What is an Exoplanet? - NASA - UNIVERSE

So far scientists have categorized exoplanets into the following types: Gas giant, Neptunian, super-Earth and terrestrial.

The planets beyond our solar system are called “exoplanets,” and they come in a wide variety of sizes, from gas giants larger than Jupiter to small, rocky planets about as big around as Earth or Mars. They can be hot enough to boil metal or locked in deep freeze. They can orbit their stars so tightly that a “year” lasts only a few days; they can orbit two suns at once. Some exoplanets are sunless rogues, wandering through the galaxy in permanent darkness.

When we describe different types of exoplanets – planets outside our solar system – what do we mean by "hot Jupiters," "warm Neptunes," and "super-Earths"? Since we're still surveying and learning about the variety of worlds out there among the stars, it's sometimes helpful to refer to characteristics they share with planets we're familiar with in our own planetary system.
NASA/JPL-Caltech

A galaxy of stars – and planets

Our galaxy, the Milky Way, is the thick stream of stars that cuts across the sky on the darkest, clearest nights. Its spiraling expanse contains at least 100 billion stars, our Sun among them. And if each of those stars has not just one planet, but, like ours, a whole system of them, then the number of planets in the galaxy is truly astronomical: We’re already heading into the trillions.

We humans have been speculating about such possibilities for thousands of years, but ours is the first generation to know, with certainty, that exoplanets are really out there. In fact, way out there. Our nearest neighboring star, Proxima Centauri, was found to possess at least one planet – probably a rocky one. It’s about 4 light-years away – more than 25 trillion miles (40 trillion kilometers). The bulk of exoplanets found so far are hundreds or thousands of light-years away.

The bad news: As yet we have no way to reach them, and won’t be leaving footprints on them anytime soon. The good news: We can look in on them, take their temperatures, taste their atmospheres and, perhaps one day soon, detect signs of life that might be hidden in pixels of light captured from these dim, distant worlds.

Exoplanet discovery – and mystery

The first exoplanets were discovered in the early 1990s, but the first exoplanet to burst upon the world stage was 51 Pegasi b, a “hot Jupiter” orbiting a Sun-like star 50 light-years away. The watershed year was 1995. Since then we’ve discovered thousands more.

Size and mass play a crucial role in determining planet types. There are also varieties within the size/mass classifications. Scientists also have noted what seems to be a strange gap in planet sizes. It’s been dubbed the “radius valley,” or the Fulton gap, after Benjamin Fulton, lead author on a paper describing it. Data from NASA’s Kepler spacecraft showed that planets of a certain size-range are rare – those between 1.5 and 2 times the size (diameter) of Earth, which would place them among the super-Earths. It’s possible that this represents a critical size in planet formation: Planets that reach this size quickly attract thick atmospheres of hydrogen and helium gas, and balloon up into gaseous planets, while planets smaller than this limit are not large enough to hold such an atmosphere and remain primarily rocky, terrestrial bodies. On the other hand, the smaller planets that orbit close to their stars could be the cores of Neptune-like worlds that had their atmospheres stripped away.

Explaining the Fulton gap will require a far better understanding of how planetary systems form.

Variety is a major theme in exoplanet discoveries over the past quarter century, as shown in this illustration. Most have been discovered by the "transit" method – watching for the tiniest of shadows as a planet crosses the face of its star.

NASA/JPL-Caltech

Types of exoplanets

Each planet type varies in interior and exterior appearance depending on composition.

Gas giants are planets the size of Saturn or Jupiter, the largest planet in our solar system, or much, much larger.

More variety is hidden within these broad categories. Hot Jupiters, for instance, were among the first planet types found – gas giants orbiting so closely to their stars that their temperatures soar into the thousands of degrees (Fahrenheit or Celsius).

Neptunian planets are similar in size to Neptune or Uranus in our solar system. They likely have a mixture of interior compositions, but all will have hydrogen and helium-dominated outer atmospheres and rocky cores. We’re also discovering mini-Neptunes, planets smaller than Neptune and bigger than Earth. No planets of this size or type exist in our solar system.

Super-Earths are typically terrestrial planets that may or may not have atmospheres. They are more massive than Earth, but lighter than Neptune.

Terrestrial planets are Earth sized and smaller, composed of rock, silicate, water or carbon. Further investigation will determine whether some of them possess atmospheres, oceans or other signs of habitability. 

