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Monday, July 29th, 2019

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`10:59a`	TESS discovers three new planets nearby, including temperate “sub-Neptune” NASA’s Transiting Exoplanet Survey Satellite, or TESS, has discovered three new worlds that are among the smallest, nearest exoplanets known to date. The planets orbit a star just 73 light-years away and include a small, rocky super-Earth and two sub-Neptunes — planets about half the size of our own icy giant. The sub-Neptune furthest out from the star appears to be within a “temperate” zone, meaning that the very top of the planet’s atmosphere is within a temperature range that could support some forms of life. However, scientists say the planet’s atmosphere is likely a thick, ultradense heat trap that renders the planet’s surface too hot to host water or life. Nevertheless, this new planetary system, which astronomers have dubbed TOI-270, is proving to have other curious qualities. For instance, all three planets appear to be relatively close in size. In contrast, our own solar system is populated with planetary extremes, from the small, rocky worlds of Mercury, Venus, Earth, and Mars, to the much more massive Jupiter and Saturn, and the more remote ice giants of Neptune and Uranus. There’s nothing in our solar system that resembles an intermediate planet, with a size and composition somewhere in the middle of Earth and Neptune. But TOI-270 appears to host two such planets: both sub-Neptunes are smaller than our own Neptune and not much larger than the rocky planet in the system. Astronomers believe TOI-270’s sub-Neptunes may be a “missing link” in planetary formation, as they are of an intermediate size and could help researchers determine whether small, rocky planets like Earth and more massive, icy worlds like Neptune follow the same formation path or evolve separately. TOI-270 is an ideal system for answering such questions, because the star itself is nearby and therefore bright, and also unusually quiet. The star is an M-dwarf, a type of star that is normally extremely active, with frequent flares and solar storms. TOI-270 appears to be an older M-dwarf that has since quieted down, giving off a steady brightness, against which scientists can measure many properties of the orbiting planets, such as their mass and atmospheric composition. “There are a lot of little pieces of the puzzle that we can solve with this system,” says Maximilian Günther, a postdoc in MIT’s Kavli Institute for Astrophysics and Space Research and lead author of a study published today in Nature Astronomy that details the discovery. “You can really do all the things you want to do in exoplanet science, with this system.” Compare and contrast worlds in the TOI 270 system with these illustrations. Temperatures given for TOI 270 planets are equilibrium temperatures, calculated without the warming effects of any possible atmospheres. Credit: NASA’s Goddard Space Flight Center A planetary pattern Günther and his colleagues detected the three new planets after looking through measurements of stellar brightness taken by TESS. The MIT-developed satellite stares at patches of the sky for 27 days at a time, monitoring thousands of stars for possible transits — characteristic dips in brightness that could signal a planet temporarily blocking the star’s light as it passes in front of it. The team isolated several such signals from a nearby star, located 73 light years away in the southern sky. They named the star TOI-270, for the 270th “TESS Object of Interest” identified to date. The researchers used ground-based instruments to follow up on the star’s activity, and confirmed that the signals are the result of three orbiting exoplanets: planet b, a rocky super-Earth with a roughly three-day orbit; planet c, a sub-Neptune with a five-day orbit; and planet d, another sub-Neptune slightly further out, with an 11-day orbit. Günther notes that the planets seem to line up in what astronomers refer to as a “resonant chain,” meaning that the ratio of their orbits are close to whole integers — in this case, 3:5 for the inner pair, and 2:1 for the outer pair — and that the planets are therefore in “resonance” with each other. Astronomers have discovered other small stars with similarly resonant planetary formations. And in our own solar system, the moons of Jupiter also happen to line up in resonance with each other. “For TOI-270, these planets line up like pearls on a string,” Günther says. “That’s a very interesting thing, because it lets us study their dynamical behavior. And you can almost expect, if there are more planets, the next one would be somewhere further out, at another integer ratio.” “An exceptional laboratory” TOI-270’s discovery initially caused a stir of excitement within the TESS science team, as it seemed, in the first analysis, that planet d might lie in the star’s habitable zone, a region that would be cool enough for the planet’s surface to support water, and possibly life. But the researchers soon realized that the planet’s atmosphere was probably extremely thick, and would therefore generate an intense greenhouse effect, causing the planet’s surface to be too hot to be habitable. But Günther says there is a good possibility that the system hosts other planets, further out from planet d, that might well lie within the habitable zone. Planet d, with an 11-day orbit, is about 10 million kilometers out from the star. Günther says that, given that the star is small and relatively cool — about half as hot as the sun — its habitable zone could potentially begin at around 15 million kilometers. But whether a planet exists within this zone, and whether it is habitable, depends on a host of other parameters, such as its size, mass, and atmospheric conditions. Fortunately, the team writes in their paper that “the host star, TOI-270, is remarkably well-suited for future habitability searches, as it is particularly quiet.” The researchers plan to focus other instruments, including the upcoming James Webb Space Telescope, on TOI-270, to pin down various properties of the three planets, as well as search for additional planets in the star’s habitable zone. “TOI-270 is a true Disneyland for exoplanet science, and one of the prime systems TESS was set out to discover,” Günther says. “It is an exceptional laboratory for not one, but many reasons — it really ticks all the boxes.” This research was funded, in part, by NASA. (Comment on this)
