ST/ Circumtriple planet discovered: a planet orbiting 3 stars

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Paradigm

32 min readOct 6, 2021

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Space biweekly vol.36, 22d September — 6th October

TL;DR

In a distant star system — a mere 1,300 light-years away from Earth — researchers may have identified the first known planet to orbit three stars. The potential discovery of a circumtriple planet has implications for bolstering understanding of planet formation.
A new study finds that next-generation telescopes used to see exoplanets could confuse Earth-like planets with other types of planets in the same solar system.
New research into metal-rich asteroids reveals information about the origins and compositions of these rare bodies that could one day be mined.
On Earth, river erosion is usually a slow-going process, but on Mars, massive floods from overflowing crater lakes had an outsized role in shaping the Martian surface, carving deep chasms and moving vast amounts of sediment, according to a new study.
Using machine learning and simulations of giant impacts, researchers found that the planets residing in the inner solar system were likely born from repeated hit-and-run collisions, challenging conventional models of planet formation.
Analyzing observations of an X-ray flare and fitting the data with theoretical models, astronomers documented a fatal encounter between an unlucky star and a black hole.
New mirror coatings will increase the volume of space LIGO can probe in its next run.
When the world’s most powerful telescope launches into space this year, scientists will learn whether Earth-sized planets in our ‘solar neighborhood’ have a key prerequisite for life — an atmosphere.
Clover plants grown in Mars-like soils experience significantly more growth when inoculated with symbiotic nitrogen-fixing bacteria than when left uninoculated, researchers report.
Every continent on Earth has dune fields, but dunes and dune-like sand patterns are also found across the solar system: on Mars, Venus, Titan, Comet 67P, and Pluto. On Earth, weather stations measure the wind speed and direction, allowing us to predict and understand airflow in the atmosphere. In the new paper, researchers using dunes to interpret wind on Mars.
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Space industry in numbers

The global smart space market size is projected to grow from USD 9.4 billion in 2020 to USD 15.3 billion by 2025, at a Compound Annual Growth Rate (CAGR) of 10.2% during the forecast period. The increasing venture capital funding and growing investments in smart space technology to drive market growth.

Analysts at Morgan Stanley and Goldman Sachs have predicted that economic activity in space will become a multi-trillion-dollar market in the coming decades. Morgan Stanley’s Space Team estimates that the roughly USD 350 billion global space industry could surge to over USD 1 trillion by 2040.

Source: Satellite Industry Association, Morgan Stanley Research, Thomson Reuters. *2040 estimates

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GW Ori: circumtriple rings and planets

by Jeremy L Smallwood, Rebecca Nealon, Cheng Chen, Rebecca G Martin, Jiaqing Bi, Ruobing Dong, Christophe Pinte in Monthly Notices of the Royal Astronomical Society

In a distant star system — a mere 1,300 light years away from Earth — UNLV researchers and colleagues may have identified the first known planet to orbit three stars.

Unlike our solar system, which consists of a solitary star, it is believed that half of all star systems, like GW Ori where astronomers observed the novel phenomenon, consist of two or more stars that are gravitationally bound to each other. But no planet orbiting three stars — a circumptriple orbit — has ever been discovered. Perhaps until now.

Disc evolution for a circumbinary disc with varying the initial inner disc radius.

Using observations from the powerful Atacama Large Millimeter/submillimeter Array (ALMA) telescope, UNLV astronomers analyzed the three observed dust rings around the three stars, which are critical to forming planets. But they found a substantial, yet puzzling, gap in the circumtriple disc.

The research team investigated different origins, including the possibility that the gap was created by gravitational torque from the three stars. But after constructing a comprehensive model of GW Ori, they found that the more likely, and fascinating, explanation for the space in the disc is the presence of one or more massive planets, Jupiter-like in nature. Gas giants, according to Jeremy Smallwood, lead author and a recent Ph.D. graduate in astronomy from UNLV, are usually the first planets to form within a star system. Terrestrial planets like Earth and Mars follow.

The planet itself cannot be seen, but the finding suggests that this is the first circumtriple planet ever discovered. Further observations from the ALMA telescope are expected in the coming months, which could provide direct evidence of the phenomenon.

“It’s really exciting because it makes the theory of planet formation really robust,” Smallwood said. “It could mean that planet formation is much more active than we thought, which is pretty cool.”

The Solar System as an Exosystem: Planet Confusion

by Dean Robert Keithly, Dmitry Savransky in The Astrophysical Journal Letters

When it comes to directly imaging Earth-like exoplanets orbiting faraway stars, seeing isn’t always believing. A new Cornell University study finds that next-generation telescopes used to see exoplanets could confuse Earth-like planets with other types of planets in the same solar system.

With today’s telescopes, dim distant planets are hard to see against the glare of their host stars, but next-generation tools such as the Nancy Grace Roman Space Telescope, currently under development by NASA, will be better at imaging Earth-like planets, which orbit stars at just the right distance to offer prime conditions for life.

Melded solar system planet phase functions.

