Researchers with the 91̽��-led are central to a group of published by NASA researchers in the journal Astrobiology outlining the history — and suggesting the future — of the search for life on exoplanets, or those orbiting stars other than the sun.

The research effort is coordinated by NASA’s Nexus for Exoplanet Systems Science, or NExSS, a worldwide network dedicated to finding new ways to study the age-old question: “Are we alone?”

A theme through the research and the discussions behind it is the need to consider planets in an integrated way, involving multiple disciplines and perspectives.

“For life to be detectable on a distant world it needs to strongly modify its planet in a way that we can detect,” said 91̽��astronomy professor , lead author of one of the papers and principle investigator of the Virtual Planetary Laboratory, or VPL for short. “But for us to correctly recognize life’s impact, we also need to understand the planet and star — that environmental context is key.”

Work done by NExSS researchers will help identify the measurements and instruments needed to search for life using future NASA flagship missions. The detection of atmospheric signatures of a few potentially habitable planets may possibly come before 2030, although whether the planets are truly habitable or have life will require more in-depth study.

The papers result from two years of effort by some of the world’s leading researchers in astrobiology, planetary science, Earth science, , astrophysics, chemistry and biology, including several from the 91̽��and the Virtual Planetary Laboratory, or VPL. The coordinated work was born of online meetings and an in-person workshop held in Seattle in July of 2016.

The pace of exoplanet discoveries has been rapid, with over 3,700 detected since 1992. NASA formed the international NExSS network to focus a variety of disciplines on understanding how we can characterize and eventually search for signs of life, called biosignatures, on exoplanets.

The NExSS network has furthered the field of exoplanet biosignatures and “fostered communication between researchers searching for signs of life on solar system bodies with those searching for signs of life on exoplanets,” said Niki Parenteau, an astrobiologist and microbiologist at NASA’s Ames Research Center, Moffett Field, California, and a VPL team member. “This has allowed for sharing of ‘lessons learned’ by both communities.”

The first of the papers reviews types of signatures astrobiologists have proposed as ways to identify life on an exoplanet. Scientists plan to look for two major types of signals: One is in the form of gases that life produces, such as oxygen made by plants or photosynthetic microbes. The other could come from the light reflected by life itself, such as the color of leaves or pigments.

Such signatures can be seen on Earth from orbit, and astronomers are studying designs of telescope concepts that may be able to detect them on planets around nearby stars. Meadows is a co-author, and lead author is , a VPL team member who earned his doctorate in astronomy and astrobiology from the 91̽��and is now a post-doctoral researcher at the University of California, Riverside.

Meadows is lead author of the second review paper, which discusses recent research on “false positives” and “false negatives” for biosignatures, or ways nature could “trick” scientists into thinking a planet without life was alive, or vice versa.

In this paper, Meadows and co-authors review ways that a planet could make oxygen abiotically, or without the presence of life, and how planets with life may not have the signature of oxygen that is abundant on modern-day Earth.

The paper’s purpose, Meadows said, was to discuss these changes in our understanding of biosignatures and suggest “a more comprehensive” treatment.  She said: “There are lots of things in the universe that could potentially put two oxygen atoms together, not just photosynthesis — let’s try to figure out what they are. Under what conditions are they are more likely to happen, and how can we avoid getting fooled?”

Schwieterman is a co-author on this paper, as well as 91̽��doctoral students , and .

With such advance thinking, scientists are now better prepared to distinguish false positives from planets that truly do host life.

Two more papers show how scientists try to formalize the lessons we have learned from Earth, and expand them to the wide diversity of worlds we have yet to discover.

, 91̽��professor of Earth and space sciences, is lead author on a paper that proposes a framework for assessing exoplanet biosignatures, considering such variables as the chemicals in the planet’s atmosphere, the presence of oceans and continents and the world’s overall climate. Doctoral student is a co-author.

By combining all this information in systematic ways, scientists can analyze whether data from a planet can be better explained statistically by the presence of life, or its absence.

“If future data from an exoplanet perhaps suggest life, what approach can distinguish whether the existence of life is a near-certainty or whether the planet is really as dead as a doornail?” said Catling. “Basically, NASA asked us to work out how to assign a probability to the presence of exoplanet life, such as a 10, 50 or 90 percent chance. Our paper presents a general method to do this.”

