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magnetar outburst

Recent evidence points to highly magnetized neutron stars as the culprits that produce fast radio bursts — brief and energetic flashes of radio emission that we’ve spotted coming from distant galaxies. But do galaxy demographics support this picture? 

fast radio burst

Artist’s impression of observatories finding and localizing a fast radio burst offset from its host galaxy’s center. [CSIRO/Andrew Howells]

An Origin Mystery

In the past decade, we’ve found around a hundred unexplained, millisecond bursts of radio light that (mostly) originate from far beyond our galaxy. We’ve developed dozens of possible explanations for what might cause these fast radio bursts (FRBs) — like the mergers of compact objects, or phenomena associated with evolved or evolving stars, or even the weird physics of cosmic strings. Still, these all remain unverified theories.

When we successfully identified the host galaxy for an FRB for the first time in 2017, we hoped that this might finally reveal the cause of these explosions. Instead, the mystery of FRBs has only deepened: the ten FRBs that have well-known localizations are hosted by galaxies with surprisingly different properties.

In September 2020, a new clue was announced: a strongly magnetized neutron star — a magnetar — in our own galaxy had been observed to emit a radio burst similar to an FRB. Was this the evidence we’d been waiting for, finally proving that magnetars are the source of FRBs?

A team of scientists led by Mohammadtaher Safarzadeh (UC Santa Cruz; Center for Astrophysics | Harvard & Smithsonian) suggests that there’s a simple way to test this scenario: by comparing the demographics of magnetar-hosting and FRB-hosting galaxies.

Exploring Demographics

Three plots showing the CDFs for log stellar mass, log SFR, and offset distribution.

Top: comparison of the cumulative distribution function of FRB host galaxies (black line) to the expected global distribution of galaxies (colored lines) for total stellar mass of galaxies. Middle: same comparison, but for galaxy star formation rate. Bottom: Offset distribution of FRBs (black line) to expected distribution of magnetars in galaxies (red line). [Adapted from Safarzadeh et al. 2020]

Suppose, Safarzadeh and collaborators say, that magnetars are the progenitors of FRBs. Since magnetars are the remnants left behind soon after massive stars collapse, these objects ought to reside in or near the places where lots of stars are being born — regions of high star formation in our universe.

With this in mind, Safarzadeh and collaborators first explore the distributions of stellar mass and star formation rates for the ten localized FRB host galaxies, to see whether FRBs are preferentially hosted in galaxies that are likely to contain magnetars. As an additional check, the authors then examine the offset of the FRBs from the centers of their host galaxies, to test whether FRB locations track the star formation rate profile within the galaxies.

Magnetars or Not?

The result? The authors find that there’s a clear inconsistency between the star formation rates of expected magnetar hosts and the star formation rates for observed FRB hosts — strongly indicating that not all FRBs are caused by magnetars. On the other hand, the offsets of FRBs from their galaxy centers are not inconsistent with the predicted locations of magnetar birth sites.

So are FRBs caused by magnetars or not? The results are still inconclusive, but we’re just getting started — our sample of FRBs is growing rapidly, and we’re soon likely to have a much larger collection of localized FRBs with which we can better explore the properties of their hosts. Keep an eye out for more developments on the FRB front!

Citation

“Confronting the Magnetar Interpretation of Fast Radio Bursts through Their Host Galaxy Demographics,” Mohammadtaher Safarzadeh et al 2020 ApJL 905 L30. doi:10.3847/2041-8213/abd03e

photograph of a rocky, icy moon

Among Jupiter’s Galilean moons, icy Europa or volcanic Io often take the spotlight — but their sibling moon Ganymede has plenty of secrets to share. Powerful new millimeter observations have now provided insight into this complex satellite’s surface.

A World Apart

photograph showing rough, cratered, dark terrain crosscut by grooved, bright terrain.

