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illustration of an exoplanet around a white dwarf with a debris disk

JWST has directly imaged what appear to be two giant exoplanets in orbit around white dwarf stars. This discovery has important implications for the fate of giant planets in our solar system as the Sun evolves into a red giant and eventually becomes a white dwarf.

planetary nebula NGC 5189

A planetary nebula appears briefly during the life cycle of most stars, including those like the Sun. Planetary nebulae tend to appear round or elliptical, but they can have dramatically varied shapes, like NGC 5189 shown here. [NASA, ESA, and the Hubble Heritage Team (STScI/AURA)]

The Fate of Most Stars

While brilliant supernova explosions demand our attention when they burst onto the scene, the vast majority of stars will end their lives more quietly, lofting their outer layers into space and forming a glowing planetary nebula that surrounds the star’s exposed core. The core, now a white dwarf containing roughly the mass of the Sun in a sphere roughly the size of Earth, starts out extremely hot and cools slowly over billions of years.

As stars evolve from main-sequence stars to red giants to white dwarfs, it’s clear that close-in planets will meet a fiery fate: as a red giant, the Sun will swell to more than 200 times its current radius, engulfing Mercury, Venus, and possibly Earth. But exactly how the transition affects planets watching things unfold from a distance isn’t yet clear. To learn more, we’ll need to study planets that survived the transformation, and recent observations with JWST may have revealed two planets that fit the bill.

Investigating Metal-Polluted White Dwarfs

Only a handful of planetary-mass objects have been discovered around white dwarfs, but many more are thought to exist; 25–50% of seemingly solo white dwarfs show metals in their spectra, which suggests that they’re collecting cast-off material from unseen planets or asteroids. If giant planets are common around these “metal-polluted” white dwarfs, it would suggest that 1) these planets are able to survive their home star’s red giant phase, and 2) they play a role in gravitationally nudging material toward the white dwarf.

images of two white dwarfs with their candidate planetary companions

The two white dwarfs and their candidate planets. The object in the upper-left corner of the top row of images is a galaxy. [Mullally et al. 2024]

Susan Mullally (Space Telescope Science Institute) and collaborators pointed JWST at four white dwarfs that may harbor planets. These white dwarfs have been shown to contain metals in their atmospheres and are young enough or close enough that their planets would be relatively bright. Even before carefully removing the white dwarfs’ light from the images, Mullally’s team spotted what they were looking for — a possible giant planet around two of the four white dwarfs.

Potential Planets on Outlying Orbits

The observations show a reddish object near two of the white dwarfs. If these objects are indeed planets and the same ages as their host white dwarfs (5.3 and 1.6 billion years old), they likely have masses of 1–7 and 1–2 Jupiter masses, respectively. They’re currently orbiting at estimated distances of 11.47 and 34.62 astronomical units (au), which correspond to orbital distances of 5.3 au and 9.7 au when their host stars were on the main sequence — similar to the present-day orbital distances of Jupiter and Saturn.

comparison of the ages and orbital separations of the two new planet candidates with the giant planets in our solar system and planetary-mass objects previously discovered through direct imaging

A comparison of the ages and orbital separations of the two new planet candidates (red triangles), the four giant planets in our solar system (blue stars), and objects with masses less than 12 Jupiter masses discovered previously through direct imaging (black circles). Click to enlarge. [Mullally et al. 2024]

While the objects appear to be associated with the white dwarfs, it’s not impossible for them to instead be small photobombing objects within our solar system or distant, reddish galaxies meandering in the background. The authors pin the likelihood of their detections being a false positive at 1 in 3,000.

If future JWST observations show that these white dwarfs and their candidate planetary companions march across the sky in step, it will mark the first direct imaging detection of planets similar to the giant planets in our solar system in age, mass, and orbital distance. More than that, it will provide evidence that widely separated planets survive their host stars ballooning into red giants, and that giant planets around white dwarfs are common and help their hosts accrete metal-rich material.

Citation

“JWST Directly Images Giant Planet Candidates Around Two Metal-Polluted White Dwarf Stars,” Susan E. Mullally et al 2024 ApJL 962 L32. doi:10.3847/2041-8213/ad2348

photograph of Earth

The chaotic interactions between solar system bodies prevent us from knowing Earth’s precise orbit billions of years in the past or future. New research shows that passing stars introduce even more uncertainty, shortening the time out to which we can reliably predict Earth’s past orbit by 10%.

Uncertain Future, Uncertain Past

a map of the solar system planets and 18,000 solar system asteroids

A map showing the positions of the planets and 18,000 asteroids on New Year’s Eve in 1999. Click for the high-resolution version of this image. [Eleanor Lutz; CC BY-NC-ND 4.0]

Though the solar system may seem stable and predictable, with the planets moving calmly along well-defined orbits, this stability may only be temporary. Planets and asteroids gravitationally nudge one another, introducing an element of chaos into the calm and making it possible — though improbable — that one or more planets in our solar system will be ejected or destroyed in the next 5 billion years.

This chaotic element doesn’t just introduce uncertainty into the future behavior of the planets. It also means we can’t look infinitely far into their past motions, either. For Earth, the limit seems to lie around 60–70 million years ago, meaning that we cannot extrapolate Earth’s orbital history beyond that point. Because changes in Earth’s orbit influence our planet’s climate, researchers interpreting the ancient climate record contained in ice cores and sediment consider the behavior of Earth’s past orbit, and it’s critical for them to know how far back our orbital simulations are reliable.

