The Planet Hunters

Milky way

The Milky Way as seen from a telescope in the Namibian desert. (Photo courtesy of Gáspár Bakos)

From Gáspár Bakos’ desk at Princeton, he can see everything that happens at his telescopes on three continents. He can see wild burros nuzzle at the cables in Chile, warthogs wander by in Namibia, and kangaroos come a bit too close for comfort in Australia.

Despite the risks to his equipment, managing telescopes in three time zones across the Southern Hemisphere has a major advantage: it is always nighttime somewhere, so Bakos’ telescope network can search around the clock for planets outside our solar system, or exoplanets.

Bakos, an assistant professor of astrophysical sciences, is one of several Princeton faculty members involved in finding and studying exoplanets. Some researchers, like Bakos, are searching for the faint dimming of starlight that happens when a planet transits in front of a star. Others are trying to achieve what some have called the Holy Grail of planet hunting: direct imaging of an exoplanet. But detecting a planet is just the beginning. Researchers hope that by studying other solar systems they can confirm theories about how planets form and perhaps even learn whether life exists on these other worlds.

“Princeton is making contributions to the search for exoplanets in a number of areas,” said David Spergel, the Charles A. Young Professor of Astronomy on the Class of 1897 Foundation and chair of the Department of Astrophysical Sciences.

Discover and characterize

Just 20 years ago, finding planets around stars other than our sun was thought to be impossible — they were too far away, too dim and too close to the glare of their star. But today, due to creative strategies and new telescopes, about 1,000 planets have been detected and confirmed, with thousands of candidates awaiting confirmation.

The vast majority of the confirmed planets are quite unlike our own, however. Some are gas giants larger than Jupiter that orbit very close to their star — so close, in fact, that they can make an entire circuit around the star in a few days. Contrast this to our solar system, where Mercury, the closest planet to the sun, has an orbital period of 88 days — that is, it takes 88 days to orbit the sun. Earth makes the journey in 365 days.

The reason we’ve found so many of these large planets, which astronomers have nicknamed hot-Jupiters, is because they are relatively easy to detect with current methods. The two most successful ways of finding planets to date are to look for the periodic dimming of starlight when the planet crosses in front of the star, as Bakos’ telescopes do, or to look for the episodic wobbling of the star in response to the planet’s gravitational pull.

Professor Gáspár Bakos

Professor Gáspár Bakos (Photo by Pal Sari)

Through findings from a space-based NASA telescope known as Kepler, which hunted planets from 2009 until earlier this year, we now know that hot-Jupiters are the exception rather than the norm. “We can detect these hot-Jupiters because they pass in front of their stars fairly often and they block a significant fraction of the starlight,” said Bakos, “not because there are more of them than there are other kinds of planets.”

These other kinds of planets could include ones with conditions capable of supporting life. These planets lie in the “habitable zone” not too close and not too far from their star, and are capable of having liquid water on their surfaces.

One of Bakos’ telescope networks, HATSouth, is looking for exoplanets, including possibly habitable ones. HATSouth can detect planets with orbital periods of 15 to 20 days, which may not seem like much, but for certain classes of stars, namely the mid- to late-M dwarf stars, planets with 15-day periods lie in the habitable zone.       Bakos started building his first HAT — Hungarian made Automated Telescope — in 1999 while a student at Eötvös Loránd University in Budapest. The automated telescopes are relatively small — close in size to amateur models — but the lower costs allow more of them to be deployed. An earlier network Bakos built, HATNet, which came online in 2003 and consists of telescopes at sites belonging to the Smithsonian Astrophysical Observatory (SAO) in Arizona and Hawaii, has discovered 43 candidate planets.

HATSouth became operational in 2009 and is a collaboration between Princeton University, the Max Planck Institute for Astronomy, Australian National University and Pontificia Universidad Católica de Chile. Originally funded by the National Science Foundation and SAO, the network consists of six robotic instruments located at Las Campanas Observatory in Chile, the High Energy Stereoscopic System site in Namibia and Siding Springs Observatory in Australia. To date, HATSouth has detected a handful of planets, and has a dozen candidates awaiting confirmation. So far, all of the planets have fairly short orbital periods — they orbit their stars in one to three days — but Bakos is optimistic about HATSouth and his new project, HATPI, for which he has received funding from the David and Lucile Packard Foundation. HATPI is a wide-field camera system that will continuously image the entire night sky at high resolution and precision for five years, with the goal of identifying planets with longer orbital periods. Bakos’ team includes Joel Hartman and Kaloyan Penev, both associate research scholars; Zoltan Csubry, an astronomical software specialist; Waqas Bhatti and Miguel de Val-Borro, both postdoctoral research associates; and Xu (Chelsea) Huang, a graduate student.

Direct imaging

While HATSouth looks for dips in starlight that indicate the presence of a planet, other researchers at Princeton are aiming to directly image exoplanets. Such imaging is possible, for example, when the planet appears on the right or left side of the star rather than directly in front of it. To date, only a handful of exoplanets have been observed this way, because the star’s light is so bright that seeing a nearby planet is like trying to see a speck of dust in the glare of a headlight.


Professor Jeremy Kasdin (Photo by Alexandra Kasdin)

Jeremy Kasdin’s group is working to develop instrumentation for direct imaging. “The idea is to block out the star’s light so that it is possible to see the planet,” said Kasdin, a professor of mechanical and aerospace engineering.

Astronomers have used this concept to study the sun since the 1930s: they place a black disc, called a coronagraph, at the center of the telescope’s image to block light from the sun so they can study solar flares on its surface.

