Aspects of an otherwise Earthlike planet’s tilt and orbital dynamics can severely affect its potential habitability — even triggering abrupt “snowball states” where oceans freeze and surface life is impossible, according to new research from astronomers at the 91̽��.

The research indicates that locating a planet in its host star’s “habitable zone” — that swath of space just right to allow liquid water on an orbiting rocky planet’s surface — isn’t always enough evidence to judge potential habitability.��

, lead author of a paper to be published in the Astronomical Journal, said he and co-authors set out to learn, through computer modeling, how two features — a planet’s obliquity or its orbital eccentricity — might affect its potential for life. They limited their study to planets orbiting in the habitable zones of “G dwarf” stars, or those like the sun.

A planet’s is its tilt relative to the orbital axis, which controls a planet’s seasons; is the shape, and how circular or elliptical — oval — the orbit is. With elliptical orbits, the distance to the host star changes as the planet comes closer to, then travels away from, its host star.

Deitrick, who did the work while with the UW, is at the University of Bern. His 91̽��co-authors are atmospheric sciences professor , astronomy professors , and and graduate student , with help from undergraduate researcher Caitlyn Wilhelm.

The Earth hosts life successfully enough as it circles the sun at an axial tilt of about 23.5 degrees, wiggling only a very little over the millennia. But, Deitrick and co-authors asked in their modeling, what if those wiggles were greater on an Earthlike planet orbiting a similar star?

Previous research indicated that a more severe axial tilt, or a tilting orbit, for a planet in a sunlike star’s habitable zone — given the same distance from its star — would make a world warmer. So Deitrick and team were surprised to find, through their modeling, that the opposite reaction appears true.

“We found that planets in the habitable zone could abruptly enter ‘snowball’ states if the eccentricity or the semi-major axis variations — changes in the distance between a planet and star over an orbit — were large or if the planet’s obliquity increased beyond 35 degrees,” Deitrick said.

The new study helps sort out conflicting ideas proposed in the past. It used a sophisticated treatment of ice sheet growth and retreat in the planetary modeling, which is a significant improvement over several previous studies, co-author Barnes said.

“While past investigations found that high obliquity and obliquity variations tended to warm planets, using this new approach, the team finds that large obliquity variations are more likely to freeze the planetary surface,” he said. “Only a fraction of the time can the obliquity cycles increase habitable planet temperatures.”

Barnes said Deitrick “has essentially shown that ice ages on exoplanets can be much more severe than on Earth, that orbital dynamics can be a major driver of habitability and that the habitable zone is insufficient to characterize a planet’s habitability.” The research also indicates, he added, “that the Earth may be a relatively calm planet, climate-wise.”

This kind of modeling can help astronomers decide which planets are worthy of precious telescope time, Deitrick said: “If we have a planet that looks like it might be Earth-like, for example, but modeling shows that its orbit and obliquity oscillate like crazy, another planet might be better for follow-up” with telescopes of the future.”

The main takeaway of the research, he added, is that “We shouldn’t neglect orbital dynamics in habitability studies.”

Other co-authors are , a former 91̽��post-doctoral researcher now with the LESIA Observatoire de Paris; and John Armstrong of Weber State University, who earned his doctorate at the UW.

The research used storage and networking infrastructure provided by the Hyak supercomputer system at the UW, funded by the UW’s Student Technology Fee. The work was funded by the NASA Astrobiology Institute through the UW-based .

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For more information, contact Deitrick at deitrr@astro.washington.edu or russell.deitrick@csh.unibe.ch; or Barnes at rory@astro.washington.edu.

Agreement number: NNA13AA93A

An atmospheric haze around a faraway planet — like the one which probably shrouded and cooled the young Earth — could show that the world is potentially habitable, or even be a sign of life itself.

Astronomers often use the Earth as a proxy for hypothetical exoplanets in computer modeling to simulate what such worlds might be like and under what circumstances they might be hospitable to life.

In new research from the 91̽��-based , 91̽��doctoral student and co-authors chose to study Earth in its era, about 2 ½ billion years back, because it is, as Arney said, “the most alien planet we have geochemical data for.”

The work builds on geological data from other researchers that suggests the early Earth was intermittently shrouded by an organic pale orange haze that came from light breaking down methane molecules in the atmosphere into more complex , organic compounds of hydrogen and carbon.

“Hazy worlds seem common both in our solar system and in the population of exoplanets we’ve characterized so far,” Arney said. “Thinking about Earth with a global haze allows us to put our home planet into the context of these other worlds, and in this case, the haze may even be a sign of life itself.”

Arney and co-authors will present their findings Nov. 11 at the American Astronomical Society’s Division of Planetary Sciences in National Harbor, Maryland.

The researchers used photochemical, climate and radiation simulations to examine the early Earth shrouded by a “fractal” hydrocarbon haze, meaning that the imagined haze particles are not spherical, as used in many such simulations, but agglomerates of spherical particles, bunched together not unlike grapes, but smaller than a raindrop. A fractal haze, they found, would have significantly lowered the planetary surface temperature.

