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 91探花.

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 91探花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 , 91探花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 91探花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.

A 91探花-led international team of astronomers has used data gathered by the Kepler Space Telescope to observe and confirm details of the outermost of seven exoplanets orbiting the star .

They confirmed that the planet, TRAPPIST-1h, orbits its star every 18.77 days, is linked in its orbital path to its siblings and is frigidly cold. Far from its host star, the planet is likely uninhabitable 鈥� but it may not always have been so.

91探花doctoral student is lead author on a published May 22 in the journal Nature Astronomy.

“TRAPPIST-1h was exactly where our team predicted it to be,” Luger said. The researchers discovered a mathematical pattern in the orbital periods of the inner six planets, which was strongly suggestive of an 18.77 day period for planet h.

“It had me worried for a while that we were seeing what we wanted to see. Things are almost never exactly as you expect in this field 鈥� there are usually surprises around every corner, but theory and observation matched perfectly in this case.”

TRAPPIST-1 is a middle-aged, ultra cool dwarf star, much less luminous than the sun and only a bit larger than the planet Jupiter. The star, which is nearly 40 light-years or about 235 trillion miles away in the constellation of Aquarius, is named after the ground-based Transiting Planets and Planetesimals Small Telescope (TRAPPIST), the facility that first found evidence of planets around it in 2015.

The TRAPPIST survey is led by of the University of Li猫ge, Belgium, who is also a coauthor on this research. In 2016, Gillon鈥檚 team announced the detection of three planets orbiting TRAPPIST-1 and this number was upped to seven in a subsequent 2017 paper. Three of
TRAPPIST-1’s planets appear to be within the star’s habitable zone, that swath of space around a star where a rocky planet could have liquid water on its surface, thus giving life a chance.

Such exoplanets are detected when they transit, or pass in front of, their host star, blocking a measurable portion of the light. Gillon’s team was able to observe only a single transit for TRAPPIST-1h, the farthest-out of the star’s seven progeny, prior to the data analyzed by Luger鈥檚 team.

Luger led a multi-institution international research team that studied the TRAPPIST-1 system more closely using 79 days of observation data from K2, the second mission of the Kepler Space Telescope. The team was able to observe and study four transits of TRAPPIST-1h across its star.

The team used the K2 data to further characterize the orbits of the other six planets, help rule out the presence of additional transiting planets, and determine the rotation period and activity level of the star. They also discovered that TRAPPIST-1’s seven planets appear linked in a complex dance known as an where their respective orbital periods are mathematically related and slightly influence each other.

“Resonances can be tricky to understand, especially between three bodies. But there are simpler cases that are easier to explain,” Luger said. For instance, closer to home, Jupiter’s moons Io, Europa and Ganymede are set in a 1:2:4 resonance, meaning that Europa’s orbital period is exactly twice that of Io, and Ganymede’s is exactly twice that of Europa.

These relationships, Luger said, suggested that by studying the orbital velocities of its neighbor planets they could predict the exact orbital velocity, and hence also orbital period, of TRAPPIST-1h even before the K2 observations. Their theory proved correct when they located the planet in the K2 data.

TRAPPIST-1’s seven-planet chain of resonances established a record among known planetary systems, the previous holders being the systems Kepler-80 and Kepler-223, each with four resonant planets. The resonances are “self-correcting,” Luger said, such that if one planet were to somehow be nudged off course, it would lock right back into resonance. “Once you’re caught into this kind of stable resonance, it’s hard to escape,” he said.

All of this, Luger said, indicates that these orbital connections were forged early in the life of the TRAPPIST-1 system, when the planets and their orbits were not fully formed.

“The resonant structure is no coincidence, and points to an interesting dynamical history in which the planets likely migrated inward in lock-step,” Luger said. “This makes the system a great testbed for planet formation and migration theories.”

It also means that while TRAPPIST-1h is now extremely cold 鈥� with an average temperature of 173 Kelvin (minus 148 F) 鈥� it likely spent several hundred million years in a much warmer state, when its host star was younger and brighter.