For planet-watching however, this light must be blocked with great precision. Because light acts as a wave, it diffracts around the edge of the telescope and, without a coronagraph, creates concentric patterns on the resulting image (see illustration, page 27), just as water makes ripples in a pond when it flows past an obstacle. These patterns obscure the planet.

To eliminate or change these patterns, researchers at Princeton’s High Contrast Imaging Laboratory, led by Kasdin, are developing a coronagraph with a distinctive shape and arrangement of slits that alter the patterns in ways that can permit detection of planets. This “shaped-pupil coronagraph” is being developed by Kasdin and his collaborators Spergel, Edwin Turner, a professor of astrophysical sciences, Michael Littman, a professor of mechanical and aerospace engineering, and Robert Vanderbei, a professor of operations research and financial engineering, along with postdoctoral research associates Tyler Groff and Alexis Carlotti and graduate students Elizabeth Jensen and A.J. Eldorado Riggs. The coronagraph could be sent up in a space-based telescope mission under consideration for later in the decade.

Pupil diagramThe Kasdin lab is also working on another light-blocking idea called an occulter. This is a giant sail that could fly in space, ahead of a space-based telescope, to block out light from a star. “It is sort of like holding your hand up to block the sun while you watch a bird in the sky,” said Kasdin. The occulter would be launched folded-up, like a flower bud with petals that would unfold in space to create a shield with a diameter of about 40 meters (131 feet) that would fly about 11,000 kilometers — or roughly 6,800 miles — ahead of the telescope to block the star’s light.

Occulter diagramAt Princeton’s Forrestal Campus three miles from the main campus, graduate student Daniel Sirbu is testing four-inch high models of occulters. The team collaborates closely with NASA’s Jet Propulsion Laboratory at the California Institute of Technology where an occulter is being built and tested with the help of engineers at Northrop Grumman and Lockheed Martin.

Video: How an occulter would unfold in space:

In addition to blocking light, Kasdin’s group is working to improve the technology for correcting faulty imaging caused by the Earth’s atmosphere, as well as heat, vibrations and imperfections in the telescopes themselves. All ground-based telescopes suffer from poor imaging quality due to atmospheric water vapor that is present even on cloudless nights, causing turbulence that makes stars appear to twinkle. Astronomers can correct these distortions using a technology known as adaptive optics which involves bendable mirrors. The Kasdin lab is working on improvements to these systems.

Coronagraphs and adaptive optics already are in use in a handful of telescopes, including the Subaru Telescope, operated by the National Astronomical Observatory of Japan (NAOJ), in Hawaii. Princeton researchers, including Kasdin and his colleagues, as well as Turner; astrophysical sciences professor Gillian Knapp; Timothy Brandt, a 2013 Ph.D. in astrophysical sciences; and others, are part of an international collaboration led by NAOJ scientist Motohide Tamura that is known as SEEDS (Strategic Explorations of Exoplanets and Disks with Subaru).

Kasdin’s group is working on designing an instrument, the Coronagraphic High Angular Resolution Imaging Spectrograph (CHARIS), to add to the Subaru Telescope to look at the different kinds of light, or spectra, emitted by a planet. Just as a prism splits white light into its rainbow of colors, CHARIS contains prisms and special filters that allow researchers to see the different wavelengths of light. These wavelengths provide signatures that can reveal the planet’s temperature and hint at which atoms and molecules are present around the planet. Graduate student Mary Anne Peters and Groff are working on CHARIS.

“The instrument makes it possible to look at the spectrum at each point in the image,” said Groff, “so you can distinguish the planet light from the star and see whether the planet’s atmosphere is uniform or cloudy, and you can get an idea of age because as the planet gets older, it cools.”

Beyond detection

Detecting exoplanets, whether by watching for their transits or by direct imaging, is just the first step in developing an understanding of these objects, said Adam Burrows, a professor of astrophysics who uses data gathered from these campaigns to construct theories about the characteristics of exoplanets and planetary systems. “Once we’ve detected planets, how do we figure out their makeup, their atmospheric compositions and temperatures, and their climates? These are the kinds of questions we are interested in answering,” Burrows said.

These questions can be addressed only by detecting and interpreting the spectral emissions that will be detected by instruments such as CHARIS, but so far these are only available for large exoplanets. “As larger ground-based telescopes and space-based missions like the James Webb Space Telescope come online,” Burrows said, “we will have data from planets that are closer in size to the Earth. The objects being studied now are but stepping stones toward the broader characterization of the planets in general in the galaxy and in the universe.”

This broader characterization includes ongoing studies by a number of other Princeton researchers, including Markus Janson, a NASA Hubble Postdoctoral Research Fellow in astrophysical sciences, who studies how planets are formed from dust and debris that orbits the star. Other researchers studying planet formation include Roman Rafikov, assistant professor in astrophysical sciences, and Ruobing Dong, who earned his Ph.D. in 2013 while working on the SEEDS project and is now a NASA fellow at the University of California-Berkeley. Emily Rauscher, a NASA Sagan Postdoctoral Fellow in astrophysical sciences, is studying the climate on these faraway worlds.

Many researchers hope that studying exoplanets will help us learn more not only about planetary formation and solar systems but also about whether other planets exist that could support life. The instruments and telescope networks being developed at Princeton could lead the way. And if a wild burro chews on a cable now and then, well, it is part of the cost of learning what lies outside our solar system.