Previous studies have focused on how planets and asteroids affect simulations of Earth’s past orbit. But the solar system doesn’t exist in a vacuum (metaphorically speaking) — could passing stars affect our ability to study Earth’s orbital history as well?

Chaos Caused by Passing Stars

In a research article published today, Nathan Kaib (Planetary Science Institute and University of Oklahoma) and Sean Raymond (National Centre for Scientific Research, France; University of Bordeaux) used numerical simulations to understand how passing stars impact our ability to trace back Earth’s orbit. Kaib and Raymond added a tiny amount of scatter in the positions of the planets and large asteroids and allowed the system to evolve backwards over 150 million years of simulation time.

plot showing the standard deviation of Earth's orbital eccentricity over time

The standard deviation of Earth’s orbital eccentricity as a function of time in the past. [Kaib & Raymond 2024]

Some of the simulations contained stars placed at random positions about 3.4 light-years from the Sun and given random velocities. (Technically, all of the simulations contained passing stars, but in the star-free runs the stars’ masses were set to minuscule values. This prevents the results from depending on the computational methods being used rather than the presence of the stars.) By marking the moment at which Earth’s orbital eccentricity, or how circular its orbit is, diverged by more than 10%, the authors determined how far back their simulations of Earth’s orbit were reliable.

Close Encounters of the Stellar Kind

plot of the change in Earth's orbital eccentricity over time, showing the effects of a close stellar encounter

The change in Earth’s orbital eccentricity, showing the effects of an HD 7977-like encounter at a time of 10 million years. Click to enlarge. [Adapted from Kaib & Raymond 2024]

Kaib and Raymond found that passing stars interfere with our ability to trace back Earth’s orbit, shortening the time we can reliably look back by about 10%. The simulations suggest that stars act on Earth by nudging the giant planets, which then nudge Earth; when the giant planets were removed from the simulation, passing stars had little effect.

To look more closely at a specific event, Kaib and Raymond considered the star HD 7977. HD 7977 currently sits about 250 light-years away, but its trajectory suggests that it passed within about 13,200 astronomical units (au) of our solar system 2.8 million years ago, making it possibly the closest recent stellar encounter. The team showed that the star’s passage likely didn’t impact Earth’s orbit much, unless the star came as close as the data allow — there’s a 5% chance that the star came within 3,900 au.

If HD 7977 did approach the solar system so closely, the encounter may have introduced a large amount of uncertainty into simulations of Earth’s past orbit. This allows for a broader range of past orbital eccentricities, opening up new possibilities to consider for Earth’s orbital and climatic past.

Citation

“Passing Stars as an Important Driver of Paleoclimate and the Solar System’s Orbital Evolution,” Nathan A. Kaib and Sean N. Raymond 2024 ApJL 962 L28. doi:10.3847/2041-8213/ad24fb

photograph of craters on the Moon's north pole

On Earth, rocks and soil are weathered by wind and water, but what weathers surfaces on the Moon? New research investigates how ice might alter lunar soil in permanently shadowed lunar craters.

The Importance of Lunar Soil

photograph of the sunlit edge of a crater on the Moon

Photograph of the sunlit rim of Shackleton crater, a crater at the Moon’s south pole. The interior of this crater never receives direct sunlight. [NASA/GSFC/Arizona State University]

The Moon is blanketed in a layer of material called regolith, made up of fragments of rock, soft lunar soil, and fines: particles less than 2 microns across (1 micron = 10-6 meter). Apollo-era astronauts encountered fines during moonwalks; the fines clung relentlessly to every surface and caused sneezing and sore throats when brought in to the lunar module. Lunar fines aren’t just annoying — studies using simulated lunar material show that fines can damage lung cells and DNA.

NASA’s upcoming Artemis missions will bring astronauts to the lunar south pole, which is dotted with steep-sided craters that lie in permanent shadow. Because some of these craters are thought to harbor water ice, it’s important to know if the ice is likely to weather the lunar surface material, creating an abundance of harmful and irritating fines.

A Lunar Experiment on Earth

Autumn Shackelford (University of Central Florida) and collaborators examined how lunar regolith might be weathered by water ice. If a tiny bit of liquid water seeps into fine cracks in lunar surface particles, the expansion of the water as it freezes will widen the cracks. The mechanical stress caused by this expansion may cause the crack system to grow, potentially reshaping the particle.

close-up image of particles in one of the samples

An image of a sample that contained 30% water by weight and was frozen for 6 months. The numbering and outlines reflect the counting and characterization of individual particles and clumps of particles. Click to enlarge. [Shackelford et al. 2024]

To understand how water ice plays a role in the weathering process, Shackelford’s team mixed samples of simulated lunar soil with water and placed the samples in a freezer. Half of the samples contained minerals similar to the lunar highlands (the lighter regions of the Moon’s surface), while the other half were similar to the lunar “seas,” or maria (the darker regions of the Moon’s surface).