`1:30p`	Removing carbon dioxide from power plant exhaust Reducing carbon dioxide (CO₂) emissions from power plants is widely considered an essential component of any climate change mitigation plan. Many research efforts focus on developing and deploying carbon capture and sequestration (CCS) systems to keep CO₂ emissions from power plants out of the atmosphere. But separating the captured CO₂ and converting it back into a gas that can be stored can consume up to 25 percent of a plant’s power-generating capacity. In addition, the CO₂ gas is generally injected into underground geological formations for long-term storage — a disposal method whose safety and reliability remain unproven. A better approach would be to convert the captured CO₂ into useful products such as value-added fuels or chemicals. To that end, attention has focused on electrochemical processes — in this case, a process in which chemical reactions release electrical energy, as in the discharge of a battery. The ideal medium in which to conduct electrochemical conversion of CO₂ would appear to be water. Water can provide the protons (positively charged particles) needed to make fuels such as methane. But running such “aqueous” (water-based) systems requires large energy inputs, and only a small fraction of the products formed are typically those of interest. Betar Gallant, an assistant professor of mechanical engineering, and her group at MIT have therefore been focusing on non-aqueous (water-free) electrochemical reactions — in particular, those that occur inside lithium-CO₂ batteries. Research into lithium-CO₂ batteries is in its very early stages, according to Gallant, but interest in them is growing because CO₂ is used up in the chemical reactions that occur on one of the electrodes as the battery is being discharged. However, CO₂ isn’t very reactive. Researchers have tried to speed things up by using different electrolytes and electrode materials. Despite such efforts, the need to use expensive metal catalysts to elicit electrochemical activity has persisted. Given the lack of progress, Gallant wanted to try something different. “We were interested in trying to bring a new chemistry to bear on the problem,” she says. And enlisting the help of the sorbent molecules that so effectively capture CO₂ in CCS seemed like a promising way to go. Rethinking amine The sorbent molecule used in CCS is an amine, a derivative of ammonia. In CCS, exhaust is bubbled through an amine-containing solution, and the amine chemically binds the CO₂, removing it from the exhaust gases. The CO₂ — now in liquid form — is then separated from the amine and converted back to a gas for disposal. In CCS, those last steps require high temperatures, which are attained using some of the electrical output of the power plant. Gallant wondered whether her team could instead use electrochemical reactions to separate the CO₂ from the amine — and then continue the reaction to make a solid, CO₂-containing product. If so, the disposal process would be simpler than it is for gaseous CO₂. The CO₂ would be more densely packed, so it would take up less space, and it couldn’t escape, so it would be safer. Better still, additional electrical energy could be extracted from the device as it discharges and forms the solid material. “The vision was to put a battery-like device into the power plant waste stream to sequester the captured CO₂ in a stable solid, while harvesting the energy released in the process,” says Gallant. Research on CCS technology has generated a good understanding of the carbon-capture process that takes place inside a CCS system. When CO₂ is added to an amine solution, molecules of the two species spontaneously combine to form an “adduct,” a new chemical species in which the original molecules remain largely intact. In this case, the adduct forms when a carbon atom in a CO₂ molecule chemically bonds with a nitrogen atom in an amine molecule. As they combine, the CO₂ molecule is reconfigured: It changes from its original, highly stable, linear form to a “bent” shape with a negative charge — a highly reactive form that’s ready for further reaction. In her scheme, Gallant proposed using electrochemistry to break apart the CO₂-amine adduct — right at the carbon-nitrogen bond. Cleaving the adduct at that bond would separate the two pieces: the amine in its original, unreacted state, ready to capture more CO₂, and the bent, chemically reactive form of CO₂, which might then react with the electrons and positively charged lithium ions that flow during battery discharge. The outcome of that reaction could be the formation of lithium carbonate (Li₂CO₃), which would deposit on the carbon electrode. At the same time, the reactions on the carbon electrode should promote the flow of electrons during battery discharge — even without a metal catalyst. “The discharge of the battery would occur spontaneously,” Gallant says. “And we’d break the adduct in a way that allows us to renew our CO₂ absorber while taking