Artist’s impression of Earth-sized planets orbiting a red dwarf star. @ NASA, ESA, and G.Bacon (STScI)

SuperComputer simulations by astrophysicists at the University of Bern of the formation of planets orbiting in the habitable zone of low mass stars such as Proxima Centauri show that these planets are most likely to be roughly the size of the Earth and to contain large amounts of water.

In August 2016, the announcement of the discovery of a terrestrial exoplanet orbiting in the habitable zone of Proxima Centauri stimulated the imagination of the experts and the general public. After all this star is the nearest star to our sun even though it is ten times less massive and 500 times less luminous. This discovery together with the one in May 2016 of a similar planet orbiting an even lower mass star (Trappist-1) convinced astronomers that such red dwarfs (as these low mass stars are called) might be hosts to a large population of Earth-like planets.

How could these objects look like? What could they be made of? Yann Alibert and Willy Benz at the Swiss NCCR PlanetS and the Center of Space and Habitability (CSH) at the University of Bern carried out the first supercomputer simulations of the formation of the population of planets expected to orbit stars ten times less massive than the sun.

"Our models succeed in reproducing planets that are similar in terms of mass and period to the ones observed recently," Yann Alibert explains the result of the study that has been accepted for publication as a Letter in the journal «Astronomy and Astrophysics". "Interestingly, we find that planets in close-in orbits around these type of stars are of small sizes. Typically, they range between 0.5 and 1.5 Earth radii with a peak at about 1.0 Earth radius. Future discoveries will tell if we are correct!» the researcher adds.

Ice at the bottom of the global ocean

In addition, the astrophysicists determined the water content of the planets orbiting their small host star in the habitable zone. They found that considering all the cases, around 90% of the planets are harbouring more than 10% of water. For comparison: The Earth has a fraction of water of only about 0,02%. So most of these alien planets are literally water worlds in comparison! The situation could be even more extreme if the protoplanetary disks in which these planets form live longer than assumed in the models. In any case, these planets would be covered by very deep oceans at the bottom of which, owing to the huge pressure, water would be in form of ice.

Water is required for life as we know it. So could these planets be habitable indeed? "While liquid water is generally thought to be an essential ingredient, too much of a good thing may be bad," says Willy Benz. In previous studies the scientists in Bern showed that too much water may prevent the regulation of the surface temperature and destabilizes the climate. "But this is the case for the Earth, here we deal with considerably more exotic planets which might be subjected to a much harsher radiation environment, and/or be in synchronous" he adds.

Following the growth of planetary embryos

To start their calculations, the scientists considered a series of a few hundreds to thousands of identical, low mass stars and around each of them a protoplanetary disk of dust and gas. Planets are formed by accretion of this material. Alibert and Benz assumed that at the beginning, in each disk there were 10 planetary embryos with an initial mass equal to the mass of the Moon. In a few day’s supercomputer time for each system the model calculated how these randomly located embryos grew and migrated. What kind of planets are formed depends on the structure and evolution of the protoplanetary disks.

"Habitable or not, the study of planets orbiting very low mass stars will likely bring exciting new results, improving our knowledge of planet formation, evolution, and potential habitability," summarizes Willy Benz. Because these stars are considerably less luminous than the sun, planets can be much closer to their star before their surface temperature becomes too high for liquid water to exist. If one considers that these type of stars also represent the overwhelming majority of stars in the solar neighbourhood and that close-in planets are presently easier to detect and study, one understands why the existence of this population of Earth-like planets is really of importance. 

Manuela Campanelli and Carlos Lousto win separate awards

Two Rochester Institute of Technology professors who are leaders in gravitational-wave astronomy have won more than $1 million in federal funding to further their research.

Carlos Lousto and Manuela Campanelli, director of RIT's Center for Computational Relativity and Gravitation, won separate multi-year grants from the National Science Foundation worth $600,000 and $435,000, respectively. 

The $600,000 grant will support Lousto's research modeling black holes orbiting and colliding with each other under extreme conditions. Technically difficult to model, these scenarios simulate on supercomputers highly spinning and energetic black holes with misaligned orbits. The waveforms from extreme black-hole mergers will become templates for actual gravitational wave searches and analysis. The three-year award will continue the well-established research program at RIT.

