Category: 7. Science

Giant impacts were common in the early evolution of the Solar System, and it is possible that Venus also experienced an impact. A giant impact on Venus could have affected its rotation rate and possibly its thermal evolution.

In this work, we explore a range of possible impacts using smoothed particle hydrodynamics (SPH). We consider the final major collision, assuming that differentiation already occurred and that Venus consists of an iron core (30% of Venus’ mass) and a forsterite mantle (70% of Venus’ mass). We use differentiated impactors with masses ranging from 0.01 to 0.1 Earth masses, impact velocities between 10 and 15 km/s, various impact geometries (head-on and oblique), different primordial thermal profiles, and a range of pre-impact rotation rates of Venus.

We analyse the post-impact rotation periods and debris disc masses to identify scenarios that can reproduce Venus’ present-day characteristics. Our findings show that a wide range of impact scenarios are consistent with Venus’ current rotation. These include head-on collisions on a non-rotating Venus and oblique, hit-and-run impacts by Mars-sized bodies on a rotating Venus.

Importantly, collisions that match Venus’ present-day rotation rate typically produce minimal debris discs residing within Venus’ synchronous orbit. This suggests that the material would likely reaccrete onto the planet, preventing the formation of long-lasting satellites – consistent with Venus’ lack of a moon.

We conclude that a giant impact can be consistent with both Venus’ unusual rotation and lack of a moon, potentially setting the stage for its subsequent thermal evolution.

Mirco Bussmann, Christian Reinhardt, Cedric Gillmann, Thomas Meier, Joachim Stadel, Paul Tackley, Ravit Helled

Comments: Accepted for publication in A&A, 12 pages, 9 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2508.03239 [astro-ph.EP] (or arXiv:2508.03239v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2508.03239
Focus to learn more
Submission history
From: Mirco Bussmann
[v1] Tue, 5 Aug 2025 09:12:48 UTC (5,186 KB)
https://arxiv.org/abs/2508.03239
Astrobiology,

A tidy textbook definition of life has never existed, yet most biology lessons still lean on the idea that living things must grow, make energy, and reproduce on their own.

That simple checklist leaves viruses outside the club, since their genetic shells activate only inside another organism and go dormant again when drifting alone.

Ryo Harada of Dalhousie University and colleagues discovered a creature that forces us to redraw those lines on the fly.

Their oddball microbe, provisionally named Sukunaarchaeum mirabile, was hiding in DNA scraped from a single plankton species off the Japanese coast.

Sukunaarchaeum virus line blurs

Biologists once leaned on André Lwoff’s dictum that “an organism is constituted of cells.” That rule pushed viruses into a separate realm of particles.

That neat boundary already looked wobbly when giant viruses surfaced in the early 2000s, flaunting genomes larger than some bacteria.

Sukunaarchaeum muddies the waters further. It is undeniably cellular, yet its playbook borrows many viral tricks. It retains the genes for building its own ribosomes and messenger RNA – parts no virus carries – but appears to outsource nearly every other task to a host cell.

A record-breaking small genome

The new organism’s entire genome fits into 238,000 base pairs – roughly the length of a medium-sized magazine article.

For comparison, the minimalist archaeon Nanoarchaeum equitans holds the previous cellular record at about 490,000 base pairs, already microscopic by prokaryotic standards.

Viruses can be both larger and smaller, yet they never lug around the full toolkit for protein synthesis. That is why Harada’s team calls Sukunaarchaeum “a cellular entity retaining only its replicative core.”

A virus-like microbe emerges

“Its genome is profoundly stripped-down, lacking virtually all recognizable metabolic pathways, and primarily encoding the machinery for its replicative core: DNA replication, transcription, and translation,” wrote Harada and co-authors in their report.

The code resembles a viral instruction manual more than a self-sufficient microbe.

Yet the creature sits within Archaea, one of the three great domains of life – not among viruses. Phylogenetic trees place it on a deep branch so distant from known groups that the authors propose creating a new phylum.

Sukunaarchaeum and borrowed genes

The team stumbled on the microbe while sequencing DNA from the dinoflagellate Citharistes regius. Nested in that plankton’s genetic debris was a tight loop of foreign DNA that refused to match any catalogued species.