CO₂ to a stable, solid form.” A process of discovery In 2016, Gallant and mechanical engineering doctoral student Aliza Khurram began to explore that idea. Their first challenge was to develop a novel electrolyte. A lithium-CO₂ battery consists of two electrodes — an anode made of lithium and a cathode made of carbon — and an electrolyte, a solution that helps carry charged particles back and forth between the electrodes as the battery is charged and discharged. For their system, they needed an electrolyte made of amine plus captured CO₂ dissolved in a solvent — and it needed to promote chemical reactions on the carbon cathode as the battery discharged. They started by testing possible solvents. They mixed their CO₂-absorbing amine with a series of solvents frequently used in batteries and then bubbled CO₂ through the resulting solution to see if CO₂ could be dissolved at high concentrations in this unconventional chemical environment. None of the amine-solvent solutions exhibited observable changes when the CO₂ was introduced, suggesting that they might all be viable solvent candidates. However, for any electrochemical device to work, the electrolyte must be spiked with a salt to provide positively charged ions. Because it’s a lithium battery, the researchers started by adding a lithium-based salt — and the experimental results changed dramatically. With most of the solvent candidates, adding the salt instantly caused the mixture either to form solid precipitates or to become highly viscous — outcomes that ruled them out as viable solvents. The sole exception was the solvent dimethyl sulfoxide, or DMSO. Even when the lithium salt was present, the DMSO could dissolve the amine and CO₂. “We found that — fortuitously — the lithium-based salt was important in enabling the reaction to proceed,” says Gallant. “There’s something about the positively charged lithium ion that chemically coordinates with the amine-CO₂ adduct, and together those species make the electrochemically reactive species.” Exploring battery behavior during discharge To examine the discharge behavior of their system, the researchers set up an electrochemical cell consisting of a lithium anode, a carbon cathode, and their special electrolyte — for simplicity, already loaded with CO₂. They then tracked discharge behavior at the carbon cathode. As they had hoped, their special electrolyte actually promoted discharge reaction in the test cell. “With the amine incorporated into the DMSO-based electrolyte along with the lithium salt and the CO₂, we see very high capacities and significant discharge voltages — almost three volts,” says Gallant. Based on those results, they concluded that their system functions as a lithium-CO₂ battery with capacities and discharge voltages competitive with those of state-of-the-art lithium-gas batteries. The next step was to confirm that the reactions were indeed separating the amine from the CO₂ and further continuing the reaction to make CO₂-derived products. To find out, the researchers used a variety of tools to examine the products that formed on the carbon cathode. In one test, they produced images of the post-reaction cathode surface using a scanning electron microscope (SEM). Immediately evident were spherical formations with a characteristic size of 500 nanometers, regularly distributed on the surface of the cathode. According to Gallant, the observed spherical structure of the discharge product was similar to the shape of Li₂CO₃ observed in other lithium-based batteries. Those spheres were not evident in SEM images of the “pristine” carbon cathode taken before the reactions occurred. Other analyses confirmed that the solid deposited on the cathode was Li₂CO₃. It included only CO₂-derived materials; no amine molecules or products derived from them were present. Taken together, those data provide strong evidence that the electrochemical reduction of the CO₂-loaded amine occurs through the selective cleavage of the carbon-nitrogen bond. “The amine can be thought of as effectively switching on the reactivity of the CO₂,” says Gallant. “That’s exciting because the amine commonly used in CO₂ capture can then perform two critical functions. It can serve as the absorber, spontaneously retrieving CO₂ from combustion gases and incorporating it into the electrolyte solution. And it can activate the CO₂ for further reactions that wouldn’t be possible if the amine were not there.” Future directions Gallant stresses that the work to date represents just a proof-of-concept study. “There’s a lot of fundamental science still to understand,” she says, before the researchers can optimize their system. She and her team are continuing to investigate the chemical reactions that take place in the electrolyte as well as the chemical makeup of the adduct that forms — the “reactant state” on which the subsequent electrochemistry is performed. They are also examining the detailed role of the salt composition. In addition, there are practical concerns to consider as they think about device design. One persistent problem is that the solid deposit quickly clogs up the carbon cathode, so further chemical reactions can’t occur. In one configuration they’re investigating — a rechargeable battery design — the cathode is uncovered during each discharge-charge cycle. Reactions during discharge deposit the solid Li₂CO₃, and reactions during charging lift it off, putting the lithium ions and CO₂ back into the electrolyte, ready to react and generate more electricity. However, the captured CO₂ is then back in its original gaseous form in the electrolyte. Sealing the battery would lock that CO₂ inside, away from the atmosphere — but only so much CO₂ can be stored in a given battery, so the overall impact of using