The team varied the amount of water added to the samples from no water — the control samples — to 30% water by weight, and left the samples in a freezer for 1, 3, or 6 months. After the samples finished hibernating, the team characterized the size and shape of the particles, counted the number of fine particles, and took spectra to search for possible spectral changes.

Water Weathering

Plots of the number of fines per particle imaged

The number of fines per particle, as a function of time spent in the freezer, amount of water, and type of particle. Click to enlarge. [Shackelford et al. 2024]

Contact with water ice changed the shapes and sizes of the simulated lunar particles, and the changes were dependent upon the amount of water, the time spent in the freezer, and whether the sample was similar to the lunar highlands or the lunar maria. Crucially, the amount of fine particulate matter — the material that was so perplexing during the Apollo missions — generally increased with both water content and time spent in the freezer. The lunar mare samples produced more fine particles than the lunar highlands samples in all cases, showing that composition plays an important role. Curiously, while the highlands simulant showed changes in its spectrum in the 5–22.5-micron range after exposure to ice, the mare samples showed no spectral changes.

With humans potentially exploring lunar regions where water ice lingers in shadowy craters, this work provides an important first look at how ice plays a role in producing fine particles. Future astronauts, take heed!

Citation

“Morphological and Spectral Characterization of Lunar Regolith Breakdown Due to Water Ice,” A. Shackelford et al 2024 Planet. Sci. J. 5 1. doi:10.3847/PSJ/ad0041

extreme-ultraviolet image of solar coronal loops

Many models of the Sun’s upper atmosphere, or corona, use magnetohydrodynamics, in which collections of particles are assumed to behave like an electrically conducting fluid. A recent research article explores how sensitive magnetohydrodynamics models are to the conditions they start with.

A Powerful Modeling Tool

image of the Sun releasing two coronal mass ejections

Two coronal mass ejections launched from the Sun in November 2000, as seen by the Solar and Heliospheric Observatory. [ESA/NASA/SOHO]

Magnetohydrodynamics simulations are a solar modeler’s bread and butter. These powerful simulations can be applied to everything from the slow shifting of the solar magnetic field as the Sun proceeds through its 11-year solar activity cycle to the rapid release of magnetic energy in solar flares or coronal mass ejections.

With so many ways to implement these models, it’s worth taking the time to consider how choices of model parameters affect the outcome. In other words, how much does what you get out depend on what you put in?

Testing Model Memory

To begin to address this question, Graham Barnes (NorthWest Research Associates) and collaborators studied the impact of initial conditions on magnetohydrodynamics simulations of the solar corona driven by electric fields. Just as the term suggests, initial conditions refer to the state of the model just before the modelers press go. For example, modelers might define the initial strength and direction of magnetic and electric fields and the density, temperature, and motion of the particles.

After a simulation begins, it will often undergo an adjustment period in which the initial quantities “relax” into a steady state. Once this steady state is reached, the model can be “driven,” such as by applying a force to the base of the corona, to study how the system changes over time. Barnes’s team sought to understand whether a model’s “memory” of the initial conditions persists through this relaxation phase. If the initial conditions are washed out during the relaxation phase, that means that any reasonable choice of parameters will do — but if the models retain a clear memory, researchers must choose their initial conditions more carefully.

Relating Relaxed Output to Initial Input

plots of the vertical magnetic field of two active regions

The two active regions used in this study. The greyscale shows the strength of the vertical component of the magnetic field. Click to enlarge. [Barnes et al. 2024]

The team focused on the initial state of the magnetic field. They modeled two solar active regions — areas with particularly strong and complex magnetic fields — using data from the Solar Dynamics Observatory’s Helioseismic and Magnetic Imager, which measures the vertical component of the solar magnetic field. They used three methods to derive the initial strength and direction of the magnetic field from the data: a commonly used potential field, a slightly more realistic representation of the corona’s magnetic field (the nonlinear force-free field), and a purely vertical field. Then, Barnes’s team allowed the simulations to relax by letting the magnetic field evolve over time while reverting other quantities, like the density of the solar plasma, back to their initial values at each time step.

comparison of initial conditions and steady state outcomes for three choices of initial conditions

Initial conditions (top row) and relaxed state (bottom row) for the modeled magnetic field of one of the active regions. This shows that the relaxed state is largely insensitive to the initial state when comparing potential field, nonlinear force-free field, and vertical field initial conditions. Click to enlarge. [Adapted from Barnes et al. 2024]

Barnes and collaborators found that the initial magnetic field conditions had little impact on the relaxed state of the magnetic field — the simple vertical magnetic field and the more realistic fields had remarkably similar outcomes. What differences did arise tended to show up at the borders of the simulation, where edge effects and boundary conditions play a larger role.

With models appearing to lack a clear memory of the conditions they started with, this work suggests that any reasonable initial magnetic field can be used for electric-field-driven models. More work is needed to understand if this result holds for other types of magnetohydrodynamics models, such as velocity-driven models.

Citation

“Are Electric-Field-Driven Magnetohydrodynamic Simulations of the Solar Corona Sensitive to the Initial Condition?” Graham Barnes et al 2024 ApJ 960 102. doi:10.3847/1538-4357/ad10a7

A projection of the night sky and La Silla observatory

Chile’s Coquimbo region is renowned for its starry skies and is home to some of astronomy’s most powerful and productive telescopes. In a recent article, researchers quantified the impact of artificial lighting on sites across the Coquimbo region for the first time and showed that we must actively preserve dark skies in remote areas and urban corridors alike.