“We could therefore be looking at a planet that was once habitable and has since frozen over, which is amazing to contemplate and great for follow-up studies,” Luger said.

Luger said he has been working with data from the K2 mission for a while now, researching ways to reduce “instrumental noise” in its data resulting from broken reaction wheels 鈥� small flywheels that help position the spacecraft 鈥� that can overwhelm planetary signals.

鈥淥bserving TRAPPIST-1 with K2 was an ambitious task,鈥� said Marko Sestovic, a doctoral student at the University of Bern and second author of the study. In addition to the extraneous signals introduced by the spacecraft鈥檚 wobble, the faintness of the star in the optical (the range of wavelengths where K2 observes) placed TRAPPIST-1h 鈥渘ear the limit of what we could detect with K2,鈥� he said. To make matters worse, Sestovic said, one transit of the planet coincided with a transit of TRAPPIST-1b, and one coincided with a stellar flare, adding to the difficulty of the observation. 鈥淔inding the planet was really encouraging,鈥� Luger said, 鈥渟ince it showed we can still do high-quality science with Kepler despite significant instrumental challenges.鈥�

Luger’s 91探花co-authors are astronomy doctoral students and , post-doctoral researcher and professor (Guggenheim Fellow). Agol separately helped confirm the approximate mass of TRAPPIST-1 planets with a technique he and colleagues devised called “” that describes planets’ gravitational tugs on one another.

Luger said the TRAPPIST-1 system’s relative nearness “makes it a prime target for follow-up and characterization with current and upcoming telescopes, which may be able to give us information about these planets’ atmospheric composition.”

Contributing to this discovery are researchers at the University of Bern in Switzerland; Paris Diderot and Paris Sorbonne Universities and the CEA Saclay in France; the University of Li猫ge in Belgium; the University of Chicago; the University of California, San Diego; California Institute of Technology; the University of Bordeaux in France; the University of Cambridge in England; NASA’s Ames Research Center, Goddard Space Flight Center, and Johnson Space Center; Massachusetts Institute of Technology; the University of Central Lancashire in England; King Abdulaziz University in Saudi Arabia; Cadi Ayyad University in Morocco; and the University of Geneva in Switzerland.

The research was funded by the via the UW-based as well as a National Science Foundation Graduate Student Research Fellowship, the Swiss National Science Foundation, the Simons Foundation, the European Research Council and the UK Science and Technology Facilities Council, among other agencies.

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For more information, visit or contact Luger at 206-543-6276 or rodluger@uw.edu

• Watch a video about the TRAPPIST-1 systems’s orbital resonances:

The animation shows a simulation of the聽planets of TRAPPIST-1 orbiting for 90 Earth-days. After 15 Earth days,聽the animation聽focuses only on the outer three planets: TRAPPIST-1f,聽TRAPPIST-1g,聽TRAPPIST-1h. The motion freezes each time two adjacent planets pass each other; an arrow appears pointing to the location of the third planet.聽This complex but predictable pattern, called an orbital resonance, occurs when planets exert a regular, periodic gravitational tug on each other as they orbit their star.聽The three-body resonance聽of the outer three planets聽causes the planets to repeat the same relative positions, and expecting such a resonance was used to predict the orbital period of TRAPPIST-1h. 聽

By Daniel Fabrycky / University of Chicago; with reference to聽Luger聽et al. 2017, Nature Astronomy

Is it life, or merely the illusion of life?

Research from the 91探花-based published Feb. 26 in Astrophysical Journal Letters will help astronomers better identify 鈥� and thus rule out 鈥� “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 鈥� a “biosignature” 鈥� 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 鈥� that when we interpret the light from these exoplanets we know exactly what we鈥檙e looking for, and what could fool us.”

Here on Earth, oxygen is produced almost exclusively by photosynthesis 鈥� 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 鈥� 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 , 91探花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 91探花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.

