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We may have seen two asteroids annihilate each other in another solar system



Enlarge / The incredible vanishing exoplanet—what models suggest we’d see as the debris of a collision diffuses away.

We’ve not actually “seen” the vast majority of exoplanets we’ve found orbiting distant stars. Instead, their existence has been inferred based on changes in the light of the stars that they orbit. That makes the 20 or so we have imaged directly exceptional. Direct imaging typically requires a very large planet, which means this sample isn’t entirely representative, but these planets do provide a unique opportunity for us to observe how bodies interact with each other and their environments in exosolar systems.

But, if two researchers at the University of Arizona are right, we can scratch one of these examples off the list. They say that the supposed planet has vanished in more recent images, which indicates it was never actually there in the first place. Instead, they argue that we’ve been observing the debris of a smash-up between two very large asteroids.

Well, it looked like a planet…

How could astronomers have identified a planet that didn’t exist? Well, it really looked like it did. Back in 2008, when scientists first announced its discovery, the planet Fomalhaut b seemed to be very much there. Consecutive images taken a couple of years apart appeared to show it orbiting, and its orbit took it through a disk of dust and frozen material that is exactly where we’d expect planet formation to take place. So, at first glance, everything looks good.

But problems became apparent pretty quickly. For one, planets have sufficient gravity to create gaps in any disks of material that they orbit within. But the disk at Fomalhaut b appears to be remarkably unperturbed by what seems to be a giant planet within it.

More problems came when scientists tried to image it in the infrared. Fomalhaut is a young system, meaning any planets there have formed relatively recently. And, based on the amount of visible light reflected by the apparent planet, it was a large gas giant, potentially far larger than Jupiter. That means the planet hasn’t had much time to release the heat generated as material streamed in to form it—and with a planet this size, there should be plenty of heat. Yet Fomalhaut b was invisible in the infrared when scientists looked for it.

Combined, these facts limit the possible size of the planet to smaller than Jupiter. Further observations took the limit down to near Earth-sized, which is basically incompatible with the amount of light we see from it. So, people started considering alternative ideas about what Fomalhaut b might be, including a smaller planet with large rings or even simply a large cloud of dust generated by local events in the ring.

The researchers behind the new work, András Gáspára and George Rieke, found a few additional images of Fomalhaut in the Hubble archive. And, in these more recent images, some strange things seem to be happening.

The non-orbital, vanishing exoplanet

The first issue was with the orbital solutions. Based on Fomalhaut b’s position in the earliest few images, researchers were able to calculate its orbit and predict where it should be in future images. And, in later images, it was close to where it was predicted to be. But differences existed and got larger over time. By the last image they had where Fomalhaut b was visible, it had deviated significantly from its predicted orbit. When the researchers tried to figure out its actual orbit, they discovered it was unstable—Fomalhaut b was apparently leaving the system.

The second was with the apparent planet itself. While it’s difficult to resolve much in the way of details at this distance, Fomalhaut b appeared to be growing increasingly diffuse over time.

The final issue was a relatively simple one: in the most recent image, the planet was gone entirely. That doesn’t mean there’s nothing there—it just means that whatever might be there has dropped below Hubble’s ability to detect it.

Assuming this data is all accurate, it’s easy to accept Gáspára and Rieke’s contention that none of this is consistent with Fomalhaut b being a planet. So what is it? Their favored explanation is a collision between two asteroids that are roughly 100km across. If the collision was energetic enough to obliterate the asteroids, it would produce a steadily expanding cloud of debris that could reflect enough sunlight to be visible to Hubble.

Since the fragments would be small, they’d quickly lose the heat of the collision to space, explaining the lack of infrared. And their gradual dispersal would explain the expansion and eventual disappearance of Fomalhaut b. Finally, because the collision would alter the orbital trajectories of the ensuing fragments, this would explain why they were seemingly on a path to exit the exosolar system entirely.

When did that happen?