“Once we have the capability of imaging Earth-like planets, we’re actually going to have to worry about confusing them with completely different types of planets,” said Dmitry Savransky, associate professor of mechanical and aerospace engineering and of astronomy.
“The future telescopes that will enable these observations will be so huge, expensive, and difficult to build and launch that we can’t afford to waste a single second of time on them,” Savransky said, “which is why it is so important to think through all of these potential issues ahead of time.”

By using Earth’s own solar system as a model of an unexplored star system, Savransky and doctoral student Dean Keithly, calculated that even with direct-imaging techniques and the increased capabilities of future, high-powered telescopes, exoplanets as different as Uranus and Earth could be mistaken for one another.

The research details how measurements estimating planet-star separation and brightness can cause “planet confusion.” The modeling finds that when two planets share the same separation and magnitude along their orbits, one planet can be confused for the other.

Δmag vs. s plots of solar system planets for varying star system inclinations with separation lines at working angles of 45 and 150 mas at 10 pc.

Keithly and Savransky identified 21 cases within their solar system model in which an individual planet had the same apparent planet-star separation and brightness as another planet. Using this data, it was calculated that an Earth-like planet could be misidentified with a Mercury-like planet in 36% of randomly generated solar systems; with a Mars-like planet in about 43% of randomly generated solar systems; and with a Venus-like planet in more than 72% of randomly generated solar systems.

In contrast, confusion between Earth-like planets and larger gas-giant planets similar to Neptune, Saturn and Uranus was less likely, and could occur in 1–4% of randomly generated solar systems.

Confusing planets for one another can be an expensive and time-consuming problem for scientists. Extensive planning and funds go into each use of a high-powered telescope, so the false identification of a habitable exoplanet wastes valuable telescope time. With this problem identified, researchers can design more efficient exoplanet direct-imaging mission surveys.

The researchers warn that further improvements to instrument contrast and inner-working angles could exacerbate the problem and advise that future exoplanet direct-imaging missions make multiple observations to more accurately differentiate between planets.

Physical Characterization of Metal-rich Near-Earth Asteroids 6178 (1986 DA) and 2016 ED85

by Juan A. Sanchez, Vishnu Reddy, William F. Bottke, Adam Battle, Benjamin Sharkey, Theodore Kareta, Neil Pearson, David C. Cantillo in The Planetary Science Journal

Metal-rich near-Earth asteroids, or NEAs, are rare, but their presence provides the intriguing possibility that iron, nickel and cobalt could someday be mined for use on Earth or in Space.

New research investigated two metal-rich asteroids in our own cosmic backyard to learn more about their origins, compositions and relationships with meteorites found on Earth.

These metal-rich NEAs were thought to be created when the cores of developing planets were catastrophically destroyed early in the solar system’s history, but little more is known about them. A team of students co-led by University of Arizona planetary science associate professor Vishnu Reddy studied asteroids 1986 DA and 2016 ED85 and discovered that their spectral signatures are quite similar to asteroid 16 Psyche, the largest metal-rich body in the solar system. Psyche, located in the main asteroid belt between the orbits of Mars and Jupiter rather than near Earth, is the target of NASA’s Psyche mission.

*Inclination vs. semimajor axis for 6178 (1986 DA), 2016 ED85, and known M/X-types (e.g., Clark et al.* *2004; Ockert-Bell et al.* *2010; Shepard et al.* *2010; Hardersen et al.* *2011; Neeley et al.* *2014) and V-types (Leith et al.* *2017; Hardersen et al.* *2018; Mansour et al.* *2020) in the middle and outer belt.*

“Our analysis shows that both NEAs have surfaces with 85% metal such as iron and nickel and 15% silicate material, which is basically rock,” said lead author Juan Sanchez, who is based at the Planetary Science Institute. “These asteroids are similar to some stony-iron meteorites such as mesosiderites found on Earth.”

Astronomers have been speculating as to what the surface of Psyche is made of for decades. By studying metal-rich NEAs that come close to the Earth, they hope to identify specific meteorites that resemble Psyche’s surface.

“We started a compositional survey of the NEA population in 2005, when I was a graduate student, with the goal of identifying and characterizing rare NEAs such as these metal-rich asteroids,” said Reddy, principal investigator of the NASA grant that funded the work. “It is rewarding that we have discovered these ‘mini Psyches’ so close to the Earth.”

“For perspective, a 50-meter (164-foot) metallic object similar to the two asteroids we studied created the Meteor Crater in Arizona,” said Adam Battle, who is a co-author of the paper along with fellow Lunar and Planetary Laboratory graduate students Benjamin Sharkey and Theodore Kareta, and David Cantillo, an undergraduate student in the Department of Geosciences.

Absolute magnitude vs. semimajor axis for 6178 (1986 DA), 2016 ED85, and asteroid families Phaeo, Brasilia, San Marcello, and 1999 CG1 from Nesvorny.

The paper also explored the mining potential of 1986 DA and found that the amount of iron, nickel and cobalt that could be present on the asteroid would exceed the global reserves of these metals.