Source: Overview - NASA Science

Study reveals the twists and turns of mammal evolution from a sprawling to upright posture

Land animals exhibit a continuum of limb postures – ranging from 'sprawled', with the limbs held out to the side of the body, like lizards, to 'upright' or 'erect', with the limbs held beneath the body and close to the animal's midline, like dogs, cats and horses. Upright posture is characteristic of most modern mammals, but when did this key trait evolve? Credit: Peter Bishop

Mammals, including humans, stand out with their distinctively upright posture, a key trait that fueled their spectacular evolutionary success. Yet, the earliest known ancestors of modern mammals more resembled reptiles, with limbs stuck out to their sides in a sprawled posture.

The shift from a sprawled stance, like that of lizards, to the upright posture of modern mammals, as in humans, dogs, and horses, marked a pivotal moment in evolution.

It involved a major reorganization of limb anatomy and function in synapsids—the group that includes both mammals and their non-mammalian ancestors—eventually leading to the therian mammals (marsupials and placentals) we know today. Despite over a century of study, the exact "how," "why," and "when" behind this evolutionary leap has remained elusive.

Now, in a study published in Science Advances, Harvard researchers provide new insights into this mystery, revealing the shift from a sprawled to upright posture in mammals was anything but straightforward.

Using cutting-edge methods that blend fossil data with advanced biomechanical modeling, the researchers found that this transition was surprisingly complex and nonlinear, and occurred much later than previously believed.

Lead author Dr. Peter Bishop, a postdoctoral fellow, and senior author Professor Stephanie Pierce, both in the Department of Organismic and Evolutionary Biology at Harvard, began by examining the biomechanics of five modern species that represent the full spectrum of limb postures, including a tegu lizard (sprawled), an alligator (semi-upright), and a greyhound (upright). 

Fossil of the early sail-backed synapsid Dimetrodon, from 290 million years ago, one of the species investigated in the study. Credit: Christina Byrd. Museum of Comparative Zoology, President and Fellows of Harvard College.

"By first studying these modern species, we greatly improved our understanding of how an animal's anatomy relates to the way it stands and moves," said Bishop. "We could then put it into an evolutionary context of how posture and gait actually changed from early synapsids through to modern mammals."

The researchers extended their analysis to eight exemplary fossil species from four continents spanning 300 million years of evolution. The species ranged from the 35g proto-mammal Megazostrodon to the 88kg Ophiacodon, and included iconic animals like the sail-backed Dimetrodon and the saber-toothed predator Lycaenops.

Using principles from physics and engineering, Bishop and Pierce built digital biomechanical models of how the muscles and bones attached to each other. These models allowed them to generate simulations that determined how much force the hindlimbs (back legs) could apply on the ground.

"The amount of force that a limb can apply to the ground is a critical determinant of locomotor performance in animals," said Bishop. "If you cannot produce sufficient force in a given direction when it's needed, you won't be able to run as fast, turn as quickly, or worse still, you could well fall over."

The study involved digitizing the fossil skeletons of extinct synapsids, creating digital biomechanical models of the musculoskeletal system of the hindlimb, and using these models to compute the limb's ability to apply force on the ground in different directions. The result is a three-dimensional 'feasible force space', which describes what the limb is capable of achieving during locomotion. Credit: Peter Bishop

The computer simulations produced a three-dimensional "feasible force space" that captures a limb's overall functional performance. "Computing feasible force spaces implicitly accounts for all the interactions that can occur between muscles, joints and bones throughout a limb," said Pierce.

"This gives us a clearer view of the bigger picture, a more holistic view of limb function and locomotion and how it evolved over hundreds of millions of years."

While the concept of a feasible force space (developed by biomedical engineers) has been around since the 1990s, this study is the first to apply it to the fossil record to understand how extinct animals once moved.

The authors packaged the simulations into new "fossil-friendly" computational tools that can aid other paleontologists in exploring their own questions. These tools could also help engineers design better bio-inspired robots that can navigate complex or unstable terrain.

The study revealed several important "signals" of locomotion, including that the overall force-generating ability in the modern species was maximal around the postures that each species used in their daily behavior. Importantly, this meant that Bishop and Pierce could be confident that the results obtained for the extinct species genuinely reflected how they stood and moved when alive.

Fossil of the mammal-like cynodont Massetognathus, from 242 million years ago, one of the species investigated in the study. Credit: Peter Bishop. Museum of Comparative Zoology, President and Fellows of Harvard College.

After analyzing the extinct species, the researchers discovered that locomotor performance peaked and dipped over millions of years, rather than progressing in a simple, linear fashion from sprawling to upright.

