Thalia Levee sat in a crimson armchair looking down at her round-faced grandchildren. She pressed her lips together, considering the request that had just left her grandsons mouth. Please Grandmother! The small boy begged from his spot on the hardwood floor. Thalia sighed. Just one last story. Then we will go to bed, I promise! The boy exclaimed. His younger sister nodded eagerly from beside him. Fine. One last story. Thats it. Thalia gave in. She knew in the shining eyes of her grandchildren she was just an old woman, a grumpy one at that. But when Thalia looked at herself in the...

Heads up, weather geeks. U.S. weather has just hit a new normal. The government has changed its reference values for temperature and precipitation. And these show that the last three decades have been the warmest on record.

People in the American West and the Pacific Northwest may not be surprised. Many cities there hit repeated record temperatures in June. They occurred during two back-to-back heat waves.

In mid-June, cities from Omaha, Neb., to Sacramento, Calif., set records of at least 105 Fahrenheit (40.6 Celsius). Phoenix, Ariz., and Death Valley, Calif., hit monster extremes on June 17 of 118 F and 128 F (47.8 and 53.3 C, respectively).

Then, in late June, another heat wave hit the Pacific Northwest. Seattle set a record high temperature of 105 F (40.6 C). Portland, Ore., reached a record 116 F (46.7 C). Even in Lytton, a village in Canadas British Columbia, temperatures soared to 121 F (49.6 C). That set a new record for the entire country. cited the director of the Arizona Burn Center in Phoenix to put Junes extreme heat in context: “If you look at hot pavement or asphalt at two o’clock in the afternoon in direct sunlight, the temperature is usually somewhere around 170 to 180 degrees Fahrenheit.” (Thats 76.7 to 82.2 C.)

Hot spots

NOAAs new climate normal shows that average temperatures across mainland United States increased nearly everywhere compared to the preceding three decades. 

U.S. mean temperature change: 19912020 compared with 19812010


The new normals

The National Oceanic and Atmospheric Administration, or NOAA, reports climate normals. These offer a standard way to compare todays weather against 30-year averages. But figuring out a new normal isnt simple. The agency compiles 30 years of observations from about 8,700 U.S. weather stations. Later, it ensures the quality of those data. Only data that past that test are used to calculate multiple measures of climate.

Over the past 30 years, the average temperature across the U.S. mainland was 11.8 C (53.3 F). The previous periods average was 11.6 C. But bump in temps varied across the United States. That likely was due in part to geography. Seasonal waverings also played a role. Some of the largest increases were in the South and Southwest. Those same regions showed a dramatic drop in rainfall.

The World Meteorological Organization requires the United States and its other member nations to update their climate normals once each decade. These allow people to view data on daily weather events against what has happened in recent history. Farmers use these data to track droughts or risk of freezes.

Keeping track of shifting averages also helps us understand the skyrocketing pace of climate change. NOAA compared the current and previous 30-year normals to the average highs in daily heat between 1901 and 2000. No part of the country is cooler now than that 20th century average. And temps in large swaths are higher by 1 to 2 degrees Fahrenheit (0.6 to 1.1 degrees Celsius).

Rising temps

The average temperatures for consecutive 30-year periods in the U.S. mainland show the country getting hotter since 1901. Here, each 30-year period is compared with the average temperature for the entire 20th century.

U.S. 30-year temperature averages compared with 20th-century average

NOAA, Jared Rennie/NCEI/North Carolina Institute for Climate Studies

NOAA, Jared Rennie/NCEI/North Carolina Institute for Climate Studies

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Every big planet begins with a pebble.

Okay, not just one. It starts with lots of pebbles a flat sea of them stretching perhaps hundreds of times wider than the distance from Earth to the sun. Their sizes vary greatly. Some may be mere dust particles. Others may be small to fairly substantial rocks.

Explainer: What is a planet?

These pebbles tumble violently within the gassy disk encircling a young star. Lurking within that disk are the ingredients not only for planets, but also for asteroids, comets and living things. What they become depends not only on those ingredients, but also on their location and the temperature of the gas. 

Like fussy chefs in a kitchen, astronomers today debate over how much of which ingredients must have been present in that early solar system. And when. And how they might have interacted and combined. And what would happen if you changed their temperature.

We all know how the planet-making process ends. It produces rocky worlds like Earth, Mars and Venus. It also leads to gas giants like Saturn and ice giants like Uranus. Outside the solar system, the planetary zoo includes stranger worlds. Scientists have spotted a world that they first thought was made of diamond but now believe has oceans flowing with lava. Theyve observed a hot gas giant where drops of iron probably fall like rain and a small hot planet enshrouded in steam.

