From University of California-Santa Cruz (US) : “Haziness of exoplanet atmospheres depends on properties of aerosol particles”

From University of California-Santa Cruz (US)

July 12, 2021
Tim Stephens
stephens@ucsc.edu

A laboratory study of haze particles produced under different conditions helps explain why some exoplanets may be obscured by hazy atmospheres.

1
Xinting Yu, a 51 Pegasi b Postdoctoral Fellow at UCSC, measured the properties of haze particles produced in the laboratory under conditions representative of exoplanet atmospheres. Photo courtesy of Heising-Simons Foundation.

2
Researchers measured the refractive indices at visible wavelengths (n) for haze samples created under a range of conditions. Image credit: Yu et al., Nature Astronomy, 2021)

Many exoplanets have opaque atmospheres obscured by clouds or hazes that make it hard for astronomers to characterize their chemical compositions. A new study shows that haze particles produced under different conditions have a wide range of properties that can determine how clear or hazy a planet’s atmosphere is likely to be.

Photochemical reactions in the atmospheres of temperate exoplanets lead to the formation of small organic haze particles. Large amounts of these photochemical hazes form in Earth’s atmosphere every day, yet our planet has relatively clear skies. The reason has to do with how easily haze particles are removed from the atmosphere by deposition processes.

“It’s not just haze production but also haze removal that determines how clear the atmosphere is,” said Xinting Yu, a postdoctoral fellow at UC Santa Cruz and lead author of the study, published July 12 in Nature Astronomy.

Yu and her colleagues measured the properties of haze particles produced in the laboratory under conditions representative of exoplanet atmospheres, including a range of gas compositions, temperatures, and energy sources. Coauthor Xi Zhang, assistant professor of Earth and planetary sciences at UC Santa Cruz, said laboratory experiments like this are essential for understanding haze formation and its impact on observations.

“We can’t bring haze samples back from exoplanets, so we have to try to mimic the atmospheric conditions in the laboratory,” he said.

According to Yu, haze removal depends on a critical material property of the particles called surface energy. “Surface energy describes how cohesive or ‘sticky’ the material is,” she said.

Sticky haze particles readily bond with each other when they collide, growing into larger particles that fall out of the atmosphere onto the surface of the planet (a process called dry deposition). They also make good condensation nuclei for cloud droplets and are easily removed by wet deposition. Hazes produced on Earth typically have high surface energy and are therefore ‘sticky’ and efficiently removed from the atmosphere.

Yu’s laboratory experiments show that the hazes produced in exoplanet atmospheres are highly diverse, with properties that depend on the conditions in which they are produced.

“Some of them are similar to the Earth haze, have high surface energy, and are easy to remove, leading to clear skies,” she said. “But some of them have very low surface energy, like a non-stick pan; they do not bond with other particles very well and remain as small particles hanging in the atmosphere for a long time.”

The study found that a critical factor is the temperature at which the haze particles are created. Hazes produced at around 400 Kelvin (260°F) tended to have the lowest surface energies, leading to less efficient removal and hazier atmospheres. This finding actually corresponds with observed trends, Yu said, noting that exoplanets at temperatures of 400 to 500 K tend to be the haziest.

Cooler planets located in the habitable zones of their host stars are more likely to have clear atmospheres, she said. “We may not have to worry about habitable exoplanets being too hazy for future observations, as hazes tend to have higher surface energies at lower temperatures,” Yu said. “So it is easy to remove these hazes, leaving relatively clear atmospheres.”

Astronomers are looking forward to having a powerful tool for characterizing exoplanet atmospheres with the upcoming James Webb Space Telescope (JWST). When an exoplanet transits across the face of its star, its atmosphere filters the light from the star, giving astronomers with a sensitive enough telescope (like JWST) an opportunity to identify the chemical components of the atmosphere using transmission spectroscopy.

A hazy atmosphere would interfere with transmission spectroscopy, but the hazes themselves may still yield valuable information, according to Zhang.

“Hazes are not featureless,” he said. “With better telescopes, we may be able to characterize the composition of exoplanet hazes and understand their chemistry. But the observations will be very hard to explain without data from laboratory experiments. This study has revealed the huge diversity of haze particles, and understanding their optical properties will be a high priority for future studies.”

In addition to Yu and Zhang, the coauthors of the paper include UCSC undergraduate Austin Dymont, astronomy professor Jonathan Fortney, and graduate student Diana Powell at UC Santa Cruz, as well as scientists at Johns Hopkins University (US), Cornell University (US), University of Texas at Austin (US), and University of Grenoble Alpes [Université Grenoble Alpes] (FR). This work was supported by National Aeronautics Space Agency (US) and the Heising-Simons Foundation.