Measuring the Impact of Light Pollution

illustration of the number of stars seen at different levels of light pollution

An illustration of the Bortle scale value, which describes the degree of light pollution experienced in different settings. Click to enlarge. [ESO/P. Horálek, M. Wallner; CC BY 4.0]

Is the Milky Way visible from where you live? If so, consider yourself lucky — light pollution prevents a third of earthlings from seeing the Milky Way, including nearly 80% of North Americans and 60% of Europeans. The luckiest few are getting fewer, with less than 1% of residents in North America and Europe enjoying pristine night skies.

While city dwellers might make do with a dwindling number of visible stars, professional astronomical observatories rely on exquisitely dark skies. To understand the impact of light pollution on professional observatories as well as cities, Rodolfo Angeloni (Gemini Observatory) and collaborators assessed the degree of light pollution at four sites across Chile’s Coquimbo region: 1) Fray Jorge National Park, a certified Starlight Reserve; 2) Las Campanas Observatory, a professional astronomical observatory atop a mountain in the Atacama Desert; 3) Collowara Astrotourism Observatory, located near a city of 11,000 people; and 4) La Serena, a city of 450,000 people.

Angeloni’s team used ground-based all-sky imaging sensors to measure the brightness of the night sky at these locations on cloudless, moonless nights. By comparing the all-sky images to models of sky brightness and cross-referencing them with the positions of artificial light sources as seen from space, the team estimated the contribution of artificial light to the night sky brightness in each area.

Map of Chile's Coquimbo region showing the locations of the four sites studied in this work

Locations of Las Campanas Observatory (LCO), La Serena (LA-CQ), Collowara Astrotourism Observatory (CAO), and Fray Jorge National Park (FJNP) on a map of artificial night sky brightness. Click to enlarge. [Angeloni et al. 2024]

From Light to Dark

The measurements confirmed that Fray Jorge National Park is an exceptional dark sky site, with just 4% of the night sky brightness coming from artificial lights. (Scattered starlight, scattered sunlight, and airglow are natural sources of night sky brightness.) Fray Jorge National Park is one of the darkest measured sites in the world, and this study highlights the immediate need to protect this site from light pollution.

At Las Campanas Observatory, the future home of the 25-meter Giant Magellan Telescope, artificial lights contributed about 11% of the observed sky brightness, with the largest contributions coming from the cities of La Serena and Vallenar (117 and 49 kilometers away, respectively). Luckily, the impact of light pollution on the observatory is currently small, but the growth of nearby cities and the ongoing Pan-American Highway project could brighten the skies at Las Campanas.

Collowara Astrotourism Observatory, situated near the 11,000-person city of Andacollo, has artificial sky brightness comparable to that of Flagstaff, AZ, a much larger city. Another way of looking at this finding is that Flagstaff, named the world’s first Dark Sky Community in 2001, has night sky brightness comparable to that of a much smaller city — in other words, active efforts to reduce light pollution work, and cities worldwide can take proven steps to address the issue.

plot of the sky brightness at Las Cumbres Observatory with major contributors of natural and artificial sky brightness labeled

The sky brightness surrounding Las Campanas Observatory, with the major sources of natural and artificial sky brightness labeled. The Milky Way (MW) dominates at about 150 degrees, with La Serena (LS-CQ) just slightly less bright but extending to a lower altitude. The city of Vallenar is also a source of sky brightness. [Angeloni et al. 2024]

The skies surrounding La Serena are overwhelmingly brightened by artificial sources, and the impact of the bright city lights was felt far away; La Serena was the largest source of artificial sky brightness at the other three sites monitored. This result makes it clear that it’s important to reduce light pollution in overly bright areas like La Serena, as these bright regions impact the sky brightness in distant locales, and it’s critical to preserve the few truly dark sites that remain. Angeloni’s team plans to continue their efforts in the region by continuously monitoring the sky brightness in La Serena and installing 40 more sensors in dark sky sites.

Bonus

Check out the video below to see a time-lapse of the night sky conditions at Collowara Astrotourism Observatory.

Citation

“Toward a Spectrophotometric Characterization of the Chilean Night Sky. A First Quantitative Assessment of ALAN Across the Coquimbo Region,” Rodolfo Angeloni et al 2024 AJ 167 67. doi:10.3847/1538-3881/ad165c

An icy surface overlaid by a liquid water ocean. Beams of light are streaming through the water from the ocean's top towards the ice below.

One would be forgiven for thinking that it’s easy to tell different types of planets apart. Unfortunately, the most interesting worlds look awfully like comparatively boring ones to our telescopes. A well-studied planet, LHS 1140 b, nicely illustrates this tension: after a re-analysis of archival data, it’s unclear if the planet has a potentially habitable surface ocean, or just a thin layer of hydrogen over a rocky surface.

Previous Work

A flux-vs-time plot showing a clean dip in the center.