A sharp boundary divides the ancient dark terrain of Nicholson Regio on Ganymede from the younger, finely grooved bright terrain of Harpagia Sulcus. [NASA]

The frozen, alien landscape of Ganymede contains a little of everything. Shadowy regions of ancient, battered dark terrain are cross-cut by newer patches of ice-rich, grooved bright terrain. Ganymede’s diverse surface features bridge the stark divide between its sibling Galilean moons, evoking both Callisto’s barren, rocky surface, and Europa’s bizarrely cracked and faulted icy landscape.

Ganymede’s complexity deepens when you look beyond its surface. Beneath its outer shell of rock and ice lurks a vast ocean that may contain more water than all of Earth’s oceans combined. What’s more, this planet-sized body (Ganymede is 26% larger than Mercury by volume!) is the only solar-system moon to produce its own, intrinsic magnetic field — which means it hosts a magnetosphere that interacts with the larger Jovian magnetosphere.

size comparison showing three bodies: Ganymede, the Earth, and the Moon.

Size comparison of the Earth, the Moon (top left), and Ganymede (bottom left). [Earth: NASA; Moon: Gregory H. Revera; Ganymede: NASA/JPL/DLR]

Digging Under the Surface

This complicated satellite’s properties means that there are many different processes — originating from both its interior and its exterior — that can modify its surface. To better understand what’s happening across Ganymede’s dramatic landscape, a recent study has now leveraged the high resolution of the Atacama Large Millimeter/submillimeter Array (ALMA) to explore the top layer of this moon’s rocky, icy surface.

Scientist Katherine de Kleer (California Institute of Technology) and collaborators observed Ganymede at several different millimeter wavelengths with ALMA and then compared these data to a thermal model, examining the thermal emission of the moon from its surface down to a depth of roughly 50 cm.

From these results, the team built global temperature maps of Ganymede and explored the vertical profile of the moon’s near-surface material to identify the physical and chemical processes at play in this region.

Taking Ganymede’s Temperature

De Kleer and collaborators found that Ganymede’s material becomes rapidly less porous and more densely packed below the surface: its porosity drops from 85% at the surface to just 10% at depth. This measurement tells us how rapidly the moon’s material responds to changes in heating (for instance, daytime illumination by the Sun): the more porous surface material loses and gains heat more quickly, whereas the deeper material responds slowly.

two plots showing places where temperature deviates from prediction for Ganymede's surface.

These maps of temperature residuals, formed by subtracting the best-fit global models, show regions where Ganymede’s surface temperature deviates from prediction. The bottom map contains the same data as the top, but is overplotted on an albedo map of Ganymede’s surface. [de Kleer et al. 2021]

From their global temperature maps, the authors identified the regions of Ganymede’s surface that deviate from best-fit models — like several bright craters that are substantially colder than predicted. Deviations like this point to variations in the local composition, porosity, and grain properties of the moon’s surface material.

De Kleer and collaborators also noted larger-scale deviations in temperature — in particular, excess heat measured at the equator and cooler temperatures than predicted at middle latitudes. These differences suggest that Ganymede’s surface is predominantly influenced by external processes, like bombardment by micrometeorites and plasma on its orbit around Jupiter.

More detailed studies of Ganymede are likely in the future, and ALMA observations of Europa and Callisto are currently being analyzed — so we can expect further insight into the surfaces of these complex, icy bodies soon.

Citation

“Ganymede’s Surface Properties from Millimeter and Infrared Thermal Emission,” Katherine de Kleer et al 2021 Planet. Sci. J. 2 5. doi:10.3847/PSJ/abcbf4

Active Galactic Nucleus

Supermassive black holes influence many aspects of our universe’s formation and evolution — yet there’s still a lot we don’t know about them! A new census of these lurking sources is helping us to answer questions. 

Hidden in Dust

epoch of reionization

In the schematic timeline of the universe, the epoch of reionization is when the first galaxies and quasars began to form and evolve. Click to enlarge. [NASA]

How many actively accreting supermassive black holes — known as active galactic nuclei, or AGN — lie scattered throughout our universe? How do these black holes grow alongside their galactic hosts? How did the powerful radiation of these AGN contribute to the reionization that shaped our universe into its current composition of low-density, ionized hydrogen?

