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Machine learning algorithms analyzed millions of galaxies to reveal where the biggest ones are.

Defending the U.S. is much more complicated in an era of climate change

Astrophotographer Mark Johnston got a front row seat to an epic fireworks show when he filmed a 100,000-mile-high tower of plasma erupting from the sun.

This week’s news roundup: Jellyfish clones are multiplying in British Columbia’s lakes, measles cases are on the rise in Oregon, and a new study finds cell phones aren’t linked to brain cancer.

The first satellite in ESA’s Cluster quartet safely came back down to Earth last night in a world-first ‘targeted reentry’, marking a brilliant end to this remarkable mission.

The spacecraft, dubbed ‘Salsa’ (Cluster 2), reentered Earth’s atmosphere at 20:47 CEST on 8 September 2024 over the South Pacific Ocean. In this region, any risk of fragments reaching land are absolutely minimised.

During the last two decades Cluster has spent in space, it has provided invaluable data on how the Sun interacts with Earth’s magnetic field, helping us better understand and forecast space weather. With this first-ever targeted reentry, Cluster will go down in history for a second reason – helping ESA become a world-leader in sustainable space exploration.

According to Nebula Theory, stars and their systems of planets form when a massive cloud of gas and dust (a nebula) undergoes gravitational collapse at the center, forming a new star. The remaining material from the nebula then forms a disk around the star from which planets, moons, and other bodies will eventually accrete (a protoplanetary disk). This is how Earth and the many bodies that make up the Solar System came together roughly 4.5 billion years ago, eventually settling into their current orbits (after a few migrations and collisions).

However, there is still debate regarding certain details of the planet formation process. On the one hand, there are those who subscribe to the traditional “bottom-up” model, where dust grains gradually collect into larger and larger conglomerations over tens of millions of years. Conversely, you have the “top-down” model, where circumstellar disk material in spiral arms fragments due to gravitational instability. Using the Atacama Large Millimeter/submillimeter Array (ALMA), an international team of astronomers found evidence of the “top-down” model when observing a protoplanetary disk over 500 light-years away.

The team was led by Jessica Speedie, an astronomy and astrophysics Ph.D. candidate at the University of Victoria. She was joined by colleagues from the Kavli Institute for Astronomy and Astrophysics (KIAA-PKU), the Center for Simulational Physics (CSP-UGA), the Cambridge Institute of Astronomy, the Centre de Recherche Astrophysique de Lyon (CNSA-CRAL), the Institute of Astronomy and Astrophysics (ASIAA), the Department of Earth, Atmospheric, and Planetary Sciences (MIT EAPS), the National Astronomical Observatory of Japan (NAOJ), the European Southern Observatory (ESO), and multiple universities and observatories.

The paper that details their research, “Gravitational instability in a planet-forming disk,” was recently published in the journal Nature.

Located in the Atacama desert in the Chilean Andes, ALMA is the largest radio telescope in the world dedicated to studying the parts of the Universe that are otherwise invisible to astronomers. This includes cold dust clouds in space, protoplanetary disks, and some of the earliest galaxies in the Universe, which are only visible at millimeter and submillimeter wavelengths. Using ALMA, Speedie and her colleagues observed the well-characterized protoplanetary disk around AB Aurigae, a young star system (4 million years old) located about 530 light-years from Earth.

The star is a pre-main sequence A-type star (blue-white) approximately 2.5 times the size of our Sun and about 2.4 times as massive. Beginning in 2017, scientists at ALMA began observing the star’s protoplanetary disk to learn more about planet formation in young star systems. Since then, astronomers have observed several developing protoplanets forming in AB Aurigae’s disk, as well as a gas giant nine times the mass of Jupiter that was confirmed in 2022. These appear as clumps within the protoplanetary disk’s spiral arms, rotating counterclockwise around the star.

The detection of these bodies around such a young star raised doubts about the “bottom-up” process. According to this model, these protoplanets did not have nearly enough time to become as large as they have. Along with her PhD advisor Ruobing Dong, Speedie and their team were determined to study how the gas in the system’s vast spiral arms is moving. ALMA’s sensitivity and high velocity resolution was crucial to that task and enabled the team to probe the gas deep within the disk and measure its motion precisely.