An artists interpretation of a binary star system, with a surrounding ring (in brown) that might give rise to a rocky, Mars-sized planet. JPL-Caltech/NASA

But how disks of gas and rocks become planet factories is still under debate. The beginnings where dust grains barely micrometers (a few ten-thousandths of an inch) across stick together to form rocky solids isnt too controversial. And thanks to powerful telescopes, researchers have ideas for how planets move once theyre fully-formed.

But in-between is a doozy. For centuries, scientists have been testing and fighting over ideas about how to connect the beginning to the end. Most of the seemingly best ideas have run into problems.

Over the last 10 years or so, however, a process called pebble accretion (Ah-KREE-shun) has gained popularity. Accretion refers to somethings gradual growth. This occurs as new bits of material join something or glom onto it. In this case, its a swirling disk of gas and pebbles that clump together to form a family of planets.

According to the theory, tiny rocks in the disk slow and heat up as they fly through the gas near a larger rock. Its a phenomenon similar to how water in a pond slows a sinking rock. These flying pebbles eventually spiral down to land on the surface of larger rocks nearby. Pebble by pebble, a giant planet is born. And compared to the age of the universe, its a fast process, only taking a few million years.

Pebble accretion really did revolutionize the way that people thought about planet formation, says Katherine Kretke. Shes an astrophysicist at the Southwest Research Institute in Boulder, Colo.

This theory would solve many of the riddles that challenged previous ideas. For example, says Seth Jacobson, It is really the only mechanism that comes close to explaining how Uranus and Neptune formed. Jacobson is an astrophysicist at Michigan State University in East Lansing.

Explainer: Stars and their families

Planet accretion also has gotten a boost from recent studies of distant stars. Observations by the largest radio telescope network in the world, perched on a lonely desert mountain in Chile, match up with some of this theorys unusual predictions.

Anders Johansen, an astronomer at Lund Observatory, in Sweden, knows a lot about pebble accretion. He has been one of the leading researchers arguing in its favor. Puzzling out how it might work consumes his days.

He compares studying the origins of planets to working through a detective story. The solar system provides clues in the planets we know, he says. Exoplanets beyond the solar system provide more clues. Scientists have to connect those clues to piece together the whole story.

It is just so much fun to work on this, he says.

The planet-making process has produced an incredible variety of worlds, such as these seven exoplanets that orbit the star TRAPPIST-1. JPL-Caltech/NASA

In the beginning

Ancient Greek philosophers believed that planets formed from the chaos that filled the universe. In the 17th century, French scientist and philosopher Ren Descartes suggested that every star sat at the center of a swirling vortex. Planets, made of darker stuff, rested in concentric bands that circled the star.

In the 18th century, a Swedish mystic named Emanuel Swedenborg proposed a different idea. He described planet formation in a way thats closer to modern ideas. The whole thing begins, he said, when the crusty shell around a star explodes. It crumbles. The debris settles into a giant ring encircling that star at its center. Material in that ring eventually clumps into what will become planets. The idea that the planets formed from a swirling cloud of star stuff is called the nebular hypothesis. 

That remains the backbone of ideas today. It also has led to the creation of a long list of new words. Dust doesnt refer to the stuff in your house that mix of dead skin cells, bits of cobwebs, dirt and more. Its the tiny particles too small to see with the eye. When scientists say pebble, they mean small rocks from about the size of a dime up to the size of a sled. They also talk about planetesimals (Plaa-neh-TES-ih muls). These are space rocks that might be as big as a city. Then there are protoplanets, a planet thats almost done forming. 

As scientists have hashed out the details, theyve also run into challenges. One idea popular in the 20th century, for example, proposed that planets formed from the collisions between ever-bigger rocks.

This is an artists depiction of a pebble-ridden disk surrounding the star Fomalhaut, 22 light-years from Earth. Data suggest this disk has begun separating from its sun as it dust (pebbles) have fallen onto inner planets. Larger giant plants imagined near the edge of the disk. David A. Hardy/

That explanation, too, starts with a disk of gas and dust, notes Alessandro Morbidelli. Then the dust sticks together to form planetesimals the size of asteroids and comets. Morbidelli is a planetary scientist at the Cote dAzur Observatory in Nice, France. Just forget about the dust, he says. Then, you produce protoplanets by colliding those planetesimals with each other.

This process may sound reasonable. However, Morbidelli also believes thats almost impossible. For one thing, planets that form far from the sun grow slowly. Worlds like Jupiter and Saturn would need tens of millions of years to get so big through smash-ups. But the dust and gas disk from which they were to form only stuck around a few million years. It seems difficult by this process to grow the cores of these planets within the lifetime of the disk, he argues.

Another problem: That model requires planetesimals to collide by crossing orbits. You have all these collisions, all this debris, says Jacobson. The current planets dont look anything like that. We have very nice orbits, almost circular. They dont look like they came from a violent, messy process.

