Astronomers could spot life signs orbiting long-dead stars

The next generation of powerful Earth- and space-based telescopes will be able to hunt distant solar systems for evidence of life on Earth-like exoplanets — particularly those that chaperone burned-out stars known as white dwarfs.

The chemical properties of those far-off worlds could indicate that life exists there. To help future scientists make sense of what their telescopes are showing them, Cornell University astronomers have developed a spectral field guide for these rocky worlds.

“We show what the spectral fingerprints could be and what forthcoming space-based and large terrestrial telescopes can look out for,” said Thea Kozakis, doctoral candidate in astronomy, who conducts her research at Cornell’s Carl Sagan Institute. Kozakis is lead author of “High-resolution Spectra and Biosignatures of Earth-like Planets Transiting White Dwarfs,” published in Astrophysical Journal Letters.

In just a few years, astronomers — using tools such as the Extremely Large Telescope, currently under construction in northern Chile’s Atacama Desert, and the James Webb Space Telescope, scheduled to launch in 2021 — will be able to search for life on exoplanets.

“Rocky planets around white dwarfs are intriguing candidates to characterize because their hosts are not much bigger than Earth-size planets,” said Lisa Kaltenegger, associate professor of astronomy in the College of Arts and Sciences and director of the Carl Sagan Institute.

The trick is to catch an exoplanet’s quick crossing in front of a white dwarf, a small, dense star that has exhausted its energy.

“We are hoping for and looking for that kind of transit,” Kozakis said. “If we observe a transit of that kind of planet, scientists can find out what is in its atmosphere, refer back to this paper, match it to spectral fingerprints and look for signs of life. Publishing this kind of guide allows observers to know what to look for.”

Kozakis, Kaltenegger and Zifan Lin assembled the spectral models for different atmospheres at different temperatures to create a template for possible biosignatures.

Chasing down these planets in the habitable zone of white dwarf systems is challenging, the researchers said.

“We wanted to know if light from a white dwarf — a long-dead star — would allow us to spot life in a planet’s atmosphere if it were there,” Kaltenegger said.

This paper indicates that astronomers should be able to see spectral biosignatures — such as methane in combination with ozone or nitrous oxide — “if those signs of life are present,” said Kaltenegger, who said this research expands scientific databases for finding spectral signs of life on exoplanets to forgotten star systems.

“If we would find signs of life on planets orbiting under the light of long-dead stars,” she said, “the next intriguing question would be whether life survived the star’s death or started all over again — a second genesis, if you will.”

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Materials provided by Cornell University. Original written by Blaine Friedlander. Note: Content may be edited for style and length.

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New measurements reveal evidence of elusive particles in a newly-discovered superconductor

Particle chasing — it’s a game that so many physicists play. Sometimes the hunt takes place inside large supercolliders, where spectacular collisions are necessary to find hidden particles and new physics. For physicists studying solids, the game occurs in a much different environment and the sought-after particles don’t come from furious collisions. Instead, particle-like entities, called quasiparticles, emerge from complicated electronic interactions that happen deep within a material. Sometimes the quasiparticles are easy to probe, but others are more difficult to spot, lurking just out of reach.

Now a team of researchers at the University of Illinois, led by physicist Vidya Madhavan, in collaboration with researchers from the National Institute of Standards and Technology, the University of Maryland, Boston College, and ETH Zurich, have used high-resolution microscopy tools to peer at the inner-workings of an unusual type of superconductor, uranium ditelluride (UTe2). Their measurements reveal strong evidence that this material may be a natural home to an exotic quasiparticle that’s been hiding from physicists for decades. The study is published in the March 26 issue of Nature.

The particles in question were theorized back in 1937 by an Italian physicist named Ettore Majorana, and since then, physicists have been trying to prove that they can exist. Scientists think a particular class of materials called chiral unconventional superconductors may naturally host Majoranas. UTe2 may have all of the right properties to spawn these elusive quasiparticles.

“We know the physics of conventional superconductors and understand how they can conduct electricity or transport electrons from one end of a wire to the other with no resistance,” said Madhavan. “Chiral unconventional superconductors are much rarer, and the physics is less well known. Understanding them is important for fundamental physics and has potential applications in quantum computing,” she said.

Inside of a normal superconductor, the electrons pair up in a way that enables the lossless, persistent currents. This is in contrast to a normal conductor, like copper wire, which heats up as current passes through it. Part of the theory behind superconductivity was formulated decades ago by three scientists at the U of I who earned a Nobel prize in physics for their work. For this conventional kind of superconductivity, magnetic fields are the enemy and break up the pairs, returning the material back to normal. Over the last year, researchers showed that uranium ditelluride behaves differently.

