From The University of Washington (US) : “‘Would you like a little ice with your exoplanet?’ For Earth-like worlds that may be a tall order”

From The University of Washington (US)

December 8, 2021
James Urton

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An artist’s depiction of Kepler-186f, an Earth-sized exoplanet, showing a hypothetical surface that includes partial ice coverage at the poles. Credit: The NASA Ames Research Center (US)/The SETI Institute (US)/JPL/Caltech-NASA(US)

Exoplanets are experiencing a stratospheric rise. In the three decades since the first confirmed planet orbiting another star, scientists have catalogued more than 4,000 of them. As the list grows, so too does the desire to find Earth-like exoplanets — and to determine whether they could be life-sustaining oases like our own globe.

The coming decades should see the launch of new missions that can gather ever-larger amounts of data about exoplanets. Anticipating these future endeavors, a team at the University of Washington and the University of Bern has computationally simulated more than 200,000 hypothetical Earth-like worlds — planets that have the same size, mass, atmospheric composition and geography as modern Earth — all in orbit of stars like our sun. Their goal was to model what types of environments astronomers can expect to find on real Earth-like exoplanets.

As they report in a paper accepted to the Planetary Science Journal and submitted Dec. 6, on these simulated exoplanets, one common feature of present-day Earth was often lacking: partial ice coverage.

“We essentially simulated Earth’s climate on worlds around different types of stars, and we find that in 90% of cases with liquid water on the surface, there are no ice sheets, like polar caps,” said co-author Rory Barnes, a UW professor of astronomy and scientist with the UW’s Virtual Planetary Laboratory. “When ice is present, we see that ice belts — permanent ice along the equator — are actually more likely than ice caps.”

The findings shed light on the complex interplay between liquid water and ice on Earth-like worlds, according to lead author Caitlyn Wilhelm, who led the study as an undergraduate student in the UW Department of Astronomy.

“Looking at ice coverage on an Earth-like planet can tell you a lot about whether it’s habitable,” said Wilhelm, who is now a research scientist with the Virtual Planetary Laboratory. “We wanted to understand all the parameters — the shape of the orbit, the axial tilt, the type of star — that affect whether you have ice on the surface, and if so, where.”

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A composite image of the ice cap covering Earth’s Arctic region — including the North Pole — taken 512 miles above our planet on April 12, 2018 by the NOAA-20 polar-orbiting satellite.Credit: The National Oceanic and Atmospheric Administration (US).

The team used a 1-D energy balance model, which computationally imitates the energy flow between a planet’s equator and poles, to simulate the climates on thousands of hypothetical exoplanets in various orbital configurations around F-, G- or K-type stars. These classes of stars, which include our own G-type sun, are promising candidates for hosting life-friendly worlds in their habitable zones, also known as the “Goldilocks” zone. F-type stars are a bit hotter and larger than our sun; K-type stars are slightly cooler and smaller.

In their simulations, the orbits of the exoplanets ranged from circular to a pronounced oval. The team also considered axial tilts ranging from 0 to 90 degrees. Earth’s axial tilt is a moderate 23.5 degrees. A planet with a 90-degree tilt would “sit on its side” and experience extreme seasonal variations in climate, much like the planet Uranus.

According to the simulations, which encompassed a 1-million-year timespan on each world, Earth-like worlds showed climates ranging from planet-wide “snowball” climates — with ice present at all latitudes — to a steaming “moist greenhouse,” which is probably similar to Venus’ climate before a runaway greenhouse effect made its surface hot enough to melt lead. But even though most environments in the simulations fell somewhere between those extremes, partial surface ice was present on only about 10% of hypothetical, habitable exoplanets.

The model included natural variations over time in each world’s axial tilt and orbit, which in part explains the general lack of ice on habitable exoplanets, according to co-author Russell Deitrick, a postdoctoral scientist at The University of Bern [Universität Bern](CH) and researcher with The University of Washington Virtual Planetary Laboratory (US).

“Orbits and axial tilts are always changing,” said Deitrick. “On Earth, these variations are called Milankovitch cycles, and are very small in amplitude. But for exoplanets, these changes can be quite large, which can eliminate ice altogether or trigger ‘snowball’ states.”

When partial ice was present, its distribution varied by star. Around F-type stars, polar ice caps — like what Earth sports currently — were found about three times more often than ice belts, whereas ice belts occurred twice as often as caps for planets around G- and K-type stars. Ice belts were also more common on worlds with extreme axial tilts, likely because seasonal extremes keep the polar climates more volatile than equatorial regions, according to Wilhelm.