To find out whether this idea is consistent with the data we have, the researchers got hold of a software packing that could model an expanding sphere of reflective particles, and they used it to produce estimates of what Hubble might observe over time. This produced images that were reasonably consistent with the actual images obtained by Hubble. Given the agreement, the researchers were able to use the model to trace things back to when the actual collision must have taken place. That indicated it had occurred shortly before the first Hubble image that indicated the planet was there.

That, of course, would suggest we’ve been extremely lucky to have imaged the collision at all. How lucky? To find out, the researchers estimated the amount of material in Fomalhaut’s disk and how many sufficiently large asteroids would be formed. From there, they calculated how often collisions might take place. While there was a pretty large range that depended on the assumptions used, it placed the frequency of sufficiently large collisions somewhere between 150,000 and half a billion years. So, if Gáspára and Rieke are right, we have indeed been very lucky.

Obviously, there are lots of strange things going on in the Universe that we can’t entirely explain, so there’s a chance that an explanation that seems to make sense will turn out to be wrong. The key test will be to have Hubble or its successor make regular diversions to the Fomalhaut system to make sure that the seemingly missing planet doesn’t reappear in the future.

PNAS, 2020. DOI: 10.1073/pnas.1912506117  (About DOIs).

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What happens if a space elevator breaks



TCD | Prod.DB | Apple TV+/ | lamy

In the first episode of the Foundation series on Apple TV, we see a terrorist try to destroy the space elevator used by the Galactic Empire. This seems like a great chance to talk about the physics of space elevators and to consider what would happen if one exploded. (Hint: It wouldn’t be good.)

People like to put stuff beyond the Earth’s atmosphere: It allows us to have weather satellites, a space station, GPS satellites, and even the James Webb Space Telescope. But right now, our only option for getting stuff into space is to strap it to a controlled chemical explosion that we usually call “a rocket.”

Don’t get me wrong, rockets are cool, but they are also expensive and inefficient. Let’s consider what it takes to get a 1-kilogram object into low Earth orbit (LEO). This is around 400 kilometers above the surface of the Earth, about where the International Space Station is. In order to get this object into orbit, you need to accomplish two things. First, you need to lift it up 400 kilometers. But if you only increased the object’s altitude, it wouldn’t be in space for long. It would just fall back to Earth. So, second, in order to keep this thing in LEO, it has to move—really fast.

Just a quick refresher on energy: It turns out that the amount of energy we put into a system (we call it work) is equal to the change in energy in that system. We can mathematically model different types of energy. Kinetic energy is the energy an object has due to its velocity. So if you increase an object’s velocity, it will increase in kinetic energy. Gravitational potential energy depends on the distance between the object and the Earth. This means that increasing an object’s altitude increases the gravitational potential energy.

So let’s say you want to use a rocket to increase the object’s gravitational potential energy (to raise it to the right altitude) and also increase its kinetic energy (to get it up to speed). Getting into orbit is more about speed than height. Only 11 percent of the energy would be in the gravitational potential energy. The rest would be kinetic.

The total energy to get just that 1-kilogram object into orbit would be about 33 million joules. For comparison, if you pick up a textbook from the floor and put it on a table, that takes about 10 joules. It would take a lot more energy to get into orbit.

But the problem is actually even more difficult than that. With chemical rockets, they don’t just need energy to get that 1-kilogram object into orbit—the rockets also need to carry their fuel for the journey to LEO. Until they burn this fuel, it’s essentially just extra mass for the payload, which means they need to launch with even more fuel. For many real-life rockets, up to 85 percent of the total mass can just be fuel. That’s super inefficient.

So what if, instead of launching atop a chemical rocket, your object could just ride up on a cable that reaches all the way into space? That’s what would happen with a space elevator.

Space elevator basics

Suppose you built a giant tower that is 400 kilometers tall. You could ride an elevator up to the top and then you would be in space. Simple, right? No, actually it’s not.

First, you couldn’t easily build a structure like this out of steel; the weight would likely compress and collapse the lower parts of the tower. Also, it would require massive amounts of material.