Observations of four transits of LHS 1140 b collected by the Spitzer Space Telescope. [Cadieux et al. 2024]

The star named LHS 1140 has become quite popular among exoplanet astronomers in the last seven years, and for good reason. While not quite as famous as its spotlight-stealing sibling TRAPPIST-1, LHS 1140 shares many of the same properties that make the former such a magnet for telescopes. At just under 50 light-years away, it’s nearby and fairly bright; at just a fifth the mass and radius of the Sun, it’s lightweight enough to be tugged around by tiny planets and small enough that those planets block a large fraction of its light when they transit. All of these characteristics combined make it an excellent host star for scientists looking to make sensitive measurements.

Consequently, LHS 1140 has been observed with a battery of instruments since the discovery of its two accompanying planets in 2017 and 2019. When analyzed one-by-one these datasets suggest that the two worlds, creatively christened in the typical exoplanet fashion as LHS 1140 b and LHS 1140 c, are a little bigger and heavier than our home world but basically made of the same combination of rock and metal. However, all of these observations hadn’t before been analyzed in a single, joint study.

Another Look

Charles Cadieux, University of Montreal, took up this challenge and led a team to reprocess nearly every byte of information collected on these worlds in an attempt to better constrain their masses and radii. They succeeded: as the authors put it, “the LHS 1140 planets are [now] among the best-characterized exoplanets to date, with relative uncertainties of only 3% for the mass and 2% for the radius.”

However, the team wasn’t aiming to shrink the old error bars just for precision’s sake. Instead, their ultimate aim was to constrain the composition of each planet. Although their analysis confirmed that LHS 1140 c is likely a generic super-Earth, their new measurements placed LHS 1140 b in a strange corner of parameter space.

A mass-vs.-radius plot. The oval-shaped contours denoting the uncertainty of LHS 1140 b parameters from this study lie between two underlying contours denoting different compositions.

The mass and radius of the LHS 1140 planets, along with contours belonging to different compositions. LHS 1140 b falls just above the rocky line, but on the very edge of the gas-envelope region. Click to enlarge. [Cadieux et al. 2024]

They found that LHS1140 b must have a bulk density less than that of Earth but still much higher than those of the giant planets. After further modeling, the team was left with two very different scenarios for how a planet could arrive in this in-between gray zone. Either LHS 1140 b has a very light, puffy atmosphere of hydrogen and helium overlaying a rocky surface, or, more exotically, LHS1140 b is a “water world,” likely covered in ice with a pocket of liquid water.

A Path Forwards

While it’d be thrilling to have a potentially habitable planet in our galactic backyard, our current data, even when collected with our best instruments and processed with the most cutting-edge techniques, is frustratingly unable to distinguish between the two scenarios. There is technically a path forward to resolving the uncertainty: the authors boldly advocate for an 18-transit observational campaign with JWST to confirm a thick, water-friendly atmosphere. However, with many planets to point to and galaxies galore to observe, there’s no guarantee that the community will choose to dedicate so much time to one target. We may have to live with just the hint that there’s a nearby alien ocean: a bitter, but awe-inspiring, ambiguity.

Citation

“New Mass and Radius Constraints on the LHS 1140 Planets: LHS 1140 b Is either a Temperate Mini-Neptune or a Water World,” Charles Cadieux et al 2024 ApJL 960 L3. doi:10.3847/2041-8213/ad1691

disk of hot gas swirling around a black hole

Sometimes, stars and black holes can happily coexist. Other times, the star gets ripped apart. In a recent research article, a team explored what can happen in between those two extremes.

Challenging Companions

Hubble Space Telescope image of the Milky Way's nuclear star cluster

Dense star clusters, like the Milky Way’s nuclear star cluster depicted at the center of this image, provide opportunities for black holes and stars to get dangerously close. [NASA, ESA, and the Hubble Heritage Team (STScI/AURA)]

Across the universe, stars are living precariously close to black holes. This is true everywhere from densely packed star clusters, where stars and stellar-mass black holes are linked together in binary systems, to the centers of galaxies, where stars circle supermassive black holes at dizzying speeds.

When stars and black holes meet, it can be spectacular: a black hole’s tidal forces can pull apart a star, resulting in a tidal disruption event that powers bright emission across the electromagnetic spectrum. Tidal disruption events can fully or partially disrupt a star, and they can occur around supermassive or stellar-mass black holes (the latter events are called micro tidal disruption events). Tidal disruption events take place when stars approach black holes on parabolic or highly elliptical orbits — what happens when the star’s orbit is nearly circular, instead?

Tidal Peeling Events

Chengcheng Xin (Columbia University) and collaborators used smoothed-particle hydrodynamics simulations to model the interaction between a star and a black hole in a close binary system. They modeled 96 combinations of starting parameters, changing the eccentricity of the system from a perfect circle to a slightly squashed one, varying the mass of the star from 1 to 15 solar masses, and exploring different values of the penetration factor: the ratio between 1) the distance from the black hole at which an object would be tidally disrupted and 2) the pericenter distance.

spiral gas structure created in a tidal peeling event

Example of the spiral gas structure created in a tidal peeling event. [Adapted from Xin et al. 2024]

Xin’s team showed that for certain orbital conditions — namely, stars on nearly circular orbits with low penetration factors, or stars on slightly more elongated orbits with penetration factors around one — the black hole gradually ensnares material from the star over the course of several orbits, leading to the star being partially or entirely disrupted. The authors dub these events tidal peeling events. Tidal peeling events leave behind a spiral of gas that can escape into space, form a disk around the black hole, or re-encounter the star as it orbits, forming a shock.