The team’s findings about ice on these simulated Earth-like worlds should help in the search for potentially habitable worlds by showing astronomers what they can expect to find, especially regarding ice distribution and the types of climates.

“Surface ice is very reflective, and can shape how an exoplanet ‘looks’ through our instruments,” said Wilhelm. “Whether or not ice is present can also shape how a climate will change over the long term, whether it goes to an extreme — like a ‘snowball Earth’ or a runaway greenhouse — or something more moderate.”

Ice alone, or its absence, does not determine habitability, though.

“Habitability encompasses a lot of moving parts, not just the presence or absence of ice,” said Wilhelm.

Life on Earth has survived snowball periods, as well as hundreds of millions of ice-free years, according to Barnes.

“Our own planet has seen some of these extremes in its own history,” said Barnes. “We hope this study lays the groundwork for upcoming missions to look for habitable signatures in exoplanet atmospheres — and to even image exoplanets directly — by showing what’s possible, what’s common and what’s rare.”

Rachel Mellman, a recent UW graduate in astronomy, is a co-author on the paper. The research was funded by NASA through grants to the Virtual Planetary Laboratory.

See the full article here .


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The University of Washington (US) is one of the world’s preeminent public universities. Our impact on individuals, on our region, and on the world is profound — whether we are launching young people into a boundless future or confronting the grand challenges of our time through undaunted research and scholarship. Ranked number 10 in the world in Shanghai Jiao Tong University rankings and educating more than 54,000 students annually, our students and faculty work together to turn ideas into impact and in the process transform lives and our world. For more about our impact on the world, every day.

So what defines us —the students, faculty and community members at the University of Washington? Above all, it’s our belief in possibility and our unshakable optimism. It’s a connection to others, both near and far. It’s a hunger that pushes us to tackle challenges and pursue progress. It’s the conviction that together we can create a world of good. Join us on the journey.

The University of Washington (US) is a public research university in Seattle, Washington, United States. Founded in 1861, University of Washington is one of the oldest universities on the West Coast; it was established in downtown Seattle approximately a decade after the city’s founding to aid its economic development. Today, the university’s 703-acre main Seattle campus is in the University District above the Montlake Cut, within the urban Puget Sound region of the Pacific Northwest. The university has additional campuses in Tacoma and Bothell. Overall, University of Washington encompasses over 500 buildings and over 20 million gross square footage of space, including one of the largest library systems in the world with more than 26 university libraries, as well as the UW Tower, lecture halls, art centers, museums, laboratories, stadiums, and conference centers. The university offers bachelor’s, master’s, and doctoral degrees through 140 departments in various colleges and schools, sees a total student enrollment of roughly 46,000 annually, and functions on a quarter system.

University of Washington is a member of the Association of American Universities(US) and is classified among “R1: Doctoral Universities – Very high research activity”. According to the National Science Foundation(US), UW spent $1.41 billion on research and development in 2018, ranking it 5th in the nation. As the flagship institution of the six public universities in Washington state, it is known for its medical, engineering and scientific research as well as its highly competitive computer science and engineering programs. Additionally, University of Washington continues to benefit from its deep historic ties and major collaborations with numerous technology giants in the region, such as Amazon, Boeing, Nintendo, and particularly Microsoft. Paul G. Allen, Bill Gates and others spent significant time at Washington computer labs for a startup venture before founding Microsoft and other ventures. The University of Washington’s 22 varsity sports teams are also highly competitive, competing as the Huskies in the Pac-12 Conference of the NCAA Division I, representing the United States at the Olympic Games, and other major competitions.

The university has been affiliated with many notable alumni and faculty, including 21 Nobel Prize laureates and numerous Pulitzer Prize winners, Fulbright Scholars, Rhodes Scholars and Marshall Scholars.

In 1854, territorial governor Isaac Stevens recommended the establishment of a university in the Washington Territory. Prominent Seattle-area residents, including Methodist preacher Daniel Bagley, saw this as a chance to add to the city’s potential and prestige. Bagley learned of a law that allowed United States territories to sell land to raise money in support of public schools. At the time, Arthur A. Denny, one of the founders of Seattle and a member of the territorial legislature, aimed to increase the city’s importance by moving the territory’s capital from Olympia to Seattle. However, Bagley eventually convinced Denny that the establishment of a university would assist more in the development of Seattle’s economy. Two universities were initially chartered, but later the decision was repealed in favor of a single university in Lewis County provided that locally donated land was available. When no site emerged, Denny successfully petitioned the legislature to reconsider Seattle as a location in 1858.