To answer all these questions,we first need to form a complete census of the AGN in our universe. This prospect is challenging, however: though AGN radiate brightly across the electromagnetic spectrum, many of these mysterious sources lie obscured within dense shrouds of dust that prevent the majority of their radiation from escaping.

High Energy to the Rescue

Fortunately, however, extremely energetic X-ray radiation can escape even heavily obscured AGN. By compiling the observations from multiple space-based X-ray observatories — like NuSTAR, the Neil Gehrels Swift Observatory, and Chandra — a team of scientists has recently built a largely unbiased survey of the AGN throughout our universe, accounting for both unobscured sources and the many sources that are hidden by dust.

plot showing the contribution of reionizing photons from galaxies and AGN, measured vs. redshift.

Ionizing photon densities for AGN (black lines) and galaxies (orange lines) for several different models. The authors’ work suggests that galaxies provide a substantially larger contribution toward reionization than AGN do, at redshifts above z = 6. [Adapted from Ananna et al. 2020]

In a new study led by Tonima Ananna (Dartmouth College and Yale University), this team is now using their census of AGN to better understand the physics of supermassive black hole growth and the impact of these beasts on our universe’s evolution.

Reionizing Our Universe

Ananna and collaborators estimate the total amount of ionizing radiation that’s emitted from all AGN in our universe as a function of redshift, applying observational constraints from both the light that we can see and estimates of the light we can’t see based on the inferred obscured AGN population.

The authors find that the total contribution of ionizing photons that escape from all AGNs is fairly small. The AGN contribution to the reionization of our universe — a process that occurred between a few hundred million and ~1 billion years after the Big Bang — is less than a quarter of the total ionizing photon density at redshifts larger than z > 6. This suggests that first-generation stars and galaxies contributed the vast majority of the radiation that drove reionization.

probability distribution plot for black hole spin shows a peak at large values.

Authors’ probability distribution for the measurement of average black-hole spin among AGN, using several different models. High spins are significantly favored. [Adapted from Ananna et al. 2020]

Dizzy Black Holes

What can we learn about the black holes themselves? Ananna and collaborators use their census to compare the total light emitted from AGNs to the amount of mass they’ve accreted over time. This measure of accretion efficiency can then tell us how fast the supermassive black holes are likely spinning.

Ananna and collaborators find a high likely accretion efficiency — which indicates that, on average, growing supermassive black holes are spinning quite rapidly. If confirmed, this may mean that supermassive black hole growth is dominated by accretion of material (which produces fast-spinning black holes) rather than by mergers (which produce a low average spin since the black holes become randomly oriented).

We still have much to learn about supermassive black holes and their influence on the universe, but this recent study provides a clear step in the right direction.

Citation

“Accretion History of AGNs. III. Radiative Efficiency and AGN Contribution to Reionization,” Tonima Tasnim Ananna et al 2020 ApJ 903 85. doi:10.3847/1538-4357/abb815

HR 8799 planets

All four planets orbiting the star HR 8799 were identified via direct imaging — a feat made possible only because of the planets’ large sizes and their wide orbits. Planetary systems with these characteristics often have difficulty holding themselves together under all of the gravitational influences involved. But could the HR 8799 system somehow stay intact?

animated photograph of four planets orbiting a masked star

An animation showing observations of the HR 8799 system taken over seven years. The observations were taken at the W.M. Keck observatory. Click to open animation. [J. Wang/C. Marois]

Subtracting Light to Find Planets

The direct imaging technique involves taking an image of a star and removing all the light associated with the star to see what remains (hopefully planets!). When astronomers used this technique on infrared observations of the star HR 8799, they discovered four planets in orbit around it.