Marine symbiosis can be intimate: some plankton rely on bacterial partners for vitamins, while others house entire algal cells that photosynthesize for them.

Sukunaarchaeum appears to take this intimacy to the extreme, shaving off every gene it can afford to lose and leaning on its host for nearly everything else a cell needs to stay alive.

Redrawing the tree of life

Because its ribosomal genes remain intact, the organism qualifies as cellular under the classic molecular litmus test.

Yet its pared-down metabolism likely prevents it from harvesting nutrients, producing ATP, or fixing carbon without help – behaviors normally reserved for viral passengers.

Scientists debate whether life is a binary label or a spectrum; Sukunaarchaeum pushes the spectrum view to the forefront.

The find suggests that many other stealth lineages may be hiding in environmental sequencing data, dismissed as contaminants or viral oddities.

Defining the meaning of “alive”

Words such as “alive” steer funding, public health policy, and even planetary protection rules for space probes.

If more organisms like Sukunaarchaeum exist, biosecurity protocols that screen only for free-living microbes could miss entire classes of symbiotic parasites.

The discovery also sharpens a practical question: What genetic load is the bare minimum for a cell to function? Synthetic biology groups pursuing engineered minimal cells may mine this archaeon’s blueprint for clues.

“The discovery of Sukunaarchaeum pushes the conventional boundaries of cellular life and highlights the vast unexplored biological novelty within microbial interactions,” wrote the researchers.

Harada’s team suspects that extreme genome pruning evolved because the host environment guaranteed nutrients, letting redundant pathways decay. 

Paleobiologists see a glimpse of early evolution, when ancient cells likely shared genes and resources more freely than today.

If so, today’s viruses and streamlined symbionts may echo an ancient lifestyle rather than representing biological outliers.

Sukunaarchaeum may not be alone

Scientists plan to investigate whether similar organisms exist in other marine ecosystems or symbiotic relationships.

These searches may involve reanalyzing existing metagenomic databases that could contain overlooked sequences resembling Sukunaarchaeum.

Another goal is to identify the specific host that enables Sukunaarchaeum to survive.

Without knowing its exact partner, researchers can’t fully explain how the symbiosis functions or what evolutionary pressures shaped such an extreme dependency.

The study is published in bioRxiv.

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British Columbia’s Burgess Shale is renowned for its exceptional preservation of soft tissues in fossils, including limbs and guts. While trilobites are abundant in the fossil record thanks to their hard exoskeleton, their soft limbs are rarely preserved and poorly understood. However, Olenoides serratus, a particularly abundant and well-preserved Burgess Shale trilobite, offers a unique opportunity to study these appendages.

In a new study published in the BMC Biology, Sarah Losso, a postdoctoral fellow in the Department of Organismic and Evolutionary Biology (OEB) at Harvard, led a team of researchers in analyzing 156 limbs from 28 O. serratus fossil specimens to reconstruct the precise movement and function of these ancient arthropod appendages —shedding light on one of the planet’s earliest animals.

“Understanding behavior and movement of fossils is challenging, because you cannot observe this activity like in living animals,” said Losso. “Instead, we had to rely on carefully examining the morphology in as many specimens as possible, as well as using modern analogues to understand how these ancient animals lived.”

Arthropods have jointed legs composed of multiple segments that can reach upwards (extend) or downwards (flex). The range of motion depends on the difference between how far each joint can reach in either direction. This range, along with the leg and shape of each segment, determines how the animal uses the limb for walking, grabbing and burrowing.

Horseshoe crabs, common arthropods found along the eastern shore of North America, are frequently compared to trilobites even though they are not closely related. Horseshoe crabs belong to a different branch of the arthropod tree, more closely related to spiders and scorpions, whereas trilobites’ family ties remain uncertain. The comparison is due to the similarity in that both animals patrol the ocean floor on jointed legs. The results, however, showed less similarity between the two animals.

Unlike horseshoe crabs, whose limb joints alternate in their specialization for flexing and extending (a pattern that facilitates both feeding and protection), O. serratus displayed a simpler, but highly functional limb design.