However, they also found the cooling would be partly countered by concentrations of greenhouse gases that tend to warm a planet. They saw that this combination would result in a moderate, possibly habitable average global temperature.

Such a haze, the researchers found, also would have absorbed ultraviolet light so well as to effectively shield the Archean Earth from deadly radiation before the rise of oxygen and the ozone layer, which now provides that protection. The haze was a benefit to just-evolving surface biospheres on Earth, as it could be to similar exoplanets.

The researchers also found that, based on the early Earth data, it’s unlikely such a haze would be formed by abiotic, or nonliving means. So for exoplanets with Earthlike amounts of carbon dioxide in their atmospheres, Arney said, “organic haze might be a novel type of biosignature. However, we know these hazes can also form without life on worlds like Saturn’s moon Titan, so we are working to come up with more ways to distinguish biological hazes from abiotic ones.”

Co-author Shawn Domagal-Goldman of the NASA Goddard Space Flight Center in Greenbelt, Maryland, said, “Giada’s work shows that the haze could have intertwined with life in more ways than we previously suspected.”

Arney added that astronomers often think of Earthlike exoplanets as “pale blue dots” — after a famous of Earth taken by the Voyager spacecraft — “but with this haze, Earth would have been a ‘pale orange dot.'”

The research was funded through the NASA Astrobiology Institute.

Arney’s 91̽��co-authors are , professor of astronomy and director of the Virtual Planetary Laboratory, and doctoral student and postdoctoral researcher . Other co-authors are Domagal-Goldman, Eric Wolf of the University of Colorado at Boulder and Mark Claire of the University of St. Andrews in the UK and Seattle’s Blue Marble Space Institute of Science.

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For more information, contact Arney at giada@uw.edu or 206-685-0403.

Scientists have catalogued nearly 2,000 around stars near and far. While most of these are giant and inhospitable, improved techniques and spacecraft have uncovered increasingly smaller worlds. The day may soon come when astrophysicists announce our planet’s twin around a distant star.

But size alone is insufficient to judge a globe. Though Earth and Venus are nearly identical in size, the latter’s surface is hot enough to melt lead. Astronomers must gather information about an exoplanet’s atmosphere, often through observing how the planet scatters or absorbs light from its parent star. But, that information is not always useful — as is the case with the exoplanet .

“When an exoplanet passes in front of its star, light can be absorbed at some wavelengths by molecules in the atmosphere, which we can analyze by looking at how light passes through the planet’s atmosphere,” said , a postdoctoral researcher in the 91̽�� Department of Astronomy. “But for this planet, with the Hubble Space Telescope, they saw almost no variation with wavelength of light.”

This “flat spectrum” for GJ1214b indicated that something in the planet’s upper atmosphere blocked light, keeping scientists in the dark regarding its atmosphere. Charnay decided to computationally model what its atmosphere could be, based on the planet’s temperature and composition. In the process, as he reports in in , he and his collaborators became the first to simulate three-dimensional exotic clouds in the atmosphere of another world.

“It’s an important step in characterizing exoplanets,” said Charnay.

GJ1214b was among the first “” exoplanets discovered, which are intermediate in size between Earth and Neptune. They’re the smallest exoplanets that can be studied with existing technology, and GJ1412b is in an ideal position.

“Most of the other mini-Neptunes that have been discovered orbit stars between 100 and 1,000 light years away,” said Charnay. “GJ1214b is quite close to Earth, just 42 light years away, and it orbits its star in just 1.6 days.”

That fast orbit gave scientists the opportunity to record the exoplanet’s flat spectrum, ruling out an atmosphere of simple hydrogen, water, carbon dioxide or methane. Instead, something high in the atmosphere was blocking light from penetrating farther down.

“There could either be high clouds in the atmosphere or an organic haze — like we see on ,” said Charnay.

Its atmospheric temperature exceeds the boiling point of water. As a result, if GJ1214b sported clouds, they would probably be some form of salt, said Charnay. But such clouds should form deep in the atmosphere, much lower than the altitude where they are observed. Charnay modeled how the clouds could form in the lower atmosphere and then rise into the upper atmosphere with sufficient circulation.

To accomplish this, Charnay, used a climate model developed by his former research group in Paris. He previously used this model for studying Titan and the early Earth, and adapted it for GJ1214b.

Charnay ran his three-dimensional cloud model on the UW’s Hyak supercomputer. It shows how GJ1214b could create, sustain and lift salt clouds into the upper atmosphere, where they would contribute to the planet’s flat spectrum that Hubble detected. His model also makes specific predictions about the effect these clouds will have on the planet’s climate and the types of information that future telescopes, like the , will be able to gather.

Charnay would next like to model the other potential cause of the exoplanet’s flat spectrum: photochemical haze, which gives Titan its shrouded orange atmosphere and Los Angeles its persistent dome of polluted air.

“Light splits chemicals in the atmosphere, creating more complex organic compounds that make the haze,” said Charnay.

