Exoplanets are experiencing a stratospheric rise. In the three decades since the first confirmed planet orbiting another star, scientists have catalogued more than 4,000 of them. As the list grows, so too does the desire to find Earth-like exoplanets — and to determine whether they could be life-sustaining oases like our own globe.

The coming decades should see the launch of new missions that can gather ever-larger amounts of data about exoplanets. Anticipating these future endeavors, a team at the 91̽�� and the University of Bern has computationally simulated more than 200,000 hypothetical Earth-like worlds — planets that have the same size, mass, atmospheric composition and geography as modern Earth — all in orbit of stars like our sun. Their goal was to model what types of environments astronomers can expect to find on real Earth-like exoplanets.

As they report in a paper accepted to the Planetary Science Journal and Dec. 6 to the preprint site arXiv, on these simulated exoplanets, one common feature of present-day Earth was often lacking: partial ice coverage.

“We essentially simulated Earth’s climate on worlds around different types of stars, and we find that in 90% of cases with liquid water on the surface, there are no ice sheets, like polar caps,” said co-author , a 91̽��professor of astronomy and scientist with the UW’s . “When ice is present, we see that ice belts — permanent ice along the equator — are actually more likely than ice caps.”

The findings shed light on the complex interplay between liquid water and ice on Earth-like worlds, according to lead author Caitlyn Wilhelm, who led the study as an undergraduate student in the 91̽��Department of Astronomy.

“Looking at ice coverage on an Earth-like planet can tell you a lot about whether it’s habitable,” said Wilhelm, who is now a research scientist with the Virtual Planetary Laboratory. “We wanted to understand all the parameters — the shape of the orbit, the axial tilt, the type of star — that affect whether you have ice on the surface, and if so, where.”

The team used a 1-D energy balance model, which computationally imitates the energy flow between a planet’s equator and poles, to simulate the climates on thousands of hypothetical exoplanets in various orbital configurations around F-, G- or K-type stars. These classes of stars, which include our own G-type sun, are promising candidates for hosting life-friendly worlds in their , also known as the “Goldilocks” zone. F-type stars are a bit hotter and larger than our sun; K-type stars are slightly cooler and smaller.

In their simulations, the orbits of the exoplanets ranged from circular to a pronounced oval. The team also considered axial tilts ranging from 0 to 90 degrees. Earth’s axial tilt is a moderate 23.5 degrees. A planet with a 90-degree tilt would “sit on its side” and experience extreme seasonal variations in climate, much like the planet Uranus.

According to the simulations, which encompassed a 1-million-year timespan on each world, Earth-like worlds showed climates ranging from planet-wide “” climates — with ice present at all latitudes — to a steaming “moist greenhouse,” which is probably similar to Venus’ climate before a made its surface hot enough to melt lead. But even though most environments in the simulations fell somewhere between those extremes, partial surface ice was present on only about 10% of hypothetical, habitable exoplanets.

The model included natural variations over time in each world’s axial tilt and orbit, which in part explains the general lack of ice on habitable exoplanets, according to co-author , a postdoctoral scientist at the University of Bern and researcher with the Virtual Planetary Laboratory.

“Orbits and axial tilts are always changing,” said Deitrick. “On Earth, these variations are called , and are very small in amplitude. But for exoplanets, these changes can be quite large, which can eliminate ice altogether or trigger ‘snowball’ states.”

When partial ice was present, its distribution varied by star. Around F-type stars, polar ice caps — like what Earth sports currently — were found about three times more often than ice belts, whereas ice belts occurred twice as often as caps for planets around G- and K-type stars. Ice belts were also more common on worlds with extreme axial tilts, likely because seasonal extremes keep the polar climates more volatile than equatorial regions, according to Wilhelm.

The team’s findings about ice on these simulated Earth-like worlds should help in the search for potentially habitable worlds by showing astronomers what they can expect to find, especially regarding ice distribution and the types of climates.

“Surface ice is very reflective, and can shape how an exoplanet ‘looks’ through our instruments,” said Wilhelm. “Whether or not ice is present can also shape how a climate will change over the long term, whether it goes to an extreme — like a ‘snowball Earth’ or a runaway greenhouse — or something more moderate.”

Ice alone, or its absence, does not determine habitability, though.