“We found that the limbs of O. serratus had a smaller range of extension and only in the part of the limb farther from the body,” explained Losso. Although their limbs were not used the same way as horseshoe crabs, Olenoides could walk, burrow, bring food towards its mouth, and even raise its body above the seafloor.

To bring their findings to life, the team created sophisticated 3D digital models based on hundreds of fossil images preserved at different angles. Because fossilized trilobite limbs are usually squashed flat, reconstructing them in three-dimensions posed a challenge.

“We relied on exceptionally well-preserved specimens, comparing limb preservation across many angles and filling in missing details using related fossils,” said senior author and OEB assistant professor Javier Ortega-Hernández.

The team compared the shape of trace fossils with the movement of the limbs. “Olenoides serratus could create trace fossils of different depths using different movements,” Losso explained. “They could raise their body above the sediment in order to walk over obstacles or to move more efficiently in fast-flowing water.”

Surprisingly, the researchers discovered that the male species also had specialized appendages used for mating, and that each leg also had a gill used for breathing.

While more than 22,000 species of trilobites have been described, less than 0.2% show any trace of legs at all. Nevertheless, lack of preservation does not imply these ancient arthropods went legless—rather, their soft limbs simply seldom survived fossilization. The rare conditions of the Burgess Shale — a fast burial by underwater landslides cutting off oxygen — were key to capturing such fleeting biological details.

The study provides a rare window into a more dynamic picture of life more than half a billion years ago, as trilobites like Olenoides serratus scuttled across the seabed with sophisticated limbs that could burrow and foraged through prehistoric seas, revealing not just how they survived, but how they thrived.

This research was partially funded by the National Science Foundation.

In the wake of a wildfire, there’s often an assumption that burned landscapes will be more susceptible to landslides. But new research from the University of Oregon suggests it’s not always that simple.

An analysis of the Columbia River Gorge, which runs along the border between Oregon and Washington, shows that steep, rocky watersheds in that area have been prone to debris flows and rockfall for thousands of years. Those events didn’t measurably increase after the Eagle Creek Fire, which scorched 47,000 acres of the gorge over the course of three months in 2017.

UO geologist Josh Roering and members of his lab published their findings Aug. 8 in Science Advances , highlighting the importance of context and geological history in landslide risk assessments. The study also could help inform safety and hazards awareness projects in the gorge, in both burned and nonburned areas.

After the Eagle Creek Fire, Oregon land managers were concerned about landslides, especially in the vicinity of the Interstate 84 transportation corridor that runs through the gorge. Their fears were, in large part, informed by what’s transpired in places like Southern California, where post-fire slides have caused devastating casualties and millions of dollars in damage.

That phenomenon can happen because as wildfire destroys vegetation and groundcover, slopes become more prone to debris movement, erosion and rock fall, Roering said, which can be more easily triggered by rain and storm events.

“When Eagle Creek burned up such a massive area of the Columbia River Gorge, the natural question was: Is that going to happen here?” Roering said. “The gorge provided a great laboratory to examine how fire affects steep and rocky landscapes.”

In his lab’s latest project, Roering and doctoral student Maryn Sanders analyzed recent debris flows in the gorge to better understand the likelihood of slope movement after a fire and to explore how to predict when debris flows will occur. Debris flows occur when loose sediment — like mud, rocks and other debris — rapidly moves down a slope, often fueled by a storm or heavy rain.

Sanders and her team turned to a remote-sensing technology known as airborne lidar, or light detection and ranging, which allows them to see through the tree cover so they can analyze physical changes on the ground below, like where erosion has occurred. That tool, alongside field observations, helped them map out debris flows so they could assess movement across the study area.

As Sanders mapped the data, she found that many debris flows were concentrated in the watersheds near Dodson, just a few miles east of Multnomah Falls on the Oregon side of the gorge. Those are some of the steepest and fastest eroding watersheds in the state.

The debris flows in that region have been especially frequent and destructive. They’ve caused fatalities and threatened additional human lives, homes and infrastructure, which make them even more vital for state agencies to understand.