Additionally, when an asteroid is catastrophically destroyed, it produces what is called an asteroid family — a bunch of small asteroids that share similar compositions and orbital paths.

The team used the compositions and orbits of asteroids 1986 DA and 2016 ED85 to identify four possible asteroid families in the outer region of the main asteroid belt, which is home to the largest reservoir of small bodies in the inner part of the solar system. This also happens to be the region where most of the largest known metallic asteroids including 16 Psyche reside.

“We believe that these two ‘mini Psyches’ are probably fragments from a large metallic asteroid in the main belt, but not 16 Psyche itself,” Cantillo said. “It’s possible that some of the iron and stony-iron meteorites found on Earth could have also come from that region in the solar system too.”

Low Mechanical Loss TiO2:GeO2 Coatings for Reduced Thermal Noise in Gravitational Wave Interferometers

by Gabriele Vajente, Le Yang, Aaron Davenport, Mariana Fazio, Alena Ananyeva, Liyuan Zhang, Garilynn Billingsley, Kiran Prasai, Ashot Markosyan, Riccardo Bassiri, Martin M. Fejer, Martin Chicoine, François Schiettekatte, Carmen S. Menoni in Physical Review Letters

Since LIGO’s groundbreaking detection, in 2015, of gravitational waves produced by a pair of colliding black holes, the observatory, together with its European partner facility Virgo, has detected dozens of similar cosmic rumblings that send ripples through space and time.

In the future, as more and more upgrades are made to the National Science Foundation-funded LIGO observatories — one in Hanford, Washington, and the other in Livingston, Louisiana — the facilities are expected to detect increasingly large numbers of these extreme cosmic events. These observations will help solve fundamental mysteries about our universe, such as how black holes form and how the ingredients of our universe are manufactured.

Measured loss angle of TiO2:GeO2, as deposited and after 10-hours-long annealing in air, at increasing temperatures. Effect of the annealing duration on the measured loss angle for the 44% TiO2:GeO2 lm.

One important factor in increasing the sensitivity of the observatories involves the coatings on the glass mirrors that lie at the heart of the instruments. Each 40-kilogram (88-pound) mirror (there are four in each detector at the two LIGO observatories) is coated with reflective materials that essentially turn the glass into mirrors. The mirrors reflect laser beams that are sensitive to passing gravitational waves.

Generally, the more reflective the mirrors the more sensitive the instrument, but there is a catch: The coatings that make the mirrors reflective also can lead to background noise in the instrument — noise that masks gravitational-wave signals of interest.

Now, a new study by the LIGO team describes a new type of mirror coating made of titanium oxide and germanium oxide and outlines how it can reduce background noise in LIGO’s mirrors by a factor of two, thereby increasing the volume of space that LIGO can probe by a factor of eight.

“We wanted to find a material at the edge of what is possible today,” says Gabriele Vajente, a LIGO senior research scientist at Caltech and lead author of a paper. “Our ability to study the astronomically large scale of the universe is limited by what happens in this very tiny microscopic space.”
“With these new coatings, we expect to be able to increase the detection rate of gravitational waves from once a week to once a day or more,” says David Reitze, executive director of LIGO Laboratory at Caltech.

Estimated bulk and shear loss angles as a function of frequency.

LIGO detects ripples in space-time using detectors called interferometers. In this setup, a powerful laser beam is split into two: each beam travels down one arm of a large L-shaped vacuum enclosure toward mirrors 4 kilometers away. The mirrors reflect the laser beams back to the source from which they originated. When gravitational waves pass by, they will stretch and squeezes space by nearly imperceptible and yet detectable amounts (much less than the width of a proton). The perturbations change the timing of the arrival of the two laser beams back at the source.

Any jiggling in the mirrors themselves — even the microscopic thermal vibrations of the atoms in the mirrors’ coatings — can affect the timing of the laser beams’ arrival and make it hard to isolate the gravitational-wave signals.

“Every time light passes between two different materials, a fraction of that light is reflected,” says Vajente. “This is the same thing that happens in your windows: you can see your faint reflection in the glass. By adding multiple layers of different materials, we can reinforce each reflection and make our mirrors up to 99.999 percent reflective.”
“What’s important about this work is that we developed a new way to better test the materials,” says Vajente. “We can now test the properties of a new material in about eight hours, completely automated, when before it took almost a week. This allowed us to explore the periodic table by trying a lot of different materials and a lot of combinations. Some of the materials we tried didn’t work, but this gave us insights into what properties might be important.”

In the end, the scientists discovered that a coating material made from a combination of titanium oxide and germanium oxide dissipated the least energy (the equivalent of reducing thermal vibrations).

“We tailored the fabrication process to meet the stringent demands in optical quality and reduced thermal noise of the mirror coatings,” says Carmen Menoni, professor at Colorado State University and member of the LIGO Scientific Collaboration. Menoni and her colleagues at Colorado State used a method called ion beam sputtering to coat the mirrors. In this process, atoms of titanium and germanium are peeled away from a source, combined with oxygen, and then deposited onto the glass to create thin layers of atoms.