Images show that the innermost planet lies roughly 16 astronomical units from the star — a bit nearer than Uranus is to the Sun — and all of the planets have orbital periods ranging from 50 to 500 years. But, given that astronomers haven’t been able to observe this system for very long, uncertainty remains about the long-term behavior of the planetary orbits in the HR 8799 system. In fact, some previous studies have suggested that the system might come apart in the distant future.

plot from a simulation showing four planets orbiting a star along with many smaller dots

The inner debris disk from an N-body simulation of the HR 8799 system. Initial positions of the planets are marked by the red circles. The gray orbits are from a different simulation of the system that was allowed to run for ten million years. The colors of points correspond to their eccentricity as indicated in the color bar. [Goździewski & Migaszewski, 2020]

But in a recent study, Krzysztof Goździewski and Cezary Migaszewski (Nicolaus Copernicus University, Poland) consider a scenario in which the HR 8799 is able to remain intact — we just need to involve mean-motion resonance (MMR) and periodic orbits.

Resonance and Periodic Orbits

When orbiting bodies are in MMR, the ratio of their orbital periods is a small integer ratio (a 5:2 resonance, for example, means that one object orbits five times in the time it takes the other to orbit twice). There are examples of MMR in the solar system: Neptune and Pluto are in a 3:2 resonance, and Jupiter’s moons Io, Europa, and Ganymede are in a 4:2:1 resonance.

Goździewski and Migaszewski have previously demonstrated that the four planets orbiting HR 8799 could be in a stable 8:4:2:1 MMR. In this study, they revisited the MMR of the HR 8799 planets in the context of periodic orbits, where particular elements associated with the planetary orbits vary periodically with time.

Mass Determination from Models

plot of mass of planet vs. observational epoch. curve stabilizes after ~3 years at 7 jupiter masses.

A demonstration of how the mass of the innermost planet can be constrained with future observations of the HR 8799 system. The authors simulated a 7-Jupiter-mass planet and then produced synthetic observations at different points in time to show that the planet’s mass could be determined from those observations. [Goździewski & Migaszewski, 2020]

Goździewski and Migaszewski used observations from a particular point in time to create an initial model of the HR 8799 system. They then let the system evolve under the constraints of MMR and periodic orbits. The resulting model not only aligned with measurements of the system made since the initial observation, but it could also be used to determine the masses of the HR 8799 planets. All we’d need is a few more future observations!

HR 8799 could have planets that are closer in than the known four. However, they might not drastically interfere with MMR in the system. In either case, HR 8799 is a good testing ground for theories of planetary formation — we just need to keep an eye on it!

Citation:

“An Exact, Generalized Laplace Resonance in the HR 8799 Planetary System,” Krzysztof Goździewski and Cezary Migaszewski 2020 ApJL 902 L40. doi:10.3847/2041-8213/abb881

Illustration of a spacecraft flying through the tail of a comet far from the Sun.

In March of 1998, a mass of plasma erupted from the Sun. As it sped out into interplanetary space, two spacecraft were perfectly situated to measure this dramatic cloud. Now, more than 20 years later, we’re still benefitting from their fortuitous alignment.

Coronal mass ejection

A coronal mass ejection caught in the act by Solar Dynamics Observatory. [NASA]

Interplanetary Journeys

Coronal mass ejections (CMEs), violent releases of plasma and magnetic fields from the Sun’s atmosphere, are by no means rare! When the Sun is at its most active, several CMEs can occur each day — and some of these ejections are so energetic that they travel great distances through our solar system, carrying embedded clouds wrapped in helical magnetic fields.

But what happens to these magnetic clouds as they journey through interplanetary space? Do they expand, unperturbed, as they travel? Or do they interact with other magnetic solar phenomena — like additional CMEs, or the solar wind — and change their shapes or behaviors?

A Fortuitous Alignment

schematic illustrating Ulysses orbit

The Ulysses spacecraft orbits on an elliptical trajectory out of the plane of the ecliptic, so its near-perfect alignment with Wind at the time of the CME was lucky! [NASA]

These questions are challenging to answer due to the difficulty of examining CMEs in interplanetary space. To gain a better understanding of CME evolution, we need repeated in situ measurements of the same propagating magnetic cloud — first when it’s close to the Sun, and again when it’s traveled farther out into the solar system.

While a number of past and current spacecraft have the ability to measure the properties of the solar wind and magnetic fields around them, the vast majority of these spacecraft orbit close to the Sun (at or within 1au distance). In 1998, however, we got lucky: the only wide-orbit spacecraft devoted to solar wind studies, Ulysses, just happened to be radially aligned with a close-in spacecraft, Wind, when a CME was launched in their direction.