Sanders noticed a few interesting characteristics of the landscape as she studied the data, which suggested fire might not be the most significant cause of slope movement in that area. It also hinted that steep, rocky terrain behaves differently than slopes in a place like Southern California.

The researchers found massive amounts of sediment accumulation in fan-like formations at the base of the rocky catchments in gorge watersheds. At first glance, those features looked unassuming because they were covered in vegetation, but with lidar imaging it was clear something more notable was going on beneath the surface.

“The size and makeup of the fans suggest that frequent debris flows have been happening in these watersheds for a really long period of time, in the magnitude of thousands of years,” Sanders said.

She also observed that the slopes were collecting sediment much faster than more stable terrain does, likely through temperature fluctuations that cause rockfall. That sets them up to produce debris flows more frequently, typically every few decades.

Sanders took a closer look and analyzed the erosion rates in the area. She found frequent debris flows throughout its geological history and saw that the landscape had behaved in a consistent manner over thousands of years, something that remained relatively unchanged after the 2017 fire.

“Because we found similar rates of erosion before and after the fire, we believe the rocky environment was not as sensitive to fire,” she said. “Our analysis suggests that fire plays a relatively small role in triggering these events and emphasizes how important it is to consider the history of place.”

Still, the frequency, size and nature of debris flows in the gorge remains an ongoing cause for concern. The researchers are in the final stages of developing a tool that could help the Oregon Department of Transportation and other stakeholders predict debris flows in the gorge. That would help them make better use of safety features like roadside warning signs and closures, alerting travelers about the heightened risk of landslides during intense storms.

“These watersheds are highly active and inherently hazardous, irrespective of fire,” Sanders said. “We want our research to help agencies like ODOT better understand this geologically-complex landscape.”

A team of astronomers says it has identified the most distant black hole ever confirmed — a cosmic heavyweight that formed just 500 million years after the Big Bang, when the universe was only about 3% of its current age.

The discovery sets a new benchmark for how early supermassive black holes can form and raises questions about their origin and growth.

The National Aeronautics and Space Administration’s (Nasa) Curiosity rover has made a striking discovery on Mars, capturing images of a rock formation resembling coral, raising intriguing questions about potential life on the planet in the past.

Images showed a light-coloured object, measuring one inch (2.5 centimetres) wide, discovered in the Gale Crater, a significant impact basin in the Martian surface, according to the New York Post.

According to a Nasa, the rock likely formed around a billion years ago when liquid water was still present on the Red Planet.

“The Red Planet once had rivers, lakes, and possibly an ocean. Although scientists aren’t sure why, its water eventually dried up and the planet transformed into the chilly desert it is today,” the statement read.

The process, which is also seen on Earth, involves water carrying dissolved minerals into rock cracks. The water then dries, leaving behind hardened minerals. Over aeons, Martian winds have eroded the surrounding rock, revealing the intricate, coral-like shapes.

The discovery was captured by the rover’s Remote Micro Imager, a high-resolution telescopic camera. However, this is not the first time the camera has captured similar formations.

Nasa has noted that similar flower-shaped objects have been found in the past, all of which point to a watery history for the planet.

This summer, the Curiosity rover has also captured images of another geological structure nicknamed ‘spiderwebs’ due to its insect-like pattern of ridges. Nasa has explained that these formations also indicate that Mars once had water, which has since hardened.

In a previous statement, Nasa said that the images and data collected are “raising new questions about how the Martian surface was changing billions of years ago.”

While the Red Planet is now a chilly desert, it once had rivers, lakes, and possibly an ocean. The ‘boxwork patterns’ of the ‘spiderwebs’ show that even as the planet was drying up, water was still present underground, creating changes that are visible today.

“Remarkably, the boxwork patterns show that even in the midst of this drying, water was still present underground, creating changes seen today,” Nasa said.

Black holes are one of the great enigmas of the universe. Even the most brilliant minds of modern physics have been unable to unravel what exactly happens inside them, where a point of infinite density known as the singularity lurks. Nor is it known what happens when they cross their boundary, the so-called event horizon. There are various theories, but almost no certainties. Known for a gravitational pull so intense that not even light can escape, these ghosts of what were once giant stars represent much more than mysterious cosmic objects: they are natural laboratories for testing the limits of physics.