"This is very timely research since black hole binary mergers are expected to be the loudest gravitational wave source for Advanced LIGO (Laser Interferometer Gravitational-wave Observatory)," said Lousto, professor in RIT's School of Mathematical Sciences and an American Physical Society Fellow.

A second NSF grant for $435,000 will develop software tools to make computational astrophysics accessible to the wider scientific community and increase productivity in gravitational wave astronomy. The Einstein Toolkit--led by Campanelli at RIT--will provide a software platform of core computational tools for research focused on astrophysical systems like black holes and that require a knowledge of Einstein's equations of general relativity.

"The idea behind the Einstein Toolkit is to create a broad and vibrant community of users and advances in the next generation of high performance computing cyberinfrastructure," said Campanelli, a professor in RIT's School of Mathematical Sciences and an American Physical Society Fellow.

The use of e-Infrastructures is enabling a new wave of collaborative scientific research through remote access to computing services, instrumentation and resources bringing “real-world” benefits that impact our daily lives. The Conference on e-Infrastructures across the Mediterranean http://www.terena.org/eumedevent3/, 30-31 March 2010 in Brussels, Belgium, builds on the success of the previous EU-Med Events and marks an important step to progressing collaboration between countries of the Mediterranean region and European Union (EU) in the field of e-Infrastructures and networking for research and education.

These advanced, multi-layered platforms spanning networking, storage, supercomputing & grids, and access to shared scientific data not only help researchers to tackle scientific problems more effectively but also engender new scientific communities to work on similar challenges irrespective of their geographical location. Economies of scale can and need to be a cornerstone in the e-Infrastructure landscape, fostering further development and reaping rewards now and in the future.

This Conference brings together invited policy makers and civil servants from relevant ministries and institutions, executives and officials from international organisations such as the European Commission, the Arab League, UN agencies, private companies and foundations, as well as representatives from the user community to deliver insights into the state of play and future perspectives, define what is needed to further collaborative work on research and education and deliberate high-level policy issues. The event is by invitation only and organised and sponsored by three European funded projects chartered with fostering the creation of e-Infrastructures through important inter-related activities:

  •  EUMEDCONNECT2 (http://www.eumedconnect2.net, a research network that connects Europe with Africa, the Mediterranean rim and the Middle East.
  • EUMEDGRID-Support (http://www.eumedgrid.org, promoting the deployment of e-Infrastructures, and supporting existing and new applications, policy development and training.
  • GÉANT (http://www.geant.net: a world-leading pan-European research network, bringing together at gigabit speeds the networks of the national entities, NRENs, which are collectively represented by TERENA (Trans-European Research & Education Networking Association – www.terena.org).


Programme agenda: http://www.terena.org/activities/development-support/eumedevent3/

Venue: International Auditorium http://www.theinternationalauditorium.be/en/contact.php


Event website:http://www.terena.org/eumedevent3/

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CAPTION This artist's illustration compares the interior structures of Earth (left) with the exoplanet Kepler-93b (right), which is one and a half times the size of Earth and 4 times as massive. New research finds that rocky worlds share similar structures, with a core containing about a third of the planet's mass, surrounded by a mantle and topped by a thin crust. CREDIT M. Weiss/CfA

Every school kid learns the basic structure of the Earth: a thin outer crust, a thick mantle, and a Mars-sized core. But is this structure universal? Will rocky exoplanets orbiting other stars have the same three layers? New research suggests that the answer is yes - they will have interiors very similar to Earth.

"We wanted to see how Earth-like these rocky planets are. It turns out they are very Earth-like," says lead author Li Zeng of the Harvard-Smithsonian Center for Astrophysics (CfA).

To reach this conclusion Zeng and his co-authors applied a supercomputer model known as the Preliminary Reference Earth Model (PREM), which is the standard model for Earth's interior. They adjusted it to accommodate different masses and compositions, and applied it to six known rocky exoplanets with well-measured masses and physical sizes.