Finally, collisions between big objects dont always produce bigger objects. Its easy to check this one: Just try smashing one stone into another.

If you take two fist-sized rocks, theres no velocity at which you can bring them together and they will stick, says Jacobson. So collisions alone, he argues, cant explain how planetesimals form.

As seen in this artists illustration of the solar system (not to scale), the planets in our solar system travel roughly circular paths around the sun. But scientists know that no orbit is perfectly circular, and some veer closer and farther from the sun during their journey. MARK GARLICK/SCIENCE PHOTO LIBRARY

Paying attention to pebbles

In 2010, two astrophysicists at the Max Planck Institute for Astronomy in Heidelberg, Germany, found a workaround. The answer, they suggested, was in the disks gas. As it moves through a fluid, a solid object slows and heats up. This is due to a force called drag. Because of drag, you need more effort to walk through water than to walk through air. The drag from gas in a disk would likely slow down the pebbles.

This idea became known as pebble accretion with a study published two years later by Johansen and Michiel Lambrechts, another astronomer at Lund Observatory. They used computer models to test their ideas. In parts of the disk having the right temperatures, those pebbles can slow enough, they found, to spiral down to the surface of a planetesimal. From there theyd stay put.

It is a really efficient process, says Kretke. If you have a ton of these pebbles around at the right size, she says, then boom! You can form a planet.

Two big objects spinning through space have only a small chance of colliding. But for one big object streaming through an ocean of pebbles and dust, a collision is likely. With pebble accretion, a planet can grow as big as Neptune or Uranus during the short lifespan of the gas disk. It doesnt need giant outer-space smashups of asteroids and comets to form. And once a planet like Jupiter has a big core, its gravity can attract the lighter elements to jacket it in a thick atmosphere of hydrogen and helium.

The project Disk Detective is recruiting citizen scientists to examine NASA data on their computers and phones. Theyll be hunting dusty debris disks that may mark the birthplaces of new planets.

Pebble accretion does a good job of explaining how big planets form, says Morbidelli. It also suggests that the gas and dust in a disk determine what type of planet forms near a star or if a planet forms at all.

Or maybe it doesnt have to be a smooth disk. With pebble accretion, planets may form from misshapen rings of dust and gas that swirl around a star. In 2018, scientists studied some gas and dust orbiting stars. They used the largest radio telescope network in the world, called ALMA. (The name stands for Atacama Large Millimeter Array.) This group of giant, silvery dishes peer into space from atop a desert mountain in Chiles Atacama Desert.

What they found was shocking. Some stars have rings of dust and gas not disks. Others have large disks, or small disks. And not all rings were smooth. Some had regions where the dust and gas got stuck and clumped. Other regions seemed sparse.

We saw such diversity at the disk level, says Morbidelli.

The challenge now, he says, is to use pebble accretion to connect those ring structures to planets that may emerge.

Problems in pebbleland

Most astronomers agree that pebble accretion is a good explanation for how big planets form. Nobody questions that, says Morbidelli. But the complete explanation of how planets form is likely much more complex. Getting pebble accretion to produce planets, even in models, requires other parameters to be just right, he cautions.

For instance, pebble accretion requires a bigger rock onto which the smaller pebbles land. So we still need collisions of planetesimals to create moon-sized objects before pebble accretion takes over, he suspects. Disks must also exist filled with plenty of matter to supply all those pebbles. ALMAs observations suggest such giant disks can exist. But scientists are still collecting evidence to prove that.

Johansen and others also caution that pebble accretion likely falls short of explaining the whole story. A study published this past February showed how planetesimal collisions could have produced Jupiter, no pebble accretion needed. Gennaro DAngelo at Los Alamos National Laboratory in New Mexico and his colleagues authored the work. Their new paper doesnt rule out pebble accretion; it just shows that it may not be the only explanation for planets. Scientists still need to explore other possibilities.

Pebble accretion may not explain the formation of all planets. One recent study, for instance, showed that the planet Jupiter could have formed from collisions of planetisimals no pebble accretion required.Image data: NASA/JPL-Caltech/SwRI/MSSS; Image processing: Tanya Oleksuik (CC BY-NC-SA 2.0)

And they are. In a paper published in 2018, scientists combined planets and planetesimals in a kind of hybrid scenario.

Pebble accretion helps explain conditions under which big planets can form, DAngelo says. But that process depends on getting the timing and temperature just right. Without that, the pebbles might drift too quickly through the disk, he says, and the growing planet might not have time to accrete them.

Plus, theres a Goldilocks issue with pebble accretion. The planet-growing time has to last long enough to allow a stream of pebbles to land onto the core. But observations of other systems show that disks dont last forever. They often last only a few million years. And that puts a deadline on planet formation. So a planet cant grow too fast or too slow or it wont form at all.