But that’s not the biggest problem—there’s still the issue with speed. (Remember, you need to move really fast to get into orbit.) If you were standing on the top of a 400-kilometer tower with the base somewhere on the Earth’s equator, you would indeed be moving, because the planet is rotating—this is just like the motion of a person on the outside of a spinning merry-go-round. Since the Earth rotates about once a day (there’s a difference between sidereal and synodic rotations), it has an angular velocity of 7.29 x 10-5 radians per second.

Angular velocity is different than linear velocity. It’s a measure of rotational speed instead of what we normally think of as velocity—movement in a straight line. (Radians are a unit of measurement to use with rotations, instead of degrees.)

If two people are standing on a merry-go-round as it spins, they will both have the same angular velocity. (Let’s say it’s 1 radian per second.) However, the person that is farther from the center of rotation will be moving faster. Let’s say one person is 1 meter from the center and the other person is 3 meters from the center. Their speeds will be 1 m/s and 3 m/s respectively. This same thing works with a rotating Earth. It’s possible to get far enough away such that the Earth’s rotation gives you the required orbital velocity to stay in orbit around the planet.

So let’s go back to our example of a person standing on the top of a 400-kilometer tower. Are they far enough away from Earth that they can stay in orbit? For one complete rotation of the Earth, their angular velocity would be 2π radians per day. That might not seem very fast, but at the equator this rotation gives you a speed of 465 meters per second. That’s over 1,000 miles per hour. However, it’s still not enough. The orbital velocity (the velocity needed to stay in orbit) at that altitude is 7.7 kilometers per second, or over 17,000 miles per hour.

Actually, there’s another factor: As you increase your distance from the Earth, the orbital velocity also decreases. If you go from an altitude of 400 to 800 kilometers above the surface of the Earth, the orbital speed decreases from 7.7 km/s to 7.5 km/s. That doesn’t seem like a large difference, but remember, it’s really the orbital radius that matters and not just the height above the surface of the Earth. Theoretically, you could build a magical tower that was high enough that you could just step off of it and be in orbit—but it would have to be 36,000 kilometers tall. That’s not going to happen.

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Study: Leidenfrost effect occurs in all three water phases: Solid, liquid, and vapor



Slow-motion video of boiling ice, a research project of the Nature-Inspired Fluids and Interfaces Lab at Virginia Tech.

Dash a few drops of water onto a very hot, sizzling skillet and they’ll levitate, sliding around the pan with wild abandon. Physicists at Virginia Tech have discovered that this can also be achieved by placing a thin, flat disk of ice on a heated aluminum surface, according to a new paper published in the journal Physical Review Fluids. The catch: there’s a much higher critical temperature that must be achieved before the ice disk will levitate.

As we’ve reported previously, in 1756, a German scientist named Johann Gottlob Leidenfrost reported his observation of the unusual phenomenon. Normally, he noted, water splashed onto a very hot pan sizzles and evaporates very quickly. But if the pan’s temperature is well above water’s boiling point, “gleaming drops resembling quicksilver” will form and will skitter across the surface. It’s called the “Leidenfrost effect” in his honor.

In the ensuing 250 years, physicists came up with a viable explanation for why this occurs. If the surface is at least 400 degrees Fahrenheit (well above the boiling point of water), cushions of water vapor, or steam, form underneath them, keeping them levitated. The Leidenfrost effect also works with other liquids, including oils and alcohol, but the temperature at which it manifests will be different. 

The phenomenon continues to fascinate physicists. For instance, in 2018, French physicists discovered that the drops aren’t just riding along on a cushion of steam; as long as they are not too big, they also propel themselves. That’s because of an imbalance in the fluid flow inside the Leidenfrost drops, acting like a small internal motor. Large drops showed a balanced flow, but as the drops evaporated, becoming smaller (about half a millimeter in diameter) and more spherical, an imbalance of forces developed. This caused the drops to roll like a wheel, helped along by a kind of “ratchet” effect from a downward tilt in the same direction the fluid in the droplet flowed. The French physicists dubbed their discovery a “Leidenfrost wheel.”