The new coating may be used for LIGO’s fifth observing run, which will begin in the middle of the decade as part of the Advanced LIGO Plus program. Meanwhile, LIGO’s fourth observing run, the last in the Advanced LIGO campaign, is expected to commence in the summer of 2022.

“This is a game changer for Advanced LIGO Plus,” says Reitze. “And this is a great example of how LIGO relies heavily on cutting-edge optics and materials science research and development. This is the biggest advance in precision optical coating development for LIGO in the past 20 years.”

The importance of lake breach floods for valley incision on early Mars

by Timothy A. Goudge, Alexander M. Morgan, Gaia Stucky de Quay, Caleb I. Fassett in Nature

On Earth, river erosion is usually a slow-going process. But on Mars, massive floods from overflowing crater lakes had an outsized role in shaping the Martian surface, carving deep chasms and moving vast amounts of sediment, according to a new study led by researchers at The University of Texas at Austin.

The study found that the floods, which probably lasted mere weeks, eroded more than enough sediment to completely fill Lake Superior and Lake Ontario.

**Valley networks and palaeolake outlet canyons on Mars.**

“If we think about how sediment was being moved across the landscape on ancient Mars, lake breach floods were a really important process globally,” said lead author Tim Goudge, an assistant professor at the UT Jackson School of Geosciences. “And this is a bit of a surprising result because they’ve been thought of as one-off anomalies for so long.”

Crater lakes were common on Mars billions of years ago when the Red Planet had liquid water on its surface. Some craters could hold a small sea’s worth of water. But when the water became too much to hold, it would breach the edge of the crater, causing catastrophic flooding that carved river valleys in its wake. A 2019 study led by Goudge determined that these events happened rapidly.

Remote sensing images taken by satellites orbiting Mars have allowed scientists to study the remains of breached Martian crater lakes. However, the crater lakes and their river valleys have mostly been studied on an individual basis, Goudge said. This is the first study to investigate how the 262 breached lakes across the Red Planet shaped the Martian surface as a whole.

The research entailed reviewing a preexisting catalog of river valleys on Mars and classifying the valleys into two categories: valleys that got their start at a crater’s edge, which indicates they formed during a lake breach flood, and valleys that formed elsewhere on the landscape, which suggests a more gradual formation over time.

From there, the scientists compared the depth, length and volume of the different valley types and found that river valleys formed by crater lake breaches punch far above their weight, eroding away nearly a quarter of the Red Planet’s river valley volume despite making up only 3% of total valley length.

“This discrepancy is accounted for by the fact that outlet canyons are significantly deeper than other ”valleys,” said study co-author Alexander Morgan, a research scientist at the Planetary Science Institute.

At 559 feet (170.5 meters), the median depth of a breach river valley is more than twice that of other river valleys created more gradually over time, which have a median depth of about 254 feet (77.5 meters).

**Example output from the progressive black top hat (PBTH) transformation.**

In addition, although the chasms appeared in a geologic instant, they may have had a lasting effect on the surrounding landscape. The study suggests that the breaches scoured canyons so deep they may have influenced the formation of other nearby river valleys. The authors said this is a potential alternative explanation for unique Martian river valley topography that is usually attributed to climate.

The study demonstrates that lake breach river valleys played an important role in shaping the Martian surface, but Goudge said it’s also a lesson in expectations. The Earth’s geology has wiped away most craters and makes river erosion a slow and steady process in most cases. But that doesn’t mean it will work that way on other worlds.

“When you fill [the craters] with water, it’s a lot of stored energy there to be released,” Goudge said. “It makes sense that Mars might tip, in this case, toward being shaped by catastrophism more than the Earth.”

L 98–59: A Benchmark System of Small Planets for Future Atmospheric Characterization

by Daria Pidhorodetska, Sarah E. Moran, Edward W. Schwieterman, Thomas Barclay, Thomas J. Fauchez, Nikole K. Lewis, Elisa V. Quintana, Geronimo L. Villanueva, Shawn D. Domagal-Goldman, Joshua E. Schlieder, Emily A. Gilbert, Stephen R. Kane, Veselin B. Kostov in The Astronomical Journal

When the world’s most powerful telescope launches into space this year, scientists will learn whether Earth-sized planets in our ‘solar neighborhood’ have a key prerequisite for life — an atmosphere.

These planets orbit an M-dwarf, the smallest and most common type of star in the galaxy. Scientists do not currently know how common it is for Earth-like planets around this type of star to have characteristics that would make them habitable.

“As a starting place, it is important to know whether small, rocky planets orbiting M-dwarfs have atmospheres,” said Daria Pidhorodetska, a doctoral student in UC Riverside’s Department of Earth and Planetary Sciences. “If so, it opens up our search for life outside our solar system.”