They found that the other planets, despite their differences from Earth, all should have a nickel/iron core containing about 30 percent of the planet's mass. In comparison, about a third of the Earth's mass is in its core. The remainder of each planet would be mantle and crust, just as with Earth.

"We've only understood the Earth's structure for the past hundred years. Now we can calculate the structures of planets orbiting other stars, even though we can't visit them," adds Zeng.

The new code also can be applied to smaller, icier worlds like the moons and dwarf planets in the outer solar system. For example, by plugging in the mass and size of Pluto, the team finds that Pluto is about one-third ice (mostly water ice but also ammonia and methane ices).

The model assumes that distant exoplanets have chemical compositions similar to Earth. This is reasonable based on the relevant abundances of key chemical elements like iron, magnesium, silicon, and oxygen in nearby systems. However, planets forming in more or less metal-rich regions of the galaxy could show different interior structures. The team expects to explore these questions in future research.

A view inside the LUX detector. (Photo by Matthew Kapust/Sanford Underground Research Facility)

The Large Underground Xenon (LUX) dark matter experiment, which operates nearly a mile underground at the Sanford Underground Research Facility (SURF) in the Black Hills of South Dakota, has already proven itself to be the most sensitive detector in the hunt for dark matter, the unseen stuff believed to account for most of the matter in the universe. Now, a new set of calibration techniques employed by LUX scientists has again dramatically improved the detector’s sensitivity.

Researchers with LUX are looking for WIMPs, or weakly interacting massive particles, which are among the leading candidates for dark matter. “We have improved the sensitivity of LUX by more than a factor of 20 for low-mass dark matter particles, significantly enhancing our ability to look for WIMPs,” said Rick Gaitskell, professor of physics at Brown University and co-spokesperson for the LUX experiment. “It is vital that we continue to push the capabilities of our detector in the search for the elusive dark matter particles,” Gaitskell said.

LUX improvements, coupled to advanced supercomputer simulations at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory’s (Berkeley Lab) National Energy Research Scientific Computing Center (NERSC) and Brown University’s Center for Computation and Visualization (CCV), have allowed scientists to test additional particle models of dark matter that now can be excluded from the search. NERSC also stores large volumes of LUX data—measured in trillions of bytes, or terabytes—and Berkeley Lab has a growing role in the LUX collaboration.

Scientists are confident that dark matter exists because the effects of its gravity can be seen in the rotation of galaxies and in the way light bends as it travels through the universe. Because WIMPs are thought to interact with other matter only on very rare occasions, they have yet to be detected directly.

The LUX dark matter detector is seen here during the assembly process in a surface laboratory in South Dakota. (Photo by Matthew Kapust/Sanford Underground Research Facility)

The LUX dark matter detector is seen here during the assembly process in a surface laboratory in South Dakota. (Photo by Matthew Kapust/Sanford Underground Research Facility)

“We have looked for dark matter particles during the experiment’s first three-month run, but are exploiting new calibration techniques better pinning down how they would appear to our detector,” said Alastair Currie of Imperial College London, a LUX researcher.

“These calibrations have deepened our understanding of the response of xenon to dark matter, and to backgrounds. This allows us to search, with improved confidence, for particles that we hadn’t previously known would be visible to LUX.”

The new research is described in a paper submitted to Physical Review Letters. The work reexamines data collected during LUX’s first three-month run in 2013 and helps to rule out the possibility of dark matter detections at low-mass ranges where other experiments had previously reported potential detections.

A view of the LUX detector during installation. (Photo by Matthew Kapust/Sanford Underground Research Facility)

A view of the LUX detector during installation. (Photo by Matthew Kapust/Sanford Underground Research Facility)

LUX consists of one-third ton of liquid xenon surrounded with sensitive light detectors. It is designed to identify the very rare occasions when a dark matter particle collides with a xenon atom inside the detector. When a collision happens, a xenon atom will recoil and emit a tiny flash of light, which is detected by LUX’s light sensors. The detector’s location at Sanford Lab beneath a mile of rock helps to shield it from cosmic rays and other radiation that would interfere with a dark matter signal.