See the full article here .


five-ways-keep-your-child-safe-school-shootings

Please help promote STEM in your local schools.

Stem Education Coalition

UC Santa Cruz (US) Lick Observatory Since 1888 Mt Hamilton, in San Jose, California, Altitude 1,283 m (4,209 ft)

UC Observatories Lick Automated Planet Finder fully robotic 2.4-meter optical telescope at Lick Observatory, situated on the summit of Mount Hamilton, east of San Jose, California, USA.

The UCO Lick C. Donald Shane telescope is a 120-inch (3.0-meter) reflecting telescope located at the Lick Observatory, Mt Hamilton, in San Jose, California, Altitude 1,283 m (4,209 ft).
UC Santa Cruz (US) campus.

The University of California-Santa Cruz (US) , opened in 1965 and grew, one college at a time, to its current (2008-09) enrollment of more than 16,000 students. Undergraduates pursue more than 60 majors supervised by divisional deans of humanities, physical & biological sciences, social sciences, and arts. Graduate students work toward graduate certificates, master’s degrees, or doctoral degrees in more than 30 academic fields under the supervision of the divisional and graduate deans. The dean of the Jack Baskin School of Engineering oversees the campus’s undergraduate and graduate engineering programs.

UCSC is the home base for the Lick Observatory.

UCO Lick Observatory’s 36-inch Great Refractor telescope housed in the South (large) Dome of main building.

Search for extraterrestrial intelligence expands at Lick Observatory
New instrument scans the sky for pulses of infrared light
March 23, 2015
By Hilary Lebow


Astronomers are expanding the search for extraterrestrial intelligence into a new realm with detectors tuned to infrared light at UC’s Lick Observatory. A new instrument, called NIROSETI, will soon scour the sky for messages from other worlds.

“Infrared light would be an excellent means of interstellar communication,” said Shelley Wright, an assistant professor of physics at UC San Diego (US) who led the development of the new instrument while at the U Toronto Dunlap Institute for Astronomy and Astrophysics (CA).

Shelley Wright of UC San Diego with (US) NIROSETI, developed at U Toronto Dunlap Institute for Astronomy and Astrophysics (CA) at the 1-meter Nickel Telescope at Lick Observatory at UC Santa Cruz

Wright worked on an earlier SETI project at Lick Observatory as a UC Santa Cruz undergraduate, when she built an optical instrument designed by University of California-Berkeley (US) researchers. The infrared project takes advantage of new technology not available for that first optical search.

Infrared light would be a good way for extraterrestrials to get our attention here on Earth, since pulses from a powerful infrared laser could outshine a star, if only for a billionth of a second. Interstellar gas and dust is almost transparent to near infrared, so these signals can be seen from great distances. It also takes less energy to send information using infrared signals than with visible light.

Frank Drake, professor emeritus of astronomy and astrophysics at UC Santa Cruz and director emeritus of the SETI Institute, said there are several additional advantages to a search in the infrared realm.

Frank Drake with his Drake Equation. Credit Frank Drake.

“The signals are so strong that we only need a small telescope to receive them. Smaller telescopes can offer more observational time, and that is good because we need to search many stars for a chance of success,” said Drake.

The only downside is that extraterrestrials would need to be transmitting their signals in our direction, Drake said, though he sees this as a positive side to that limitation. “If we get a signal from someone who’s aiming for us, it could mean there’s altruism in the universe. I like that idea. If they want to be friendly, that’s who we will find.”

Scientists have searched the skies for radio signals for more than 50 years and expanded their search into the optical realm more than a decade ago. The idea of searching in the infrared is not a new one, but instruments capable of capturing pulses of infrared light only recently became available.

“We had to wait,” Wright said. “I spent eight years waiting and watching as new technology emerged.”

Now that technology has caught up, the search will extend to stars thousands of light years away, rather than just hundreds. NIROSETI, or Near-Infrared Optical Search for Extraterrestrial Intelligence, could also uncover new information about the physical universe.

“This is the first time Earthlings have looked at the universe at infrared wavelengths with nanosecond time scales,” said Dan Werthimer, UC Berkeley SETI Project Director. “The instrument could discover new astrophysical phenomena, or perhaps answer the question of whether we are alone.”

NIROSETI will also gather more information than previous optical detectors by recording levels of light over time so that patterns can be analyzed for potential signs of other civilizations.

“Searching for intelligent life in the universe is both thrilling and somewhat unorthodox,” said Claire Max, director of UC Observatories and professor of astronomy and astrophysics at UC Santa Cruz. “Lick Observatory has already been the site of several previous SETI searches, so this is a very exciting addition to the current research taking place.”

NIROSETI will scan the skies several times a week on the Nickel 1-meter telescope at Lick Observatory, located on Mt. Hamilton east of San Jose.