To help fill this gap in understanding, Pidhorodetska and her team studied whether the soon-to-launch James Webb Space Telescope, or the currently-in-orbit Hubble Space Telescope, are capable of detecting atmospheres on these planets. They also modeled the types of atmospheres likely to be found, if they exist, and how they could be distinguished from each other. Study co-authors include astrobiologists Edward Schwieterman and Stephen Kane from UCR, as well as scientists from Johns Hopkins University, NASA’s Goddard Space Flight Center, Cornell University and the University of Chicago.

The star at the center of the study is an M-dwarf called L 98–59, which measures only 8% of our sun’s mass. Though small, it is only 35 light years from Earth. It’s brightness and relative closeness make it an ideal target for observation.

Shortly after they form, M-dwarfs go through a phase in which they can shine two orders of magnitude brighter than normal. Strong ultraviolet radiation during this phase has the potential to dry out their orbiting planets, evaporating any water from the surface and destroying many gases in the atmosphere.

“We wanted to know if the ablation was complete in the case of the two rocky planets, or if those terrestrial worlds were able to replenish their atmospheres,” Pidhorodetska said.

One transit of an H2-/He-dominated atmosphere simulated with HST/WFC is shown for L 98–59 c (top) and L 98–59 d (middle) in gray.

The researchers modeled four different atmospheric scenarios: one in which the L 98–59 worlds are dominated by water, one in which the atmosphere is mainly composed of hydrogen, a Venus-like carbon dioxide atmosphere, and one in which the hydrogen in the atmosphere escaped into space, leaving behind only oxygen and ozone.

They found that the two telescopes could offer complementary information using transit observations, which measure a dip in light that occurs as a planet passes in front of its star. The L 98–59 planets are much closer to their star than Earth is to the sun. They complete their orbits in less than a week, making transit observations by telescope faster and more cost effective than observing other systems in which the planets are farther from their stars.

“It would only take a few transits with Hubble to detect or rule out a hydrogen- or steam-dominated atmosphere without clouds,” Schwieterman said. “With as few as 20 transits, Webb would allow us to characterize gases in heavy carbon dioxide or oxygen-dominated atmospheres.”

Of the four atmospheric scenarios the researchers considered, Pidhorodetska said the dried-out oxygen-dominated atmosphere is the most likely.

“The amount of radiation these planets are getting at that distance from the star is intense,” she said.

The O3 concentrations for each modeled O2/O3 atmosphere as a function of altitude [km].

Though they may not have atmospheres that lend themselves to life today, these planets can offer an important glimpse into what might happen to Earth under different conditions, and what might be possible on Earth-like worlds elsewhere in the galaxy.

The L 98–59 system was only discovered in 2019, and Pidhorodetska said she is excited to get more information about it when Webb is launched later this year.

“We’re on the precipice of revealing the secrets of a star system that was hidden until very recently,” Pidhorodetska said.

Soil fertility interactions with Sinorhizobium-legume symbiosis in a simulated Martian regolith; effects on nitrogen content and plant health

by Franklin Harris, John Dobbs, David Atkins, James A. Ippolito, Jane E. Stewart in PLOS ONE

Clover plants grown in Mars-like soils experience significantly more growth when inoculated with symbiotic nitrogen-fixing bacteria than when left uninoculated.

As Earth’s population grows, researchers are studying the possibility of farming Martian soils, or “regolith.” However, regolith is lacking in some essential plant nutrients, including certain nitrogen-containing molecules that plants require to live. Therefore, agriculture on Mars will require strategies to increase the amount of these nitrogen compounds in regolith.

Harris and colleagues hypothesize that bacteria could play a cost-effective role in making Martian soils more fertile. On Earth, bacteria in soils help convert or “fix” atmospheric nitrogen into the molecules that plants need. Some of these microbes have symbiotic relationships with plants, in which they fix nitrogen within nodules found on plant roots.

Linear regression of shoot biomass by nodules (A), root biomass by nodules (B) and remaining and added nitrogen in soil by quantity of nodules © for either regolith or potting soil.

To explore a possible role for symbiotic nitrogen-fixing bacteria in astroagriculture, the researchers grew clover in man-made regolith that closely matches that of Mars. They inoculated some of the plants with the microbe Sinorhizobium meliloti, which is commonly found in clover root nodules on Earth. Previous research had shown that clover can be grown in regolith, but had not explored inoculation with nitrogen- fixers.

The researchers found that the inoculated clover experienced 75% more root and shoot growth compared to the uninoculated clover. However, the regolith surrounding the inoculated plants showed no signs of elevated NH4 — an essential nitrogen-containing molecule for plants — compared to the regolith surrounding uninoculated plants.

These findings suggest that the symbiotic microbes boosted clover growth, but did not result in excess production of nitrogen compounds that other plants growing nearby could hypothetically use. The researchers also grew some clover in potting soil and noted certain differences in the symbiotic relationship when comparing the plants grown in regolith versus soil.

Sweet clover (*Melilotus officinalis)* shoot length (A), shoot biomass (B), and root biomass (c) as measured either above or below the root sheath.