In this context, renowned theoretical physicist Cosimo Bambi, a researcher at Fudan University in Shanghai, China, proposes an idea that is, to say the least, bold: sending tiny interstellar spacecraft to the black hole closest to Earth, with the mission of collecting firsthand data and verifying whether what Albert Einstein predicted in his theory of general relativity more than 100 years ago remains valid under extreme conditions. His proposal was published this Thursday in the journal iScience, part of the Cell Press group.

“It’s true,” Bambi begins via video call from China. “It sounds like science fiction, but I don’t think it is. It’s something realistic, not for now, but for the next few years.” The plan outlined by the physicist is based on an already risky assumption: the existence of a black hole 20 or 25 light-years from Earth. The first challenge would be finding it. Detecting one of these cosmic monsters in isolation is difficult because they don’t emit light or other forms of radiation, so their existence can only be inferred based on interactions with other objects, as if they were invisible giants. The closest known black hole is 1,560 light-years from our planet. But Bambi insists that probability and the use of radio telescopes or the detection of gravitational waves could help locate one closer. Thus, according to the author’s calculations, a nanoprobe would take between 60 and 75 years to reach the black hole, and data transmission would add another 25 years. In total, the project would last 80 to 100 years.

Once that initial challenge is overcome, the technological race would begin. Bambi’s proposal includes the use of nanocraft, tiny special probes weighing just a few grams, equipped with all the necessary instruments to take measurements, and sails that would be propelled by powerful laser beams from Earth at one-third the speed of light.

Then comes the third challenge: what to do once the mission reaches its objective. The physicist proposes several verification processes, including the functioning of space-time around the black hole, testing the existence of the event horizon, and investigating whether the fundamental constants of physics change in the presence of such intense gravitational fields. To do this, Bambi thinks the ideal approach would be to send two nanocraft. One — let’s call it A — would remain at a certain distance and monitor the other — B — which would orbit near the black hole, emitting periodic signals. The alterations in these signals would reveal whether or not the metric follows theoretical predictions. A second experiment would study the existence of the event horizon. Craft B would drop toward the black hole, and A would measure how long its signal remains active before disappearing.

This is, of course, if the spacecraft manage to enter orbit and survive such a hostile environment. “I’m not saying it’s possible to do this now, but it deserves to be discussed within the community,” the scientist explains. He adds that the spirit of his idea is to “stimulate” his colleagues and test whether the pillars on which modern physics has stood for more than a century are still solid enough.

The oldest black hole

“In the end, it’s not just about answering scientific questions, but also philosophical ones like what exactly time is, or the origin of the universe,” Bambi points out. These are mysteries that current equipment can’t resolve, even if they try. However, some tools like the James Webb Space Telescope keep throwing up surprises. The latest, published this week in The Astrophysical Journal Letters, detected the oldest active black hole to date. Analysis of the light emitted by the CAPERS-LRD-z9 galaxy — located more than 13.3 billion light-years from Earth — has allowed scientists to identify the fingerprint of a black hole 300 million times more massive than the Sun, lodged in its center and already there wreaking havoc when the universe was barely 500 million years old.

Some of the postulates raised throughout the Bambi article are based on known technologies. For example, the use of nanocraft propelled by light beams. Back in 2016, Stephen Hawking and other astronomers presented Breakthrough Starshot, a project to reach Alpha Centauri, the closest star system to the Sun, using these devices. But beyond that, the experts consulted have been rather skeptical of Bambi’s plans.

“This project doesn’t seem realistic to me,” notes Pablo Pérez González, a researcher at the Spanish National Research Council’s Center for Astrobiology. The scientist agrees that the interest in studying an astronomical object up close is “extraordinary” due to the amount of information that can be obtained. “It’s like comparing seeing a cat on a cell phone with touching, smelling, or hearing it,” he asserts. But he believes feasibility must be considered and a realistic plan established before rushing into writing theories. “Thinking about something like this is commendable and interesting; it’s like going from reading about visiting the Moon in a Jules Verne book to actually doing it. But is that science?”

