Ilic et al

A paper describing these results appeared in the July 2022 issue of the Monthly Notices of the Royal Astronomical Society, and appears online. The authors are Nikoleta Ilic (Leibniz Institute for Astrophysics Potsdam (AIP) in Germany), Katja Poppenhaeger (AIP), and S. Marzieh Hosseini (AIP). NASA's Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.

Source: NASA’s Chandra X-ray Observatory

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Fast Facts for one of 3 dozen objects in the survey, HD 189733:

Scale: Image of HD 189733 is about 30 arcsec (0.00929 light-years or 3.39 light-days) across.
Category: Normal Stars & Star Clusters
Coordinates (J2000): RA 20h 00m 43.7s | Dec +22° 42´ 39.1"
Constellation: Vulpecula
Observation Date: July 5, 2011
Observation Time: 5 hours 21 minutes
Obs. ID: 12340
Instrument: ACIS
References: Ilic, N., et al., 2022, MNRAS, 513, 4380. arXiv:2203.13637
Color Code: X-ray: purple
Distance Estimate: About 64.5 light-years

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Hubble observations used to answer key exoplanet questions

PR Image heic2206a

Artist’s Impression of 25 Hot Jupiters

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Videos

PR Video heic2206a

Hubble Helps Answer Key Exoplanet Questions 

 

PR Video hubblecast121b

Hubblecast 121: What can we learn from exoplanet transits?

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Archival observations of 25 hot Jupiters by the NASA/ESA Hubble Space Telescope have been analysed by an international team of astronomers, enabling them to answer five open questions important to our understanding of exoplanet atmospheres. Amongst other findings, the team found that the presence of metal oxides and hydrides in the hottest exoplanet atmospheres was clearly correlated with the atmospheres' being thermally inverted.

The field of exoplanet science has long since shifted its focus from just detection onto characterisation [1], although characterisation remains extremely challenging. Thus far, the majority of research into characterisation has been directed towards modelling, or studies focusing on one or a few exoplanets. This new work, led by researchers based at University College London (UCL), used the largest amount of archival data ever examined in a single exoplanet atmosphere survey to analyse the atmospheres of 25 exoplanets. The majority of the data came from observations taken with the NASA/ESA Hubble Space Telescope. The lead author, Quentin Changeat, explains: "Hubble enabled the in-depth characterisation of 25 exoplanets, and the amount of information we learnt about their chemistry and formation — thanks to a decade of intense observing campaigns — is incredible."

The science team sought to find answers to five open questions about exoplanet atmospheres — an ambitious goal that they succeeded in reaching.  Their questions probed what H– [2] and certain metals [3] can tell us about the chemistry and circulation of exoplanet atmospheres, and about planet formation. They chose to investigate a wide range of hot Jupiters [4], with the intention of identifying trends within their sample population that might provide insight into exoplanet atmospheres more generally. The study’s co-leader, Billy Edwards of UCL and the Commissariat à l'énergie atomique et aux énergies alternatives (CEA) said: "Our paper marks a turning point for the field: we are now moving from the characterisation of individual exoplanet atmospheres to the characterisation of atmospheric populations."

In order to investigate their sample of 25 exoplanets, the team reanalysed an enormous amount of archival data [5], consisting of 600 hours of Hubble observations, which they complemented with more than 400 hours of observations from the Spitzer Space Telescope. Their data contained eclipses for all 25 exoplanets, and transits for 17 of them. An eclipse occurs when an exoplanet passes behind its star as seen from Earth, and a transit occurs when a planet passes in front of its star. Eclipse and transit data can both provide crucial information about an exoplanet’s atmosphere.

The large-scale survey yielded results, with the team able to identify some clear trends and correlations between the exoplanets’ atmospheric constitutions and observed behaviour. Some of their key findings related to the presence or absence of thermal inversions [6] in the atmospheres of their exoplanet sample. They found that almost all the exoplanets with a thermally inverted atmosphere were extremely hot, with temperatures over 2000 Kelvins. Importantly, this is sufficiently hot that the metallic species TiO (titanium oxide), VO (vanadium oxide) and FeH (iron hydride) are stable in an atmosphere. Of the exoplanets displaying thermal inversions, almost all of them were found to have H–, TiO, VO or FeH in their atmospheres.

It is always challenging to draw inferences from such results, because correlation does not necessarily equal causation. However, the team were able to propose a compelling argument for why the presence of H–, TiO, VO or FeH might lead to a thermal inversion — namely that all these metallic species are very efficient absorbers of stellar light. It might be that exoplanet atmospheres hot enough to sustain these species tend to be thermally inverted because they then absorb so much stellar light that their upper atmospheres heat up even more. Conversely, the team also found that colder hot Jupiters (with temperatures less than 2000 Kelvins, and thus without H–, TiO, VO or FeH in their atmospheres) almost never had thermally inverted atmospheres.