“Habitability encompasses a lot of moving parts, not just the presence or absence of ice,” said Wilhelm.

Life on Earth has survived snowball periods, as well as hundreds of millions of ice-free years, according to Barnes.

“Our own planet has seen some of these extremes in its own history,” said Barnes. “We hope this study lays the groundwork for upcoming missions to look for habitable signatures in exoplanet atmospheres — and to even image exoplanets directly — by showing what’s possible, what’s common and what’s rare.”

Rachel Mellman, a recent 91̽��graduate in astronomy, is a co-author on the paper. The research was funded by NASA through grants to the Virtual Planetary Laboratory.

For more information, contact Barnes at rkb9@uw.edu and Wilhelm at cwilhelm@uw.edu.

Grant numbers: NNA13AA93A, 80NSSC18K0829.

91̽�� astrobiologist has created software that simulates multiple aspects of planetary evolution across billions of years, with an eye toward finding and studying potentially habitable worlds.

Barnes, a 91̽��assistant professor of astrobiology, astronomy and data science, released the first version of VPLanet, his virtual planet simulator, in August. He and his co-authors described it in a accepted for publication in the Publications of the Astronomical Society of the Pacific.

“It links different physical processes together in a coherent manner,” he said, “so that effects or phenomena that occur in some part of a planetary system are tracked throughout the entire system. And ultimately the hope is, of course, to determine if a planet is able to support life or not.”

VPLanet’s mission is three-fold, Barnes and co-authors write. The software can:

• simulate newly discovered exoplanets to assess their potential to possess surface liquid water, which is a key to life on Earth and indicates the world is a viable target in the search for life beyond Earth
• model diverse planetary and star systems regardless of potential habitability, to learn about their properties and history, and
• enable transparent and open science that contributes to the search for life in the universe

The first version includes modules for the internal and magnetic evolution of terrestrial planets, climate, atmospheric escape, tidal forces, orbital evolution, rotational effects, stellar evolution, planets orbiting binary stars and the gravitational perturbations from passing stars.

It’s designed for easy growth. Fellow researchers can write new physical modules “and almost plug and play them right in,” Barnes said. VPLanet can also be used to complement more sophisticated tools such as machine learning algorithms.

An important part of the process, he said, is validation, or checking physics models against actual previous observations or past results, to confirm that they are working properly as the system expands.

“Then we basically connect the modules in a central area in the code that can model all members of a planetary system for its entire history,” Barnes said.

And though the search for potentially habitable planets is of central importance, VPLanet can be used for more general inquiries about planetary systems.

“We observe planets today, but they are billions of years old,” he said. This is a tool that allows us to ask: ‘How do various properties of a planetary system evolve over time?’”

The project’s history dates back almost a decade to a Seattle meeting of astronomers called “Revisiting the Habitable Zone” convened by , principal investigator of the UW-based , with Barnes. The habitable zone is the swath of space around a star that allows for orbiting rocky planets to be temperate enough to have liquid water at their surface, giving life a chance.

They recognized at the time, Barnes said, that knowing if a planet is within its star’s habitable zone simply isn’t enough information: “So from this meeting we identified a whole host of physical processes that can impact a planet’s ability to support and retain water.”

Barnes discussed VPLanet and presented a tutorial on its use at the recent AbSciCon19 worldwide astrobiology conference, held in Seattle.

The research was done through the Virtual Planetary Laboratory and the source code is available .

Barnes’s other faculty co-authors are astronomy professor ; , professor of atmospheric sciences; and research scientist . Other 91̽��co-authors are doctoral students , , and ; and undergraduate researchers Caitlyn Wilhelm, Benjamin Guyer and Diego McDonald.

Other co-authors are of the Carnegie Institution for Science; of the Flatiron Institute, of the Max Planck Institute for Astronomy in Heidelberg, Germany, of the University of Bern, of the NASA Goddard Space Flight Center and of Weber State University.

The research was funded by a grant from the NASA Astrobiology Program’s Virtual Planetary Laboratory team, as part of the research coordination network, or NExSS.

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For more information, contact Barnes at 206-543-8979 or rkb9@uw.edu.

