An atmospheric peculiarity the Earth shares with Jupiter, Saturn, Uranus and Neptune is likely common to billions of planets, 91Ě˝»¨ astronomers have found, and knowing that may help in the search for potentially habitable worlds.

First, some history: It’s known that air grows colder and thinner with altitude, but in 1902 a scientist named LĂ©on Teisserenc de Bort, using instrument-equipped balloons, found a point in Earth’s atmosphere at about 40,000 to 50,000 feet where the air stops cooling and begins growing warmer.

He called this invisible turnaround a “tropopause,” and coined the terms “stratosphere” for the atmosphere above, and “troposphere” for the layer below, where we live — terms still used today.

Then, in the 1980s, NASA spacecraft discovered that tropopauses are also present in the atmospheres of the planets Jupiter, Saturn, Uranus and Neptune, as well as Saturn’s largest moon, Titan. And remarkably, these turnaround points all occur at roughly the same level in the atmosphere of each of these different worlds — at a pressure of about 0.1 bar, or about one-tenth of the air pressure at Earth’s surface.

Now, a paper by 91Ě˝»¨astronomer and planetary scientist published online Dec. 8 in the journal Nature Geoscience uses basic physics to show why this happens, and suggests that tropopauses are probably common to billions of thick-atmosphere planets and moons throughout the galaxy.

“The explanation lies in the physics of infrared radiation,” said Robinson. Atmospheric gases gain energy by absorbing infrared light from the sunlit surface of a rocky planet or from the deeper parts of the atmosphere of a planet like Jupiter, which has no surface.

Using an analytic model, Catling, professor of Earth and space sciences,  and Robinson, a postdoctoral researcher in astronomy, show that at high altitudes atmospheres become transparent to thermal radiation due to the low pressure. Above the level where the pressure is about 0.1 bar, the absorption of visible, or ultraviolet, light causes the atmospheres of the giant planets — and Earth and Titan — to grow warmer as altitude increases.

The physics, they write, provides a rule of thumb — that the pressure is around 0.1 bar at the tropopause turnaround — which should apply to the vast number of planetary atmospheres with stratospheric gases that absorb ultraviolet or visible light.

Astronomers could use the finding to extrapolate temperature and pressure conditions on the surface of planets and work out whether the worlds are potentially habitable — the key being whether pressure and temperature conditions allow liquid water on the surface of a rocky planet.

“Then we have somewhere we can start to characterize that world,” Robinson said. “We know that temperatures are going to increase below the tropopause, and we have some models for how we think those temperatures increase — so given that leg up, we can start to extrapolate downward toward the surface.”

He added, “It’s neat that common physics not only explains what’s going on in solar system atmospheres, but also might help with the search for life elsewhere.”

Funding for the research came from the NASA Astrobiology Institute’s Virtual Planetary Laboratory.

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This news release is based in part on writing by Catling and Robinson. For more information, contact Robinson at 520-907-8369 or robinson@astro.washington.edu; or Catling at 206-543-8653 or dcatling@uw.edu.

The mystery of how the surface of Mars, long dead and dry, could have flowed with water billions of years ago may have been solved by research that included a 91Ě˝»¨ astronomer.

There is evidence that Mars had water at its surface 3.8 billion years ago or before, but scientists are divided on how that might have happened, especially since the sun was about 30 percent fainter back then, thus less able to melt water ice on Mars.

Earlier efforts with computer models to simulate a warm, wet Mars using only carbon dioxide and water have not been successful. Now, researchers at Pennsylvania State University and the 91Ě˝»¨have published a paper arguing that the presence of a third ingredient alters that outcome, strengthening the greenhouse effect — where heat energy from sunlight is trapped in the atmosphere — enough to warm the surface and allow liquid water.

“We found that sprinkling a small amount of molecular hydrogen into a thick atmosphere composed primarily of carbon dioxide does the trick,” said coauthor , a 91Ě˝»¨postdoctoral researcher. “While carbon dioxide is a strong greenhouse gas, it can’t solve the early Mars problem on its own. But carbon dioxide and hydrogen make a great team.”

The paper was published Nov. 24 in the journal Nature Geoscience. The lead author is Ramses Ramirez, a doctoral student at Penn State working with planetary scientist James Kasting, a coauthor.

For the paper, Ramirez and postdoctoral researcher and coauthor Ravi Kopparapu of Penn State developed a computer climate model demonstrating that gases from early Martian volcanic activity could have sent enough hydrogen and carbon dioxide into the atmosphere to create a strong greenhouse effect and raise temperatures enough to allow liquid water on the surface.

“This is exciting because explaining how early Mars could have been warm and wet enough to form the ancient valleys had scientists scratching their heads for the past 30 years,” Ramirez said. “We think we have a credible solution to this great mystery.”

Robinson of the 91Ě˝»¨worked with Ramirez and Kopparapu to help ensure that radiation physics in the Penn State model was as accurate as possible.

“A warm and wet Mars brings to mind a climate like modern Earth,” Robinson said. “Maybe, also like Earth, a warm and wet early Mars could have had life.”

Other coauthors of the paper are Michael E. Zugger of Penn State and Richard Freedman of the SETI Institute.

Funding for the research came from the NASA Astrobiology Institute’s Virtual Planetary Laboratory.

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This news release was based in part on a release by Anne Danahy of Penn State. For more information, contact Robinson at 520-907-8369, or robinson@astro.washington.edu; or Ramirez at 814-863-7689 or rmr5265@psu.edu.