Carlos Barceló, a researcher in the theoretical physics group at the Institute of Astrophysics of Andalusia, points in the same direction. “I think the idea is a guided speculation. The researcher is well-known in the field of gravitation, so he really knows what he’s talking about, but his article is highly speculative,” he comments. For Barceló, each of the points developed in the Bambi plan involves extremely high levels of technological development, so it’s difficult to know if it will be viable for the next 100 years. “The margins of error are enormous. This is more appropriate for an essay than a scientific article,” he reflects.

Bambi isn’t discouraged. “In science, experiments or ideas often aren’t for a single generation and often take years,” he explains. For him, “the real challenge in all of this is luck.” Astrophysicists don’t know where the nearest black hole is. “If it were 20 light-years away, the mission would be possible; if it were 40 or 50, it would become much more costly and complicated, and the project would have to be abandoned,” he asserts.

Discovering such a nearby black hole would already be a major breakthrough and would attract the scientific community’s interest in studying it. “The real challenge is convincing colleagues that this is worthwhile. If there’s motivation, the technology can be developed,” Bambi says. In this regard, Pérez González emphasizes that interstellar missions to visit nearby stars “are much more viable” and that “part of the objectives presented by the author could be addressed.”

Barceló agrees: “Stimulating the creation of microsatellites that can be sent to distant places, in a more reasonable timeframe, is very interesting and will be developed sooner or later.” In fact, some research is already underway to send micro-ships powered by these technologies to planets near the edge of the solar system. “That seems to me to be the closest thing to being feasible,” he adds.

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With spectroscopy, astronomers can look for telltale signs of stars, galaxies and other celestial objects. Black holes gobble up dust and matter around them, compressing and heating the material as it swirls around and falls into the black hole. All of that can be seen with spectroscopy, said study co-author Steven Finkelstein, a professor of astronomy at the University of Texas at Austin.

“We look for these signatures of very fast-moving gas,” Finkelstein said. “We’re talking about velocities of 1,000, 2,000, sometimes even 3,000 kilometers per second. Nothing else in the universe moves that fast, so we know it has to be gas around a black hole.”

Scientists have identified possible black hole candidates that are more distant, but this is the oldest one that has been confirmed with spectroscopy, he added.

The galaxy that harbors the newfound black hole was also a fascinating discovery, the researchers said. It’s part of a class of galaxies nicknamed “Little Red Dots” because they emit red wavelengths of light and are very compact and unexpectedly bright, according to Taylor.

Not much is known yet about Little Red Dots, but they were first spotted by the James Webb Space Telescope. Though some have been spotted relatively nearby, Finkelstein said they were likely more common in the early universe.

Studying the CAPERS-LRD-z9 galaxy may yield clues about how Little Red Dots came to be and what causes their distinct red color, the researchers said. It may also provide clues about how such an old black hole came to be so large early on in the universe’s evolution.

In follow-up studies, the researchers are hoping to find other black holes in the distant universe that are just as old — if not older.

“We only ever survey very tiny areas of the sky with the James Webb Space Telescope,” Finkelstein said. “So, if we find one thing, there’s got to be a lot more out there.”

White dwarfs are remnants of main sequence stars like our Sun that have depleted their hydrogen. Their lives of fusion have come to an end, and they’ll simmer with residual heat for trillions of years. Their lifespans are longer than the current age of the Universe.

White dwarfs are fascinating objects for several reasons. About 97% of stars will become white dwarfs so they have a lot to teach us about stellar evolution. Since they cool at predictable rates, they serve as timekeepers, helping astronomers determine the ages of the stellar clusters they reside in. They also sometimes explode as Type 1a supernovae, which serve as standard candles in the cosmic distance ladder. Last but not least, they’re extreme objects that allow astrophysicists to test things like theories of quantum mechanics.

White dwarfs aren’t all the same. The Chandrasekhar limit sets the upper mass limit for white dwarfs at about 1.44 solar masses. Above that, they can’t support their own mass and either explode or collapse into neutron stars. Most white dwarfs are well below that, and only rare ones are above one solar mass. These are called high-mass white dwarfs.

One known high-mass white dwarf is called WD 0525+526. It’s about 130 light-years away and is 20% more massive than the Sun. New research based on UV data from the Hubble Space Telescope suggests that the white dwarf is the result of a merger with another star.