Grant numbers

VPL under cooperative agreement #NNA13AA93A

NASA grants #NNX15AN35G, #13-13-NA17 0024, and #80NSSC18K0829

NASA Earth and Space Science Fellowship Program grant #80NSSC17K0482

inflated a big black fabric tent in the lobby of the Hyatt Regency Hotel on Wednesday and stood outside it inviting passers-by: “Come on in and watch the show — we’re talking about astrobiology.”

Barnes, a 91̽�� research assistant professor of astronomy, was showing off the department’s to colleagues at , the national conference on astrobiology, . The conference is happening all week at the Hyatt, and dozens of 91̽��faculty and students are involved.

The tent is about 10 feet tall and 20 feet across and stays upright with the help of a high-powered fan. Its graphics come via Microsoft’s World Wide Telescope. 91̽��astronomy faculty and students to conduct outreach about astronomy to area schools, and have been educating (and entertaining) thousands of students about the cosmos since.

But recently, Barnes and graduate students have been using the Mobile Planetarium to tell K-12 students about astrobiology. It’s a hit with middle school students especially, he said.

“They get excited about it. It’s a very visceral experience, very immersive, and there’s often a lot of screaming as they move through the universe,” Barnes said, smiling. “You can see they’re engaged.”

Astrobiology graduate students were conducting the shows inside the tent on Wednesday, starting a new presentation every so often as people filed by and stepped — climbed, really — through a fabric doorway into the dark interior.

There mid-Wednesday, astronomy doctoral students David Graham, then , gave engaging, illustrated lectures to visitors huddled in the darkness inside. Aided by graphics displayed in color against the rounded tent ceiling, they in turn took their audience from scenes of “extremophile” creatures living on Earth out through the solar system and into deep space — so far out, whole galaxies appear as mere dots.

Garcia talked of the “big questions” astrobiologists want to answer, leading with the most basic: “Are we alone in the universe?”

He added: “Personally I think this also tells me that life is precious. Even if planets are common we haven’t seen life on them yet, so the life on our own planet is really precious as well.

“So, it’s not just a scientific pursuit — astrobiology — but it’s also, how do we relate to our environment? And I think it’s really beautiful in that way.”

Barnes does assessments before and after his school visits and said they show the students enjoy the presentation, even if a little part of that might be just being away from class.

“But it’s all just about getting them to remember this experience. They remember they had a good experience — that’s still a win.”

He said one thing he definitely sees is that “people from all over, whatever their background, they do ‘get’ this — they go, ‘Whoa, there’s no life out there that we know, but that’s interesting and maybe I can think about this.’

“And the best, of course, is every time you go you get two or three kids who say, ‘This is really cool — maybe I want to study planets.'”

Barnes also reported on the 91̽��Mobile Planetarium to the conference in a separate session Thursday. He hopes to keep it going and is looking for further funding for the project.

Soon he was back out front, looking for the next audience.

“We’re talking astrobiology. Want to come in and see the show?”

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For more information, contact Barnes at rkb9@uw.edu.

is the study of life in the universe and of the terrestrial environments and planetary and stellar processes that support it. To study astrobiology is to ask questions that cut across multiple disciplines and could take lifetimes to answer. The field gathers expertise from a host of other disciplines including biology, chemistry, geology, oceanography, atmospheric and Earth science, aeronautical engineering and of course astronomy itself.

These questions also include: What can Earth’s own species, and its chemical past, tell us about how to spot life elsewhere? How did the first cells arise? Can we map the surfaces of exoplanets? How can we motivate students to be curious about space?

Every two years, researchers gather from around the world to share and discuss their latest findings in a weeklong conference. Called for short, this year’s conference will be held June 24-28 at the Hyatt Regency Hotel in Bellevue. It’s the biggest meeting of astrobiologists in the world and dozens of 91̽�� researchers will attend and participate.

Public attitudes have warmed greatly toward astrobiology in the 21^st century, prompted by exoplanet discoveries and exploration of other worlds in the solar system. Study of extraterrestrial life remains a hopeful science wryly aware that, as an old joke goes, it has yet to prove that its very subject matter exists.

The 91̽��founded its own program in 1999, involving roughly 30 faculty and about as many students a year. “The program is a leader in both training the next generation of astrobiologists and in fundamental astrobiology research,” said , 91̽��professor of astronomy and principal investigator for the UW-based , which explores computer models of planetary environments and will be the subject of a .