Charnay will have to wait until the James Webb Space Telescope launches later this decade to find out which theory — clouds or haze — gives GJ1214b a flat spectrum. In the meantime, in addition to his quest to simulate haze on this exoplanet, Charnay would like to model what the atmosphere was like on Earth before life evolved.

“Worlds like Titan and this exoplanet have complex atmospheric chemistry that might be closer to what early Earth’s atmosphere was like,” said Charnay. “We can learn a lot about how planetary atmospheres like ours form by looking at them.”

Charnay’s co-authors on the paper include 91̽��astronomy professor , recent 91̽��astronomy doctoral graduate Amit Misra, 91̽��astronomy graduate student and University of Toronto researcher Jérémy Leconte. Their work was funded by NASA and the UW’s .

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For more information, contact Charnay at 206-553-9139 or bcharnay@uw.edu.

NASA cooperative agreement NNA13AA93A.

With its thick, hazy atmosphere and surface rivers, mountains, lakes and dunes, , Saturn’s largest moon, is one of the most Earthlike places in the solar system.

As the examines Titan over many years, its discoveries bring new mysteries. One of those involves the seemingly wind-created sand dunes spotted by Cassini near the moon’s equator, and the contrary winds just above.

Here’s the problem: Climate simulations indicate that Titan’s near-surface winds — like Earth’s trade winds — blow toward the west. So why do the surface dunes, reaching a hundred yards high and many miles long, point to the east?

The direction of the dunes has at times been attributed to the effects of Saturn’s gravitational tides or various land features or wind dynamics, but none quite explained their eastward slant.

Violent methane storms high in Titan’s dense atmosphere, where winds do blow toward the east, might be the answer, according to new research by 91̽�� astronomer and co-authors in a published today in the journal Nature Geoscience.

Using computer models, Charnay, a 91̽��post-doctoral researcher, and co-authors hypothesize that the attitude of Titan’s sand dunes results from rare methane storms that produce eastward gusts much stronger than the usual westward surface winds.

“These fast eastward gusts dominate the sand transport, and thus dunes propagate eastward,” Charnay said.

The storm winds reach up to 10 meters a second (22 mph), about 10 times faster than Titan’s gentler near-surface winds. And though the storms happen only when Titan is in equinox and its days and nights are of equal length — about every 14.75 years — they are of sufficient power to realign Titan’s dunes. Titan was last in equinox in August 2009.

It probably helps that, according to Cassini’s observations, Titan’s atmosphere is in “super-rotation” above about 5 miles, meaning that it rotates a lot faster than the surface itself. Their model, Charnay said, suggests that these methane storms “produce strong downdrafts, flowing eastward when they reach the surface,” thus rearranging the dunes.

Charnay said he tried first, without success, to solve the problem with a global climate model that didn’t factor in methane clouds, then realized that it was impossible, hinting that methane could be part of the solution.

“It was a kind of detective game, as often is the case in planetary sciences, where we have many mysteries and a few clues to solve them,” he said.

The dunes in question, which are linear and run parallel to Titan’s equator, are probably not composed of silicates like Earth sand, Charnay said, but of hydrocarbon polymers — a kind of soot resulting from the decomposition of methane in the atmosphere.

Charnay noted a December reported in Nature showing that it would take winds of at least 3.2 mph to lift and transport sand across Titan’s surface — that’s 40 percent to 50 percent stronger wind than previous estimates.

The measurement of such a high wind speed threshold was a pleasant surprise, Charnay said: “That means that only fast winds transport Titan’s sand, compatible with our hypothesis of strong storm gusts controlling the orientation and propagation of dunes.”

Titan, discovered in 1655 by Christiaan Huygens, has long intrigued astronomers. Its atmosphere is 98.4 percent nitrogen and most of the rest is methane, and a bit of hydrogen. Its gravity is one-sixth that of Earth’s and its air density is four- to five-times higher, meaning that flight will be relatively easy for visiting spacecraft. The European Space Agency’s Huygens probe, which rode along on Cassini, on Titan in 2005 and sent back of the moon’s stone-strewn surface.

Charnay said direct observation by Cassini would be the way to confirm his hypothesis. Unfortunately, the Cassini mission will end in 2017 and Titan’s next equinox is not until 2023.

“But there will be other missions,” he said. “There are still a lot of mysteries about Titan. We still don’t know how a thick nitrogen atmosphere formed, where the methane comes from nor how Titan’s sand forms.

“And it is not completely excluded that , perhaps in its . So Titan really is a fascinating and evolving world, which has to be understood as a whole.”

Charnay’s co-authors are Erika Barth and Scot Rafkin of the Southwest Research Institute in Boulder, Colorado; Sébastien Lebonnois of the Laboratory of Dynamic Meteorology; and Sylvain Courrech du Pont, Clément Narteau, Sebastian Rodriguez and Antoine Lucas of Paris Diderot University.

The research was done in part through the , a UW-based interdisciplinary research group, and funded by the NASA Postdoctoral Program and the French National Research Agency.

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��For more information, contact Charnay at bcharnay@uw.edu or benjamin.charnay@lmd.jussieu.fr. Grant #ANR-12-BS05-001-03/EXO-DUNES.

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