The new research is titled “A hot white dwarf merger remnant revealed by an ultraviolet detection of carbon.” It’s published in Nature Astronomy and the lead author is Dr. Snehalata Snahu, a Research Fellow at the University of Warwick in the UK.

WD 0525+526 is different from other white dwarfs not only because of its higher mass. It’s chemistry is also different.

White dwarfs are typically composed of carbon and oxygen cores, with atmospheres dominated by either hydrogen or helium. When astronomers detect metals in a white dwarf’s atmosphere, meaning any element heavier than hydrogen or helium, they take notice. It can mean that the white dwarf has engulfed a planetesimal.

But astronomers know of six high-mass white dwarfs that have some puzzling features. They have hydrogen-dominated atmospheres with signs of only tiny amounts of carbon that’s likely dredged up from the core by convection. These stars could be the results of stellar mergers and are called DAQ white dwarfs.

“Finding clear evidence of mergers in individual white dwarfs is rare.” – Professor Boris Gänsicke, Department of Physics, University of Warwick.

“These rare white dwarfs likely originate from stellar mergers, making them ‘smoking guns’ for one of the binary evolution channels leading to thermonuclear supernovae,” the authors write. (A thermonuclear supernova is a Type 1a supernova.) “However, optical spectroscopy can uncover only the most carbon-enriched objects, suggesting that many more merger remnants may masquerade as normal pure-hydrogen-atmosphere white dwarfs.”

While optical spectroscopy struggles to detect fainter carbon signals, UV does not. In this research, the astronomers used the Hubble’s UV-sensing capability to examine WD 0525+526.

“In optical light (the kind of light we see with our eyes), WD 0525+526 looks like a heavy but otherwise ordinary white dwarf,” said first author Dr. Snehalata Sahu in a press release. “However, through ultraviolet observations obtained with Hubble, we were able to detect faint carbon signatures that were not visible to optical telescopes.”

This figure places WD 0525+526 in context with the six other known hydrogen-rich ultra-massive white dwarfs. WD 0525+526 is indicated by a red star, while the six published white dwarfs with spectral type DAQ (hydrogen and carbon lines present) are shown as blue circles. Panel b shows the carbon abundances of the stars, measured by log(C/H) in spectroscopy. It represents the logarithmic ratio of carbon to hydrogen. WD 0525+526 is significantly lower in carbon. Image Credit: Snahu et al. 2025. NatAst.

“Finding small amounts of carbon in the atmosphere is a tell-tale sign that this massive white dwarf is likely to be the remnant of a merger between two stars colliding,” said Dr. Snahu. “It also tells us there may be many more merger remnants like this masquerading as common pure-hydrogen atmosphere white dwarfs. Only ultraviolet observations would be able to reveal them to us.”

A white dwarf’s thick hydrogen and helium atmospheres keep the core tightly-wrapped. There’s very little convection in white dwarfs, since they’re supported not by thermal pressure but by electron degeneracy pressure. What convection they do experience is weak and limited to the thin atmospheric levels near their surfaces. This means that it’s very difficult for a white dwarf to dredge carbon up from its core.

But when a white dwarf merges with another star, the hydrogen and helium layers are almost completely burned off. The resulting star has only a very thin layer of hydrogen and helium, and carbon can now be dredged up from the star’s interior. This describes WD 0525+526.

This figure shows the spectrum of WD 0525+526. It corroborates the white dwarf’s hydrogen atmosphere, and also shows the presence of carbon. Blue lines represent strong carbon transitions and green dashed lines represent weaker ones or ones with less convincing data. The insets provide close-ups of the C III (left) and C II (right) lines that were used to determine the photospheric carbon abundance. Image Credit: Snahu et al. 2025. NatAst.

Co-first author Antoine Bédard, also from Warwick University, said “We measured the hydrogen and helium layers to be ten-billion times thinner than in typical white dwarfs. We think these layers were stripped away in the merger, and this is what now allows carbon to appear on the surface.”