Dr. Cassandra Hall, an Assistant Professor of Computational Astrophysics at the University of Georgia was also a co-author on the research. Four years ago, Hall led a study where she and her colleagues (which included Dong and other members of Speedie’s team) simulated how a gravitationally unstable disk would behave. As she indicated in a NRAO press release:

“Disks that are gravitationally unstable should have distinctive ‘wiggles’ in their velocity field, unlike disks that are stable. Back in 2020, we performed some of the most advanced simulations in the world to predict the existence of this hallmark signature of gravitational instability. It was clear, it was testable, and it was a bit scary – if we didn’t find it, then something had to be very, very wrong with our understanding of these disks.”

Spiral arms form in a protoplanetary disk when the disk-to-star mass ratio is sufficiently high. Over time, changes in density lead to changes in gravity, which causes variations in the velocities of gas in and around the spiral arms. These variations in velocity are seen as “wiggles,” and the magnitude can be used to infer the mass ratio between the host star and the material in its disk. Using ALMA’s array of radio antennas, Speedie and her team mapped the velocity of carbon monoxide isotopes within the disk’s spiral arms and looked for indications of the predicted “wiggles.”

These measurements yielded a three-dimensional rectangular “data cube” that mapped gas velocity and position within the protoplanetary disk along the observatory’s line of sight. As is customary with ALMA’s interferometry measurements, the data was parsed into “slices” (or strategically oriented cuts), allowing Speedie and her team to conclusively identify the velocity wiggle indicating gravitational instability. This constitutes the first direct observational confirmation that the “top-down” pathway to planet formation is correct.

What’s more, it indicates that planetary systems may form much faster than previously thought, which could have significant implications for astrogeology and exoplanet research. As Speedie explained, Hall’s work, ALMA’s sensitivity, and the quality data products it created for them were what made this discovery possible:

“This is a classic science story of, ‘we predicted it, and then we found it’. The Hall-mark of gravitational instability. We worked with one of the deepest ALMA observations taken with such high-velocity resolution toward a single protoplanetary disk to date. The ALMA data provides a clear diagnosis of gravitational instability in action. There is no other mechanism we know of that can create the global architecture of spiral structure and velocity patterns that we observe.”

In the near future, Speedie and her colleagues plan to continue using ALMA to learn more about how planetary systems form around young stars. As part of the NFS/NRAO ALMA ambassador program, Speedie is training alongside other postdoctoral students and early career astronomers to share ALMA’s resources and capabilities with the wider astronomical community.

Further Reading: NRAO, Nature

The post ALMA Detects Hallmark “Wiggle” of Gravitational Instability in Planet-Forming Disk appeared first on Universe Today.

In 2012, two previous dark matter detection experiments—the Large Underground Xenon (LUX) and ZonEd Proportional scintillation in Liquid Noble gases (ZEPLIN)—came together to form the LUX-ZEPLIN (LZ) experiment. Since it commenced operations, this collaboration has conducted the most sensitive search ever mounted for Weakly Interacting Massive Particles (WIMPs) – one of the leading Dark Matter candidates. This collaboration includes around 250 scientists from 39 institutions in the U.S., U.K., Portugal, Switzerland, South Korea, and Australia.

On Monday, August 26th, the latest results from the LUX-ZEPLIN project were shared at two scientific conferences. These results were celebrated by scientists at the University of Albany‘s Department of Physics, including Associate Professors Cecilia Levy and Matthew Szydagis (two members of the experiment). This latest result is nearly five times more sensitive than the previous result and found no evidence of WIMPs above a mass of 9 GeV/c2. These are the best-ever limits on WIMPS and a crucial step toward finding the mysterious invisible mass that makes up 85% of the Universe.

Led by the Department of Energy’s (DoE) Lawrence Berkeley National Laboratory, the LZ experiment is located at the Sanford Underground Research Facility in South Dakota, about 1,500 meters (nearly a mile) beneath the surface. The experiment relies on an ultra-sensitive detector made of 10 tonnes (11 U.S. tons) of liquid xenon to hunt for the elusive signals caused by WIMP-nucleus interactions. While direct detections are yet to be made, these latest results have helped scientists narrow the search.

As Levy explained in a recent UofA press release:

“Dark matter interacts very, very rarely with normal matter, but we don’t know exactly how rarely. The way we measure it is through this cross-section or how probable an interaction is within our detector. Depending on the mass of a dark matter particle, which we don’t know yet, an interaction within the detector is more or less probable. What the new LZ results tell us is that dark matter interacts with normal matter even more rarely than we thought, and the only instrument in the world that is sensitive enough to measure that is LZ.”