A New Kind of Transient Phenomenon?

From our distant vantage point, would we be able to distinguish a tidal peeling event from a micro tidal disruption event? Both phenomena exhibit intense electromagnetic radiation powered by accretion of stellar gas onto a black hole. The rates of accretion are similar between tidal peeling events and micro tidal disruption events, but their timescales and time evolution are different. Tidally peeled stars are stripped apart over the course of several short orbits — likely a few to a few tens of hours — while micro tidal disruption events can be slower to play out.

As this study is an initial exploration of tidal peeling events, there is still much to learn about their physical and observational characteristics. Future avenues for investigation include the impact of outflowing jets and winds, shocks that form when the star collides with its own tidal debris, and what changes might arise when simulations are started sooner, long before the star and the black hole are already positioned close together and poised for the peeling to begin.

Citation

“‘Tidal Peeling Events’: Low-eccentricity Tidal Disruption of a Star by a Stellar-mass Black Hole,” Chengcheng Xin et al 2024 ApJ 961 149. doi:10.3847/1538-4357/ad11d3

a narrow ribbon of gas that is part of the Cygnus Loop supernova remnant

A supernova is a spectacular way for a star to die. Massive stars meet this fate when the outward pressure exerted by core nuclear fusion can no longer hold off the gravity of the star’s outer layers, and the remnants of lower-mass stars can attain this honor through accretion or collisions. Today we’ll introduce four research articles that examine various aspects of supernova science, from attempts to determine how lightweight a supernova progenitor star can be to exploring why some massive stars don’t produce supernovae at all.

Probing the Smallest Supernova Stars

stellar evolution schematic

The main stellar evolution pathways. It’s not yet clear exactly which stars explode as supernovae and which become white dwarfs. Click to enlarge. [ESA]

Where is the dividing line between stars that end their lives in core-collapse supernovae and those that are fated to become white dwarfs? The least massive stars that undergo core collapse lie somewhere in the 8–12-solar-mass range, and refining the estimate further requires researchers to track down the faintest, most rapidly evolving supernovae.

Luckily, increasing coverage by transient-hunting surveys has generated a growing sample of these faint, fast events. Kaustav Das (California Institute of Technology) and coauthors studied nine supernovae detected by the Zwicky Transient Facility that were found to have certain chemical abundance ratios that hint at the progenitor stars being low mass. These supernovae are calcium-rich Type IIb supernovae, which have much larger [Ca II]/[O I] ratios than typical core-collapse supernovae.

Das’s team used spectra of each of these supernovae to measure the amount of oxygen present, which can be used in theoretical models to estimate the mass of the star that exploded. The mass estimates for all stars in the sample were less than 12 solar masses, suggesting that this type of supernova tends to arise from stars near the low-mass end of the progenitor mass range. The current sample of known calcium-rich Type IIb supernovae is still small, but future detections should allow researchers to refine models and improve estimates.

supernova remnant W49B

Not all massive stars end their lives as supernovae, leaving behind a supernova remnant like W49B shown here. [X-ray: NASA/CXC/MIT/L.Lopez et al.; Infrared: Palomar; Radio: NSF/NRAO/VLA]

Focusing on Failed Supernovae

When massive stars extinguish their core nuclear fusion and collapse, do they always generate luminous supernovae? Both observations and theory suggest that the answer is no, with some would-be supernovae forming a black hole with no accompanying supernova. Up to a third of massive stars might fail to generate a supernova!

Eric Coughlin (Syracuse University) performed a mathematical exploration of failed supernovae, focusing on the creation and propagation of a shock between the collapsing core and the outward-moving outer layers of the star. Coughlin showed that while some dying massive stars don’t produce supernovae per se, they still undergo an explosion that marks their impending demise. The strength of the explosion from a failed supernova depends on the properties of the star and how much of its mass is lost in the form of neutrinos: chargeless, nearly massless subatomic particles that rarely interact with matter.

In addition to the mathematical solutions that described the explosions, the equations also permitted a solution in which the matter settles near the central object. While we’ll have to wait for future work for a full examination of these solutions, it’s possible that they’ll apply to smaller stellar outbursts that do not destroy the star.

Prospects for Detecting Supernova Neutrinos

When a massive star’s core collapses, it can form a black hole or a neutron star: an extremely dense, rapidly spinning, city-sized sphere made almost entirely of neutrons. As protons and electrons are crushed into neutrons in the star’s core, the transformation produces neutrinos that push the star’s collapsing outer layers outward. While most of the star’s outer layers escape, forming the glowing, complex structures of a supernova remnant, a small fraction of the material falls back onto the protoneutron star, generating even more neutrinos.

calculated neutrino event rates for two detectors and two neutrino mass hierarchies

The event rates for a 1.98-solar-mass protoneutron star with various accretion rates as seen by Super-Kamiokande (left) and DUNE (right) and for normal (top) and inverted (bottom) neutrino mass hierarchies. [Akaho et al. 2024]

Ryuichiro Akaho (Waseda University) and collaborators calculated the likelihood of detecting the neutrinos that are produced when material rebounding from the collapsed stellar core falls back onto the core. Using detailed neutrino radiation–hydrodynamics simulations, the team modeled the fluxes and flavors of the neutrinos produced about ten seconds after the supernova occurs.