Two phenomena known to inhibit the potential habitability of planets 鈥� tidal forces and vigorous stellar activity 鈥� might instead help chances for life on certain planets orbiting low-mass stars, 91探花 astronomers have found.

In a published this month in the journal Astrobiology, 91探花doctoral student and co-author , research assistant professor, say the two forces could combine to transform uninhabitable “mini-Neptunes” 鈥� big planets in outer orbits with solid cores and thick hydrogen atmospheres 鈥� into closer-in, gas-free, potentially habitable worlds.

Most of the stars in our galaxy are low-mass stars, also called M dwarfs. Smaller and dimmer than the sun, with close-in habitable zones, they make good targets for finding and studying potentially habitable planets. Astronomers expect to find many Earthlike and “super-Earth” planets in the habitable zones of these stars in coming years, so it’s important to know if they might indeed support life.

Super-Earths are planets greater in mass than our own yet smaller than gas giants such as Neptune and Uranus. The habitable zone is that swath of space around a star that might allow liquid water on an orbiting rocky planet’s surface, perhaps giving life a chance.

“There are many processes that are negligible on Earth but can affect the habitability of M dwarf planets,” Luger said. “Two important ones are strong tidal effects and vigorous stellar activity.”

A tidal force is a star’s gravitational tug on an orbiting planet, and is stronger on the near side of the planet, facing the host star, than on the far side, since gravity weakens with distance. This pulling can stretch a world into an ellipsoidal or egg-like shape as well as possibly causing it to migrate closer to its star.

“This is the reason we have ocean tides on Earth, as tidal forces from both the moon and the sun can tug on the oceans, creating a bulge that we experience as a high tide,” Luger said. “Luckily, on Earth it’s really only the water in the oceans that gets distorted, and only by a few feet. But close-in planets, like those in the habitable zones of M dwarfs, experience much stronger tidal forces.”

This stretching causes friction in a planet’s interior that gives off huge amounts of energy. This can drive surface volcanism and in some cases even heat the planet into a runaway greenhouse state, boiling away its oceans, and all chance of habitability.

Vigorous stellar activity also can destroy any chance for life on planets orbiting low-mass stars. M dwarfs are very bright when young and emit lots of high-energy X-rays and ultraviolet radiation that can heat a planet’s upper atmosphere, spawning strong winds that can erode the atmosphere away entirely. In a , Luger and Barnes showed that a planet’s entire surface water can be lost due to such stellar activity during the first few hundred million years following its formation.

“But things aren’t necessarily as grim as they may sound,” Luger said. Using computer models, the co-authors found that tidal forces and atmospheric escape can sometimes shape planets that start out as mini-Neptunes into gas-free, potentially habitable worlds.

How does this transformation happen?

Mini-Neptunes typically form far from their host star, with ice molecules joining with hydrogen and helium gases in great quantity to form icy/rocky cores surrounded by massive gaseous atmospheres.

“They are initially freezing cold, inhospitable worlds,” Luger said. “But planets need not always remain in place. Alongside other processes, tidal forces can induce inward planet migration.” This process can bring mini-Neptunes into their host star’s habitable zone, where they are exposed to much higher levels of X-ray and ultraviolet radiation.

This can in turn lead to rapid loss of the atmospheric gases to space, sometimes leaving behind a hydrogen-free, rocky world smack dab in the habitable zone. The co-authors call such planets “habitable evaporated cores.”

“Such a planet is likely to have abundant surface water, since its core is rich in water ice,” Luger said. “Once in the habitable zone, this ice can melt and form oceans,” perhaps leading to life.

Barnes and Luger note that many other conditions would have to be met for such planets to be habitable. One is the development of an atmosphere right for creating and recycling nutrients globally.

Another is simple timing. If hydrogen and helium loss is too slow while a planet is forming, a gaseous envelope would prevail and a rocky, terrestrial world may not form. If the world loses hydrogen too quickly, a runaway greenhouse state could result, with all water lost to space.

“The bottom line is that this process 鈥� the transformation of a mini-Neptune into an Earthlike world 鈥� could be a pathway to the formation of habitable worlds around M dwarf stars,” Luger said.