Box: Data mining for planets

Xu (Chelsea) Huang

Xu (Chelsea) Huang (Photo by Keren Fedida)

Data mining for planets Xu (Chelsea) Huang remembers the thrill of finding her first planet. “It was exciting,” said the graduate student in astrophysical sciences. Huang found that planet and many more in 2012 while looking through a publicly available data set from NASA’s space-based Kepler mission, which scans for dips in starlight as the planet crosses in front of the star. Using techniques developed for analyzing HATNet findings under the guidance of Associate Research Scholar Joel Hartman and Assistant Professor Gáspár Bakos, Huang found 150 potential planets — many of which were hot “super-Earths” that are slightly large than Earth but orbiting their host stars much more closely — that the Kepler team and others had missed. The paper was published earlier this year in the journal Monthly Notices of the Royal Astronomy Society. When the Kepler mission later released an updated list of possible planets, about half the ones that Huang had found were on it.

Box: Forecasting the climate on other worlds

Emily Rauscher

Emily Rauscher, a NASA Sagan Postdoctoral Fellow at Princeton’s Department of Astrophysical Sciences, is modeling the climate on exoplanets. (Photo by Andrew Howard)

Emily Rauscher, a NASA Sagan Postdoctoral Fellow in the Department of Astrophysical Sciences, said that new acquaintances don’t believe her when she says she does climate modeling for exoplanets. Rauscher uses what is known about our solar system, plus the laws of fluid dynamics and the exoplanets’ orbital period and mass, to try to understand the climate on these faraway worlds. “If you watch a planet throughout its orbit, you can see the change in the amount of light emitted from the planet’s night side versus from its day side,” Rauscher said. “Because it is very hot in the day and very cold at night, we expect winds to blow around the planet, and by measuring the difference in brightness coming from the planet, we can detect how the wind affects the planet’s temperature.” Rauscher is fascinated by the idea of life on other planets but said that there is plenty to discover even on uninhabitable hot-Jupiters. “There is a big push to discover Earth-like planets,” she said, “but there is a lot we can learn from studying the planets we know about already.”

Box: Exploring how planets are formed

Studying exoplanets also could help researchers learn more about how planets are formed from dust and debris that orbits the star, said Markus Janson, a NASA Hubble Postdoctoral Research Fellow in the Department of Astrophysical Sciences. Some of the material that doesn’t end up in planets is collected in rings called debris disks, he explained. Our solar system has two such debris disks: an asteroid belt between Mars and Jupiter and the Kuiper Belt beyond Neptune.

“Our conventional theory of how planets form— that dust sticks together and forms into planets like Earth, and that sometimes large amounts of gas accumulate onto a rocky core to form gas giants like Jupiter — is based on what we’ve observed in our solar system.” Janson said. “Now we can study other solar systems, so we can test this theory.” A recent study by Janson and colleagues, accepted for publication by The Astrophysical Journal, indicates that the majority of exoplanetary systems probably did form in this manner.

-By Catherine Zandonella

Italian Master Drawings: Exhibition goes beneath the surface

Michaelangelo picture

On display will be an architectural sketch depicting a floor plan for an unrealized chapel (bottom). It was only through the use of infrared reflectography in the mid-1990s that the sketch, located on the reverse side of a study of profile heads (top) that had been tentatively associated with the artist, was confirmed as the work of Michelangelo. Michelangelo, Bust of a Youth and Character Head of an Old Man, 1520s. Black chalk on tan laid paper. Gift of Frank Jewett Mather Jr. (Photos courtesy of the Princeton University Art Museum)

A new exhibition, 500 Years of Italian Master Drawings from the Princeton University Art Museum, on view from Jan. 25 through May 11, 2014, explores the mental process behind creation through nearly 100 rarely seen highlights by such masters as Vittore Carpaccio, Michelangelo, Luca Cambiaso, Gianlorenzo Bernini, Guercino, Salvator Rosa, Giambattista and Giandomenico Tiepolo, and Amedeo Modigliani.

The creative process is captured in the Italian word disegno, which translates as “drawing” or “design.” But the term is far richer, said Laura Giles, the Heather and Paul G. Haaga Jr., Class of 1970, Curator of Prints and Drawings at the Princeton University Art Museum. She defined the term as “encompassing both the mental formulation and the physical act of creation,” a construct that was, she added, “embedded into the Italian drawing process by the 15th century.”

The exhibition represents the culmination of more than 35 years of scholarship on the museum’s Italian drawings, including the acquisition of more than 125 works that have entered the collection through gift, bequest or purchase.

Many of the drawings have benefited from new insights concerning attribution, iconography, dating, function and provenance. Among the many noteworthy findings is the discovery, first made in the 1990s, of an architectural sketch by Michelangelo on the reverse side of a study of profile heads that had been tentatively associated with the artist. The drawing, depicting a floor plan for an unrealized chapel, is obscured from view by an 18th-century collector’s mount. Only through the utilization of infrared reflectography was the floor plan revealed.

The exhibition celebrates the publication of a new scholarly catalogue, Italian Master Drawings from the Princeton University Art Museum, authored and edited by Giles, Postdoctoral Research Associate Lia Markey and Renaissance art specialist Claire Van Cleave, with contributions from many leading scholars.

Laura Giles

Laura Giles, the Heather and Paul G. Haaga Jr., Class of 1970, Curator of Prints and Drawings, is curating the exhibition. (Photo by Henry Vega)

The catalogue is the first academic exploration of the collection since 1977. The research for the catalogue received significant support from The Getty Foundation’s Cataloguing of Museum Collections Grant Program.

In tandem with the publication of the exhibition catalogue, the museum will add updated research and high-resolution images to its online collections catalogue, allowing global access to the Italian drawings collection.

The museum’s collection of over 80,000 works includes more than 1,000 Italian drawings from the 15th through the early 20th century, encompassing the history of Italian art from the early Renaissance to early modernism.

–By Erin Firestone

Globalization raises new ethical questions

Eric Gregory

Eric Gregory, a professor of religion, studies our obligations to strangers in a world of ever-growing connectedness.
(Photo by Brian Dorsey)

As strangers become more accessible to us through global markets and new media, so too do questions of our obligations to them. For Eric Gregory, who examines religious and philosophical ethics, our ever-growing connectedness to people around the world has necessitated a closer look at how and why we should help those in need.