In 1861, scouting began for an appropriate 10 acres (4 ha) site in Seattle to serve as a new university campus. Arthur and Mary Denny donated eight acres, while fellow pioneers Edward Lander, and Charlie and Mary Terry, donated two acres on Denny’s Knoll in downtown Seattle. More specifically, this tract was bounded by 4th Avenue to the west, 6th Avenue to the east, Union Street to the north, and Seneca Streets to the south.

John Pike, for whom Pike Street is named, was the university’s architect and builder. It was opened on November 4, 1861, as the Territorial University of Washington. The legislature passed articles incorporating the University, and establishing its Board of Regents in 1862. The school initially struggled, closing three times: in 1863 for low enrollment, and again in 1867 and 1876 due to funds shortage. University of Washington awarded its first graduate Clara Antoinette McCarty Wilt in 1876, with a bachelor’s degree in science.

19th century relocation

By the time Washington state entered the Union in 1889, both Seattle and the University had grown substantially. University of Washington’s total undergraduate enrollment increased from 30 to nearly 300 students, and the campus’s relative isolation in downtown Seattle faced encroaching development. A special legislative committee, headed by University of Washington graduate Edmond Meany, was created to find a new campus to better serve the growing student population and faculty. The committee eventually selected a site on the northeast of downtown Seattle called Union Bay, which was the land of the Duwamish, and the legislature appropriated funds for its purchase and construction. In 1895, the University relocated to the new campus by moving into the newly built Denny Hall. The University Regents tried and failed to sell the old campus, eventually settling with leasing the area. This would later become one of the University’s most valuable pieces of real estate in modern-day Seattle, generating millions in annual revenue with what is now called the Metropolitan Tract. The original Territorial University building was torn down in 1908, and its former site now houses the Fairmont Olympic Hotel.

The sole-surviving remnants of Washington’s first building are four 24-foot (7.3 m), white, hand-fluted cedar, Ionic columns. They were salvaged by Edmond S. Meany, one of the University’s first graduates and former head of its history department. Meany and his colleague, Dean Herbert T. Condon, dubbed the columns as “Loyalty,” “Industry,” “Faith”, and “Efficiency”, or “LIFE.” The columns now stand in the Sylvan Grove Theater.

20th century expansion

Organizers of the 1909 Alaska-Yukon-Pacific Exposition eyed the still largely undeveloped campus as a prime setting for their world’s fair. They came to an agreement with Washington’s Board of Regents that allowed them to use the campus grounds for the exposition, surrounding today’s Drumheller Fountain facing towards Mount Rainier. In exchange, organizers agreed Washington would take over the campus and its development after the fair’s conclusion. This arrangement led to a detailed site plan and several new buildings, prepared in part by John Charles Olmsted. The plan was later incorporated into the overall University of Washington campus master plan, permanently affecting the campus layout.

Both World Wars brought the military to campus, with certain facilities temporarily lent to the federal government. In spite of this, subsequent post-war periods were times of dramatic growth for the University. The period between the wars saw a significant expansion of the upper campus. Construction of the Liberal Arts Quadrangle, known to students as “The Quad,” began in 1916 and continued to 1939. The University’s architectural centerpiece, Suzzallo Library, was built in 1926 and expanded in 1935.

After World War II, further growth came with the G.I. Bill. Among the most important developments of this period was the opening of the School of Medicine in 1946, which is now consistently ranked as the top medical school in the United States. It would eventually lead to the University of Washington Medical Center, ranked by U.S. News and World Report as one of the top ten hospitals in the nation.

In 1942, all persons of Japanese ancestry in the Seattle area were forced into inland internment camps as part of Executive Order 9066 following the attack on Pearl Harbor. During this difficult time, university president Lee Paul Sieg took an active and sympathetic leadership role in advocating for and facilitating the transfer of Japanese American students to universities and colleges away from the Pacific Coast to help them avoid the mass incarceration. Nevertheless many Japanese American students and “soon-to-be” graduates were unable to transfer successfully in the short time window or receive diplomas before being incarcerated. It was only many years later that they would be recognized for their accomplishments during the University of Washington’s Long Journey Home ceremonial event that was held in May 2008.