In 2019, an international team of scientists finally identified the source of the accompanying cracking sound Leidenfrost reported. The scientists found that it depends on the size of the droplet. Smaller drops will skitter off the surface and evaporate, while larger drops explode with that telltale crack. The culprit is particle contaminants, present in almost any liquid. Larger drops will start out with a higher concentration of contaminants, and that concentration increases as the droplets shrink. They end up with such a high concentration that the particles slowly form a kind of shell around the droplet. That shell interferes with the vapor cushion holding the drop aloft, and it explodes when it hits the surface.

And last year, MIT scientists determined why the droplets are propelled across a heated oily surface 100 times faster than on bare metal. Under the right conditions, a thin coating formed outside each droplet, like a cloak. As the droplet got hotter, minuscule bubbles of water vapor began to form between the droplet and the oil, then moved away. Subsequent bubbles typically formed near the same spots, forming a single vapor trail that served to push the droplet in a preferred direction. 

But can you achieve the Leidenfrost effect with ice? That’s what the Virginia Tech team set out to discover. “There are so many papers out there about levitating liquid, we wanted to ask the question about levitating ice,” said co-author Jonathan Boreyko. “It started as a curiosity project. What drove our research was the question of whether or not it was possible to have a three-phase Leidenfrost effect with solid, liquid, and vapor.”

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Two cannabinoids have opposing effects on SARS-CoV-2 in culture



Enlarge / Don’t try this at home. Seriously. We mean it.

Over the course of the COVID-19 pandemic, researchers have tested a wide range of drugs to see if they inhibit the virus. Most of these tests didn’t end up going anywhere; even the few drugs that did work typically required concentrations that would be impossible to achieve inside human cells. And a few (looking at you, ivermectin and chloroquine) took off with the public despite iffy evidence for effectiveness, seemingly causing nearly as many problems as they would have solved if they actually worked.

Nevertheless, two years on, word of yet another one of these drug experiments caused a bit of a stir, as the drug in question was a cannabinoid. Now, the full data has gone through peer review, and it looks better than you might expect. But the number of caveats is pretty staggering: the effect is small, it hasn’t been tested in patients, the quality assurance of commercial cannabidiol (CBD) products is nearly nonexistent, and—probably most importantly—another cannabinoid blocks the effect entirely.

With that out of the way, on to the data.

Why test cannabinoids?

One of the big focuses of the drug testing was to look for chemicals that were already approved for use in humans, which would simplify their use as treatments for a separate disorder since all the safety data should be available already. And CBD is approved for use in people with seizure disorders, although the biochemical basis of its effectiveness is unclear.

In any case, the researchers behind the new work (primarily at the University of Chicago) started with lung cancer cells that produce the protein that SARS-CoV-2 uses to infect cells and dumped both the virus and CBD on the cells. And it worked. At non-toxic doses, the reproduction of the virus was strongly inhibited by CBD. The team went on to confirm the result in other lung cell lines. They also demonstrated that a partly metabolized derivative had a similar effect, but a range of additional cannabinoids did not.

And this is where we get to one of the downsides. THC, the most potent mind-altering substance in cannabis, did not have an effect on its own. But when given at the same time as CBD, it reversed CBD’s inhibition of viral growth. So simply trying to use cannabis for viral protection will fail pretty miserably.

In any case, this is where the work starts to move beyond the hundreds of similar “let’s throw drugs on some cells” studies that have been done: the researchers do their best to figure out how CBD works. They checked whether it stopped human cells from producing the protein that the virus latches onto when infecting them, but that wasn’t the cause. And they confirmed that viruses could still get inside cells by using the SARS-CoV-2 spike protein.

But once the virus gets inside, not a lot seems to happen. Very little of the spike protein gets made in infected cells treated with CBD, and levels stay low for up to 15 hours after infection.

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