“But this remnant is also unusual: it has about 100,000 times less carbon on its surface compared to other merger remnants. The low carbon level, together with the star’s high temperature (nearly four times hotter than the Sun), tells us WD 0525+526 is much earlier in its post-merger evolution than those previously found. This discovery helps us build a better understanding of the fate of binary star systems, which is critical for related phenomena like supernova explosions.”

Astrophysicists don’t expect carbon to reach the surface of a star so hot. That can happen later in the post-merger, when the star has cooled and convection can act more efficiently to dredge carbon to the surface where it’s visible. Instead, the researchers identified a type of convective mixing called semi-convection taking place in the star. Semi-convection can allow some carbon to penetrate the outer layers and reach the surface, and this is the first time it’s been observed in a white dwarf. It creates much slower mixing than regular convection, and is only partial; it doesn’t create homogenization. Semi-convection takes place in stars other than white dwarfs, too, but is very difficult to model.

This figure shows semi-convection delivers less carbon to the white dwarf’s upper layers than regular convection. Red represents hydrogen, blue represents helium, and green represents carbon. Semi-convection allows a thin hydrogen-helium shell to float on WD 0525+526’s surface, which explains why this red dwarf has less surface carbon than cooler DAQ stars observed long after their mergers. Image Credit: Snahu et al. 2025. NatAst.

“Finding clear evidence of mergers in individual white dwarfs is rare,” added Professor Boris Gänsicke, Department of Physics, University of Warwick, who obtained the Hubble data for this study. “But ultraviolet spectroscopy gives us the ability to detect these signs early, when the carbon is still invisible at optical wavelengths. Because the Earth’s atmosphere blocks ultraviolet light, these observations must be carried out from space, and currently only Hubble can do this job.”

The authors note that there are other potential explanations for the carbon observed on WD 0525+526′ surface. “Another possible source of photospheric carbon is accretion of material from the ISM, a disrupted planetary system or a close (sub)stellar companion,” they write. “However, in all cases, other heavy elements would be detected alongside carbon in the UV spectrum of the white dwarf.” Astronomers would expect to find things like silicon, nitrogen, phosphorous, and sulfur would be present, depending on the accretion source.

After testing their data against all of these scenarios, they rejected all of them.

“We did not detect photospheric silicon or any heavy element other than carbon in the COS spectrum of WD 0525+526,” the authors concluded. “Therefore, all variants of the accretion scenario seem highly unlikely.”

The National Aeronautics and Space Administration’s (Nasa) Curiosity rover has captured images of a rock formation that closely resembles coral, in a discovery that suggests there might have been life on Mars. 

The object, which is approximately one inch (2.5 centimetres) wide and light-coloured, was found in the Gale Crater, a large impact basin on the planet’s surface, the New York Post reported.

According to a Nasa statement, the rock was likely formed a billion years ago, a time when liquid water was still present on the Red Planet. 

“The Red Planet once had rivers, lakes, and possibly an ocean. Although scientists aren’t sure why, its water eventually dried up and the planet transformed into the chilly desert it is today,” the statement read.

The process, which is also seen on Earth, involves water carrying dissolved minerals into rock cracks. The water then dries, leaving behind hardened minerals. Over aeons, Martian winds have eroded the surrounding rock, revealing the intricate, coral-like shapes.

The discovery was captured by the rover’s Remote Micro Imager, a high-resolution telescopic camera. However, this is not the first time the camera has captured similar formations. 

Nasa has noted that similar flower-shaped objects have been found in the past, all of which point to a watery history for the planet.

This summer, the Curiosity rover has also captured images of another geological structure nicknamed ‘spiderwebs’ due to its insect-like pattern of ridges. Nasa has explained that these formations also indicate that Mars once had water, which has since hardened.

In a previous statement, Nasa said that the images and data collected are “raising new questions about how the Martian surface was changing billions of years ago.” 

While the Red Planet is now a chilly desert, it once had rivers, lakes, and possibly an ocean. The ‘boxwork patterns’ of the ‘spiderwebs’ show that even as the planet was drying up, water was still present underground, creating changes that are visible today.

“Remarkably, the boxwork patterns show that even in the midst of this drying, water was still present underground, creating changes seen today,” Nasa said.