Some extinct species also appeared to be more flexible—able to shift back and forth between more sprawled or more upright postures, like modern alligators and crocodiles do. While others showed a strong reversal towards more sprawled postures before mammals evolved.

Paired with the study's other results, this indicated that the traits associated with upright posture in today's mammals evolved much later than previously thought, most likely close to the common ancestor of therians.

These findings also help reconcile several unresolved problems in the fossil record. For example, it explains the persistence of asymmetric hands, feet, and limb joints in many mammal ancestors, traits typically associated with sprawling postures among modern animals.

It can also help explain why fossils of early mammal ancestors are frequently found in a squashed, spread-eagle pose—a pose more likely to be achieved with sprawled limbs, while modern placental and marsupial fossils are typically found lying on their sides.

"It is very gratifying as a scientist, when one set of results can help illuminate other observations, moving us closer to a more comprehensive understanding," Bishop said.

Evolutionary interrelationships of the modern (black silhouettes) and extinct (gray silhouettes) species investigated. The study revealed a complex history of posture evolution in synapsids, and that a fully 'upright' posture typical of modern placentals and marsupials was late to evolve. Credit: Peter Bishop

Pierce, whose lab has studied the evolution of the mammalian body plan for nearly a decade, notes that these findings are consistent with patterns seen in other parts of the synapsid body, like the vertebral column.

"The picture is emerging that the full complement of quintessentially therian traits was assembled over a complex and prolonged period, with the full suite attained relatively late in synapsid history," she said.

Beyond mammals, the study suggests that some major evolutionary transitions, like the shift to an upright posture, were often complex and potentially influenced by chance events. For instance, the strong reversal in synapsid posture, back toward more sprawled poses, appears to coincide with the Permian-Triassic mass extinction—when 90% of life was wiped out.

This extinction event led to other groups like the dinosaurs becoming the dominant animal groups on land, pushing synapsids back into the shadows. The researchers speculate that due to this "ecological marginalization," the evolutionary trajectory of synapsids may have changed so much that it altered the way they moved.

Whether this hypothesis turns out to be supported or not, understanding the evolution of mammal posture has long been a complex puzzle. Pierce emphasized how advances in computing power and digital modeling have provided scientists new perspectives to address these ancient mysteries.

"Using these new techniques with ancient fossils allows us to have a better perspective of how these animals evolved, and that it wasn't just this simple, linear evolutionary story," she said. "It was really complicated and these animals were probably living and moving in their environments in ways that we hadn't appreciated before. There was a lot happening and mammals today are really quite special." 

by Harvard University

Source: Study reveals the twists and turns of mammal evolution from a sprawling to upright posture

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NASA Helps Find Thawing Permafrost Adds to Near-Term Global Warming - EARTH

The Permafrost Tunnel north of Fairbanks, Alaska, was dug in the 1960s and is run by the U.S. Army’s Cold Regions Research and Engineering Laboratory. It is the site of much research into permafrost — ground that stays frozen throughout the year, for multiple years.

NASA/Kate Ramsayer

Earth’s far northern reaches have locked carbon underground for millennia. New research paints a picture of a landscape in change.

A new study, co-authored by NASA scientists, details where and how greenhouse gases are escaping from the Earth’s vast northern permafrost region as the Arctic warms. The frozen soils encircling the Arctic from Alaska to Canada to Siberia store twice as much carbon as currently resides in the atmosphere — hundreds of billions of tons — and most of it has been buried for centuries.

An international team, led by researchers at Stockholm University, found that from 2000 to 2020, carbon dioxide uptake by the land was largely offset by emissions from it. Overall, they concluded that the region has been a net contributor to global warming in recent decades in large part because of another greenhouse gas, methane, that is shorter-lived but traps significantly more heat per molecule than carbon dioxide. 

Greenhouse gases shroud the globe in this animation showing data from 2021. Carbon dioxide is shown in orange; methane is shown in purple. Methane traps heat 28 times more effectively than carbon dioxide over a 100-year timescale. Wetlands are a significant source of such emissions.

NASA’s Scientific Visualization Studio

The findings reveal a landscape in flux, said Abhishek Chatterjee, a co-author and scientist at NASA’s Jet Propulsion Laboratory in Southern California. “We know that the permafrost region has captured and stored carbon for tens of thousands of years,” he said. “But what we are finding now is that climate-driven changes are tipping the balance toward permafrost being a net source of greenhouse gas emissions.”