The existence and nature of Dark Matter are among the greatest mysteries in modern astrophysics. Originally proposed to explain the rotational curves of galaxies, the existence of Dark Matter is vital to the most widely accepted cosmological model – the Lambda Cold Dark Matter (LCDM) model. Unfortunately, according to the prevailing theories, DM only interacts with normal (aka. “luminous”) matter via gravity, the weakest of the four fundamental forces. Detecting these interactions requires incredibly sensitive instruments and an environment free of electromagnetic energy (including heat and light).

While no direct detections have been made, the latest results from LZ have narrowed the range of possibilities for one of the leading DM candidates. As Szydagis said:

“It’s often misunderstood what is meant by the phrase ‘world’s best dark matter experiment’ since no one has made a conclusive, unambiguous discovery yet. However, new, stricter null results like LZ’s are still extremely valuable for science. UAlbany, as one part of the multinational collaboration that is LZ, has been making important contributions ensuring the robustness of LZ’s results, going back to the very beginning of the experiment.”

Although DM remains “invisible” to us, the presence of its gravitational pull is fundamental to our understanding of the Universe. For example, the formation and movement of galaxies are attributed to DM, and its existence is vital for explaining the large-scale structure and evolution of the Universe. If DM does not exist, then our understanding of gravity – as described by Einstein’s Theory of General Relativity – is essentially wrong and needs revision. However, General Relativity has been experimentally validated again and again over the past century.

Therefore, narrowing the search for its constituent particle is vital to proving that our foundational theories about the Universe are correct. As Levy noted, UAlbany scientists have been making integral contributions to LZ for over a decade, and their work is far from done! “Working on LZ is always so exciting, even if we still have not made a discovery yet,” she said. “We all know that if it were easy, someone else would have done it already! I think right now what we need to take out of this result is that LZ is a great team of scientists, our detector is working superbly, our analysis is extremely robust, and we are nowhere near done taking data.”

Further Reading: University at Albany

The post Largest Dark Matter Detector is Narrowing Down Dark Matter Candidate appeared first on Universe Today.

Why is there a triangle hovering over the Sun?

Medical clowns, who play with children in hospitals, may help them be discharged sooner by reducing their heart rates

Medical clowns, who play with children in hospitals, may help them be discharged sooner by reducing their heart rates

Video: 00:06:50

The first of four satellites that make up ESA’s Cluster mission is coming safely back down to Earth, marking a brilliant end to this remarkable mission.

The satellite’s orbit was tweaked back in January to target a region as far as possible from populated regions. This ensures that any spacecraft parts that survive the reentry will fall over open ocean.

During 24 years in space, Cluster has sent back precious data on how the Sun interacts with Earth’s magnetic field, helping us better understand and forecast potentially dangerous space weather. 

With this first ever targeted reentry, Cluster goes down in history for a different reason, taking ESA well beyond international space safety standards and helping ensure the long-term sustainability of space activities.

A new clip for the upcoming Disney+ miniseries, "Lego Star Wars: Rebuild the Galaxy," which premieres on Disney+ on Sept. 13.

Get ready to rip and tear with our ranked list of all the Doom games.

SpaceX plans to start launching uncrewed Mars missions with its Starship megarocket in 2026 and crewed flights to the Red Planet two years after that, Elon Musk said.

Throughout Earth’s history, the planet’s surface has been regularly impacted by comets, meteors, and the occasional large asteroid. While these events were often destructive, sometimes to the point of triggering a mass extinction, they may have also played an important role in the emergence of life on Earth. This is especially true of the Hadean Era (ca. 4.1 to 3.8 billion years ago) and the Late Heavy Bombardment, when Earth and other planets in the inner Solar System were impacted by a disproportionately high number of asteroids and comets.

These impactors are thought to have been how water was delivered to the inner Solar System and possibly the building blocks of life. But what of the many icy bodies in the outer Solar System, the natural satellites that orbit gas giants and have liquid water oceans in their interiors (i.e., Europa, Enceladus, Titan, and others)? According to a recent study led by researchers from Johns Hopkins University, impact events on these “Ocean Worlds” could have significantly contributed to surface and subsurface chemistry that could have led to the emergence of life.

The team was led by Shannon M. MacKenzie, a planetary scientist, and her colleagues at Johns Hopkins University Applied Physics Laboratory (JHUAPL). They were joined by researchers from Dartmouth’s Thayer School of Engineering, the University of Western Ontario, Curtin University’s School of Earth and Planetary Sciences, the Planetary Habitability Laboratory (PHL) at UPR at Arecibo, Jacobs Technology, NASA’s Jet Propulsion Laboratory, and the Astromaterials Research and Exploration Science (ARES) at NASA Johnson Space Center. The paper that details their findings recently appeared in The Planetary Science Journal.