“We have all sorts of relationships to our family, our friends, our fellow citizens,” he said. “Is it justified to show preference to family and friends in terms of how you treat them versus the person with whom you don’t have that kind of relationship? How do we think about those who might be suffering in terms of more distant relations?”

Gregory, a professor of religion, explained that Christian theologians such as St. Augustine have tackled the question of what people owe strangers in their writings, but lived in a time when strangers were far more distant than they are at present. Today, an individual can plausibly interact not just with their next-door neighbor, or with someone one village over, but with an entire globe. Yet global realities such as nationalism and religion can impose constraints on our dealings with others whom we have yet to meet, Gregory said. This tension, he explained, means we now need to balance our understanding of concern for distant others with the particularity of our relations with more immediate communities.

The calculus is a complex one, and the debate typically centers on efficiency. But Gregory, with support from The Tikvah Center for Law & Jewish Civilization at New York University School of Law and the National Endowment for the Humanities, is examining classical texts by authors from the religious and secular traditions to move past a utilitarian approach for investigating the question. “I think one of the virtues or hopes of the humanities is that there’s an ongoing conversation about how best to live a life,” he said.

-By Tara Thean

Small RNAs fight cancer’s spread

Tumor cells spread toward bone

Breast cancer cells (right) spread toward the hindlimb bone (left), using natural bone-destroying cells (osteoclasts) to continue their advance. (Image courtesy of Yibin Kang)

Cancer patients may benefit from a dual strategy for tackling their disease in a class of molecules called microRNAs. Molecular biology graduate student Brian Ell has revealed that microRNAs — small bits of genetic material capable of repressing the expression of certain genes — may serve as both therapeutic targets and predictors of metastasis, or a cancer’s spread from its initial site to other parts of the body.

MicroRNAs are specifically useful for tackling bone metastasis, which occurs in about 70 percent of late-stage cancer patients. During bone metastasis, tumors invade the tightly regulated bone environment and take over the osteoclasts, cells that break down bone material. These cells then go into overdrive and dissolve the bone far more quickly than they would during normal bone turnover, leading to bone lesions and ultimately pathological conditions such as fracture, nerve compression and extreme pain.

“The tumor uses the osteoclasts as forced labor,” explained cancer metastasis expert in the Department of Molecular Biology Yibin Kang, who is Ell’s adviser. Their research is supported by the National Institutes of Health, the Department of Defense, the Susan G. Komen for the Cure Foundation, the Brewster Foundation and the Champalimaud Foundation.

MicroRNAs can reduce that forced labor by inhibiting osteoclast proteins and thus limiting the number of osteoclasts present, as Kang’s lab observed when mice with bone metastasis injected with microRNAs developed significantly fewer bone lesions. Their findings suggest that microRNAs could be effective treatment targets for tackling bone metastasis. And that’s not all: microRNAs may also help doctors detect the cancer’s spread to the bone, with trials in human patients demonstrating a strong correlation between elevated levels of another group of microRNAs and the occurrence of bone metastasis.

Kang, the Warner-Lambert/Parke- Davis Professor of Molecular Biology, said he ultimately hopes to extend mice experimentation to clinical trials. “In the end, we want to help the patients,” he said.

–By Tara Thean

Fragile families, fragile children

Sara McLanahan

Sara McLanahan is the principal investigator on the Fragile Families and Child Wellbeing Study, which found that children of unmarried parents encounter a great deal of instability. (Photo by Larry Levanti)

Relationships are complicated in the best of times, but even more so for unmarried parents and their children. Children born to unmarried parents encounter considerable instability in their family life when their biological parents end relationships and form relationships with new partners, according to data from the Fragile Families and Child Wellbeing Study, an initiative spearheaded by family demography expert in the Woodrow Wilson School Sara McLanahan.

The study found that just over a third of unmarried parents who are romantically involved at birth are still together by the time their child is 5 years old, compared to 80 percent of married parents. More than 60 percent of unmarried mothers have by then also changed residential partners — that is, had one or more new partners move in or out of the household.

Children encounter an even wider cast of characters if researchers take account of mothers’ more casual dating partnerships, with more than 75 percent of unmarried mothers experiencing a change in either a co-residential or short-term dating relationship. Half-siblings are also part of the picture: nearly 50 percent of children born to unmarried mothers live with a half-sibling by the time they reach age 5.

“The bottom line is that very few children born to unmarried parents are living in stable single-mother families,” said McLanahan, the William S. Tod Professor of Sociology and Public Affairs. The Fragile Families study is funded by the Eunice Kennedy Shriver National Institute of Child Health & Human Development, the National Science Foundation, the U.S. Department of Health and Human Services, and several foundations.

That unmarried parents are more likely to have low education and income levels means that their children often fare worse as well, reporting more physical and mental health problems. Children of unmarried parents also tend to score lower on reading and math tests. McLanahan explained that while economic adversity accounts for much of this disadvantage, a high level of instability and family complexity may contribute to these negative outcomes.

–By Tara Thean

Green roofs’ energy savings hinge on climate

Green roofs

Green roofs, such as these above the dormitory at Princeton’s Butler College, must be designed so that they take advantage of local climate conditions. (Photo by Brian Green)

Urban planners who want green roofs in their cities need to remember that the roofs may not work the same way in different climates. Green roofs, which are covered with a layer of a vegetation to keep the building cool, perform differently according to the amount of solar radiation and precipitation present, according to Elie Bou-Zeid, an assistant professor of civil and environmental engineering.