In 2019, Sheng Ran, Nicholas Butch (both co-authors on this study) and their collaborators announced that UTe2 remains superconducting in the presence of magnetic fields up to 65 Tesla, which is about 10,000 times stronger than a refrigerator magnet. This unconventional behavior, combined with other measurements, led the authors of that paper to surmise that the electrons were pairing up in an unusual way that enabled them to resist break-ups. The pairing is important because superconductors with this property could very likely have Majorana particles on the surface. The new study from Madhavan and collaborators strengthens the case for this.

The team used a high-resolution microscope called a scanning tunneling microscope to look for evidence of the unusual electron pairing and Majorana particles. This microscope can not only map out the surface of uranium ditelluride down to the level of atoms but also probe what’s happening with the electrons. The material itself is silvery with steps jutting up from the surface. These step features are where evidence for Majorana quasiparticles is best seen. They provide a clean edge that, if predictions are correct, should show signatures of a continuous current that moves in one direction, even without the application of a voltage. The team scanned opposite sides of the step and saw a signal with a peak. But the peak was different, depending on which side of the step was scanned.

“Looking at both sides of the step, you see a signal that is a mirror image of each other. In a normal superconductor, you cannot find that,” said Madhavan. “The best explanation for seeing the mirror images is that we are directly measuring the presence of moving Majorana particles,” said Madhavan. The team says that the measurements indicate that free-moving Majorana quasiparticles are circulating together in one direction, giving rise to mirrored, or chiral, signals.

Madhavan says the next step is to make measurements that would confirm that the material has broken time-reversal symmetry. This means that the particles should move differently if the arrow of time were theoretically reversed. Such a study would provide additional evidence for the chiral nature of UTe2.

If confirmed, uranium ditelluride would be the only material, other than superfluid He-3, proven to be a chiral unconventional superconductor. “This is a huge discovery that will allow us to understand this rare kind of superconductivity, and maybe, in time, we could even manipulate Majorana quasiparticles in a useful way for quantum information science.”

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Cesium vapor aids in the search for dark matter

The hunt for dark matter is one of the most exciting challenges facing fundamental physics in the 21st century. Researchers have long known that it must exist, as many astrophysical observations would otherwise be impossible to explain. For example, stars rotate much faster in galaxies than they would if only ‘normal’ matter existed.

In total, the matter we can see only accounts for, at the most, 20 percent of the total matter in the universe — meaning that a remarkable 80 percent is dark matter. “There’s an elephant in the room but we just can’t see it,” said Professor Dmitry Budker, a researcher at the PRISMA+ Cluster of Excellence of Johannes Gutenberg University Mainz (JGU) and the Helmholtz Institute Mainz (HIM), explaining the problem he and many of his colleagues worldwide are contending with.

Dark matter could consist of extremely light particles

But so far no one knows what dark matter is made of. Scientists in the field are considering and researching a whole range of possible particles that might theoretically qualify as candidates. Among these are extremely lightweight bosonic particles, currently considered to be one of the most promising prospects. “These can also be regarded as a classical field oscillating at a specific frequency. But we can’t yet put a figure on this — and therefore the mass of the particles,” explained Budker. “Our basic assumption is that this dark matter field is coupled to visible matter and has an extremely subtle influence on certain atomic properties that would normally be constant.”

Budker and his team in Mainz have now developed a new method which they describe in the current issue of the leading specialist journal Physical Review Letters. It employs atomic spectroscopy and involves the use of cesium atom vapor. Only on exposure to laser light of a very specific wavelength do these atoms become excited. The conjecture is that minute changes in the corresponding observed wavelength would indicate coupling of the cesium vapor to a dark matter particle field.

“In principle, our work is based on a particular theoretical model, the hypotheses of which we are experimentally testing,” added the paper’s principal author, Dr. Dionysis Antypas. “In this case, the concept underlying our work is the relaxion model developed by our colleagues and co-authors at the Weizmann Institute in Israel.” According to the relaxion theory, there must be a region in the vicinity of large masses such as the Earth in which the density of dark matter is greater, making the coupling effects easier to observe and detect.

Previously inaccessible frequency range searched

With their new technique, the scientists have now accessed a hitherto unexplored frequency range in which, as postulated in relaxion theory, the effects of certain forms of dark matter on the atomic properties of cesium should be relatively easy to spot. The results also allow the researchers to formulate new restrictions as to what the nature of dark matter is likely to be. Dmitry Budker likens this meticulous search to the hunt for a tiger in a desert. “In the frequency range that we’ve explored in our current work, we still have not pinpointed dark matter. But at least, now that we’ve searched in this range, we know we don’t have to do it again.” The researchers still don’t know where dark matter — the tiger in his metaphor — is lurking, but they now know where it is not. “We just keep on targeting in more closely on the part of the desert where the tiger is most likely to be. And, at some point, we will catch him,” maintained Budker with confidence.

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Materials provided by Johannes Gutenberg Universitaet Mainz. Note: Content may be edited for style and length.

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IEEE Spectrum

DARPA Subterranean Challenge: The Scoring Rules

How autonomous robots in the underground scavenger hunt pick up points for their teams