“The Astrobiology Science Conference is the biggest meeting of astrobiologists in the world, and this year, members of the 91̽��Astrobiology Program are playing a major role in conference organization, as well as presenting our research at the meeting,” said Meadows, who chaired the science committee for AcSciCon2019.

Here are several 91̽��presentations and papers scheduled for the weeklong conference. Though the lead presenter is listed here only, most projects involve the work of several colleagues.

• A study of water vapor and ice particles emitting from the plume on Saturn’s moon Enceladus, leading to a better understanding of the moon’s subsurface ocean. With Earth and space sciences doctoral student and colleagues. ()
• An examination of whether the coming James Webb Space Telescope will be able to detect atmospheres for all worlds in the intriguing, seven-planet system TRAPPIST-1, and finding that clouds and water vapor in the planets’ atmospheres might make such study more challenging. With astronomy and astrobiology doctoral student and colleagues. ()
• Description of a new open-source computer software package called VPLanet that simulates a wide range of planetary systems across billions of years, simulating atmospheres, orbits and stellar phenomena that can affect a planet’s ability to sustain liquid water on its surface, which is key to life. With Rory Barnes and colleagues. ()
• An exploration of how viruses and hosts co-evolved, enabling microbial life in extremely cold brines. With oceanography professor ().
• Modeling Earth’s atmosphere 2.7 billion years ago and the effect of iron-rich micrometeorites that rained down, melted and interacted with the surrounding gases, leading to a better understanding of carbon dioxide levels at that time. With Earth and space sciences graduate student and colleagues. ()
• A presentation on the 91̽��Astronomy Department’s successful outreach to students through its that visits K-12 schools, enabling them to create shows of their own. With astronomy research assistant professor and several colleagues. and .)
• An exploration of how to determine if oxygen detected on an exoplanet is really produced by life, using high-resolution planetary spectra from ground-based telescopes. With , an astronomy doctoral student, and colleagues. ()
• A discussion of how studying a giant Pacific Octopus might help us learn more about different forms of cognition and better know and understand life beyond Earth — if we ever find it. With , a doctoral student in psychology. ()
• A study of microbial life in extremely cold brines within unfrozen subsurface areas of permafrost, and their possible relevance to similar environments on Mars or icy moons in the solar system. With , a doctoral student in biological oceanography, and colleagues. (.)

Many other 91̽��faculty members will participate, either with reports on their own research or in support of colleagues or graduate students. These include ESS professors , , , , , astronomy professors , and , among others.

Astrobiologists such as Sullivan point out that the field’s focus and scientific benefit is about more than simply hunting for life, though that is the key motivator.

“It’s about thinking about life in a cosmic context. And about the origin and evolution of life,” Sullivan said.

“Even if you only care about Earth life, astrobiology is a viable — fundamental, I would say — interdisciplinary science that thrives independently of the existence of extraterrestrial life.”

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

Planets orbiting “short-period” binary stars, or stars locked in close orbital embrace, can be ejected off into space as a consequence of their host stars’ evolution, according to new research from the 91̽��.

The findings help explain why astronomers have detected few — which orbit stars that in turn orbit each other — despite observing thousands of short-term binary stars, or ones with orbital periods of 10 days or less.

It also means that such binary star systems are a poor place to aim future ground- and space-based telescopes to look for habitable planets and life beyond Earth.

There are several different types of , such as and binaries, named for the ways astronomers are able to observe them. In a accepted for publication in , lead author , a 91̽��astronomy doctoral student, studies binaries, or those where the orbital plane is so near the line of sight, both stars are seen to cross in front of each other. Fleming will present the paper at the Division on Dynamical Astronomy conference April 15-19.

When eclipsing binaries orbit each other closely, within about 10 days or less, Fleming and co-authors wondered, do tides — the gravitational forces each exerts on the other — have “dynamical consequences” to the star system?

“That’s actually what we found” using computer simulations, Fleming said. “Tidal forces transport from the stellar rotations to the orbits. They slow down the stellar rotations, expanding the orbital period.”

This transfer of angular momentum causes the orbits not only to enlarge but also to circularize, morphing from being eccentric, or football-shaped, to perfect circles. And over very long time scales, the spins of the two stars also become synchronized, as the moon is with the Earth, with each forever showing the same face to the other.