In a study published in the journal Building and Environment, Bou-Zeid and his team found that the green roofs on the campuses of Princeton and Tsinghua University in Beijing performed similarly when the researchers controlled for the radiation and precipitation levels in the two areas, indicating the levels’ importance in green roof function. With support from the U.S. Department of Energy through Pennsylvania State University’s Energy Efficiency Building Hub and the National Science Foundation of China, the researchers used surface temperature, heat convection from the Earth’s surface to the atmosphere, and the amount of incident energy conducted through the roof as performance measures.


Elie Bou-Zeid, an assistant professor in civil and environmental engineering, stands with a wireless sensing station that measures wind speed and direction, air temperature, relative humidity, surface temperature, and incoming and reflected solar radiation from black and white roofs. (Photo by Elle Starkman)

Bou-Zeid said he hopes his work will help city planners account for the specific climatic conditions in their cities when integrating rooftop gardens into their building decisions, and assess the potential benefits of irrigation that improves green roof performance in dry periods.

Highly effective green roofs are important in cities, which suffer from the “urban heat island” phenomenon: a sustained period of excessive heat in metropolitan areas caused by buildings that absorb heat and release it into the atmosphere, a lack of vegetation, and high human activity. Increasing the number of green spaces will trap rainwater, Bou-Zeid explained, thereby providing a “heat sink” in which evaporation of that water encourages heat loss and cools things down.

The New York City Office of the Mayor is taking the heat waves of the city particularly seriously, Bou-Zeid said. New York’s asphalt and concrete roads and buildings actively absorb heat, making the area sometimes up to seven degrees warmer than its neighbors. Bou-Zeid is working with representatives from the NYC Cool Roofs program, a citywide initiative to promote the use of reflective, white rooftop coating, to examine which areas of the city will suffer most during a heat wave. He later hopes to relate physical maps of area-specific heat stress in the city to physical health indicators.

“Heat waves are the deadliest natural disasters,” Bou-Zeid said. He noted that the 2003 European heat wave, which produced the Northern Hemisphere’s hottest-ever August, caused up to 70,000 deaths in the region. “They are silent killers.”

–By Tara Thean

Secrets of the Southern Ocean


Marine geochemistry specialist Robert Key doesn’t consider himself particularly prone to depression. Yet emails to his wife from a research vessel on the freezing waters of the Southern Ocean depicted an emotional slump amid harsh conditions and brutal working hours.

“It’s wet and windy and miserable, and if you’re down there in the winter, then it’s dark the whole time as well,” said Key, a research oceanographer in the Program in Atmospheric and Oceanic Sciences. “You’re away from contact with people. Essentially all you do is eat and work.”

Robert Key

Robert Key collects water samples during a research voyage aboard the NOAA vessel, the Ronald H. Brown. (Photo courtesy of Robert Key)

But Key and other Princeton researchers push through the challenging conditions because they want to learn more about the waters at the bottom of the globe, which have a significant impact on the Earth’s ecosystems and climate. By collecting and analyzing samples of seawater, and using the results to help construct computer models of the ocean and atmosphere, the scientists aim to understand the Southern Ocean’s major influence on the world’s carbon and nutrient cycles. In doing so, they hope to provide insight into what our planet will look like in an era of human-driven climate change.

Though it makes up less than a third of the world’s ocean coverage, the Southern Ocean surrounding Antarctica soaks up about half of the man-made carbon dioxide absorbed by the world’s oceans from the atmosphere each year. Its waters act as a giant pump, with currents that carry carbon dioxide down into the deep recesses of the ocean where the carbon can remain for roughly 1,000 years. In return, currents bring up frigid water from the deep, water that has never been exposed to today’s elevated levels of carbon dioxide and therefore is able to absorb more of the gas than today’s surface waters.


This frigid water also absorbs heat: the Southern Ocean has helped prevent the planet from warming up as much as it might have by now from human activity, according to Jorge Sarmiento, the George J. Magee Professor of Geoscience and Geological Engineering. Because its waters are so cold, the Southern Ocean absorbs about 60 percent of the excess heat that moves annually from the atmosphere into the ocean.

Along with absorbing carbon dioxide and heat, the Southern Ocean regulates the movement of nutrients such as nitrogen and phosphorus. The ocean’s patterns of circulation transport nutrient-rich water from the deep ocean back to the surface, where currents carry the nutrients to the north. These nutrients provide three-quarters of the ocean’s biological productivity, spurring the production of plant matter called phytoplankton that serves as the basis of the aquatic food chain.

Slideshow of the summer voyage of the research vessel R/V S.A. Agulhas II:

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“It’s old, it’s cold and it’s rich,” said Sarmiento. These traits, he explained, enable the Southern Ocean to have the influence that it does over global climate and nutrient regulation — and challenges scientists to find out how this massive storage vessel for carbon, nutrients and heat might react to rising carbon emissions and climate change.

Southern Ocean diagramA key question is whether rising carbon emissions will boost or hamper the Southern Ocean’s ability to sponge up carbon dioxide. Changing wind and rainfall patterns due to a warming Earth could shift how much carbon and heat the Southern Ocean can store in either direction, according to Daniel Sigman, the Dusenbury Professor of Geological and Geophysical Sciences. If winds pick up, for example, then mixing between the Southern Ocean’s deep and shallow waters may pick up as well. If high-latitude rainfall increases, more freshwater on the polar ocean’s surface may mean a higher density difference between the surface and deep waters, leading to less mixing. Depending on the response of ocean biology to this range of possible changes in ocean circulation, the rate of carbon uptake may either rise or fall.