These findings suggest the possibility that symbiosis between plants and nitrogen-fixing bacteria could aid agriculture on Mars. Future research could continue to explore such relationships with other crops and address issues with plant toxicity in regolith.

The authors add: “This study shows that nodule forming bacteria Sinorhizobium meliloti has been shown to nodulate in Martian regolith, significantly enhancing growth of clover (Melilotus officinalis) in a greenhouse assay. This work increases our understanding of how plant and microbe interactions will help aid efforts to terraform regolith on Mars.”

Collision Chains among the Terrestrial Planets. II. An Asymmetry between Earth and Venus

by Alexandre Emsenhuber, Erik Asphaug, Saverio Cambioni, Travis S. J. Gabriel, Stephen R. Schwartz in The Planetary Science Journal

Planet formation — the process by which neat, round, distinct planets form from a roiling, swirling cloud of rugged asteroids and mini planets — was likely even messier and more complicated than most scientists would care to admit, according to new research led by researchers at the University of Arizona Lunar and Planetary Laboratory.

The findings challenge the conventional view, in which collisions between smaller building blocks cause them to stick together and, over time, repeated collisions accrete new material to the growing baby planet.

Instead, the authors propose and demonstrate evidence for a novel “hit-and-run-return” scenario, in which pre-planetary bodies spent a good part of their journey through the inner solar system crashing into and ricocheting off of each other, before running into each other again at a later time. Having been slowed down by their first collision, they would be more likely to stick together the next time. Picture a game of billiards, with the balls coming to rest, as opposed to pelting a snowman with snowballs, and you get the idea.

“We find that most giant impacts, even relatively ‘slow’ ones, are hit-and-runs. This means that for two planets to merge, you usually first have to slow them down in a hit-and-run collision,” Asphaug said. “To think of giant impacts, for instance the formation of the moon, as a singular event is probably wrong. More likely it took two collisions in a row.”

One implication is that Venus and Earth would have had very different experiences in their growth as planets, despite being immediate neighbors in the inner solar system. In the paper, led by Alexandre Emsenhuber, who did this work during a postdoctoral fellowship in Asphaug’s lab and is now at Ludwig Maximilian University in Munich, the young Earth would have served to slow down interloping planetary bodies, making them ultimately more likely to collide with and stick to Venus.

“We think that during solar system formation, the early Earth acted like a vanguard for Venus,” Emsenhuber said.

The solar system is what scientists call a gravity well, the concept behind a popular attraction at science exhibits. Visitors toss a coin into a funnel-shaped gravity well, and then watch their cash complete several orbits before it drops into the center hole. The closer a planet is to the sun, the stronger the gravitation experienced by planets. That’s why the inner planets of the solar system on which these studies were focused — Mercury, Venus, Earth and Mars — orbit the sun faster than, say, Jupiter, Saturn and Neptune. As a result, the closer an object ventures to the sun, the more likely it is to stay there. So when an interloping planet hit the Earth, it was less likely to stick to Earth, and instead more likely to end up at Venus, Asphaug explained.

“The Earth acts as a shield, providing a first stop against these impacting planets,” he said. “More likely than not, a planet that bounces off of Earth is going to hit Venus and merge with it.”

Emsenhuber uses the analogy of a ball bouncing down a staircase to illustrate the idea of what drives the vanguard effect: A body coming in from the outer solar system is like a ball bouncing down a set of stairs, with each bounce representing a collision with another body.

Cumulative distribution across 1000 simulations of the hit-and-run chain resolution time (the time at which the runner was lost by accretion, either with a planet or with the Sun, or ejection) for four series of dynamical evolutions: slow runner from proto-Venus (solid red), slow runner from proto-Earth (solid black), fast runner from proto-Venus (dashed red), and fast runner from proto-Earth (dashed black). The vertical dashed line at 50 Myr is the epoch where we compute the analyses of the chains and collision statistics.

“Along the way, the ball loses energy, and you’ll find it will always bounce downstairs, never upstairs,” he said. “Because of that, the body cannot leave the inner solar system anymore. You generally only go downstairs, toward Venus, and an impactor that collides with Venus is pretty happy staying in the inner solar system, so at some point it is going to hit Venus again.”

Earth has no such vanguard to slow down its interloping planets. This leads to a difference between the two similar-sized planets that conventional theories cannot explain, the authors argue.

“The prevailing idea has been that it doesn’t really matter if planets collide and don’t merge right away, because they are going to run into each other again at some point and merge then,” Emsenhuber said. “But that is not what we find. We find they end up more frequently becoming part of Venus, instead of returning back to Earth. It’s easier to go from Earth to Venus than the other way around.”

To track all these planetary orbits and collisions, and ultimately their mergers, the team used machine learning to obtain predictive models from 3D simulations of giant impacts. The team then used these data to rapidly compute the orbital evolution, including hit-and-run and merging collisions, to simulate terrestrial planet formation over the course of 100 million years. In the second paper, the authors propose and demonstrate their hit-and-run-return scenario for the moon’s formation, recognizing the primary problems with the standard giant impact model.