A significant aspect of this research was that the team were able to use a large sample of exoplanets and an extremely large amount of data to determine trends, which can be used to predict behaviour in other exoplanets. This is extremely useful, because it provides insight into how planets may form, and also because it allows other astronomers to more effectively plan future observations. Conversely, if a paper studies a single exoplanet in great detail, whilst that is valuable it is much harder to extrapolate trends from. An improved understanding of exoplanet populations could also bring us closer to solving open mysteries about our own Solar System. As Changeat says: "Many issues such as the origins of the water on Earth, the formation of the Moon, and the different evolutionary histories of Earth and Mars, are still unsolved despite our ability to obtain in-situ measurements. Large exoplanet population studies, such as the one we present here, aim at understanding those general processes."

 

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Notes

[1] Exoplanet characterisation involves investigating the physical properties (such as radius) and chemical properties (such as atmospheric composition) of an exoplanet. It is crucial both for better understanding planet formation and evolution, and for determining whether complex processes — such as the evolution and maintenance of life — could be possible on an exoplanet.

[2] H– is a negative hydrogen ion that has been formed by the dissociation of a molecule such as H2 (hydrogen) or H2O (water). These molecules dissociate at very high temperatures (over 2500 Kelvins, or 2227 °C).

[3] In astronomy, a ‘metal’ is defined as any element with more protons in its nucleus than hydrogen or helium (which have one and two protons respectively). Thus, ‘metallicity’ is a measure of how many elements or molecules are present that are not hydrogen or helium.

[4] Hot Jupiters are an informal class of exoplanets with short-period orbits (orbiting their parent star in roughly ten days or less), and large, inflated gassy atmospheres. They are of particular interest because i) they are relatively easy to detect and ii) there is no hot Jupiter within our Solar System, so we have to look to exoplanets to study them.

[5] Archival data are data that the team did not collect specifically for this research, but were collected by another team(s) for a different initial purpose and are now publicly available. The reanalysis and repurposing of archival data can be an extremely cost and environmentally effective way of getting new results.

[6] A thermal inversion is a natural phenomenon where a planet’s or exoplanet’s atmosphere does not steadily cool with increased altitude, but instead reverses from cooling to heating at a higher altitude. Thermal inversions are thought to occur because of the presence of certain metallic species in the atmosphere. For example, the Earth’s atmosphere has a thermal inversion that is due to the presence of ozone (O3).

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More Information

The Hubble Space Telescope is a project of international cooperation between ESA and NASA.

These results have been published in the Astrophysical Journal Supplement Series.

The international team of astronomers in this study consists of: Q. Changeat (University College London, United Kingdom [UCL]), B. Edwards (UCL and Commissariat à l'énergie atomique et aux énergies alternatives [CEA], Université Paris-Saclay, Université de Paris, France), A. F. Al-Refaie (UCL), A. Tsiaras (UCL and Osservatorio Astrofisico di Arcetri, Firenze, Italy), J. W. Skinner (Queen Mary University of London, United Kingdom), J. Y-K. Cho (Center for Computational Astrophysics, Flatiron Institute, New York, USA), K. H. Yip (UCL), L. Anisman (UCL), M. Ikoma (National Astronomical Observatory of Japan, Tokyo, Japan and The Graduate University for Advanced Studies [SOKENDAI], Tokyo, Japan), M. F. Bieger (University of Exeter, United Kingdom), O. Venot (Université de Paris and Université Paris Est Creteil, France), S. Shibata (University of Zurich, Switzerland), I. P. Waldmann (UCL), G. Tinetti (UCL).

Image credit: ESA/Hubble, N. Bartmann

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Links 

• Images of Hubble
• Science paper
• UCL press release
• NVIDIA blog
• ESA's New and Future Exoplanet Missions

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Contacts

Quentin Changeat

University College London
United Kingdom
Email: quentin.changeat.18@ucl.ac.uk

Bethany Downer
ESA/Hubble Chief Science Communications Officer
Email: Bethany.Downer@esahubble.org

Source: ESA/Hubble/News

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Astronomers Provide 'Field Guide' to Exoplanets Known as Hot Jupiters By Daniel Stolte, University Communications

This artist’s impression shows a hot Jupiter planet orbiting close to one of the stars in the rich old star cluster Messier 67, located between 2,500 and 3,000 light-years from Earth in the constellation of Cancer (The Crab).ESO/L. Calçada

The turbulent atmosphere of a hot, gaseous planet known as HD 80606b is shown in this simulation based on data from NASA's Spitzer Space Telescope. The planet spends most of its time far away from its star, but every 111 days, it swings extremely close to the star, experiencing a massive burst of heat.NASA/JPL-CalTech

By combining Hubble Space Telescope observations with theoretical models, a team of astronomers has gained insights into the chemical and physical makeup of a variety of exoplanets known as hot Jupiters. The findings provide a new and improved "field guide" for this group of planets and inform ideas about planet formation in general.

Hot Jupiters – giant gas planets that race around their host stars in extremely tight orbits – have become a little bit less mysterious thanks to a new study combining theoretical modeling with observations by the Hubble Space Telescope.