In the runaway greenhouse stage, a planet absorbs more solar energy than it can give off to retain equilibrium. As a result, the world overheats, boiling its oceans and filling its atmosphere with steam, which leaves the planet glowing-hot and forever uninhabitable, as Venus is now.

One estimate of the inner edge of a star’s “habitable zone” is where the runaway greenhouse process begins. The habitable zone is that ring of space around a star that’s just right for water to remain in liquid form on an orbiting rocky planet’s surface, thus giving life a chance.

Revisiting this classic planetary science scenario with new computer modeling, the astronomers found a lower thermal radiation threshold for the runaway greenhouse process, meaning that stage may be easier to initiate than had been previously thought.

“The habitable zone becomes much narrower, in the sense that you can no longer get as close to the star as we thought before going into a runaway greenhouse,” said , a 91Ě˝»¨astronomy postdoctoral researcher and second author on the paper. The lead author is of the University of Victoria.

Though further research is called for, the findings could lead to a recalibration of where the habitable zone begins and ends, with some planets having their candidacy as possible habitable worlds revoked.

“These worlds on the very edge got ‘pushed in,’ from our perspective — they are now beyond the runaway greenhouse threshold,” Robinson said.

Subsequent research, the astronomers say, is needed in part because their computer modeling was done in a “single-column, clear-sky model,” or a one-dimensional measure averaged around a planetary sphere that does not account for the atmospheric effect of clouds.

The findings apply to planet Earth as well. As the sun increases in brightness over time, Earth, too, will move into the runaway greenhouse stage — but not for a billion and a half years or so. Still, it inspired the astronomers to write, “As the solar constant increases with time, Earth’s future is analogous to Venus’s past.”

Other co-authors are Kevin J. Zahnle of the NASA Ames Research Center in Moffett Field, Calif.; and David Crisp of the Jet Propulsion Laboratory in Pasadena, Calif.

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For more information, contact Robinson at 520-907-8369, or robinson@astro.washington.edu; or Goldblatt at 250-721-7641 or czg@uvic.ca.

In a bit of cosmic irony, planets orbiting cooler stars may be more likely to remain ice-free than planets around hotter stars. This is due to the interaction of a star’s light with ice and snow on the planet’s surface.

Stars emit different types of light. Hotter stars emit high-energy visible and ultraviolet light, and cooler stars give off infrared and near-infrared light, which has a much lower energy.

It seems logical that the warmth of terrestrial or rocky planets should depend on the amount of light they get from their stars, all other things being equal. But new climate model research led by , a doctoral student in the , has added a surprising new twist to the story: Planets orbiting cool stars actually may be much warmer and less icy than their counterparts orbiting much hotter stars, even though they receive the same amount of light.

That’s because the ice absorbs much of the longer wavelength, near-infrared light predominantly emitted by these cooler stars. This is counter to what we experience on Earth, where ice and snow strongly reflect the visible light emitted by the Sun.

Around a cooler (M-dwarf) star, the more light the ice absorbs, the warmer the planet gets. The planet’s atmospheric greenhouse gases also absorb this near-infrared light, compounding the warming effect.

The researchers found that planets orbiting cooler stars, given similar amounts of light as those orbiting hotter stars, are therefore less likely to experience so-called “snowball states,” icing over from pole to equator.

However, around a hotter star such as an F-dwarf, the star’s visible and ultraviolet light is reflected by planetary ice and snow in a process called ice-albedo feedback. The more light the ice reflects, the cooler the planet gets.

This feedback can be so effective at cooling that terrestrial planets around hotter stars appear to be more susceptible than other planets to entering snowball states. That’s not necessarily a bad thing, in the scheme of time — Earth itself is believed to have experienced several snowball states during the course of its 4.6 billion year history.

Shields and co-authors found that this interaction of starlight with a planet’s surface ice is less pronounced near the outer edge of the habitable zone, where carbon dioxide is expected to build up as temperatures decrease. The habitable zone is the swath of space around a star that’s just right to allow an orbiting planet’s surface water to be in liquid form, thus giving life a chance.

That is the case because planets at that zone’s outer edge would likely have a thick atmosphere of carbon dioxide or other greenhouse gases, which blocks the absorption of radiation at the surface, causing the planet to lose any additional warming advantage due to the ice.

The researchers’ findings are documented in a published in the August issue of the journal Astrobiology, and published online ahead of print July 15.

Shields said that astronomers hunting for possible life will prioritize planets less vulnerable to that snowball state — that is, planets other than those orbiting hotter stars. But that doesn’t mean they will rule out the cooler planets.

“The last snowball episode on Earth has been linked to the explosion of multicellular life on our planet,” Shields said. “If someone observed our Earth then, they might not have thought there was life here — but there certainly was.

“So though we’d look for the non-snowball planets first, we shouldn’t entirely write off planets that may be ice-covered, or headed for total ice cover. There could be life there too, though it may be much harder to detect.”

Shields’ 91Ě˝»¨co-authors are , associate professor of astronomy; , associate professor of atmospheric sciences; and , an astronomy research associate. Other co-authors are Raymond T. Pierrehumbert of the University of Chicago and Manoj Joshi of the University of East Anglia.

The work was funded by a National Science Foundation Graduate Research Fellowship and performed as part of the NASA Astrobiology Institute’s Virtual Planetary Laboratory Lead Team.

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For more information, contact Shields at aomawa@astro.washington.edu.