Very occasionally, Earth gets bombarded by a large meteorite. But every day, our planet gets pelted by space dust, micrometeorites that collect on Earth’s surface.

A 91̽�� team looked at very old samples of these small meteorites to show that the grains could have reacted with carbon dioxide on their journey to Earth. Previous work suggested the meteorites ran into oxygen, contradicting theories and evidence that the Earth’s early atmosphere was virtually devoid of oxygen. The new was published this week in the open-access journal Science Advances.

“Our finding that the atmosphere these micrometeorites encountered was high in carbon dioxide is consistent with what the atmosphere was thought to look like on the early Earth,” said first author , a 91̽��doctoral student in Earth and space sciences.

At 2.7 billion years old, these are the oldest known micrometeorites. They were collected in limestone in the Pilbara region of Western Australia and fell during the Archean eon, when the sun was weaker than today. A 2016 paper by the team that discovered the samples suggested they at the time they fell to Earth.

That interpretation would contradict current understandings of our planet’s early days, which is that oxygen rose during the “,” almost half a billion years later.

Knowing the conditions on the early Earth is important not just for understanding the history of our planet and the conditions when life emerged. It can also help inform the search for life on other planets.

“Life formed more than 3.8 billion years ago, and how life formed is a big, open question. One of the most important aspects is what the atmosphere was made up of — what was available and what the climate was like,” Lehmer said.

The new study takes a fresh look at interpreting how these micrometeorites interacted with the atmosphere, 2.7 billion years ago. The sand-sized grains hurtled toward Earth at up to 20 kilometers per second. For an atmosphere of similar thickness to today, the metal beads would melt at about 80 kilometers elevation, and the molten outer layer of iron would then oxidize when exposed to the atmosphere. A few seconds later the micrometeorites would harden again for the rest of their fall. The samples would then remain intact, especially when protected under layers of sedimentary limestone rock.

The previous paper interpreted the oxidization on the surface as a sign that the molten iron had encountered molecular oxygen. The new study uses modeling to ask whether carbon dioxide could have provided the oxygen to produce the same result. A computer simulation finds that an atmosphere made up of from 6% to more than 70% carbon dioxide could have produced the effect seen in the samples.

“The amount of oxidation in the ancient micrometeorites suggests that the early atmosphere was very rich in carbon dioxide,” said co-author , a 91̽��professor of Earth and space sciences.

For comparison, carbon dioxide concentrations today are rising and are currently at about 415 parts per million, or 0.0415% of the atmosphere’s composition.

High levels of carbon dioxide, a heat-trapping greenhouse gas, would counteract the sun’s weaker output during the Archean era. Knowing the exact concentration of carbon dioxide in the atmosphere could help pinpoint air temperature and and acidity of the oceans during that time.

More of the ancient micrometeorite samples could help narrow the range of possible carbon dioxide concentrations, the authors wrote. Grains that fell at other times could also help trace the history of Earth’s atmosphere through time.

“Because these iron-rich micrometeorites can oxidize when they are exposed to carbon dioxide or oxygen, and given that these tiny grains presumably are preserved throughout Earth’s history, they could provide a very interesting proxy for the history of atmospheric composition,” Lehmer said.

Other co-authors are , a 91̽��professor emeritus of astronomy; , a 91̽��professor of Earth and space sciences; and , a former 91̽��undergraduate who is now at Rutgers University. The research was funded by NASA, the 91̽��Astrobiology Program, the 91̽��Virtual Planetary Laboratory and the Simons Foundation’s Collaboration on the Origins of Life.

For more information, contact Lehmer at olehmer@uw.edu or Catling at 206-543-8653 or dcatling@uw.edu.

, 91̽�� professor of astronomy and leader of NASA’s UW-based (VPL), has been named recipient of the 2018 Frank Drake Award from the SETI Institute. She is the first woman to receive the award.

The mission of the nonprofit , founded in 1984, is to “explore, understand and explain the origin and nature of life in the universe.” The Frank Drake award is named for the pioneering astronomer who in 1961 created the , which addressed the relative likelihood of advanced civilizations in space. Many view the equation still as a road map for astrobiology.

Meadows directs the UW’s and is principal investigator for the VPL, which is administered by the NASA Astrobiology Institute and includes researchers from the 91̽��as well as about two dozen institutions and NASA centers.