The two spacecrafts’ observations of the resulting magnetic cloud have now been re-examined in a study led by Daniele Telloni (INAF Astrophysical Observatory of Turin, Italy).

Colliding Clouds

The magnetic cloud contained in this interplanetary CME arrived at Wind, which orbits at 1 au, in early March of 1998. 18 days later, the cloud arrived at Ulysses, which happened to be in line at the time at 5.4 au (out at Jupiter’s orbit). Telloni and collaborators conducted new analysis of Ulysses’s and Wind’s combined data sets for the interplanetary magnetic field and the solar wind plasma during the magnetic cloud’s passage of the two spacecraft.

Two plots showing spatial positions of Wind, Ulysses, and the Sun at the time of the CME.

Alignment of Wind (blue) and Ulysses (red) with respect to the Sun (yellow star) in March 1998. [Telloni et al. 2020]

The authors’ work shows that the cloud did not travel undisturbed through the space between Earth’s and Jupiter’s orbits, but instead experienced significant erosion and restructuring along the way, likely due to interaction with another magnetic cloud from a previous solar ejection. This erosion stripped away some of the helical magnetic field encasing the cloud, converting stored magnetic energy into kinetic energy as magnetic field lines rearranged at the interface between the two clouds.

This valuable insight was made possible due to two spacecraft being aligned at the right time — so what are the chances of similar good luck in the future? With the wealth of recently launched planetary and solar missions like BepiColombo, Solar Orbiter, and Parker Solar Probe, odds are good for future radial alignments that will allow us to further measure ejections from the Sun.

Citation

“Magnetohydrodynamic Turbulent Evolution of a Magnetic Cloud in the Outer Heliosphere,” Daniele Telloni et al 2020 ApJL 905 L12. doi:10.3847/2041-8213/abcb03

Binary neutron star merger

On 17 August 2017, an alert went out roughly 40 minutes after the LIGO observatory detected gravitational waves from a pair of colliding neutron stars. This alert sent telescopes worldwide slewing rapidly in an all-hands-on-deck effort to image the fireworks show accompanying the merger.

But what if that alert had gone out before the collision?

When Stars Collide

neutron-star merger

Artist’s impression of the electromagnetic signal from the merger of two neutron stars. [National Science Foundation/LIGO/Sonoma State University/A. Simonnet]

When neutron stars merge, they are expected to produce both a gravitational-wave signal from their inspiral and a host of spectrum-spanning electromagnetic signatures from before, during, and after the collision.

The event captured in August 2017, known as GW170817, is one of just two binary neutron star mergers we’ve observed with LIGO and its European sister observatory Virgo so far. But these collisions are likely to become a common detection in the future, particularly as LIGO and Virgo continue to upgrade and approach their design sensitivity.

Reducing the Lag

In the case of GW170817, interferometer glitches and data transfer issues prevented an alert from going out until 40 minutes after the merger. By the time follow-up telescopes had been advised where to search for the collision, the relevant region of the sky was below the horizon; the first manual follow-up couldn’t be conducted until nearly 8 hours after the merger.

To capture a neutron star merger without this delay, we’ll clearly need reduce the alert lag time — but could LIGO send out alerts even before the neutron stars collide? A study led by Surabhi Sachdev (The Pennsylvania State University) has recently demonstrated that a LIGO data analysis pipeline will be capable of providing advance warning for some future mergers.

Early Warning

Sachdev and collaborators analyze the performance of GstLAL, an early warning gravitational-wave detection pipeline for LIGO/Virgo that looks for signals of neutron star binaries approaching merger.

sky localizations

Cumulative distributions of the sky localizations of injected binary neutron star merger signals recovered by the authors’ pipeline. Results show that at least one event per year will be detected before merger and localized to within 100 deg2. [Sachdev et al. 2020]

By injecting merger signals into a simulated dataset, the authors show that the pipeline is able to recover many of these signals 10–60 seconds before the merger occurs. These early detections are made possible when mergers happen nearby, so that a large signal-to-noise ratio can accumulate as the neutron stars inspiral in their last few moments collision.

