天美影视传媒 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 UW 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.

鈥淚t 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.鈥�

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鈥檚 designed for easy growth. Fellow researchers can write new physical modules 鈥渁nd 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.

鈥淭hen 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.

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

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鈥檚 other faculty co-authors are astronomy professor ; , professor of atmospheric sciences; and research scientist . Other UW 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鈥檚 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

Aspects of an otherwise Earthlike planet鈥檚 tilt and orbital dynamics can severely affect its potential habitability 鈥� even triggering abrupt 鈥渟nowball states鈥� where oceans freeze and surface life is impossible, according to new research from astronomers at the 天美影视传媒.

The research indicates that locating a planet in its host star鈥檚 鈥渉abitable zone鈥� 鈥� that swath of space just right to allow liquid water on an orbiting rocky planet鈥檚 surface 鈥� isn鈥檛 always enough evidence to judge potential habitability.聽

, 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 鈥� a planet鈥檚 obliquity or its orbital eccentricity 鈥� 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 鈥� oval 鈥� 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 UW 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 鈥� given the same distance from its star 鈥� 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 鈥� changes in the distance between a planet and star over an orbit 鈥� 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 UW 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鈥檚 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 天美影视传媒.

The findings help explain why astronomers have detected few 鈥� which orbit stars that in turn orbit each other 鈥� 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 UW 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 鈥� the gravitational forces each exerts on the other 鈥� 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 鈥� and they get thrown out of the system,” said , UW 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 鈥� 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 鈥� “or non-detections” 鈥� 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 UW 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.