Webb reveals skull-shaped nebula and Uranus auroras

The James Webb Space Telescope delivered two striking discoveries in quick succession this month: the first three-dimensional map of Uranus's upper atmosphere and auroras, and detailed new infrared portraits of a little-known nebula that looks like a brain floating inside a transparent skull.

A New View of Uranus's Auroras

An international team led by Paola Tiranti, a PhD student at Northumbria University in England, used Webb's Near-Infrared Spectrograph (NIRSpec) to observe Uranus for 15 hours — nearly a full rotation of the planet — on January 19, 2025. The resulting data, published February 19 in Geophysical Research Letters, allowed scientists for the first time to map the temperature and density of ions stretching up to 5,000 kilometers above the planet's cloud tops.

The observations revealed two bright auroral bands near Uranus's magnetic poles, along with a distinct low-emission zone between them likely shaped by the planet's magnetic field lines. While the Hubble Space Telescope first captured images of auroras on Uranus in 2011, Webb's data represent the most detailed picture yet of how those auroras form and how the planet's tilted magnetic field influences them.

"This is the first time we've been able to see Uranus's upper atmosphere in three dimensions," Tiranti said in a statement released by the European Space Agency. "With Webb's sensitivity, we can trace how energy moves upward through the planet's atmosphere and even see the influence of its lopsided magnetic field."

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The team also confirmed that Uranus's upper atmosphere continues to cool, a trend first observed in the early 1990s. Webb measured an average temperature of about 426 kelvins — roughly 150 degrees Celsius — lower than values recorded by ground-based telescopes or the Voyager 2 spacecraft, which performed the only close flyby of Uranus in 1986.

The "Exposed Cranium" Nebula

On February 25, NASA and ESA released Webb's new infrared images of nebula PMR 1, nicknamed the "Exposed Cranium" for its resemblance to a brain inside a translucent skull. The nebula is being created by an aging star shedding its outer layers, and Webb captured its features using both its NIRCam and MIRI instruments.

The images reveal distinct phases of the star's evolution — an outer shell of hydrogen blown off first, and a more structured inner cloud containing heavier gases. A dark lane running vertically through the nebula gives it the look of left and right brain hemispheres and may be linked to twin jets of material erupting from the central star. NASA's now-retired Spitzer Space Telescope first glimpsed the nebula in infrared more than a decade ago, but Webb's resolution has exposed detail that Spitzer could not resolve.

What Comes Next

Scientists say much remains unknown about PMR 1, including whether the dying star at its center is massive enough to end in a supernova or will instead cool into a dense white dwarf. As for Uranus, upcoming Webb observation cycles will monitor seasonal changes as the planet's north pole tilts toward the sun, with peak summer expected around 2028 to 2030. "By revealing Uranus's vertical structure in such detail, Webb is helping us understand the energy balance of the ice giants," Tiranti said. "This is a crucial step towards characterizing giant planets beyond our solar system."

New gravitational-wave method tackles cosmic expansion mystery

A team of astrophysicists from the University of Illinois Urbana-Champaign and the University of Chicago has developed a new technique to measure the rate at which the universe is expanding, leveraging the faint hum of gravitational waves produced by merging black holes across the cosmos. The research, accepted for publication in Physical Review Letters, introduces what the team calls the "stochastic siren" method — an approach that could help resolve one of the most persistent puzzles in modern cosmology.

A New Tool for Cosmology

The Hubble constant, which quantifies the universe's present-day expansion rate, has been measured using two broad approaches: observations of the early universe, such as the cosmic microwave background, and observations of the nearby universe, such as supernovae. These approaches yield values that disagree — roughly 67 km/s/Mpc from early-universe data versus approximately 73 km/s/Mpc from late-universe measurements — a discrepancy that has reached a statistical confidence exceeding five sigma.

The stochastic siren method offers an entirely independent path. Rather than relying solely on individually detected black hole mergers or electromagnetic observations, it incorporates information from the gravitational-wave background — a collective signal from the many distant black hole collisions too faint for current detectors to resolve individually.

"Because we are observing individual black hole collisions, we can determine the rates of those collisions happening across the universe," said Bryce Cousins, a physics graduate student at Illinois and lead author of the study. "Based on those rates, we expect there to be a lot more events that we can't observe, which is called the gravitational-wave background."

​The method's logic rests on a relationship between the expansion rate and spatial volume. A slower expansion rate implies a larger cosmic volume, which means more mergers and a stronger background signal. The current non-detection of the background therefore rules out the lowest expansion rates. When the team combined this constraint with data from resolved mergers recorded during the first three observing runs of the LIGO-Virgo-KAGRA Collaboration, they produced a more precise estimate of the Hubble constant than resolved mergers alone could provide.

A Promising Path Forward

Daniel Holz, a professor at the University of Chicago and co-author, called the technique "an entirely new tool for cosmology," adding that it opens "an exciting and completely new direction" for constraining the Hubble constant and other cosmological quantities.

​The gravitational-wave background is expected to be detected within the next six years as detector sensitivity improves. Even before that milestone, the stochastic siren method would progressively tighten the lower bound on the Hubble constant with each successive observing run that fails to detect the background — gradually probing the heart of the Hubble tension.

"This should pave the way for applying this method in the future as we continue to increase the sensitivity, better constrain the gravitational-wave background, and maybe even detect it," Cousins said. "By including that information, we expect to get better cosmological results and be closer to resolving the Hubble tension."

Astronomers detect radio echo of an unseen gamma-ray burst

 A team of astronomers has identified what they describe as the most convincing example yet of an "orphan afterglow" — the fading radio echo of a gamma-ray burst whose original explosion was never observed from Earth. The discovery, detailed in a paper accepted for publication in The Astrophysical Journal and posted to arXiv on February 24, opens a new window into some of the most powerful and elusive events in the universe.

The radio source, designated ASKAP J005512-255834, was found using the Australian SKA Pathfinder (ASKAP), a 36-antenna radio telescope in Western Australia. It sits in a small, actively star-forming galaxy roughly 1.7 billion light-years from Earth.

Ruling Out Alternatives

The signal was visible almost exclusively at radio wavelengths, with no counterpart in visible light or X-rays — a hallmark of an orphan afterglow. Follow-up observations spanning frequencies from 0.3 to 9 GHz, using instruments including the Australia Telescope Compact Array, the upgraded Giant Metrewave Radio Telescope, and MeerKAT, revealed an evolving spectrum consistent with synchrotron emission.

The team systematically ruled out other explanations, including pulsars, supernovae, and active galactic nuclei. Only two scenarios fit the observed behavior: the late-time afterglow of a long gamma-ray burst viewed off-axis, or a star being torn apart by an intermediate-mass black hole — a rare and still-hypothetical class of black holes.

Opening a Hidden Population

Either explanation would represent an exceptionally rare detection. The paper notes that if ASKAP J005512-255834 is confirmed as an orphan afterglow, it would be only the second such discovery made through radio observations. The team expressed hope that ASKAP and future radio survey instruments could uncover many more of these hidden events, offering a fuller census of gamma-ray bursts across the cosmos.