The expanding stellar orbit “engulfs planets that were originally safe, and then they are no longer safe — and they get thrown out of the system,” said , 91̽��assistant professor of astronomy and a co-author on the paper. And the ejection of one planet in this way can perturb the orbits of other orbiting worlds in a sort of cascading effect, ultimately sending them out of the system as well.

Making things even more difficult for circumbinary planets is what astronomers call a “region of instability” created by the competing gravitational pulls of the two stars.

“There’s a region that you just can’t cross — if you go in there, you get ejected from the system,” Fleming said. “We’ve confirmed this in simulations, and many others have studied the region as well.”

This is called the “dynamical stability limit.” It moves outward as the stellar orbit increases, enveloping planets and making their orbits unstable, and ultimately tossing them from the system.

Another intriguing characteristic of such binary systems, detected by others over the years, Fleming said, is that planets tend to orbit just outside this stability limit, to “pile up” there. How planets get to the region is not fully known; they may form there, or they may migrate inward from farther out in the system.

Applying their model to known short-period binary star systems, Fleming and co-authors found that this stellar-tidal evolution of binary stars removes at least one planet in 87 percent of multiplanet circumbinary systems, and often more. And even this is likely a conservative estimate; Barnes said the number may be as high as 99 percent.

The researchers have dubbed the process the Stellar Tidal Evolution Ejection of Planets, or STEEP. Future detections — “or non-detections” — of circumbinary around short-period binary stars, the authors write, will “will provide the best indirect observational test of the STEEP process.

The shortest-period binary star system around which a circumbinary planet has been discovered was , with a period of about 7.45 days. The co-authors suggest that future studies looking to find and study possibly habitable planets around short-term binary stars should focus on those with longer orbital periods than about 7.5 days.

Fleming and Barnes’ co-authors are 91̽��astronomy professor , post-doctoral researcher and undergraduate student David E. Graham. This work used storage and networking infrastructure provided by the Hyak supercomputer system at the UW, funded by the UW’s Student Technology Fee.

The research was funded by the NASA Astrobiology Institute through the UW-based . Fleming is supported by funding from the NASA Space Science Fellowship Program.

As for habitability and the search for life, Fleming said planets orbiting short-term eclipsing binaries might otherwise be attractive targets for closer study, with their edge-on angle showing eclipses, and more, to the distant viewer.

“But this mechanism tends to kill them,” he added. “So, it’s not a good place to look.”

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For more information, contact Fleming at dflemin3@uw.edu, or Barnes at 206-543-8979 or rory@astro.washington.edu.

Grant numbers: NSF IGERT DGE-1258485 fellowship; NASA Earth and Space Science Fellowship Program # 80NSSC17K0482; Virtual Planetary Laboratory, under Cooperative Agreement # NNA13AA93A, NNX14AK26G.

of Queen Mary University of London, will give a lecture titled “Proxima Centauri b: A World of Possibilities” at 7 p.m. Wednesday, May 3. in 120 Kane Hall. This will be followed by brief comments by 91̽��astronomy professors and as well as professor of astronomy at the University of Arizona, and a panel discussion.

Anglada-Escude was principal investigator for the team that in August 2016 detected Proxima Centauri b orbiting its star 4.22 light years, or about 25 trillion miles away in the constellation of Centaurus. Not only the exoplanet orbiting the closest star to the sun, Proxima Centauri b is also probably similar in mass to Earth and receives about the same amount of starlight from its host star, raising the possibility that it could be habitable.

Following Anglada-Escudé, Barnes will talk about the planet’s formation and evolution with an eye toward water being possible there today. Meadows, lead investigator of the UW-based , will discuss the ramifications of the planet’s evolution on the possibility of life on Proxima Centauri b, and how we might look for it.

Guyon, an expert on exoplanet imaging who is also project scientist for Japan’s telescope, will then discuss technology coming online in the next decade to observe Proxima Centauri b with massive telescopes.

He will also talk about Starshot, one of the supported by Russian entrepreneur Yuri Milner, Facebook founder Mark Zuckerberg and physicist Stephen Hawking, which proposes to use a high-powered, Earth-based laser to send a swarm of lightweight, sail-like probes to the planet at about one-fifth the speed of light.

The event is sponsored by the and the NASA Astrobiology Institute. Admission is free but online to the event is required.