From 1958 to 1973, the University of Washington saw a tremendous growth in student enrollment, its faculties and operating budget, and also its prestige under the leadership of Charles Odegaard. University of Washington student enrollment had more than doubled to 34,000 as the baby boom generation came of age. However, this era was also marked by high levels of student activism, as was the case at many American universities. Much of the unrest focused around civil rights and opposition to the Vietnam War. In response to anti-Vietnam War protests by the late 1960s, the University Safety and Security Division became the University of Washington Police Department.

Odegaard instituted a vision of building a “community of scholars”, convincing the Washington State legislatures to increase investment in the University. Washington senators, such as Henry M. Jackson and Warren G. Magnuson, also used their political clout to gather research funds for the University of Washington. The results included an increase in the operating budget from $37 million in 1958 to over $400 million in 1973, solidifying University of Washington as a top recipient of federal research funds in the United States. The establishment of technology giants such as Microsoft, Boeing and Amazon in the local area also proved to be highly influential in the University of Washington’s fortunes, not only improving graduate prospects but also helping to attract millions of dollars in university and research funding through its distinguished faculty and extensive alumni network.

21st century

In 1990, the University of Washington opened its additional campuses in Bothell and Tacoma. Although originally intended for students who have already completed two years of higher education, both schools have since become four-year universities with the authority to grant degrees. The first freshman classes at these campuses started in fall 2006. Today both Bothell and Tacoma also offer a selection of master’s degree programs.

In 2012, the University began exploring plans and governmental approval to expand the main Seattle campus, including significant increases in student housing, teaching facilities for the growing student body and faculty, as well as expanded public transit options. The University of Washington light rail station was completed in March 2015, connecting Seattle’s Capitol Hill neighborhood to the University of Washington Husky Stadium within five minutes of rail travel time. It offers a previously unavailable option of transportation into and out of the campus, designed specifically to reduce dependence on private vehicles, bicycles and local King County buses.

University of Washington has been listed as a “Public Ivy” in Greene’s Guides since 2001, and is an elected member of the American Association of Universities. Among the faculty by 2012, there have been 151 members of American Association for the Advancement of Science, 68 members of the National Academy of Sciences(US), 67 members of the American Academy of Arts and Sciences, 53 members of the National Academy of Medicine(US), 29 winners of the Presidential Early Career Award for Scientists and Engineers, 21 members of the National Academy of Engineering(US), 15 Howard Hughes Medical Institute Investigators, 15 MacArthur Fellows, 9 winners of the Gairdner Foundation International Award, 5 winners of the National Medal of Science, 7 Nobel Prize laureates, 5 winners of Albert Lasker Award for Clinical Medical Research, 4 members of the American Philosophical Society, 2 winners of the National Book Award, 2 winners of the National Medal of Arts, 2 Pulitzer Prize winners, 1 winner of the Fields Medal, and 1 member of the National Academy of Public Administration. Among UW students by 2012, there were 136 Fulbright Scholars, 35 Rhodes Scholars, 7 Marshall Scholars and 4 Gates Cambridge Scholars. UW is recognized as a top producer of Fulbright Scholars, ranking 2nd in the US in 2017.

The Academic Ranking of World Universities (ARWU) has consistently ranked University of Washington as one of the top 20 universities worldwide every year since its first release. In 2019, University of Washington ranked 14th worldwide out of 500 by the ARWU, 26th worldwide out of 981 in the Times Higher Education World University Rankings, and 28th worldwide out of 101 in the Times World Reputation Rankings. Meanwhile, QS World University Rankings ranked it 68th worldwide, out of over 900.

U.S. News & World Report ranked University of Washington 8th out of nearly 1,500 universities worldwide for 2021, with University of Washington’s undergraduate program tied for 58th among 389 national universities in the U.S. and tied for 19th among 209 public universities.

In 2019, it ranked 10th among the universities around the world by SCImago Institutions Rankings. In 2017, the Leiden Ranking, which focuses on science and the impact of scientific publications among the world’s 500 major universities, ranked University of Washington 12th globally and 5th in the U.S.

In 2019, Kiplinger Magazine’s review of “top college values” named University of Washington 5th for in-state students and 10th for out-of-state students among U.S. public colleges, and 84th overall out of 500 schools. In the Washington Monthly National University Rankings University of Washington was ranked 15th domestically in 2018, based on its contribution to the public good as measured by social mobility, research, and promoting public service.