Will they truly be habitable? That remains for future research to learn, Luger said.

“Either way, these evaporated cores are probably lurking out there in the habitable zones of these stars, and many may be discovered in the coming years.”

Luger is lead author of the paper, with Barnes and Victoria Meadows his 91探花co-authors. Other co-authors are E. Lopez and Jonathan Fortney of the University of California, Santa Cruz, and Brian Jackson of Boise State University.

The research was done through the , a UW-based interdisciplinary research group, and funded through the NASA Astrobiology Institute under Cooperative Agreement Number NNA13AA93A .

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This release is based on an by Luger. View a for the research. For more information, contact Luger at 206-543-6276 or rodluger@uw.edu; or Barnes at 206-543-8979 or rory@astro.washington.edu.

Planets orbiting close to low-mass stars 鈥� easily the most common stars in the universe 鈥� are prime targets in the search for extraterrestrial life.

But new research led by an astronomy graduate student at the 91探花 indicates some such planets may have long since lost their chance at hosting life because of intense heat during their formative years.

Low-mass stars, also called M dwarfs, are smaller than the sun, and also much less luminous, so their habitable zone tends to be fairly close in. The habitable zone is that swath of space that is just right to allow liquid water on an orbiting planet’s surface, thus giving life a chance.

Planets close to their host stars are easier for astronomers to find than their siblings farther out. Astronomers discover and measure these worlds by studying the slight reduction in light when they transit, or pass in front of their host star; or by measuring the star’s slight “wobble” in response to the planet’s gravity, called the radial velocity method.

But in a to be published in the journal Astrobiology, doctoral student and co-author , a 91探花research assistant professor, find through computer simulations that some planets close to low-mass stars likely had their water and atmospheres burned away when they were still forming.

“All stars form in the collapse of a giant cloud of interstellar gas, which releases energy in the form of light as it shrinks,” Luger said. “But because of their lower masses, and therefore lower gravities, M dwarfs take longer to fully collapse 鈥� on the order of many hundreds of millions of years.”

“Planets around these stars can form within 10 million years, so they are around when the stars are still extremely bright. And that’s not good for habitability, since these planets are going to initially be very hot, with surface temperatures in excess of a thousand degrees. When this happens, your oceans boil and your entire atmosphere becomes steam.”

Also boding ill for the atmospheres of these worlds is the fact that M dwarf stars emit a lot of X-ray and ultraviolet light, which heats the upper atmosphere to thousands of degrees and causes gas to expand so quickly it leaves the planet and is lost to space, Luger said.

“So, many of the planets in the habitable zones of M dwarfs could have been dried up by this process early on, severely decreasing their chance of actually being habitable.”

A side effect of this process, Luger and Barnes write, is that ultraviolet radiation can split up water into its component hydrogen and oxygen atoms. The lighter hydrogen escapes the atmosphere more easily, leaving the heavier oxygen atoms behind. While some oxygen is clearly good for life, as on Earth, too much oxygen can be a negative factor for the origin of life.

“Rodrigo has shown that this prolonged runaway greenhouse phase can produce huge atmospheres full of oxygen 鈥� like, 10 times denser than that of Venus and all oxygen,” said Barnes. “Searches for life often rely on oxygen as a tracer of extraterrestrial life 鈥� so the abiological production of such huge quantities of oxygen could confound our search for life on exoplanets.”

Luger said the working title of their paper was “Mirage Earths.”

“Because of the oxygen they build up, they could look a lot like Earth from afar 鈥� but if you look more closely you’ll find that they’re really a mirage; there’s just no water there.”

The research was funded by NASA’s Astrobiology Institute, through the , headquartered at the UW.

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For more information, contact Luger at 206-543-6276 or rodluger@gmail.com; or Barnes at 206-543-8979 or rory@astro.washington.edu. Funded under NASA Astrobiology Institute Cooperative Agreement聽 NNA13AA93A.