Carbon Stockpile

Permafrost is ground that has been permanently frozen for anywhere from two years to hundreds of thousands of years. A core of it reveals thick layers of icy soils enriched with dead plant and animal matter that can be dated using radiocarbon and other techniques. When permafrost thaws and decomposes, microbes feed on this organic carbon, releasing some of it as greenhouse gases.

Unlocking a fraction of the carbon stored in permafrost could further fuel climate change. Temperatures in the Arctic are already warming two to four times faster than the global average, and scientists are learning how thawing permafrost is shifting the region from being a net sink for greenhouse gases to becoming a net source of warming.

They’ve tracked emissions using ground-based instruments, aircraft, and satellites. One such campaign, NASA’s Arctic-Boreal Vulnerability Experiment (ABoVE), is focused on Alaska and western Canada. Yet locating and measuring emissions across the far northern fringes of Earth remains challenging. One obstacle is the vast scale and diversity of the environment, composed of evergreen forests, sprawling tundra, and waterways.

This map, based on data provided by the National Snow and Ice Data Center, shows the extent of Arctic permafrost. The amount of permafrost underlying the surface ranges from continuous — in the coldest areas — to more isolated and sporadic patches.

NASA Earth Observatory

Cracks in the Sink

The new study was undertaken as part of the Global Carbon Project’s RECCAP-2 effort, which brings together different science teams, tools, and datasets to assess regional carbon balances every few years. The authors followed the trail of three greenhouse gases — carbon dioxide, methane, and nitrous oxide — across 7 million square miles (18 million square kilometers) of permafrost terrain from 2000 to 2020.

Researchers found the region, especially the forests, took up a fraction more carbon dioxide than it released. This uptake was largely offset by carbon dioxide emitted from lakes and rivers, as well as from fires that burned both forest and tundra.

They also found that the region’s lakes and wetlands were strong sources of methane during those two decades. Their waterlogged soils are low in oxygen while containing large volumes of dead vegetation and animal matter — ripe conditions for hungry microbes. Compared to carbon dioxide, methane can drive significant climate warming in short timescales before breaking down relatively quickly. Methane’s lifespan in the atmosphere is about 10 years, whereas carbon dioxide can last hundreds of years.

The findings suggest the net change in greenhouse gases helped warm the planet over the 20-year period. But over a 100-year period, emissions and absorptions would mostly cancel each other out. In other words, the region teeters from carbon source to weak sink. The authors noted that events such as extreme wildfires and heat waves are major sources of uncertainty when projecting into the future.

Bottom Up, Top Down

The scientists used two main strategies to tally greenhouse gas emissions from the region. “Bottom-up” methods estimate emissions from ground- and air-based measurements and ecosystem models. Top-down methods use atmospheric measurements taken directly from satellite sensors, including those on NASA’s Orbiting Carbon Observatory-2 (OCO-2) and JAXA’s (Japan Aerospace Exploration Agency)Greenhouse Gases Observing Satellite.

Regarding near-term, 20-year, global warming potential, both scientific approaches aligned on the big picture but differed in magnitude: The bottom-up calculations indicated significantly more warming.

“This study is one of the first where we are able to integrate different methods and datasets to put together this very comprehensive greenhouse gas budget into one report,” Chatterjee said. “It reveals a very complex picture.” 

By: Jet Propulsion Laboratory

Source: NASA Helps Find Thawing Permafrost Adds to Near-Term Global Warming - NASA

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Circumhorizontal arc - Fire Rainbow (Massimo - A circumhorizontal arc )

 

A circumhorizontal arc is an optical phenomenon that belongs to the family of ice halos formed by the refraction of sunlight or moonlight in plate-shaped ice crystals suspended in the atmosphere, typically in actual cirrus or cirrostratus clouds. In its full form, the arc has the appearance of a large, brightly spectrum-coloured band (red being the topmost colour) running parallel to the horizon, located far below the Sun or Moon. The distance between the arc and the Sun or Moon is twice as far as the common 22-degree halo. Often, when the halo-forming cloud is small or patchy, only fragments of the arc are seen. As with all halos, it can be caused by the Sun as well as (but much more rarely) the Moon.^[1]

Other currently accepted names for the circumhorizontal arc are circumhorizon arc or lower symmetric 46° plate arc.^[2] The misleading term "fire rainbow" is sometimes used to describe this phenomenon, although it is neither a rainbow, nor related in any way to fire. The term, apparently coined in 2006,^[3] may originate in the occasional appearance of the arc as "flames" in the sky, when it occurs in fragmentary cirrus clouds.^[4]

Source: For more information: Circumhorizontal arc - Wikipedia  

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