Researchers affiliated with the VPL, under Meadows’ guidance, use theoretical modeling to determine whether distant exoplanets might be able to support life. Recent research from the team has explored possible exoplanet and signs of life in distant atmospheres and for possibly habitable exoplanets.

“Vikki Meadows is a truly outstanding awardee for the Frank Drake Award,” said John Rummel, chair of the SETI Institute’s science advisory board, whose members choose the award. “She is a leader in the scientific estimation of environments on extrasolar planets, and in the search for signs of habitability and life. As a professor and mentor, she has infused others with her enthusiasm and research expertise — leading from the front.”

Bill Diamond, SETI Institute president and CEO, added: “It is an honor to recognize Professor Meadows for her innumerable contributions to astrobiology and for her inspiring leadership to students and colleagues alike. Vikki is pioneering our understanding of planetary habitability and the development of technologies and methodologies for biosignature detection and we are delighted to name her as the 2018 recipient of the Drake Award.”

Meadows in turn praised the collaboration and interdisciplinary nature of the work she directs.

“Over the past 18 years the Virtual Planetary Laboratory has transcended interdisciplinary boundaries to inform the upcoming telescopic search for habitable exoplanets and life beyond the solar system,” said Meadows. “Astrobiology addresses questions so big, they can’t be answered by a single researcher or even a single field.

“Instead, it takes a community with a staggering breadth of expertise and techniques, and the willingness to work with and learn from each other. It has been my very great honor to lead this spectacular team of interdisciplinary researchers, and a privilege to engage in such exciting and impactful research!”

Meadows is only the fourth recipient of the Frank Drake Award, the first of which was given to Drake himself in 2001. In 2002 the award went to physicist , a Nobel Prize winner for his work developing masers and lasers; in 2015 , astronomer and principal investigator for NASA’s planet-hunting Kepler Mission, received the honor.

Before coming to the 91̽��in 2007, Meadows was a research scientist at the (JPL) and an associate research scientist at the at the California Institute of Technology. She has been a scientist for the , the and the .

Meadows also is a member of the Science and Technology Definition Team for the concept and chairs . She is the 2001 recipient of the for scientific leadership and has received several NASA group achievement awards. She currently serves on the National Academy of Sciences committees for astrobiology and exoplanets, and the . She earned her Ph.D. in physics from the University of Sydney.

She will receive the award and discuss her ongoing work in a public event June 14 at SRI International in Menlo Park, California.

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Based on . For more about the SETI Institute, contact Rebecca McDonald, director of communications, at rmcdonald@seti.org; to learn more about Meadows and her work, contact her at vsm@astro.washington.edu.

The amount of biomass – life – in Earth’s ancient oceans may have been limited due to low recycling of the key nutrient , according to new research by the 91̽�� and the University of St. Andrews in Scotland.

The research, published Nov. 22 in the journal Science Advances, also comments on the role of volcanism in supporting Earth’s early biosphere — and may even apply to the search for life on other worlds.

The paper’s lead author is , a 91̽��doctoral student in Earth and space sciences; coauthor is , a research fellow at the University of St. Andrews and former 91̽��postdoctoral researcher. , 91̽��professor of Earth and space sciences, advised the researchers.

Their aim, Kipp said, was to use theoretical modeling to study how ocean phosphorus levels have changed throughout Earth’s history.

“We were interested in phosphorus because it is thought to be the nutrient that limits the amount of life there is in the ocean, along with carbon and nitrogen,” said Kipp. “You change the relative amount of those and you change, basically, the amount of biological productivity.”

Kipp said their model shows the ability of phosphorus to be recycled in the ancient ocean “was much lower than today, maybe on the order of 10 times less.”

All life needs abundant food to thrive, and the chemical element phosphorus – which washes into the ocean from rivers as phosphate — is a key nutrient. Once in the ocean, phosphorus gets recycled several times as organisms such as plankton or eukaryotic algae that “eat” it are in turn consumed by other organisms.

“As these organisms use the phosphorus, they in turn get grazed upon, or they die and other bacteria decompose their organic matter,” said Kipp, “and they release some of that phosphorus back into the ocean. It actually cycles through several times,” allowing the liberated phosphorus to build up in the ocean. The amount of recycling is a key control on the amount of total phosphorus in the ocean, which in turn supports life.