So far LUX hasn’t detected a dark matter signal, but its exquisite sensitivity has allowed scientists to all but rule out vast mass ranges where dark matter particles might exist. These new calibrations increase that sensitivity even further.

One calibration technique used neutrons as stand-ins for dark matter particles. Bouncing neutrons off the xenon atoms allows scientists to quantify how the LUX detector responds to the recoiling process.

“It is like a giant game of pool with a neutron as the cue ball and the xenon atoms as the stripes and solids,” Gaitskell said. “We can track the neutron to deduce the details of the xenon recoil, and calibrate the response of LUX better than anything previously possible.”

The nature of the interaction between neutrons and xenon atoms is thought to be very similar to the interaction between dark matter and xenon. “It’s just that dark matter particles interact very much more weakly—about a million-million-million-million times more weakly,” Gaitskell said.

The neutron experiments help to calibrate the detector for interactions with the xenon nucleus. But LUX scientists have also calibrated the detector’s response to the deposition of small amounts of energy by struck atomic electrons. That’s done by injecting tritiated methane—a radioactive gas—into the detector.

“In a typical science run, most of what LUX sees are background electron recoil events,” said Carter Hall a University of Maryland professor. “Tritiated methane is a convenient source of similar events, and we’ve now studied hundreds of thousands of its decays in LUX. This gives us confidence that we won’t mistake these garden-variety events for dark matter.”

Another radioactive gas, krypton, was injected to help scientists distinguish between signals produced by ambient radioactivity and a potential dark matter signal.

“The krypton mixes uniformly in the liquid xenon and emits radiation with a known, specific energy, but then quickly decays away to a stable, non-radioactive form,” said Dan McKinsey, a UC Berkeley physics professor and co-spokesperson for LUX who is also an affiliate with Berkeley Lab. By precisely measuring the light and charge produced by this interaction, researchers can effectively filter out background events from their search.

“And so the search continues,” McKinsey said. “LUX is once again in dark matter detection mode at Sanford Lab. The latest run began in late 2014 and is expected to continue until June 2016. This run will represent an increase in exposure of more than four times compared to our previous 2013 run. We will be very excited to see if any dark matter particles have shown themselves in the new data.”

McKinsey, formerly at Yale University, joined UC Berkeley and Berkeley Lab in July, accompanied by members of his research team.

The Sanford Lab is a South Dakota-owned facility. Homestake Mining Co. donated its gold mine in Lead to the South Dakota Science and Technology Authority (SDSTA), which reopened the facility in 2007 with $40 million in funding from the South Dakota State Legislature and a $70 million donation from philanthropist T. Denny Sanford. The U.S. Department of Energy (DOE) supports Sanford Lab’s operations.

Kevin Lesko, who oversees SURF operations and leads the Dark Matter Research Group at Berkeley Lab, said, “It’s good to see that the experiments installed in SURF continue to produce world-leading results.”

The LUX scientific collaboration, which is supported by the DOE and National Science Foundation (NSF), includes 19 research universities and national laboratories in the United States, the United Kingdom and Portugal.

“The global search for dark matter aims to answer one of the biggest questions about the makeup of our universe. We’re proud to support the LUX collaboration and congratulate them on achieving an even greater level of sensitivity,” said Mike Headley, Executive Director of the SDSTA.

Planning for the next-generation dark matter experiment at Sanford Lab is already under way. In late 2016 LUX will be decommissioned to make way for a new, much larger xenon detector, known as the LUX-ZEPLIN (LZ) experiment. LZ would have a 10-ton liquid xenon target, which will fit inside the same 72,000-gallon tank of pure water used by LUX. Berkeley Lab scientists will have major leadership roles in the LZ collaboration.

“The innovations of the LUX experiment form the foundation for the LZ experiment, which is planned to achieve over 100 times the sensitivity of LUX. The LZ experiment is so sensitive that it should begin to detect a type of neutrino originating in the Sun that even Ray Davis’ Nobel Prize-winning experiment at the Homestake mine was unable to detect,” according to Harry Nelson of UC Santa Barbara, spokesperson for LZ.

 

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