Voyager 1 image of Valhalla, a multi-ring impact structure 3,800 km (2,360 mi) in diameter.
Credit: NASA/JPL Exogenesis

As indicated in their paper, impacts from asteroids, comets, and large meteors are more often associated with destruction and extinction-level events. However, multiple lines of evidence indicate that these same types of impacts may have supported the emergence of life on Earth roughly 4 billion years ago. These events not only delivered volatiles (such as water, ammonia, and methane) and organic molecules, but modern research indicates that they also created new substrates and compounds essential to life.

Moreover, they created a variety of environments that were essential to the emergence and sustainment of life on Earth. As they wrote:

“Exogenously delivered materials have been estimated to be an important source of organics on early Earth. Shockwaves could provide the energy for organic synthesis of important precursors like HCN or amino acids. The iron and heat from very large impactors can facilitate the reducing atmospheric conditions necessary for abundant HCN production. Impacts fracture and, in typical terrestrial events, melt the target: the more permeable substrates and excavation of deeper rock layers promote hydrothermal activity and endolithic habitats.”

According to the latest fossilized evidence, the earliest life forms emerged on Earth roughly 4.28 billion years ago. These fossils were recovered from hydrothermal vent precipitates in the Nuvvuagittuq Greenstone Belt in northern Quebec, Canada, confirming that hydrothermal activity played a vital role in the emergence of life on Earth. But what about the many “Ocean Worlds” that reside in the outer Solar System? This includes bodies like Europa, Ganymede, Enceladus, and Titan, as well as Uranus’ moons Ariel and Titania, Neptune’s moon Triton, and Trans-Neptunian bodies like Pluto, Charon, and possibly more.

Ocean Worlds

This term refers to bodies predominantly composed of volatile elements such as water and differentiated between an icy crust and a rocky and metallic core. At the core-mantle boundary, tidal flexing (the result of gravitational interaction with another body) causes a buildup of heat and energy released via hydrothermal vents into the ice. This allows these worlds to maintain oceans of liquid water in their interiors. In short, these worlds have all the necessary ingredients for life: water, the requisite chemical compounds, and energy.

Impact velocity and first contact pressure estimates for potential icy and rocky impactors on “Ocean Worlds.” Credit: Mackenzie, S.M. et al. (2024)

Furthermore, data from the NASA/ESA Cassini–Huygens mission confirmed that the plumes regularly erupting from Enceladus’ southern polar region contain organic molecules. Last but not least, the presence of surface craters indicates that these bodies have experienced surface impacts throughout their history. The question naturally arises: could impacts have delivered the necessary building blocks of life to “Ocean Worlds” the same way they delivered them to the inner Solar System? And if so, what does that mean about their potential habitability today? As the team wrote in their paper:

“Impact processes are likely an important part of the answers to these questions, as impacts can drive exchange through the ice crust—either through direct seeding or flushing through the crust—and therefore drive episodic influxes of organic and inorganic materials from the surface and/or from the impactor itself. Impacts can also generate ephemeral microcosms: any liquid water melted during impact freezes out over timescales commensurate with the impact energy.”

“The exciting potential for chemistry within these pockets has been established, from concentrating salts to driving amino acid synthesis. Furthermore, shock-driven chemistry of icy, sometimes organic-rich (in the case of Titan especially) target materials may generate new “seed” compounds (e.g., amino acids or nucleotides) in the melt pool.”

Investigation

The first step for MacKenzie and her team was to investigate the initial shock levels created by the most common impacts for Ocean Worlds—comets that likely originated from the Kuiper Belt and Oort Cloud. To do this, the team calculated the velocities and maximum pressure that would be achieved by impacts involving icy and rocky bodies. They also considered how this would vary based on different families (primary or secondary impacts) and which systems were involved – i.e., Jupiter or Saturn. Whereas primary impacts involve comets or asteroids, secondary impacts are caused by the ejecta they create.

In the case of the Jupiter and Saturn systems, secondary impactors may be icy or rocky depending on where they originated (an icy body like Europa, Enceladus, and Titan, a rocky body like Io and larger asteroids). Whereas primary impacts have higher velocities and produce larger melt volumes), secondary impacts are more frequent. To determine melt sizes, the team consulted observed crater sizes on Europa, Enceladus, and Titan, and dynamic models that calculate the cumulative rate of cratering over time. They then compared the peak pressures at impact to thresholds for the survivability of elements essential to life, organic molecules, amino acids, and even microbes identified in previous studies.