Akaho’s team found that the mass of the protoneutron star and the rate at which it gathers material from its surroundings both have an impact on the output neutrino luminosity and the average energy of the neutrinos. For a supernova happening about 33,000 light-years away, the neutrino flux should rise above the background measured by the existing Super-Kamiokande and under-construction Deep Underground Neutrino Experiment (DUNE) detectors. The exact strength of the signal depends on several factors, including neutrino oscillation — the process through which a neutrino born in a certain “flavor” morphs to a different flavor as it travels through space.

Investigating Type Ia Supernova Diversity

Supernovae aren’t always the result of massive stars collapsing. Many arise from white dwarfs, which are the exposed cores of low- to intermediate-mass stars that have finished fusing hydrogen in their cores and lost their outer layers. When a white dwarf accretes gas from a companion star, the white dwarf gains mass and heats up, eventually triggering a supernova. Alternatively, the collision of two white dwarfs can generate a supernova. Supernovae arising from white dwarfs are called Type Ia or thermonuclear runaway supernovae.

illustrations of the two main Type Ia supernova pathways

Illustrations of the two main Type Ia supernova pathways: the single-degenerate model (top) and the double-degenerate model (bottom). [Both images from NASA’s Goddard Space Flight Center Conceptual Image Lab]

Observations show that Type Ia supernovae have substantial variety in their light curves and properties, leading Mao Ogawa (Kyoto University) and collaborators to investigate the origins of this diversity. Ogawa’s team focused on the division between normal-velocity and high-velocity supernovae, which are differentiated by the velocity of their ejecta.

The team selected a sample of 14 Type Ia supernovae for which spectra were collected within one week of the explosion being detected at Earth. The sample included high-velocity supernovae, normal-velocity supernovae, and some that were similar to the peculiar supernova SN 1999aa. The team then used radiative transfer modeling to model the spectra and extract the properties of the supernova ejecta. Ultimately, they found that the supernovae fell into two groups: one with high-density, carbon-poor ejecta, which makes up the high-velocity sample and some of the normal-velocity sample and one that has low-density, carbon rich ejecta, which makes up the remaining normal-velocity sample and those like SN 1999aa. While more work remains to be done, the team suspects these two groups might be the result of different formation mechanisms.

Citation

“Probing the Low-Mass End of Core-Collapse Supernovae Using a Sample of Strongly Stripped Calcium-Rich Type IIb Supernovae from the Zwicky Transient Facility,” Kaustav K. Das et al 2023 ApJ 959 12. doi:10.3847/1538-4357/acfeeb

“The Division Between Weak and Strong Explosions from Failed Supernovae,” Eric R Coughlin 2023 ApJ 955 110. doi:10.3847/1538-4357/acf313

“Detectability of Late-Time Supernova Neutrinos with Fallback Accretion onto Protoneutron Star,” Ryuichiro Akaho et al 2024 ApJ 960 116. doi:10.3847/1538-4357/ad118c

“Systematic Investigation of Very-early-phase Spectra of Type Ia Supernovae,” Mao Ogawa et al 2023 ApJ 955 49. doi:10.3847/1538-4357/acec74

ultraviolet image of the spiral galaxy Messier 81

By studying the formation and evolution of galaxies in the early universe, researchers seek to test the predictions of our leading theory of cosmology. New research suggests that the ultraviolet luminosity of low-mass galaxies just a few hundred million years after the Big Bang may provide a way to differentiate between cosmological models.

Supersonic Flows in the Early Universe

snapshot of simulation output showing the dark matter and gas in two galaxy clusters

Simulation output showing the distribution of dense dark matter (orange), diffuse dark matter (blue), and high-velocity gas (white) in and surrounding two galaxy clusters. [TNG Collaboration]

The leading theory of cosmology, called ΛCDM, describes a universe in which the matter that we see and touch every day (so-called “normal” or baryonic matter), makes up just 15% of all matter. The remaining 85% is dark matter, the nature of which is still unknown. While dark matter doesn’t have much of an impact on daily life today — a little dark matter probably passed through you while you read this sentence, and I’m guessing you didn’t notice! — it played a leading role in the early universe, dictating when, where, and at what masses the first galaxies formed.

While galaxies today are nestled within dark matter halos and arrayed along strands of dark matter scaffolding, dark matter and luminous matter weren’t always so aligned; ΛCDM predicts that in the very early universe, just after protons and electrons came together to form atoms, there were places where normal matter moved relative to dark matter with immense speed — faster than the local speed of sound. This supersonic relative motion complicated the formation of the first stars and galaxies, potentially altering early galaxy formation in a measurable way.

simulation results with and without a difference in the velocities of dark and normal matter in the early universe

Comparison of the gas and stellar structures that form when the relative velocity between dark and normal matter is supersonic (left) and when there is no velocity difference between dark and normal matter (right). Click to enlarge. [Williams et al. 2024]

Suppression of Small-Scale Structure

Claire Williams (University of California, Los Angeles) and collaborators set out to understand the impact of this supersonic relative motion on the formation of low-mass galaxies. Using high-resolution fluid dynamics simulations, Williams’s team modeled the evolution of galaxies from a redshift of 200 to a redshift of 12. Their simulations tackled the cooling and condensing of molecular clouds, the ignition of the first stars, and the assembling of galaxies and star clusters.