Scientists have built models to predict how the Southern Ocean’s carbon sink will behave over the next several decades. But these researchers lack sufficient observations of the Southern Ocean to adequately inform high-resolution models of the ocean’s circulation, to assess the predictive powers of their models, or even to understand which processes are the most important for the models to provide accurate simulations. Data for winter at the Southern Ocean is especially sparse. A major reason: the brutal working conditions in the winter.

“I thought I knew something about winds and bad waters and blizzards and storms,” said climate modeler Joellen Russell, who grew up in an Eskimo village 31 miles above the Arctic Circle. “I couldn’t get my head around it.” Russell, an associate professor at the University of Arizona who collaborates with Sarmiento, remembers being thrown across her cabin one night when a wave slammed against the side of the ship, and how buckets of water would crash onto the deck in such great volume that the water looked black. “You’re like, ‘I want to go home now,’” she said.

Slideshow of the winter voyage of the research vessel R/V S.A. Agulhas II:

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Field research in the Southern Ocean, which typically lasts between 30 to 70 days, holds scientists to a punishing schedule. They can work up to 18 hours a day, seven days a week. Russell described a typical night working in a chemistry lab during one of her research cruises: she slept on a bean bag on the lab’s floor, waking up every 15 minutes to a beeping noise reminding her to perform certain laboratory tasks. The workhorse nature of the job stems mainly from the high cost of being out on the water: tens of thousands of dollars per day. “Because it’s so expensive to get out there, you feel the pressure,” Russell said. “You want every last bit of data you can get.”

On board the ship

Researchers aboard the National Oceanic and Atmospheric Administration (NOAA)’s ship, the Ronald H. Brown, performed experiments in the ship’s laboratory to analyze water samples for nutrient levels, alkalinity and other factors during this research cruise in March and April 2010. Clockwise from front left are: Benjamin Botwe, assistant lecturer at the University of Ghana; Charles Fischer, oceanographer at NOAA Atlantic Oceanographic and Meteorological Laboratory; Calvin Mordy, associate at NOAA Pacific Marine Environmental Laboratory; Yui Takeshita, graduate student at Scripps Institution of Oceanography; and Laura Fantozzi, staff research associate at Scripps Institution of Oceanography. (Photo by Ivy Frenger)

Fortunately, researchers now can use robotic battery-powered floats that provide salinity and temperature measurements for up to five years. About 3,500 of these floats, called Argo floats, are making measurements around the world’s seas. Funding and management for the floats come from the contributions of 23 countries.

But there are measurements that Argo floats leave out, and Associate Professor of Geosciences Frederik Simons hopes to fill in some of the gaps with an instrument of his own. Simons has spent the past few years developing an autonomous buoy, in collaboration with University of Rhode Island professor Harold Vincent, that detects GPS position, time and ocean depth while measuring seismic waves generated by distant earthquakes that are converted to water pressure waves at the ocean floor. They call their instrument the Son-O-Mermaid.
Simons hopes the Son-O-Mermaid will overcome the paucity of oceanic data, particularly in the Southern Ocean. “We are making pictures of the interior of the Earth using waves recorded through the Earth,” said Simons. “When we do that, we can learn about mantle plumes, subduction zones, mid-ocean ridge earthquakes — basic questions that people wonder about.”

Though the Son-O-Mermaid, which can stay in the water in a range of ocean conditions, is set up for use in seismology, researchers could easily adapt it for their own use in physical, chemical or biological data collection, according to Simons.

A three-decade legacy

Much of the groundwork for today’s understanding of the Southern Ocean comes from earlier work by Sarmiento, whose interest in the Earth’s carbon cycle began around 1984. This was when he co-wrote — with J. Robert Toggweiler, an oceanographer at the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory in Princeton — one of the first papers to point out that the Southern Ocean’s waters dictate the carbon dioxide content of the atmosphere. His paper was one of three to become so influential in the field as to be given a nickname: the “Harvardton Bears” papers, so named because their authors were affiliated with Princeton, Harvard or the University of Bern.

Jorge Sarmiento

Professor Jorge Sarmiento (Photo by Denise Applewhite)

This early work revealed that the Southern Ocean acts as a leak in the ocean’s biological pump. The Southern Ocean actually releases some carbon dioxide into the atmosphere because phytoplankton, which consume carbon dioxide as they grow, cannot keep up with the rapid supply of nutrients and carbon from the underlying deep ocean and leave much of it unused.

Sarmiento has continued to investigate the ocean’s role in global climate over the three decades since, with a particular focus on the Southern Ocean. In 1996, he constructed a model that predicted how future global warming would affect the ocean’s ability to absorb carbon dioxide: he suggested that warming would weaken the ocean’s circulation, which would in turn compromise the movement of carbon dioxide to deep waters. His further research proposed new ideas about why atmospheric carbon dioxide was lower during ice ages, and he identified pathways that nutrients follow as they migrate from the Southern Ocean to seas farther north.

“You just fall into something, a fascinating problem that you can’t let go of,” said Sarmiento. “Suddenly things come together and you get an answer, and it’s different from anything that anyone else has come up with.”

Sarmiento, who began his Princeton career as the University’s only biogeochemist, is now surrounded by several colleagues examining the oceans’ biogeochemical processes: Michael Bender, professor of geosciences; François Morel, the Albert G. Blanke Jr. Professor of Geosciences; Stephen Pacala, the Frederick D. Petrie Professor in Ecology and Evolutionary Biology; and Bess Ward, the William J. Sinclair Professor of Geosciences.

Sarmiento also has good company in researchers such as Sigman, who has made major contributions to scientists’ knowledge about the Southern Ocean. Sigman is trying to understand the role of the Southern Ocean in global climate by studying past climate changes and reconstructing the strength of the carbon dioxide leak back through Earth history.