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For more information on their work, contact Barnes at 206-543-8979 or rory@astro.washington.edu; or Meadows at 206-543-0206 or vsm@astro.washington.edu. To learn more about the event, contact astrobio@uw.edu.

The world’s attention is now on Proxima Centauri b, a possibly Earth-like planet orbiting the closest star, 4.22 light-years away. The planet’s orbit is just right to allow liquid water on its surface, needed for life. But could it in fact be habitable?

If life is possible there, the planet evolved very differently than Earth, say researchers at the 91̽��-based , where astronomers, geophysicists, climatologists, evolutionary biologists and others team to study how distant planets might host life.

Astronomers at Queen Mary University in London have announced of Proxima Centauri b, a planet orbiting close to a star 4.22 light years away. The find has been called “the biggest exoplanet discovery since the discovery of exoplanets.”

, 91̽��research assistant professor of astronomy, published a blog post about the discovery at palereddot.org, a website dedicated to the search for life around Proxima Centauri. His essay describes research underway through the 91̽��planetary lab — part of the NASA Astrobiology Institute — to answer the question, is life possible on this world?

“The short answer is, ‘It’s complicated,’ Barnes writes. “Our observations are few, and what we do know allows for a dizzying array of possibilities” — and almost as many questions.

The Virtual Planetary Laboratory is directed by , 91̽��professor of astronomy. UW-affiliated researchers include , and . Using computer models, the researchers studied clues from the orbits of the planet, its system, its host star and apparent companion stars Alpha Centauri A and B  — plus what is known of stellar evolution to begin evaluating Proxima b’s chances.

Relatively little is known about Proxima:

• It’s at least as massive as Earth and may be several times more massive, and its “year” — the time it takes to orbit its star — is only 11 days.
• Its star is only 12 percent as massive as our sun and much dimmer (so its habitable zone, allowing liquid water on the surface, is much closer in) and the planet is 25 times closer in than Earth is to our Sun.
• The star may form a third part of the Alpha Centauri binary star system, separated by a distance of 15,000 “,” which could affect the planet’s orbit and history
• The new data hint at the existence of a second planet in the system with an orbital period near 200 days, but this has not been proven

Perhaps the biggest obstacle to life on the planet, Barnes writes, is the brightness of its host star. Proxima Centauri, a red dwarf star, is comparatively dim, but wasn’t always so.

“Proxima’s brightness evolution has been slow and complicated,” Barnes writes. “Stellar evolution models all predict that for the first one billion years Proxima slowly dimmed to its current brightness, which implies that for about the first quarter of a billion years, planet b’s surface would have been too hot for Earth-like conditions.”

Barnes notes that he and 91̽��graduate student Rodrigo Luger recently showed that had modern Earth been in such a situation, “it would have become a Venus-like world, in a that can destroy all of the planet’s primordial water,” thus extinguishing any chance for life.

Next come a host of questions about the planet’s makeup, location and history, and the team’s work toward discerning answers.

• Is the planet “rocky” like Earth? Most orbits simulated by the planetary lab suggest it could be — and thus can host water in liquid form, a prerequisite for life.
• Where did it form, and was there water? Whether it formed in place or further from its star, where ice is more likely, planetary lab researchers believe it “entirely possible” Proxima b could be water-rich, though they are not certain.
• Did it start out as a hydrogen-enveloped Neptune-like planet and then lose its hydrogen to become Earth-like? Planetary laboratory research shows this is indeed possible, and could be a viable pathway to habitability.
• Proxima Centauri flares more often than our sun; might such flares have long-since burned away atmospheric ozone that might protect the surface and any life? This is possible, though a strong magnetic field, as Earth has, could protect the surface. Also, any life under even a few meters of liquid water would be protected from radiation.

Another concern is that the planet might be tidally locked, meaning one side permanently faces its star, as the moon does Earth. Astronomers long thought this to mean a world could not support life, but now believe planetwide atmospheric winds would transport heat around the planet.

“These questions are central to unlocking Proxima’s potential habitability and determining if our nearest galactic neighbor is an inhospitable wasteland, an inhabited planet, or a future home for humanity,” Barnes writes.

Planetary lab researchers also are developing techniques to determine whether Proxima b’s atmosphere is amenable to life.