The data that astronomers collect on exoplanets will be sparse. They will not have samples from these distant worlds, and in many cases will study the planet as a single point of light. By analyzing these fingerprints of atmospheric gases and surfaces embedded in that light, they will discern as much as possible about the properties of that exoplanet.

“Because life, planet, and parent star change with time together, a biosignature is no longer a single target but a suite of system traits,” said , a biometeorologist at NASA’s Goddard Institute for Space Studies in New York and a VPL team member. She said more biologists and geologists will be needed to interpret observations “where life processes will be adapted to the particular environmental context.”

The final article discusses the ground-based and space-based telescopes that astronomers will use to search for life beyond the solar system. This includes a variety of observatories, from those in operation today to ones that will be built decades in the future.

Taken together, this cluster of papers explains how the exoplanet community will evolve from their current assessments of the sizes and orbits of these faraway worlds, to thorough analysis of their chemical composition and eventually whether they harbor life.

“I’m excited to see how this research progresses over the coming decades,” said , an astrobiologist at NASA’s Goddard Space Flight Center, Greenbelt, Maryland, and a VPL team member. He is also a co-author on four of the five papers.

“NExSS has created a diverse network of scientists. That network will allow the community to more rigorously assess planets for biosignatures than would have otherwise been possible.”

NExSS is an interdisciplinary, cross-divisional NASA research coordination network.

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Based on a . For more information, contact Meadows at vsm@astro.washington.edu or Catling at dcatling@uw.edu.

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

As telescopes of ever-greater power scan the cosmos looking for life, knowing where to look — and where not to waste time looking — will be of great value.

New research by 91̽�� astronomer and co-authors describes possible planetary systems where a gravitational nudge from one planet with just the right orbital configuration and tilt could have a mild to devastating effect on the orbit and climate of another, possibly habitable world.

Their have been accepted for publication in the Astrophysical Journal.

The magnitude of the chaos can range widely, Barnes said, from planets whose orbits remain largely circular to those “whose orbits get so elongated that a planet could slam into its host star — an extreme form of climate change!”

Even if the effect isn’t that dramatic, the orbit — thus the climate, as orbit is a primary driver of climate — could still be severe enough to inhibit life, or sterilize the planet if life has already begun, Barnes said.

The particular effect they studied is called a “” and it comes into play when two planets’ orbital periods are an integer ratio of each other, such as Neptune orbiting the sun three times for every time Pluto orbits twice. A repetitive force, like a gravitational nudge, happens at the same place in the planets’ orbits around the star, the effect of which grows slowly over millions of years.

This can happen to a planet in its star’s habitable zone, the swath of space around it that’s just right to allow an orbiting rocky planet to have water in liquid form on its surface, thus giving life a chance. Barnes calls such worlds “chaotic Earths” and suggests making them lower priorities in the search for life.

Another condition for this orbital bullying is “mutual inclination,” meaning that the two planets are angled toward each other in space. Planets in our solar system all lie along the same plane in space, and are called coplanar, but not all planetary systems are like that. So Barnes and colleagues decided to “kick up” inclinations between planets in computer models and study the result.

“That was the basic idea,” he said. “What happens when you have planets that are in this resonance and with mutual inclinations?

“And what we found was that things go all haywire. Those little perturbations that keep happening at the same point cause one of the orbits to do some crazy things — even flip over entirely — and then kind of come back to where it was before. It was pretty unexpected for us.”

If the fluctuations are small, such worlds might yet retain their chance of life and be worth further study. But if they are dramatic, astronomers should probably look elsewhere.

“Planets in systems that drive orbits to near-misses with the host stars are less promising targets and should be skipped over for other candidates,” Barnes said, “even if they are found today on circular orbits in the habitable zone.”

Further computer modeling will help researchers distinguish between these two possibilities, he said.

Powerful tools such as the will come online in a few years, able to determine the atmospheres of exoplanets, or those outside the solar system. But the work will be expensive, so astronomers will need to choose their objects of study wisely, Barnes said.

Barnes is lead author of the study. Co-authors are and , 91̽��astronomy professor and graduate student, respectively; Richard Greenberg of the University of Arizona and Sean Raymond of Laboratoire d’Astrophysique de Bordeaux in France.

The research was done through the , a UW-based interdisciplinary research group, and funded by NASA and the National Science Foundation.

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For more information, contact Barnes at 206-543-8979 or rory@astro.washington.edu. NASA Cooperative Agreement No. NNA13AA93A, NSF grant AST-1108882.