In particular, Sigman is exploring the hypothesis that the Southern Ocean carbon dioxide leak was reduced during the ice ages.

Daniel Sigman

Professor Daniel Sigman (Photo by Denise Applewhite)

Sigman uses marine sediments to identify how iron — which is carried to the Southern Ocean in dust originating in Africa, Australia and South America — has affected phytoplankton growth in the Southern Ocean and contributed to the global climate cycles of the Earth’s last 1 million years.

An essential nutrient for phytoplankton, iron is relatively scarce in the Southern Ocean. The lack of iron prevents phytoplankton populations from growing to numbers large enough for them to fully consume the ocean’s nutrients, including carbon dioxide. This means that a lot of carbon dioxide can leak right back into the atmosphere.

This may not have always been the case, however. By studying marine sediments, Sigman and his colleagues found evidence that more dust deposits in the past may have enabled phytoplankton in the subAntarctic zone — the Southern Ocean region roughly 40 to 50 degrees above the South Pole — to consume more carbon dioxide, which may have helped hold down the global temperature. That lower temperature would have caused stronger ice ages starting 1 million years ago, the scientists wrote in a 2011 paper published in Nature. With more dust — and thus more iron — in the water, phytoplankton growth escalated and ensured that excess carbon in the water was consumed before it escaped into the atmosphere as carbon dioxide gas. This allowed heat-trapping carbon dioxide to stay in the ocean.

“Our hypothesis is that iron supply to the sub- Antarctic zone is one of the two key ingredients that lowers atmospheric carbon dioxide during ice ages,” Sigman said.

The other ingredient is changes in the mixing of water between the surface and the deep ocean. With colleagues from the Swiss Federal Institute of Technology in Zurich, Sigman found evidence that the subAntarctic zone changes were complemented by circulation changes in the Antarctic zone, the Southern Ocean region adjacent to the Antarctic continent and south of the subAntarctic zone. At the beginning of each ice age of the last million years, the Antarctic zone appeared to reduce its vertical mixing, which may have slowed the leakage of carbon dioxide to the atmosphere and trapped more of it in the ocean, providing the first cooling step of the impending ice age. The cooling, in turn, encouraged more dust to be deposited in the ocean, in part because continents became drier and dustier. And the phytoplankton fertilized by this dust may then have caused further carbon dioxide drawdown and global cooling.

These two factors — more dust-borne iron and more mixing of waters — could have allowed the Southern Ocean to absorb more carbon dioxide in the past. These findings may hint at how the Southern Ocean will change as the Earth warms in the next few years, Sigman said, with the possibility of more mixing and less iron input.

With the Southern Ocean playing a considerable role in what the climate might look like in years to come, scientists such as Sarmiento see it as their responsibility to uncover knowledge that will enable others to understand the systems at work. But though Sarmiento and Princeton’s other Southern Ocean specialists have made considerable strides in investigating the ocean’s place in the global climate, challenges remain — from increasing public awareness of the Southern Ocean’s critical importance to uncovering cost-effective, automated methods of obtaining more data from this poorly sampled region.

Key especially looks forward to the latter: he hopes that new technologies will afford him a break from the rigors of Southern Ocean fieldwork.

“It gets harder and harder to work a 72-hour week,” he said. “You get too old to go out to sea as much as I used to.”

-By Tara Thean

Telescopes take the universe’s temperature

Two telescope projects are measuring cosmic microwave background radiation with the goal of understanding more about the universe’s early history. The telescopes (pictured) are located on a peak in the Atacama Desert in Chile. (Image courtesy of ACT Collaboration)

Two telescope projects are measuring cosmic microwave background radiation with the goal of understanding more about the universe’s early history. The telescopes (pictured) are located on a peak in the Atacama Desert in Chile. (Image courtesy of ACT Collaboration)

Two telescopes on a Chilean mountaintop are poised to tell us much about the universe in its infancy. They are surveying the faint temperature fluctuations left over from the explosive birth of the universe, with the goal of piecing together its early history and understanding how clusters of galaxies evolved.

The telescopes are measuring these temperature fluctuations, known as cosmic microwave background radiation or CMB for short, from their perch 17,000 feet above sea level in Chile’s desolate Atacama Desert, where a dry atmosphere permits radiation to reach Earth with relatively little attenuation. In contrast to backyard telescopes that help us see visible light from stars and planets, these telescopes collect invisible microwave radiation.

Lyman Page

Lyman Page

These invisible waves are mostly uniform but contain slight differences in intensity and polarization that hold a wealth of information for cosmologists, said Lyman Page, the Henry De Wolf Smyth Professor of Physics. Page and Professor of Physics Suzanne Staggs co-lead two telescope projects, the Atacama Cosmology Telescope (ACT) and the Atacama B-mode Search telescope (ABS), which are funded by the National Science Foundation.

“If you imagine the temperature perturbations as a distant mountain range, the peaks and valleys correspond to the temperature variations,” Page said. “By looking at the patterns — the spacing between peaks, and whether they are narrow or fat — we are able to answer questions about the composition and evolution of the universe,” Page said.

ACT, which is about 18 feet across and looks like a giant metal bowl, has already made new discoveries, and confirmed and extended the findings of other CMB surveys, including two space-based telescopes, NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) and the European Space Agency’s Planck mission. A new, more sensitive receiver is currently being mounted on the ACT telescope, which is a collaborative effort with David Spergel, Princeton’s Charles A. Young Professor of Astronomy on the Class of 1897 Foundation, along with researchers at the University of Pennsylvania, National Institutes of Standards and Technology, the University of British Columbia, and 10 other institutions contributing significantly to the instruments and analysis.