“Nearly all the components of an atmosphere imprint their presence in a spectrum (of light),” Barnes writes. “So with our knowledge of the possible histories of this planet, we can begin to develop instruments and plan observations that pinpoint the critical differences.”

At high enough pressures, he notes, oxygen molecules can momentarily bind to each other to produce in the light spectrum.

“Crucially, the pressures required to be detectable are large enough to discriminate between a planet with too much oxygen, and one with just the right amount for life.

As we learn more about the planet and the system, we can build a library of possible spectra from which to quantitatively determine how likely it is that life exists on planet b.”

Our own Sun is expected to burn out in about 4 billion years, but Proxima Centauri has a much better forecast, perhaps burning for 4 trillion years longer.

“If Proxima b is habitable, then it might be an ideal place to move. Perhaps we have just discovered a future home for humanity. But in order to know for sure, we must make more observations, run many more computer simulations and, hopefully, send probes to perform the first direct reconnaissance of an exoplanet,” Barnes writes. “The challenges are huge, but Proxima b offers a bounty of possibilities that fills me with wonder.”

Proxima Centauri b may be the first exoplanet to be directly characterized by powerful ground- and space-based telescopes planned, and its atmosphere spectroscopically probed for active biology.

The research was funded by the NASA Astrobiology Institute.

“Whether habitable or not,” Barnes concluded, “Proxima Centauri b offers a new glimpse into how the planets and life fit into our universe.”

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For more information, contact Barnes at 206-543-8979 or rory@astro.washington.edu; or Meadows at 206-543-0206 or vsm@astro.washington.edu.

Cooperative agreement No. NNA13AA93.

Is it life, or merely the illusion of life?

Research from the 91̽��-based published Feb. 26 in Astrophysical Journal Letters will help astronomers better identify — and thus rule out — “false positives” in the search for life beyond Earth.

Powerful devices such as the , set for launch in 2018, may help astronomers look for life on a handful of faraway worlds by searching for, among other things, evidence of oxygen — a “biosignature” — in their atmospheres. This is done by transit spectroscopy, or studying the spectral features of light visible through a planet’s atmosphere when it transits or passes in front of its host star.

“We wanted to determine if there was something we could observe that gave away these ‘false positive’ cases among exoplanets,” said lead author , a doctoral student in astronomy. “We call them ‘biosignature impostors’ in the .

“The potential discovery of life beyond our solar system is of such a huge magnitude and consequence, we really need to be sure we’ve got it right — that when we interpret the light from these exoplanets we know exactly what we’re looking for, and what could fool us.”

Here on Earth, oxygen is produced almost exclusively by photosynthesis — plants and algae converting the sun’s rays into energy to sustain life. And so Earth’s oxygen biosignature is indeed evidence of life. But that may not be universally true.

from the Virtual Planetary Laboratory has found that some worlds can create oxygen “abiotically,” or by nonliving means. This is more likely in the case of planets orbiting low-mass stars, which are smaller and dimmer than our sun and the most common in the universe.

The first abiotic method they identified results when the star’s ultraviolet light splits apart carbon dioxide (CO₂) molecules, freeing some of the oxygen atoms to form into O_2, the kind of oxygen present in Earth’s atmosphere.

The giveaway that this particular oxygen biosignature might not indicate life came when the researchers, through computer modeling, found that the process produces not only oxygen but also significant and potentially detectable amounts of carbon monoxide. “So if we saw carbon dioxide and carbon monoxide together in the atmosphere of a rocky planet, we would know to be very suspicious that future oxygen detections would mean life,” Schwieterman said.

The team also found an indicator for abiotic oxygen resulting from starlight similarly breaking down atmospheric water, H₂O, allowing hydrogen to escape and leaving vast quantities of oxygen — far more than the Earth has ever had in its atmosphere.

In such cases, Schwieterman said, oxygen molecules collide with each other frequently, producing short-lived pairs of oxygen molecules that become O₄ molecules, with their own unique signature.

“Certain O₄ features are potentially detectable in transit spectroscopy, and many more could be seen in reflected light,” Schwieterman said. “Seeing a large O₄ signature could tip you off that this atmosphere has far too much oxygen to be biologically produced.”

“With these strategies in hand, we can more quickly move on to more promising targets that may have true oxygen biosignatures,” he said.