Cumulative cratering rates assuming heliocentric, cometary impactors. Credit: Mackenzie, S.M. et al. (2024)

From this, they determined that most impacts at Europa and Enceladus experience peak pressures greater than what bacterial spores can survive. However, they also determined that a significant amount of material still survives these impacts and that higher first-contact pressures could also facilitate the synthesis of organic compounds in the meltwater that fills the craters. Meanwhile, on average, Titan and Enceladus experienced impacts with lower impact velocities, creating peak pressures that fall within the tolerance range for both bacterial spores and amino acids.

The next step was to consider how long fresh craters would survive and whether this would be sufficient for synthesizing biological materials. Based on the observed crater sizes on Enceladus and Europa, they determined that the longest-lived craters last only a few hundred years, whereas Titan could take centuries to tens of thousands of years for fresh craters to freeze. While Europa and Enceladus experience more high-velocity impacts (due to Titan’s dense atmosphere), the long-lived nature of Titan’s craters means that all three bodies have a chance for organic chemistry experiments to occur.

They also considered resurfacing rates on Europa, Enceladus, and Titan and how these would cycle biological material to their interiors. In all three cases, the satellites have relatively “young” terrain, implying regular resurfacing events.

Results

Based on these considerations, Mackenzie and her team determined that melts produced by comet impacts on Europa, Enceladus, and Titan have been frequent and long-lived enough to be of astrobiological interest. However, this varies based on the composition of the comets and the surface ice in question. As they summarized:

“At Europa and Enceladus, the survival and deposition of impactor organics is more important as there are fewer surface organics within the ice crust to seed the melt pool. On Titan, the survival of elements like phosphorous may be more important. Thus, even the small, more frequent impact events contribute to the astrobiological potential by delivering less modified compounds to the surface that are available either for immediate reaction if melt is produced or for future processing (including in subsequent impact events).”

Total melt production for observed craters on Enceladus (cyan) and Titan (orange), binned by observed crater diameter. Credit: Mackenzie, S.M. et al. (2024)

For instance, they found that a comet impacting Europa at the average impact velocity would create a 15 km (9.3 mi) crater and provide ~1 km3 (0.24 mi3)of meltwater. Based on the abundance of glycine (an essential amino acid) found on the comet 67P Churyumov–Gerasimenko, they determined that several parts per million would survive – roughly three orders of magnitude higher than what has been observed forming around hydrothermal vents here on Earth. “Thus, impactors seed whatever chemistry happens in the melt, providing organic and other essential elements depending on the impactor composition,” they added.

While this does not necessarily mean that these and other “Ocean Worlds” are currently habitable or actively support life, they demonstrate potential for future study. In the coming years, missions like the ESA’s JUpiter ICy moons Explorer (JUICE), and NASA’s Europa Clipper and Dragonfly missions will reach Ganymede, Europa, and Titan (respectively). There are also plans to create an Enceladus Orbiter to pick up where the Cassini-Huygens probe left off by examining Enceladus’ plume activity more closely.

Therefore, conducting in-situ sampling and analysis on these moons could provide powerful insight into prebiotic chemical pathways and determine under what conditions life can emerge. These sample studies will also address the larger question of whether or not life could exist in the interiors of “Ocean Worlds,” providing a preview of what future missions prepared to explore beneath the ice will find.

Further Reading: The Planetary Science Journal

The post Could Comets have Delivered the Building Blocks of Life to “Ocean Worlds” like Europa, Enceladus, and Titan too? appeared first on Universe Today.

When you walk across your lawn or down the street, you move on the surface of a surprisingly layered world. Some of those layers are rock, others are molten. A surprising amount of water is mixed into those layers, as well. It turns out that most planets have more of it “deep down” than we imagined.

Most of a planet’s water isn’t on the surface, even though we see oceans, lakes, and rivers here on Earth. The heart of our planet is iron, and covered by silicate rock layers. Scientists have long used our planet’s makeup as a sort of “model” for rocky exoplanets around other stars. That model may be outdated and too simplistic, according to Professor Caroline Dorn at ETH Zurich. “It is only in recent years that we have begun to realize that planets are more complex than we had thought,” she said. Dorn has been collaborating with Haiyang Luo and Jie Deng from Princeton University to understand the distribution of water mixed with silicates and iron inside a planet. They used computer simulations to come up with a robust model of the distribution of water on exoplanets.

Recent investigations of Earth’s water content triggered the team’s work. It turned out that our oceans contain only a small fraction of the overall water budget. The interior could be hiding the equivalent of 80% of the surface oceans. That raised a big question: could other planets have similarly hidden reservoirs?