Suzanne Staggs

Suzanne Staggs

The CMB originated in the hot plasma soon after the Big Bang, which cosmologists consider to be the birth of the universe. As the universe expanded, the radiation propagated, carrying the secrets of the early universe with it. One of the questions is why the CMB on opposite sides of the universe is so similar in temperature. The leading explanation of this observation is the inflation model, which posits that the universe underwent a rapid acceleration of its expansion just after the Big Bang.

This is where the lower-resolution, second telescope comes in. Co-led by Staggs, the ABS is looking for signs of inflation. “Inflation should produce gravitational waves which create patterns in the CMB called ‘B modes,’” said Staggs. B modes are extremely faint — to measure them requires an instrument that can detect temperature changes of just billionths of a degree. To obtain these sensitivities, ABS mirrors, which are relatively small at about two feet across, sit inside a cryogenically cooled barrel.

The two telescopes can be operated remotely, but require frequent trips to the Chilean peak, which often include Princeton students and postdocs. The team at Princeton includes Senior Research Physicist Norm Jarosik, Associate Research Scholar Jonathan Sievers, postdoctoral researchers Matthew Hasselfield, Rénee Hložek, Akito Kusaka and Laura Newburgh, and graduate students Farzan Beroz, Kang Hoon (Steve) Choi, Emily Grace, Colin Hill, Shuay-Pwu (Patty) Ho, Christine Pappas, Lucas Parker, Blake Sherwin, Sara Simon, Katerina Visnjic and Sophie Zhang.

–By Catherine Zandonella

Manuscripts spark dialogue on authorship

Martin Kern

Martin Kern (Photo by Frank Wojciechowski)

Hundreds of early Chinese bamboo, silk and wood manuscripts excavated in the last 40 years are challenging the idea of the author as the sole creator of literary work.

Not one of the manuscripts, which include philosophical and literary essays as well as administrative and technical writings from pre-Imperial or ancient China (before 221 B.C.), mentions its author, according to Martin Kern, a Chinese literature expert in the Department of East Asian Studies. The texts are also difficult to attribute to any one author, as individual manuscripts can sometimes have little to do with each other and are thus difficult to categorize. The absence of explicit author credits and the composite nature of the early Chinese texts, Kern said, calls into question the idea that for ancient China, scholars can equate a written artifact with the original thought of a sole individual.

“We have to rethink the idea that for a text to be credible it has to be tied to a certain person from the beginning,” Kern said. He explained that in the Chinese tradition beginning in early Imperial times, the dominant approach to “saying what a text is” involves attributing entire books to one prominent figure even though they contain a range of disparate chapters most likely not composed by a particular individual. The “author” then becomes a principal actor for imbuing a text with meaning, anchoring a text in a particular time, place and literary or philosophical niche according to the traditional perception of his — and in this time period the author could be assumed to be male — ideas and biography.

Chinese literature expert Martin Kern studies ancient texts such as this one, a bamboo strip (presented in segments in image) containing a manuscript dating from around 300 B.C. and probably from a tomb in southern China. (Image courtesy of the Shanghai Museum.)

Chinese literature expert Martin Kern studies ancient texts such as this one, a bamboo strip (presented in segments in image) containing a manuscript dating from around 300 B.C. and probably from a tomb in southern China. (Image courtesy of the Shanghai Museum.)

“We start out with the assumption of a unifying person with a unifying set of ideas,” Kern said, referring to interpretations of ancient Chinese literature. “Now we look at these texts, and we see it’s not at all like that.”

Kern, who publishes in both English and Chinese, pointed to fifth-century B.C. Greece to further highlight the contrast between pre- Imperial Chinese texts and those of early Greece. The writings of several Greek philosophers explicitly identify Homer as the creator of poetry, and other Greek figures such as Herodotus and Thucydides established their creator roles at the outset of their texts. In making their authorship explicit, Kern argued, the writers marked their works with a well-defined historical, geographical and cultural context that would later be instrumental in guiding the texts’ reception.

By contrast, the classics of ancient China went for centuries without authorial attribution — long after they had spread across the vast Chinese realm. This is not to say that authorship had no place in Chinese literature at all — the Imperial Chinese tradition is rife with examples of texts that were intimately linked to individual personalities. However, Kern suggests that these are only later developments, beginning in the second century B.C. with the newly established empire, and must not be projected backwards in time to the formative pre-Imperial period of China.

“The textual attributions for the pre-Imperial authors come into being retrospectively,” he said. Kern, the Greg (’84) and Joanna (P13) Zeluck Professor in Asian Studies and chair of the East Asian studies department, hopes to encourage others in his field to rethink prevailing assumptions about authorship in antiquity. He also wants to spread his ideas to colleagues beyond Chinese studies.

“I want to say, ‘look, you have your situation in Western classical antiquity, and … you think everything about that is normal — in the same way that the Chinese tradition considers itself the normal case,’” he said. Yet the Chinese evidence for composite texts complicates the notion of an “author” and shows it as “a totally cultural decision,” he said.

–By Tara Thean

A. M. Homes wins Women’s Prize for Fiction

A. M. Homes

A. M. Homes (Photo by Marion Ettinger)

The 2013 Women’s Prize for Fiction was awarded to A.M. Homes, a lecturer in creative writing and the Lewis Center for the Arts, for her novel May We Be Forgiven. The £30,000 ($46,000) prize rewards excellence, originality and accessibility in women’s writing worldwide. Homes received the prize in London.

A contributing editor at Vanity Fair, Homes has written the novels This Book Will Save Your Life, Music For Torching, The End of Alice, In a Country of Mothers and Jack. Her short-story collections include Things You Should Know and The Safety of Objects. Homes has also written for television: she helped write and produce the television show The L Word, and adapted her first novel, Jack, for Showtime