Buick explained: “Every gardener knows that their plants grow only small and scraggly without phosphate fertilizer. The same applies for photosynthetic life in the oceans, where the phosphate ‘fertilizer’ comes largely from phosphorus liberated by the degradation of dead plankton.”

But all of this requires oxygen. In today’s oxygen-rich oceans, nearly all phosphorus gets recycled in this way and little falls to the ocean floor.

Several billion years ago, in the Precambrian era, however, there was little or no oxygen in the environment.

“There are some alternatives to oxygen that certain bacteria could use, said co-author Stüeken. “Some bacteria can digest food using sulfate. Others use iron oxides.” Sulfate, she said, was the most important control on phosphorus recycling in the Precambrian era.

“Our analysis shows that these alternative pathways were the dominant route of phosphorus recycling in the Precambrian, when oxygen was very low,” Stüeken said. “However, they are much less effective than digestion with oxygen, meaning that only a smaller amount of biomass could be digested. As a consequence, much less phosphorus would have been recycled, and therefore total biological productivity would have been suppressed relative to today.”

Kipp likened early Earth’s low-oxygen ocean to a kind of “canned” environment, with oxygen sealed out: “It’s a closed system. If you go back to the early Precambrian oceans, there’s not very much going on in terms of biological activity.”

Stüeken noted that volcanoes were the biggest source of sulfate in the Precambrian, unlike now, and so they were necessary for sustaining a significant biosphere by enabling phosphorus recycling.

In fact, minus such volcanic sulfate, Stüeken said, Earth’s biosphere would have been very small, and may not have survived over billions of years. The findings, then, illustrate “how strongly life is tied to fundamental geological processes such as volcanism on the early Earth,” she said.

Kipp and Stüeken’s modeling may have implications as well for the search for life beyond Earth.

Astronomers will use upcoming ground- and space-based telescopes such as the James Webb Space Telescope, set for launch in 2019, to look for the impact of a marine biosphere, as Earth has, on a planet’s atmosphere. But low phosphorus, the researchers say, could cause an inhabited world to appear uninhabited — making a sort of “false negative.”

Kipp said, “If there is less life — basically, less photosynthetic output — it’s harder to accumulate atmospheric oxygen than if you had modern phosphorus levels and production rates. This could mean that some planets might appear to be uninhabited due to their lack of oxygen, but in reality they have biospheres that are limited in extent due to low phosphorus availability.

”These ‘false negatives’ are one of the biggest challenges facing us in the search for life elsewhere,” said , 91̽��astronomy professor and principal investigator for the NASA Astrobiology Institute’s , based at the UW.

“But research on early Earth’s environments increases our chance of success by revealing processes and planetary properties that guide our search for life on nearby exoplanets.”

The work was funded by grants from NASA and the National Science Foundation.

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For more information, contact Kipp at kipp@uw.edu, Buick at 206-543-1913 or buick@ess.washington.edu or Stüeken at
ees4@st-andrews.ac.uk.

NASA Exobiology grant NNX16AI37G to Prof. Buick.

A 91̽��-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 — but it may not always have been so.

91̽��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 — 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 Liège, Belgium, who is also a coauthor on this research. In 2016, Gillon’s 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’s 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 — with an average temperature of 173 Kelvin (minus 148 F) — 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 — small flywheels that help position the spacecraft — that can overwhelm planetary signals.

“Observing TRAPPIST-1 with K2 was an ambitious task,” 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’s wobble, the faintness of the star in the optical (the range of wavelengths where K2 observes) placed TRAPPIST-1h “near the limit of what we could detect with K2,” 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. “Finding the planet was really encouraging,” Luger said, “since it showed we can still do high-quality science with Kepler despite significant instrumental challenges.”

Luger’s 91̽��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 Liège 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 planets of TRAPPIST-1 orbiting for 90 Earth-days. After 15 Earth days, the animation focuses only on the outer three planets: TRAPPIST-1f, TRAPPIST-1g, TRAPPIST-1h. The motion freezes each time two adjacent planets pass each other; an arrow appears pointing to the location of the third planet. 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. The three-body resonance of the outer three planets causes the planets to repeat the same relative positions, and expecting such a resonance was used to predict the orbital period of TRAPPIST-1h.  

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