Planets and Water

To answer that question, the science team simulated how water behaves in the conditions present when planets are young. Many known exoplanets orbit close to their stars, which means they’re likely to be hot worlds. They probably have oceans of molten magma that haven’t yet solidified to make silicate bedrock mantles.

Artist’s impression of a lava world. The exoplanet K2-141b is so close to its host star that it likely has magma oceans and surface temperatures over 3000 degrees. Water may be mixed in with the magma. c. ESO

As it turns out water dissolves very well in these magma oceans. The iron core takes time to develop,” she said. “A large share of the iron is initially contained in the hot magma soup in the form of droplets,” she explained, noting that water sequestered in this soup combines with the iron droplets and sinks with them to the core. “The iron droplets behave like a lift that is conveyed downwards by the water,” Dorn said.

That kind of mixing of iron and water happened in the moderate pressure environment in Earth’s interior. Larger planets with higher interior pressures presented a challenge to understand. It turns out they mix water and iron, too. “The larger the planet and the greater its mass, the more the water tends to go with the iron droplets and become integrated in the core,” said Dorn. “Under certain circumstances, iron can absorb up to 70 times more water than silicates. However, owing to the enormous pressure at the core, the water no longer takes the form of H2O molecules but is present in hydrogen and oxygen.”

Evolving Planets over Time

This result is a big deal if you want to understand how planets form and develop. That’s because the water never escapes the planet’s core. However, under the right conditions, water mixed in with the magma ocean can “de-gas” under the right conditions. Essentially, it separates and rises to the surface as the magma cools and forms the mantle. “So if we find water in a planet’s atmosphere, there is probably a great deal more in its interior,” explained Dorn.

That gives a lot of new information to use as scientists search for planets around other stars and look for habitable worlds. In particular, astronomers using the JWST can track the types of molecules in exoplanet atmospheres and use that information to find habitable worlds. “Only the composition of the upper atmosphere of exoplanets can be measured directly,” said Dorn. “Our group wishes to make the connection from the atmosphere to the inner depths of celestial bodies.”

TOI-270d appears to be a super-Earth or Earth-type planet, as shown in this artists’ concept. Could it have water hidden in its core that could boost its habitability. Courtesy Martin Vargic CC BY 3.0

Currently, the team studies exoplanet TOI-270d. “Evidence has been collected there of the actual existence of such interactions between the magma ocean in its interior and the atmosphere,” said Dorn. It’s at the top of her list of interesting objects to examine more closely for water, along with another one called K2-18b. It seems to be a promising candidate for habitability as well.

So, Does Deep Water Imply Life or Habitability?

Since water is important in the search for life-bearing worlds, looking for wet Earth-type and super-Earth worlds is the next step in searching out life. Dorn’s team found that planets with these deep water layers are likely to be fairly rare. That’s because most of their water is not on the surface. In other words, they may not be ocean worlds, but places with water trapped in their cores.

That’s not all bad. The science team assumes that even planets with a relatively high water content could have the potential to develop Earth-like habitable conditions. Dorn’s team may give scientists new ways to look for water-abundant worlds.

For More Information

Planets Contain More Water Than Thought
The Interior as the Dominant Water Reservoir in Super-Earths and Sub-Neptunes

The post There’s More Water Inside Planets Than We Thought appeared first on Universe Today.

Popular science history paints a picture of the Greek geocentric model dominating astronomical thought beginning around the 3rd century BCE, and being the favored model for ~1,500 years. Then, suddenly (it suggests), astronomical thought was overhauled at the birth of the Renaissance by brilliant astronomers such as Copernicus, Kepler, and Galileo, all of whom rejected placing the Earth at the center of the cosmos.

But these sources are generally quiet on why this shift occurred. If mentioned at all, sources generally suggest that it was because the Ptolemaic geocentric model was too complicated – overly burdened with epicycle and equants. Heliocentrism, in comparison, was simple – elegant, even.

Yet, Copernicus’ heliocentric model was still rooted in the Greek philosophical principles of uniform circular motion. Thus, it too was forced to adopt many of the complications we’re regularly told were the reason for rejecting Ptolemy’s model – epicycles included.

So, why then, did Copernicus actually turn his back on over 1,500 years of astronomical thought?

The answers are an interesting glimpse into the astronomical paradigm of the 16th century.

To find out Copernicus’ thoughts, we can examine the first book of his masterwork, De Revolutionibus.