Can pebbles lead to Earths?

Another lingering question about pebble accretion is whether it can form small, rocky worlds such as Earth and Mars.

Johansen thinks it can. In a February paper in Science Advances, he and his colleagues describe a model that shows how a stream of pebbles might do this. Pebbles collect on planetesimals at about the same distance from the sun as Mars is today. After the planets form, they migrate or drift over time into their current positions.

Computer simulations allow researchers to visualize how planetesimals formed, giving rise to planets, billions of years ago. Later, these visualizations suggest the neighborhood temporarily got very violent as the giant outer planets changed their orbits, wreaking tumult everywhere.

Morbidelli has his doubts. Personally, I think that somehow the giant planets grew by pebble accretion. But the terrestrial [rocky] plants, he believes, were mostly immune. Jupiter is the solar systems oldest planet. And as it grew, it blocked the flow of pebbles toward the inner solar system. That would have left no more pebbles to build the rocky planets. The solar systems inner planets, including Earth, could instead have formed through big collisions, he suspects.

Finding a way for planets to form is only the first step, says Kretke. The next question is, do we actually have a situation where pebble accretion dominates the process? In our solar system, or some other planetary system?

Scientists need better evidence before anyone declares pebble accretion to be the main planet-forming process in the cosmos. But given the wild diversity of planets both close to home and far away, he suspects scientists will find a range of explanations.

The physics is not different from here to there, he says, But the planets and the processes will depend on the conditions where they form.

Understanding those processes, says Johanson, promises two major rewards. First, scientists could understand each step in how to make a world, from dust to planet. Second, such studies could help point out where to look for life beyond the solar system.

If we want to understand habitable exoplanets, he says, we have to understand our own habitable planet.

Four of your senses are located just on your head. Taste is in your mouth. Smell is in your nose. Sight in your eyes and hearing in your ears. But touch? Touch is all over your body. Your fingertips and face can sense touch, and so can the bottoms of your feet and the backs of your knees. Its completely essential. Without it we wouldnt know if we stubbed our toes or burned our skin. 

See all the entries from our Lets Learn About series

Your skin (and your organs, bones and muscle) is full of receptor cells for different aspects of touch. These cells might respond to pressure or heat. They could respond to something that causes pain. Some of these receptor cells can also respond to cold and different chemicals. Each of the receptor cells connects to a sensory neuron. These are cells that send information back to the spinal cord and brain. There, your brain processes the touch your receptors felt, and determines whether you just tried to pet a cat or a cactus. Some areas of your body are more sensitive to touch than others, which is why you pet a cat with your hand and not with your back.

Its pretty easy to fool our eyes, ears or noses with sights, sounds and scents. But touch? Thats tougher. Scientists are working on haptic devices technologies that can mimic our sense of touch. Some use stretchy fabrics to make our skin feel something thats not there. Others are using sound waves that we cant hear to make illusions real to the touch.

Virtual reality is mostly sight and sound right now. But with the power of haptics, it could be touch, too.

Want to know more? Weve got some stories to get you started:

Touching allows octopuses to pre-taste their food: Special sensory cells in suckers in the animals arms sense chemicals. Those cells allow them to taste the difference between food and poison. (1/4/2021) Readability: 7.1

This artificial skin feels ghosts things you wish were there: Engineers have developed a wearable device that simulates the sense of touch. It may benefit robotic surgery and deep-sea exploration. (11/20/2020) Readability: 6.4

Testing the power of touch: We pet dogs with our fingers, not our arms or backs. Our fingers are more sensitive to touch. But how do we know? Here’s how you can test that. (2/5/2020) Readability: 6.3

Explore more

Scientists Say: Neuron

Explainer: What is skin?

Explainer: What is a neuron?

Shaking hands could transfer your DNA leaving it on things you never touched

Feeling objects that arent there

A do-it-yourself map of touch


Word Find

Some areas of our bodies are more sensitive to touch than others. In fact, you can measure your skin sensitivity and draw a map of it with a free program. The resulting misshapen body is called cortical homunculus. Its a representation of how our brain perceives touch all over our bodies.
Acollection of 1,000 prehistoricstructures dubbed mustatils the plural form of the Arabic term for rectangles scattered across 124,274 miles(200,000 kilometers) innorthwest Saudi Arabiamay be theworld's oldest monuments. A team of archeologistsfromthe University of Western Australia (UWA)reached this conclusion afterradiocarbon dating of charcoalfound inside the courtyardsindicatedtheywere constructed in 5,000 BC or about 2,000 years before theEgyptian pyramids or monuments likeStonehenge in southern England.
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