“The standard model for the moon requires a very slow collision, relatively speaking,” Asphaug said, “and it creates a moon that is composed mostly of the impacting planet, not the proto-Earth, which is a major problem since the moon has an isotopic chemistry almost identical to Earth.”

In the team’s new scenario, a roughly Mars-sized protoplanet hits the Earth, as in the standard model, but is a bit faster so it keeps going. It returns in about 1 million years for a giant impact that looks a lot like the standard model.

“The double impact mixes things up much more than a single event,” Asphaug said, “which could explain the isotopic similarity of Earth and moon, and also how the second, slow, merging collision would have happened in the first place.”

The researchers think the resulting asymmetry in how the planets were put together points the way to future studies addressing the diversity of terrestrial planets. For example, we don’t understand how Earth ended up with a magnetic field that is much stronger than that of Venus, or why Venus has no moon. Their research indicates systematic differences in dynamics and composition, according to Asphaug.

“In our view, Earth would have accreted most of its material from collisions that were head-on hits, or else slower than those experienced by Venus,” he said. “Collisions into the Earth that were more oblique and higher velocity would have preferentially ended up on Venus.”

This would create a bias in which, for example, protoplanets from the outer solar system, at higher velocity, would have preferentially accreted to Venus instead of Earth. In short, Venus could be composed of material that was harder for the Earth to get ahold of.

“You would think that Earth is made up more of material from the outer system because it is closer to the outer solar system than Venus. But actually, with Earth in this vanguard role, it makes it actually more likely for Venus to accrete outer solar system material,” Asphaug said.

Mass, Spin, and Ultralight Boson Constraints from the Intermediate-mass Black Hole in the Tidal Disruption Event 3XMM J215022.4–055108

by Sixiang Wen, Peter G. Jonker, Nicholas C. Stone, Ann I. Zabludoff in The Astrophysical Journal

While black holes and toddlers don’t seem to have much in common, they are remarkably similar in one aspect: Both are messy eaters, generating ample evidence that a meal has taken place.

But whereas one might leave behind droppings of pasta or splatters of yogurt, the other creates an aftermath of mind-boggling proportions. When a black hole gobbles up a star, it produces what astronomers call a “tidal disruption event.” The shredding of the hapless star is accompanied by an outburst of radiation that can outshine the combined light of every star in the black hole’s host galaxy for months, even years.

In a paper, a team of astronomers led by Sixiang Wen, a postdoctoral research associate at the University of Arizona Steward Observatory, use the X-rays emitted by a tidal disruption event known as J2150 to make the first measurements of both the black hole’s mass and spin. This black hole is of a particular type — an intermediate-mass black hole — which has long eluded observation.

The evolution of unabsorbed X-ray flux (top panel) and mass accretion rate (bottom panel).

“The fact that we were able to catch this black hole while it was devouring a star offers a remarkable opportunity to observe what otherwise would be invisible,” said Ann Zabludoff, UArizona professor of astronomy and co-author on the paper. “Not only that, by analyzing the flare we were able to better understand this elusive category of black holes, which may well account for the majority of black holes in the centers of galaxies.”

By re-analyzing the X-ray data used to observe the J2150 flare, and comparing it with sophisticated theoretical models, the authors showed that this flare did indeed originate from an encounter between an unlucky star and an intermediate-mass black hole. The intermediate black hole in question is of particularly low mass — for a black hole, that is — weighing in at roughly 10,000 times the mass of the sun.

“The X-ray emissions from the inner disk formed by the debris of the dead star made it possible for us to infer the mass and spin of this black hole and classify it as an intermediate black hole,” Wen said.

Dozens of tidal disruption events have been seen in the centers of large galaxies hosting supermassive black holes, and a handful have also been observed in the centers of small galaxies that might contain intermediate black holes. However, past data has never been detailed enough to prove that an individual tidal disruption flare was powered by an intermediate black hole.

“Thanks to modern astronomical observations, we know that the centers of almost all galaxies that are similar to or larger in size than our Milky Way host central supermassive black holes,” said study co-author Nicholas Stone, a senior lecturer at Hebrew University in Jerusalem. “These behemoths range in size from 1 million to 10 billion times the mass of our sun, and they become powerful sources of electromagnetic radiation when too much interstellar gas falls into their vicinity.”

The mass of these black holes correlates closely with the total mass of their host galaxies; the largest galaxies host the largest supermassive black holes.

“We still know very little about the existence of black holes in the centers of galaxies smaller than the Milky Way,” said co-author Peter Jonker of Radboud University and SRON Netherlands Institute for Space Research, both in the Netherlands. “Due to observational limitations, it is challenging to discover central black holes much smaller than 1 million solar masses.”

Despite their presumed abundance, the origins of supermassive black holes remain unknown, and many different theories currently vie to explain them, according to Jonker. Intermediate-mass black holes could be the seeds from which supermassive black holes grow.