While previous studies mostly focused on individual worlds classified as "hot Jupiters" due to their superficial similarity to the gas giant in our own solar system, the new study is the first to look at a broader population of the strange worlds. Published in Nature Astronomy, the study, led by a University of Arizona researcher, provides astronomers with an unprecedented "field guide" to hot Jupiters and offers insight into planet formation in general.

Although astronomers think that only about 1 in 10 stars host an exoplanet in the hot Jupiter class, the peculiar planets make up a sizeable portion of exoplanets discovered to date, due to the fact that they are bigger and brighter than other types of exoplanets, such as rocky, more Earthlike planets or smaller, cooler gas planets. Ranging in size from about one-third the size of Jupiter to 10 Jupiter masses, all hot Jupiters orbit their host stars at an extremely close range, usually much closer than Mercury – the innermost planet in our solar system – is to the sun. A "year" on a typical hot Jupiter lasts hours, or at most a few days. For comparison, Mercury takes almost three months to complete a trip around the sun.

Because of their close orbits, most, if not all, hot Jupiters are thought to be locked in a high-speed embrace with their host stars, with one side eternally exposed to the star's radiation and the other shrouded in perpetual darkness. The surface of a typical hot Jupiter can get as hot as almost 5,000 degrees Fahrenheit, with "cooler" specimens reaching 1,400 degrees – hot enough to melt aluminum.

The research, which was led by Megan Mansfield, a NASA Sagan Fellow at the University of Arizona's Steward Observatory, used observations made with the Hubble Space Telescope that allowed the team to directly measure emission spectra from hot Jupiters, despite the fact that Hubble can't image any of these planets directly.

"These systems, these stars and their hot Jupiters, are too far away to resolve the individual star and its planet," Mansfield said. "All we can see is a point – the combined light source of the two."

Mansfield and her team used a method known as secondary eclipsing to tease out information from the observations that allowed them to peer deep into the planets' atmospheres and gain insights into their structure and chemical makeup. The technique involves repeated observations of the same system, catching the planet at various places in its orbit, including when it dips behind the star.

"We basically measure the combined light coming from the star and its planet and compare that measurement with what we see when the planet is hidden behind its star," Mansfield said. "This allows us to subtract the star's contribution and isolate the light emitted by the planet, even though we can't see it directly."

The eclipse data provided the researchers with insight into the thermal structure of the atmospheres of hot Jupiters and allowed them to construct individual profiles of temperatures and pressures for each one. The team then analyzed near-infrared light, which is a band of wavelengths just beyond the range humans can see, coming from each hot Jupiter system for so-called absorption features. Because each molecule or atom has its own specific absorption profile, like a fingerprint, looking at different wavelengths allows researchers to obtain information about the chemical makeup of hot Jupiters. For example, if water is present in the planet's atmosphere, it will absorb light at 1.4 microns, which falls into the range of wavelengths that Hubble can see very well.

"In a way, we use molecules to scan through the atmospheres on these hot Jupiters," Mansfield said. "We can use the spectrum we observe to get information on what the atmosphere is made of, and we can also get information on what the structure of the atmosphere looks like."

The team went a step further by quantifying the observational data and comparing it to models of the physical processes believed to be at work in the atmospheres of hot Jupiters. The two sets matched very well, confirming that many predictions about the planets' nature – based on theoretical work – appear to be correct, according to Mansfield, who said the findings are "exciting because they were anything but guaranteed."

The results suggest that all hot Jupiters, not just the 19 included in the study, are likely to contain similar sets of molecules, like water and carbon monoxide, along with smaller amounts of other molecules. The differences among individual planets should mostly amount to varying relative amounts of these molecules. The findings also revealed that the observed water absorption features varied slightly from one hot Jupiter to the next.

"Taken together, our results tell us there is a good chance we have the big picture items figured out that are happening in the chemistry of these planets," Mansfield said. "At the same time, each planet has its own chemical makeup, and that also influences what we see in our observations."

According to the authors, the results can be used to guide expectations of what astronomers might be able to see when looking at a hot Jupiter that hasn't been studied before. The launch of NASA's news flagship telescope, the James Webb Space Telescope, slated for Dec. 18, has exoplanet hunters excited because Webb can see in a much broader range of infrared light, and will allow a much more detailed look at exoplanets, including hot Jupiters.

"There is a lot that we still don't know about how planets form in general, and one of the ways we try to understand how that could happen is by looking at the atmospheres of these hot Jupiters and figuring out how they got to be where they are," Mansfield said. "With the Hubble data, we can look at trends by studying the water absorption, but when we are talking about the composition of the atmosphere as a whole, there are many other important molecules you want to look at, such as carbon monoxide and carbon dioxide, and JWST will give us a chance to actually observe those as well."

Resources for the Media 

Media contact:

Daniel Stolte
Science Writer, University Communications
stolte@arizona.edu
520-626-4402

Researcher contact:

Megan Mansfield
NASA Sagan Fellow, Steward Observatory
meganmansfield@arizona.edu

Source: The Arizona University/News

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