A Well-Notified Future

Sachdev and collaborators predict that when LIGO and Virgo reach their design sensitivity, nearly 50% of total detectable mergers will be spotted at least 10 seconds before merger — a total that amounts to perhaps 6 to 60 events per year, depending on the neutron star merger rate.

With rapid localization and quick relay times for alerts, this early-warning system could provide follow-up telescopes with the opportunity to capture neutron-star mergers in real time, as they happen. Such observations would provide insight into what magnetic conditions are like around the neutron stars, how heavy elements are synthesized, and whether binary neutron stars are the source of fast radio bursts.

Citation

“An Early-warning System for Electromagnetic Follow-up of Gravitational-wave Events,” Surabhi Sachdev et al 2020 ApJL 905 L25. doi:10.3847/2041-8213/abc753

16 Psyche

Editor’s note: In these last two weeks of 2020, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

HST UV Observations of Asteroid (16) Psyche

Published October 2020

Main takeaway:

A study led by Tracy Becker (Southwest Research Institute) presents new ultraviolet images of the large asteroid (16) Psyche captured with the Hubble Space Telescope. These images reveal potential weathering of the asteroid’s metallic surface by the solar wind.

Why it’s interesting:

At more than 200 km in diameter, Psyche is one of the largest asteroids known in our solar system; it’s thought to be the exposed core of a failed protoplanet. One of this body’s most unusual properties is its apparent composition: its surface appears to be predominantly composed of metals like iron and nickel. The new ultraviolet observations of Psyche’s surface may help us to better understand its composition and how this asteroid holds up under the physical processes of our solar system, like bombardment by the solar wind.

Psyche

Illustration of a spacecraft orbiting the asteroid Psyche. [NASA/JPL-Caltech/Arizona State Univ./Space Systems Loral/Peter Rubin]

Why these observations are timely:

We’re going to Psyche! NASA will launch a spacecraft in 2022 that will arrive at Psyche in 2026. This mission’s goal is to spend nearly 2 years orbiting the asteroid and observing its topography, surface features, gravity, magnetism, and other characteristics to better understand the properties and origins of planetary cores. Carefully examining Psyche now will help us to get more out of this future mission.

Citation

Tracy M. Becker et al 2020 Planet. Sci. J. 1 53. doi:10.3847/PSJ/abb67e

coronal strands

Editor’s note: In these last two weeks of 2020, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Is the High-Resolution Coronal Imager Resolving Coronal Strands? Results from AR 12712

Published April 2020

Main takeaway:

NASA’s High-Resolution Coronal Imager, or Hi-C mission, captured high-resolution images of the fine strands that make up the loops within the Sun’s outer atmosphere, the corona. The images, presented in a publication led by Thomas Williams (University of Central Lancaster, UK), demonstrate resolution on scales all the way down to ~200 km (~125 miles).

Why it’s interesting:

coronal strands

Top: The location of the active region explored with new Hi-C images (identified by the dotted box). Bottom: Comparison of SDO/AIA (left) and Hi-C (right) images of the coronal strands in the active region. Click for a closer look. [Williams et al. 2020]

These images show that the loops of plasma that extend above the Sun’s surface have even finer structure than previously observed. The coronal loops are constructed from collections of strands that have typical widths of ~500 km, although some are even narrower. Examining the structure of this plasma could help us to understand how the solar corona is so unexpectedly hot — the corona clocks in at more than a million degrees Fahrenheit, hundreds of times hotter than the Sun’s surface.

Why these views are unique:

Perhaps surprisingly, Hi-C is not a space telescope parked in orbit and beaming these spectacularly high resolution solar images back to Earth. Instead, this imager was launched on a sounding rocket for its third time in 2018, and it produced these remarkable observations from just 329 seconds of data taken while the rocket was high in its arc through our atmosphere. The high quality of these images — even when compared with those of established space telescopes like the Solar Dynamics Observatory — highlights the need for a permanent solar observatory with the resolving power of Hi-C.