“Therefore, if we get a better handle of how many bona fide intermediate black holes are out there, it can help determine which theories of supermassive black hole formation are correct,” he said.

*For our fiducial fc, the effects of using different observed epochs to constrain M and a are shown.*

Even more exciting, according to Zabludoff, is the measurement of J2150’s spin that the group was able to obtain. The spin measurement holds clues as to how black holes grow, and possibly to particle physics. This black hole has a fast spin, but not the fastest possible spin, Zabludoff explained, begging the question of how the black hole ends up with a spin in this range.

“It’s possible that the black hole formed that way and hasn’t changed much since, or that two intermediate-mass black holes merged recently to form this one,” she said. “We do know that the spin we measured excludes scenarios where the black hole grows over a long time from steadily eating gas or from many quick gas snacks that arrive from random directions.”

In addition, the spin measurement allows astrophysicists to test hypotheses about the nature of dark matter, which is thought to make up most of the matter in the universe. Dark matter may consist of unknown elementary particles not yet seen in laboratory experiments. Among the candidates are hypothetical particles known as ultralight bosons, Stone explained.

“If those particles exist and have masses in a certain range, they will prevent an intermediate-mass black hole from having a fast spin,” he said. “Yet J2150’s black hole is spinning fast. So, our spin measurement rules out a broad class of ultralight boson theories, showcasing the value of black holes as extraterrestrial laboratories for particle physics.”

In the future, new observations of tidal disruption flares might let astronomers fill in the gaps in the black hole mass distribution, the authors hope.

“If it turns out that most dwarf galaxies contain intermediate-mass black holes, then they will dominate the rate of stellar tidal disruption,” Stone said. “By fitting the X-ray emission from these flares to theoretical models, we can conduct a census of the intermediate-mass black hole population in the universe,” Wen added.

To do that, however, more tidal disruption events have to be observed. That’s why astronomers hold high hopes for new telescopes coming online soon, both on Earth and in space, including the Vera C. Rubin Observatory, also known as the Legacy Survey of Space and Time, or LSST, which is expected to discover thousands of tidal disruption events per year.

Interaction bounding surfaces exposed in migrating transverse aeolian ridges on Mars

by Mackenzie Day in Geology

Dunes develop when wind-blown sand organizes into patterns, most often in deserts and arid or semi-arid parts of the world. Every continent on Earth has dune fields, but dunes and dune-like sand patterns are also found across the solar system: on Mars, Venus, Titan, Comet 67P, and Pluto. On Earth, weather stations measure the wind speed and direction, allowing us to predict and understand airflow in the atmosphere.

On other planets and planetary bodies, we do not yet have weather stations measuring the winds (with a few recent exceptions on Mars only). Without a way to directly measure wind on the surface of another planet, we can use the patterns in dunes to interpret what the wind must be doing, based on our knowledge of dunes on Earth. Furthermore, by studying dunes across planets, we can get a better understanding of how wind and sand behave in general. In the new paper, Mackenzie Day of the University of California Los Angeles focuses on what happens when two dunes collide.

“On Earth, we know that dunes collide, combine, link, and merge all the time,” says Day. This is what drives changes in dune-field patterns over time. When this happens, the dune-dune interaction leaves behind a particular pattern in the sand, but that pattern is usually covered by actively moving sand and difficult to see without special tools.”

Sand on Mars forms dunes and dune-like features, as seen here in a satellite photo from the Iapygia Quadrangle of Mars. Banding on the back of these dune-like features indicated that they are migrating toward the lower right in the image and the wind is form the upper left. HiRISE camera image ESP_020782_1610. Photo credit NASA/U of A.

On Mars, many dunes look and behave similar to dunes on Earth, but in addition Mars hosts patterns of organized sand that are dune-like but have some differences that have yet to be explained by the scientific community. Whether or not these unusual features, sometimes called “transverse aeolian ridges” or “megaripples,” are formed like dunes has been long debated.

“In this work, says Day, I show that these unusual wind-blown sand ridges sometimes show on their surfaces the pattern that forms when two dunes combine.”

In the Iapygia region of Mars, transverse aeolian ridges incorporated both light and dark sands, leading to light-dark banding in the upwind side of the ridges. Banding occurring only on one side of the ridges suggests that the banding formed as the ridges migrated. Furthermore, the dune-interaction pattern known from Earth can be seen in some ridges where the banding is truncated and then reconnects, just like two dunes touching and then combining downwind.

The pattern associated with dune-interactions only forms when two dunes combine, therefore seeing it in these martian sand ridges demonstrates that these enigmatic features (like those shown in the image attached) behave like dunes on Earth. “Just like dunes on Earth, transvers aeolian ridges on Mars migrate, combine, and develop complex patterns in response to the wind.” Transverse aeolian ridges are incredibly common on Mars, and the results of this work allow us to better interpret the wind at the surface of Mars using these dune-like features.

“Overall,” Day says, “this work leverages both knowledge of Mars and knowledge of Earth to understand the other planet and opens the door to improving how we interpret wind across planetary bodies further into the solar system.”