“It’s one thing to detect a biosignature gas, but another thing to be able to interpret what you are looking at, said , 91̽��professor of astronomy and principal investigator of the Virtual Planetary Laboratory. “This research is important because biosignature impostors may be more common for planets orbiting low-mass stars, which will be the first places we look for life outside our solar system in the coming decade.”

Schwieterman’s other 91̽��co-authors are astronomy professor and doctoral students and .

Other co-authors are Shawn Domagal-Goldman of the NASA Goddard Space Flight Center in Greenbelt, Maryland; Drake Deming of the University of Maryland; and Chester Harman of Pennsylvania State’s Center for Exoplanets and Habitable Worlds.

The research was funded by the NASA Astrobiology Institute.

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For more information, contact Schwieterman at 321-505-1605 or eschwiet@uw.edu. Follow him on Twitter at @nogreenstars.

Cooperative agreement # NNA13AA93A.

We know the Earth is habitable because — well, here we are. But would it look like a good candidate for life from hundreds of light-years away?

Good, but perhaps not great, according to astronomer of the 91̽��-based . It’s a question, among many others, that he and co-authors asked in a recent .

Barnes, a research assistant professor of astronomy, colleagues are drawing up a “” that ranks exoplanets to help prioritize the search for life.

Astronomers spot possible exoplanets, or those beyond the solar system, not through direct observation but by the dimming of light that happens when the worlds pass in front of, or “transit” their host star. Many factors go into judging a world’s possible habitability, including the amount of energy it gets from its star, the distance and radius of its orbital path and the behavior of its neighbor planets. Spectrometry is used to estimate the mass and radius of the host star, from which astronomers can the estimate the size of the planet itself.

They use this data to create a model of a planet — “an idea of a planet,” Barnes said, which is then compared with information about other worlds. “And you basically try and sort out, do I think that could reasonably be a planet that’s habitable?”

But validating, or confirming planets is methodical, time-consuming work, and access to the big telescopes needed is expensive. The habitability index helps astronomers rank and prioritize planets to help determine which are worthy of closer study.

Managing these myriad calculations, the index gives the Earth, if observed from afar as we now observe faraway planets, about an 82 percent chance of being right for life.

But wait — only 82 percent?

Why wouldn’t the Earth — the single example of a life-hosting world in all our experience — score a perfect, 100 percent rating?

“Basically, where we lose some of the probability, or chance for life, is that we could be too close to the star,” Barnes said. “We actually are kind of close to the inner edge of the habitable zone. If we spotted Earth with our current techniques, we would reasonably conclude that it could be too hot for life.”

The habitable zone is that swath of space around a star where an orbiting rocky planet might be able to keep liquid water on its surface, thus giving life a chance.

But distance to the host star is only one of many data points Barnes and colleagues account for with the habitability index. Others are the composition of the planet, the details of its orbital path and the behavior of nearby worlds.

In the paper, Barnes and co-authors argue that potential habitability could as effectively be thought of as “a cooling problem.” That is, just as there is a habitable zone or “Goldilocks” sweet spot in distance, so too is there one in how successfully a planet sheds energy to maintain the right conditions for liquid water on a planet’s surface.

So, why doesn’t our presence on the Earth, all things considered, earn a perfect, 100 percent score? Because again, here we are, living proof. But the astronomers would not know that, if Earth were spotted hundreds or thousands of light-years away in the Kepler field of vision.

“Remember, we have to think about the Earth as if we don’t know anything about it,” Barnes said. “We don’t know that it’s got oceans, and whales and thing like that — imagine it’s just this thing that dims some of the light around a nearby star when it passes.”

It becomes a sort of sociology question, Barnes said. People would get pretty excited if astronomers did spot an exact Earth twin orbiting an exact Sun twin out there, Barnes allowed.

But if there came a choice between spending money and time to study the Earth twin — so close to the superheated inner edge of its habitable zone — or another planet located by Kepler with a higher habitability index rating, which should we choose to spend millions on and study?

Sorry, Earth twin.

“The point of the paper is that the other one is the best to spend our time on. Because it’s less in danger.”

“But,” Barnes added, “it’s obviously based on this very limited information.”

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For more information, contact Barnes at 206-543-8979 or rory@astro.washington.edu.