The team found that having dark matter and normal matter moving at supersonic velocities relative to one another makes it harder for small galaxies to form; an order of magnitude fewer galaxies formed with stellar masses less than about 3 million solar masses when a velocity difference was present. Larger galaxies formed in the same numbers regardless of the velocity difference.

Clues from Ultraviolet Photons

measured and predicted ultraviolet luminosity functions

Modeled ultraviolet luminosity functions in the case of a supersonic velocity difference (pink line) and no velocity difference (blue line). The remaining colored data points are values derived from observations and the gray and black lines are model predictions. [Adapted from Williams et al. 2024]

What effect do these findings have on quantities that we could potentially measure? The velocity difference affects the simulated galaxies’ star formation, which impacts their ultraviolet luminosity. This in turn determines the number of galaxies present at each ultraviolet luminosity, otherwise known as the ultraviolet luminosity function.

Williams’s team found key differences in these quantities at a redshift of 12 in areas with the velocity difference and areas without. In regions without the supersonic flow, small galaxies and star clusters form readily and begin to churn out stars. In regions with the supersonic flow, small galaxies are uncommon, and the gas that would have gone toward the smallest galaxies is instead captured by and spurs star formation in slightly more massive galaxies, in the 10-million-solar-mass range. This means that the ultraviolet luminosity function is lower for the smallest, faintest galaxies but is boosted for slightly brighter and more massive galaxies at a redshift of 12.

Because ultraviolet photons emitted in the distant past are shifted to longer wavelengths when they reach us today, the ultraviolet luminosity function is potentially measurable at infrared wavelengths with JWST. Ultra-deep JWST observations such as the NGDEEP survey may reach ultraviolet magnitudes as faint as −14, potentially revealing the small galaxies that can probe early-universe cosmology.

Citation

“The Supersonic Project: Lighting Up the Faint End of the JWST UV Luminosity Function,” Claire E. Williams et al 2024 ApJL 960 L16. doi:10.3847/2041-8213/ad1491

artist's impression of colliding black holes

When researchers scour the detections of merging black holes made by gravitational wave observatories, they use models and statistics to make careful inferences about the population of black holes in our universe. In a recent article, researchers explored whether an emerging trend in gravitational wave data is real or an artifact of previous analysis methods.

A New Window on the Universe

Illustration of the first black hole merger detected by LIGO

Illustration of the first black hole merger detected by LIGO. [Aurore Simmonet (Sonoma State University)]

The detection of gravitational waves from merging black holes in 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO) gave scientists a new way to investigate black holes. By analyzing the spacetime ripples from colliding black holes, researchers hope to understand how the black holes formed (through the collapse of massive stars or the successive mergers of existing black holes?) and how they came to exist in binary systems (by first belonging to a stellar binary system or by forming solo and linking up with another black hole later?).

One potential result that has emerged from several analyses of gravitational wave signals is that the effective spins and the ratios of the masses of merging black hole binaries appear to be anticorrelated. But as with all results that are extracted delicately, statistically from complex data sets, it’s important to ask if this is a real feature of the data, with real implications for how black hole binary systems are assembled, or if it’s a result of our models or statistical analyses.

a diagram illustrating positive and negative effective spin

Top: The black hole spins are aligned with the system’s orbital angular momentum (positive effective spin). Bottom: The black hole spins are misaligned with the system’s orbital angular momentum (negative effective spin). [Kerry Hensley]

Statistical Investigation

Christian Adamcewicz (Monash University and OzGrav) and collaborators approached this question by applying a new statistical treatment to detections of black hole mergers. This new treatment features a new model for effective spin and allows for a subpopulation of black hole binaries with zero effective spin, which hasn’t yet been ruled out and might have an impact that hasn’t been accounted for.

The team applied their population model to the third catalog of gravitational wave signals from the LIGO and Virgo detectors and used Bayesian statistical methods to extract the properties of the overarching population of black holes. They found that the previously reported anticorrelation between effective spin and mass ratio is likely real, ruling out the possibility of there being no correlation at 99.7% probability.

More Work, a Paradox, and Astrophysical Possibilities

Adamcewicz and collaborators acknowledge that this work doesn’t provide a final verdict on this question (as they put it, “a modeler’s job is never done”), and that other statistical effects need to be rooted out. One lingering possibility is that this result is due to the amalgamation paradox, which arises when trends present in different factors disappear or flip when the factors are considered together.

If the observed anticorrelation holds up to further statistical scrutiny, a number of astrophysical phenomena could be responsible for this effect. Extensive mass transfer between black hole progenitor stars, stars evolving within a common envelope and accreting matter at a high rate, or even black hole binary systems assembled within the accretion disks of active galactic nuclei should all be investigated with future black hole population models.

Citation

“Evidence for a Correlation Between Binary Black Hole Mass Ratio and Black Hole Spins,” Christian Adamcewicz et al 2023 ApJ 958 13. doi:10.3847/1538-4357/acf763

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