Citation

Thomas Williams et al 2020 ApJ 892 134. doi:10.3847/1538-4357/ab6dcf

'Oumuamua

Editor’s note: In these last two weeks of 2020, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Destruction of Molecular Hydrogen Ice and Implications for 1I/2017 U1 (‘Oumuamua)

Published August 2020

Main takeaway:

One more potential explanation for the strange behavior of interstellar asteroid 1I/2017 U1 (‘Oumuamua) has been dashed thanks to a study by Thiem Hoang (Korea Astronomy and Space Science Institute; Korea University of Science and Technology) and Abraham Loeb (Harvard University). Hoang and Loeb show that an ‘Oumuamua-sized body made out of molecular hydrogen ice could not have survived the interstellar journey from its birth site to our solar system.

Why it’s interesting:

outgassing 'Oumuamua

Illustration of outgassing from the surface of interstellar asteroid ‘Oumuamua. [ESA/Hubble, NASA, ESO, M. Kornmesser]

A number of mysteries were raised by the 2017 discovery of ‘Oumuamua, the first interstellar object detected visiting our solar system. Chief among these: what caused ‘Oumuamua’s unexpected non-gravitational acceleration as it sped away from the Sun and off into space? One posited theory was that this interstellar asteroid was actually a hydrogen iceberg. A composition of hydrogen ice could explain the asteroid’s unusual, elongated shape, and sublimation of this ice could drive its extra acceleration. But Hoang and Loeb’s analysis shows that an ‘Oumuamua-sized hydrogen iceberg would be destroyed within 10 million years by starlight heating. Collisional heating would also rapidly wear at such an object in its birthplace, potentially destroying it before it could escape into the interstellar medium and journey to our solar system. And even the formation of such a hydrogen iceberg seems unlikely: Hoang and Loeb show that it’s difficult to form icy grains rich in molecular hydrogen in the dense clouds that could birth an interstellar asteroid.

What this means for dark matter:

The implications of this hydrogen challenge extend beyond ‘Oumuamua. Primordial snowballs — theorized bodies made of molecular hydrogen ice in the early universe — have been proposed as one potential component of dark matter. But Hoang and Loeb’s work shows that such snowballs are unlikely to have been able to form, and even if they had, they would be unable to survive to present day. This suggests we must look elsewhere — both for an explanation for dark matter and for an explanation for ‘Oumuamua’s mysteries.

Citation

Thiem Hoang and Abraham Loeb 2020 ApJL 899 L23. doi:10.3847/2041-8213/abab0c

K2-18b

Editor’s note: In these last two weeks of 2020, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

The Interior and Atmosphere of the Habitable-zone Exoplanet K2-18b

Published February 2020

Main takeaway:

A team of scientists led by Nikku Madhusudhan (University of Cambridge, UK) investigated the atmospheric and interior conditions of K2-18b, an exoplanet located ~124 light-years away that is roughly 2.6 Earth radii in size and 8.6 Earth masses in weight. They found that this planet could have the right conditions to support liquid water beneath its atmosphere on its surface.

Why it’s interesting:

super-Earth

Artist’s impression of a super-Earth exoplanet orbiting its host. [Jack Madden/Cornell University]

Most studies of potentially habitable worlds have focused on Earth-like, rocky planets. K2-18b represents a different category: a super-Earth that’s between Earth and Neptune in its properties. Madhusudhan and collaborators’ analysis demonstrates that the potential for habitable conditions aren’t restricted to Earth-like planets — larger, less dense bodies might also present the right conditions to support life.

What kind of world K2-18b might be:

Madhusudhan and collaborators use observations of K2-18b’s atmosphere and its bulk properties to constrain this planet’s structure. The resulting possible scenarios for K2-18b range from  a rocky world surrounded by an enormous hydrogen and helium atmospheric envelope, to a water world with a very thin atmosphere. Future observations with observatories like the upcoming James Webb Space Telescope should help us to learn more about this potentially habitable world.

Citation

Nikku Madhusudhan et al 2020 ApJL 891 L7. doi:10.3847/2041-8213/ab7229

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