The Force Needed to Sustain Geocentrism

The first reason he gives applies to the forces involved:

Surely if [Ptolemy’s reasoning for the geocentric model] were tenable, the magnitude of the heavens would extend infinitely. For the farther the movement is borne upward by the vehement force, the faster will the movement be, on account of the ever-increasing circumference which must be traversed every twenty-four hours.

– Copernicus, De Revolutionibus, Book I, Chapter 8

Copernicus’ writing of De Revolutionibus predated Newton’s Principa by over 140 years. The notion that “an object in motion tends to stay in motion” was, therefore, not yet one in the scientific consciousness.

Instead, natural philosophers believed that the natural tendency of objects was that of rest and the only way an object could be kept in motion was through an application of force.

In the Ptolemaic geocentric model, the Earth did not rotate on an axis. Instead, the stars were all affixed to the surface of a sphere at an immense distance which rotated about the Earth every day along with the rest of the cosmos. Copernicus criticizes the absurd amount of force he supposed would be necessary since, “things to which force or violence is applied get broken up and are unable to subsist for a long time.”

In other words, Copernicus believed that the force that should keep Ptolemy’s geocentric model going would necessarily destroy it.

The heliocentric model avoids this by making the motion of the stars and planets around the sky every night not actual motion, but apparent motion caused by the rotation of the Earth about its poles. This would require a far smaller force since the Earth is smaller than the stellar sphere. Indeed, this completely removes the need for the motion of the stellar sphere, and now the planets and Sun can move far more slowly, and thus would have a much reduced force on them.

To be fair, various astronomers had considered the possibility that the cosmos was geocentric, but did allow for the Earth to rotate on its axis. However, the Ptolemaic cosmos with its static Earth was still the predominant model of the day, which is why Copernicus attacks it with little mention of other authors.

But, if you’re willing to accept that the Earth rotates on its axis, why wouldn’t you accept that it has other motions too?

Early Musings on Gravity

I myself think that gravity or heaviness is nothing except a certain natural appetency implanted in the parts by the divine providence of the universal Artisan, in order that they should unite with one another in their oneness and wholeness and come together in the form of a globe. It is believable that this affect is present in the sun, moon, and the other bright planets and that through its efficacy they remain in the spherical figure in which they are visible, though they nevertheless accomplish their circular movements in many different ways.

-Copernicus, De Revolutionibus, Book I, Chapter 9

To understand this, we should briefly examine Ptolemy’s thinking on gravity. In the Almagest, Ptolemy opines that there is some point in the universe towards which all things fall unless they are supported. Thus, the Earth, being unsupported by a celestial sphere, must fall towards this point and thus, is the center of the cosmos; ergo, geocentrism.

Copernicus suggests that, perhaps gravity is just an innate force, and it would have the property to make things round. And since the Sun and moon are obviously round, perhaps they too have gravity. This removes the need for the central point to the cosmos that Ptolemy relies on, undercutting Ptolemy’s argument.

Elongation of Inferior vs Superior planets

How unconvincing is Ptolemy’s argument that the sun must occupy the middle position between those planets which have the full range of angular elongation from the sun [i.e., Mercury and Venus] and those which do not [i.e., Mars, Jupiter, and Saturn] is clear from the fact that the moon’s full range of angular elongation proves its falsity.

– Copernicus, De Revolutionibus, Book I, Chapter 10

Here, Copernicus is taking aim at the argument that the Sun must be between Venus and Mars due to a division in the angular elongation (the distance from the Sun) inferior and superior planets are able to have. Specifically, Mercury and Venus are never more than 24º and 45º away from the Sun respectively. Meanwhile, Mars, Jupiter, and Saturn can be any angular distance from the Sun (although they are always found along the ecliptic).

Ptolemy explains this by matching the mean (or average) speeds of Mercury and Venus to that of the Sun. Therefore, their getting ahead of and falling behind the Sun’s motion is due only to their epicycles. The other three planets had mean speeds unrelated to the Sun, allowing their centers of motion to drift anywhere along the ecliptic relative to the Sun.

The Ptolemaic order of the planets was largely correct; Ptolemy had ordered them according to speed. Ignoring the Sun and moon momentarily, this meant the planets, in increasing distance from the Earth, were ordered Mercury, Venus, Mars, Jupiter, and Saturn.

The Sun was inserted between Venus and Mars, again based on its speed. But, this conveniently meant that the Sun’s sphere provided a division between planets which were fixed to the Sun (Mercury and Venus), and those that could obtain any elongation (Mars, Jupiter, and Saturn). And astronomers of the day used this division as evidence that that positioning of the Sun among the planets must be correct.