batteries to capture CO₂ emissions would be limited in this scenario. The second configuration the researchers are investigating — a discharge-only setup — addresses that problem by never allowing the gaseous CO₂ to re-form. “We’re mechanical engineers, so what we’re really keen on doing is developing an industrial process where you can somehow mechanically or chemically harvest the solid as it forms,” Gallant says. “Imagine if by mechanical vibration you could gently remove the solid from the cathode, keeping it clear for sustained reaction.” Placed within an exhaust stream, such a system could continuously remove CO₂ emissions, generating electricity and perhaps producing valuable solid materials at the same time. Gallant and her team are now working on both configurations of their system. “We don’t know which is better for applications yet,” she says. While she believes that practical lithium-CO₂ batteries are still years away, she’s excited by the early results, which suggest that developing novel electrolytes to pre-activate CO₂ could lead to alternative CO₂ reaction pathways. And she and her group are already working on some. One goal is to replace the lithium with a metal that’s less costly and more earth-abundant, such as sodium or calcium. With seed funding from the MIT Energy Initiative, the team has already begun looking at a system based on calcium, a material that’s not yet well-developed for battery applications. If the calcium-CO₂ setup works as they predict, the solid that forms would be calcium carbonate — a type of rock now widely used in the construction industry. In the meantime, Gallant and her colleagues are pleased that they have found what appears to be a new class of reactions for capturing and sequestering CO₂. “CO₂ conversion has been widely studied over many decades,” she says, “so we’re excited to think we may have found something that’s different and provides us with a new window for exploring this topic.” This research was supported by startup funding from the MIT Department of Mechanical Engineering. Mingfu He, a postdoc in mechanical engineering, also contributed to the research. Work on a calcium-based battery is being supported by the MIT Energy Initiative Seed Fund Program. This article appears in the Spring 2019 issue of Energy Futures, the magazine of the MIT Energy Initiative. (Comment on this)
`4:00p`	Health effects of China’s climate policy extend across Pacific Improved air quality can be a major bonus of climate mitigation policies aimed at reducing greenhouse gas emissions. By cutting air pollution levels in the country where emissions are produced, such policies can avoid significant numbers of premature deaths. But other nations downwind from the host country may also benefit. A new MIT study in the journal Environmental Research Letters shows that if the world’s top emitter of greenhouse gas emissions, China, fulfills its climate pledge to peak carbon dioxide emissions in 2030, the positive effects would extend all the way to the United States, where improved air quality would result in nearly 2,000 fewer premature deaths. The study estimates China’s climate policy air quality and health co-benefits resulting from reduced atmospheric concentrations of ozone, as well as co-benefits from reduced ozone and particulate air pollution (PM2.5) in three downwind and populous countries: South Korea, Japan, and the United States. As ozone and PM2.5 give a well-rounded picture of air quality and can be transported over long distances, accounting for both pollutants enables a more accurate projection of associated health co-benefits in the country of origin and those downwind. Using a modeling framework that couples an energy-economic model with an atmospheric chemistry model, and assuming a climate policy consistent with China’s pledge to peak CO₂ emissions in 2030, the researchers found that atmospheric ozone concentrations in China would fall by 1.6 parts per billion in 2030 compared to a no-policy scenario, and thus avoid 54,300 premature deaths — nearly 60 percent of those resulting from PM2.5. Total avoided premature deaths in South Korea and Japan are 1,200 and 3,500, respectively, primarily due to PM2.5; for the U.S. total, 1,900 premature deaths, ozone is the main contributor, due to its longer lifetime in the atmosphere. Total avoided deaths in these countries amount to about 4 percent of those in China. The researchers also found that a more stringent climate policy would lead to even more avoided premature deaths in the three downwind countries, as well as in China. The study breaks new ground in showing that co-benefits of climate policy from reducing ozone-related premature deaths in China are comparable to those from PM2.5, and that co-benefits from reduced ozone and PM2.5 levels are not insignificant beyond China’s borders. “The results show that climate policy in China can influence air quality even as far away as the U.S.,” says Noelle Eckley Selin, an associate professor in MIT’s Institute for Data, Systems, and Society and Department of Earth, Atmospheric and Planetary Sciences (EAPS), who co-led the study. “This shows that policy action on climate is indeed in everyone’s interest, in the near term as well as in the longer term.” The other co-leader of the study is Valerie Karplus, the assistant professor of global economics and management in MIT’s Sloan School of Management. Both co-leaders are faculty affiliates of the MIT Joint Program on the Science and Policy of Global Change. Their co-authors include former EAPS graduate student and lead author Mingwei Li, former Joint Program research scientist Da Zhang, and former MIT postdoc Chiao-Ting Li. (Comment on this)

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