ASU astrophysicist helps discover that ultrahot planets have starlike atmospheres

August 9, 2018

Recent observations by NASA's Hubble and Spitzer space telescopes of ultrahot Jupiter-like planets have perplexed theorists. The spectra of these planets have suggested they have exotic — and improbable — compositions.

However, a new study just published by a research team that includes Arizona State University astrophysicist Michael Line, an assistant professor in ASU's School of Earth and Space Exploration, proposes an explanation — that these gas-rich planets have compositions that are basically normal, going by what is known about planet formation. What’s different about them is that the atmospheres on their daysides look more like the atmosphere of a star than a planet. 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, each illuminated to different degrees by its parent star. The images were made with a computer simulation being used to help scientists understand the atmospheres of these planets. Ultrahot Jupiters reflect almost no light, much like charcoal. However, their daysides have temperatures of between 3,600 F and 5,400 F, so they produce their own glow like a hot ember. The orange color in this simulated image thus comes from the planet's own heat. Image credit: NASA/JPL-Caltech/Vivien Parmentier/Aix-Marseille University (AMU) Download Full Image

"Interpreting the spectra of the hottest of these Jupiter-like planets has posed a thorny puzzle for researchers for years," Line said.

The biggest puzzle is why water vapor appears to be missing from these worlds' atmospheres, when it is abundant in similar but slightly cooler planets.

According to the new study, ultrahot Jupiters do in fact possess the ingredients for water (hydrogen and oxygen atoms). But due to the strong radiation on the planet's daysides, temperatures there go high enough that water molecules are completely torn apart.

With ultrahot Jupiters orbiting extremely close to their stars, one side of the planet faces the star perpetually, while the nightside is gripped by endless darkness.

Dayside temperatures reach between 3,600 to 5,400 degrees Fahrenheit (2,000 to 3,000 degrees Celsius), ranking ultrahot Jupiters among the hottest exoplanets known. And nightside temperatures are around 1,800 degrees Fahrenheit cooler.

Star-planet hybrids

Among the growing catalogue of planets outside our solar system — known as exoplanets — ultrahot Jupiters have stood out as a distinct class for about a decade.

"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 published in Astronomy and Astrophysics. "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, their nightsides are difficult for current instruments to probe.

Jupiter-like exoplanets are 99 percent molecular hydrogen and helium with smaller amounts of water and other molecules. But what their spectra show depends strongly on temperature. Warm-to-hot planets form clouds of minerals, while hotter planets make starlight-absorbing molecules of titanium oxide. Yet to understand ultrahot Jupiter spectra, the research team had to turn to processes more commonly found in stars. Image credit: Michael Line/ASU

The new paper proposes a model for what might be happening on both the illuminated and dark sides of these planets. The model is based largely on observations and analysis from three recently published studies, coauthored by Parmentier, Line, and others, that focus on three ultrahot Jupiters, WASP-103b, WASP-18b, and HAT-P-7b.

The new study suggests that fierce winds driven by heating may blow the torn-apart water molecules into the planets' cooler nightside hemispheres. There the atoms can recombine into molecules and condense into clouds, all before drifting back into the dayside to be ripped apart again.

Family resemblance?

Hot Jupiters were the first widely discovered kind of exoplanet, starting back in the mid-1990s. These are cooler cousins to ultrahot Jupiters, with dayside temperatures below 3,600 degrees Fahrenheit (2,000 Celsius).

Water has proven to be common in their atmospheres, and thus when ultrahot Jupiters began to be found, astronomers expected them to show water in their atmospheres as well. But water turned out to be missing on their easily observed daysides, which got theorists looking at alternative, even exotic, compositions.

One hypothesis for why water appeared absent in ultrahot Jupiters has been that these planets must have formed with very high levels of carbon instead of oxygen. Yet this idea could not explain the traces of water sometimes detected at the dayside-nightside boundary.

To break the logjam, the research team took a cue from well-established physical models of stellar atmospheres, as well as "failed stars," known as brown dwarfs, whose properties overlap somewhat with hot and ultrahot Jupiters.

"Unsatisfied with exteme compositions, we thought harder about the problem," Line said. "Then we realized that many earlier interpretations were missing some key physics and chemistry that happens at these ultrahot temperatures."

The team adapted a brown dwarf model developed by Mark Marley, one of the paper's co-authors 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," Parmentier said.

"Our role in this research has been to take the observed spectra of these planets and model their physics carefully," Line said. "This showed us how to produce the observed spectra using gases that are more likely to be present under the extreme conditions. These planets don't need exotic compositions or unusual pathways to make them."

Robert Burnham

Science writer, School of Earth and Space Exploration


NPR social media editor Lori Todd joins ASU Cronkite News

August 9, 2018

Lori Todd, the senior social media editor for NPR, is joining Arizona State University to lead the digital desk of Cronkite News, the student-operated, faculty-led news division of Arizona PBS at the Walter Cronkite School of Journalism and Mass Communication.

Todd will teach and guide advanced Cronkite students to create multimedia content that integrates text, video, images, statistical information and other content into usable, shareable products such as interactives, infographics and data visualizations for She will hold the faculty rank of professor of practice. NPR social media editor Lori Todd is the new director of the digital desk at Cronkite News. Download Full Image

At NPR, Todd worked across news and programming divisions to set social strategy, define best practices and develop training and experimentation on emerging platforms. She was responsible for developing NPR’s Social Leads, a cohort of 30-plus digital and audio producers responsible for the organization’s social media accounts.

“Lori’s expertise in digital audience strategy, design and cross-team collaboration are tremendous assets for our students,” said Assistant Dean Rebecca Blatt. “She has a deep commitment to journalistic values that we uphold as well as a passion for working with young journalists. We’re thrilled to have her joining the Cronkite School.”

Todd, who starts Aug. 13, launched her career in newspaper design at the Austin American-Statesman and the Miami Herald. She also served as a social media manager at Tribune Publishing in Florida before arriving at NPR. There, she managed social producers at the South Florida Sun Sentinel and the Orlando Sentinel, helping develop the Sun Sentinel’s first live interactive web series.

She also served as the community manager for the John S and James L. Knight Foundation and outreach coordinator for the public radio series “Under The Sun” on WLRN in Miami.

Todd said she is excited to do innovative work with Cronkite students. At NPR, she mentored interns, teaching them how to tell stories across platforms. With her students and team, she led a push to bring more diverse audiences into the fold at NPR.

Todd said she was attracted to the Cronkite School’s teaching hospital model of journalistic education in which students work and learn from faculty in a professional setting.

“The opportunity to work with students and shape newsrooms is going to be really exciting,” Todd said. “I am excited to join the Cronkite News team and help students get the courage and confidence they need to start a career in journalism.”