But the moon, Copernicus tells us, upends this argument, because the moon is the innermost sphere and it is able to have any elongation, just like the outer planets.

Keep in mind, the nature of the moon, Sun, and planets was still quite uncertain at this time. Quite frequently, the term “planet” can include all of them. Hence why Copernicus considered their nature all together in this point.

Apogee & Perigee are Aligned with the Sun

For, it is manifest that the planets are always nearer the Earth at the time of their evening rising, i.e., when they are opposite to the sun and the Earth is in the middle between them and the sun. But, they are farthest away from the Earth at the time of their evening setting, i.e., when they are occulted in the neighbourhood of the sun, namely when we have the sun between them and the Earth. All that shows clearly enough that their center is more directly related to the sun and is the same as that to which Venus and Mercury refer their revolutions.

– Copernicus, De Revolutionibus, Book I, Chapter 10

Copernicus’ next argument has to do with the position of the planets when at their farthest points to Earth versus their closest points. These are known as apogee and perigee, respectively.

What Copernicus is indicating is that planets always seem to have their apogee when they are nearest to the Sun. This is a natural consequence of a heliocentric model (because the planet is on the opposite side of the Sun), but the geocentric model has no special cause for this.

This is easiest to understand if we think about a superior planet, like Mars, in the context of the heliocentric model. If we think of the closest Mars can be to Earth (perigee), it occurs when the Sun, Earth, and Mars are all in a straight line, in that order. When that occurs, Mars would be rising in the evening, being highest in the sky around midnight.

Conversely, the furthest Mars could be from us, is when it is on the opposite side of the Sun. It’s still on a straight line, but this time the order would be Mars, Sun, then Earth. When this occurs, Mars is setting in the evening (although we couldn’t see it because it would be too close to the Sun to be visible).

What Copernicus is pointing out is that this is true for every planet – they’re all tied to the Sun in this manner. Thus, he tells us, the Sun clearly has some special privilege.

Venus’ Massive Epicycle

Moreover, there is the fact that the diameter of the epicycle of Venus – by reason of which Venus has an angular distance of approximately 45º on either side of the sun – would have to be six times greater than the distance from the center of the Earth to its perigee, as will be shown in the proper place. Then what will they say is contained in all this space, which is so great as to take in the Earth, air, ether, moon, and Mercury, and which moreover the vast epicycle of Venus would occupy if it revolved around the immovable Earth?

– Copernicus, De Revolutionibus, Book I, Chapter 10

Epicycles are often cited as one of the biggest problems with the Ptolemaic geocentric model. And that is precisely what Copernicus is taking aim at here. That’s not to say that Copernicus was fundamentally against epicycles. Indeed, his own adherence to uniform circular motion forced him to include epicycles in his model. But what Copernicus is criticizing here is the size demanded by the Ptolemaic model for Venus in particular.

As discussed above, the mean motion of Venus is tied to that of the Sun. So it can only deviate from that position based on its epicycle. Thus, to get 45º away from the Sun, it was going to need a massive epicycle. One so large, it would take Venus crashing through the spheres of both Mercury and the Moon. The latter was particularly problematic because of a belief about the nature of matter.

The natural philosophy of the time was still alchemical, with four terrestrial elements (earth, fire, air, and water) and one celestial element (æther, or quintessence). It was held that the celestial element was eternal and unchanging. “Incorruptible,” as they would phrase it, which is why the heavens were so pure and consistent. It was only on Earth that we had the other four classical elements, which were “mutable” or “corruptible”. But where does that division between the incorruptible and corruptible take place? Greek astronomers placed it at the sphere of the moon which was the closest to Earth in the geocentric model.

Conclusion

However, because Venus’ epicycle would be so big, it would cross into this realm. Thus, there becomes a logical contradiction as you’d have the celestial matter diving in and out of the terrestrial realm which was not something that was considered acceptable.

Ultimately, these arguments were only partially convincing to astronomers of the time. We know that Copernicus’ work was widely read. However, it was not quickly adopted.

Even after Kepler revised it, sweeping away the Ptolemaic equants and epicycles and replacing them with ellipses, geocentrism still took quite a bit of time to be fully dislodged. Newton’s theory of gravity gave a compelling theoretical reason to give centrality to the larger object, but it was the discovery of the aberration of starlight and the parallaxes of stars that finally disproved the geocentric model.

The post Why Did Copernicus Reject Geocentrism? appeared first on Universe Today.

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