Bad Astronomy
The entire universe in blog form

June 28 2016 1:24 PM

Small Meteorite Punches Through Roof of House in Thailand

In the early morning of Tuesday, June 28, 2016, a small rock that is very likely a meteorite fell onto a house in Phitsanulok's Muang district, punching a hole in the roof and doing some minor damage inside.

Apparently many people in the area, including the owner of the home, heard a loud explosion some time before, which may have been the shock wave from the meteorite entering the atmosphere.

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I don’t speak or read Thai, but there’s a video from the Matichon TV YouTube channel with some pretty interesting footage.

I’ll note that in September 2015 a somewhat larger meteor burned up in the same area and was caught on video, but that’s certainly a coincidence.

The rock from this fall will have to be tested to make sure it’s a meteorite, but it certainly looks like one to my eye; the gray interior is common in stony meteorites, and the outer dark shell may be a “fusion crust”, caused by the few seconds of intense heat as the meteoroid (what the solid piece of rock or metal is called before it hits the ground) passes through the air at high speed.

One part of the story gives me pause, though: The homeowner says the rock was hot to the touch when she picked it up. In general, a meteoroid is only hot for a few seconds as it rams through the air, decelerating from hypersonic speeds down to below the speed of sound. As it slams through the upper atmosphere around 80 – 100 km above the Earth’s surface, the intense pressure of its passage compresses the air violently, and that heats the air up, which in turn heats up the meteoroid. It gets hot enough to glow (the air glows too) and we call that part the actual meteor.

But it slows in just seconds, falling the rest of the way at a few hundred kilometers per hour, taking a few minutes to fall the rest of the way to Earth. Air is very cold that high up, so many meteorites should be very cold to the touch if they’re found immediately after hitting the ground. Perhaps the homeowner made a mistake, thinking the intense cold was heat (if you’ve ever touched something extremely cold, the sensations are very similar).

roof hole
Light shines through the hole in the roof made by the rock.

Matichon TV, from the video

I’ll be very curious to hear about any tests run on the fragments. The homeowner says she’ll keep it, and that it will bring her good luck. I don’t know about that, but meteorites from known falls that hit objects on the ground can be extremely valuable to collectors. They’re called “hammers”, and this one has a mass of around 300 grams, so it’s easily worth thousands of dollars.

She’s lucky that no one was injured; at a hundred or more kph that would’ve hurt (it weighs twice as much as a baseball and was moving at least as rapidly as a fastball). Still, if I could choose to have one hit my house, I’d probably take that chance! What a story!

The odds are extremely low, of course. The Earth is ¾ water, and even the land area is mostly uninhabited. Meteorite falls over populated areas are rare, and having them hit objects even rarer. Very few injuries have been reported, and fewer verified (though a woman named Ann Hodges was hit by a meteorite in the 1950s in Sylacauga, Alabama, and that’s well worth your time to read about). It’s not something I worry about, to be honest.

Perhaps more meteorites from this fall will be found, too. That would be nice. A meteorite is in some ways a gift from the Universe to us, a piece of it that we can hold in our hands and examine in our labs, instead of just seeing from some huge distance away. Sometimes the Universe is pretty cool that way.

Want to learn more about meteors? Here's my episode of Crash Course Astronomy about them!

Tip o’ the Whipple Shield to Ron Baalke.

June 28 2016 9:15 AM

Stealthy Black Hole Spotted by Accident

Using a fleet of telescopes both on the ground and orbiting the Earth, astronomers have discovered what is very likely to be a black hole only a few thousand light-years from Earth.

On its own that’s not terribly newsworthy; we know of lots of black holes in the Milky Way galaxy. What makes this one so very interesting is that it has been hiding in plain sight, located in the sky near a gorgeous globular cluster, masquerading as a distant galaxy.

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And even that’s not the most interesting thing. The most interesting thing is what this implies for the total number of such black holes in the Milky Way. There could be a lot more than we first thought. A lot more.*

The object has the handy nickname of VLA J213002.08+120904 (a combination of VLA for Very Large Array, the radio observatory that discovered it, and its coordinates on the sky). It’s also called M15 S2, which I find amusing. Let me explain, because this is fun.

Globular clusters are collections of hundreds of thousands of stars in close proximity to each other. M15 is a glorious example of such a beast; about 30,000 light-years away, quite bright and well-studied. In 1996, a group of astronomers scanned the cluster using the VLA, looking for gas flowing into it. By accident they found a point source of radio emission in the outskirts of the cluster, but they weren’t sure what it was. Some astronomers thought it might be a distant galaxy coincidentally aligned in the sky with the cluster.

In 2014, another team of astronomers observed this object (which they designated S2, for the second unidentified point source of radio emission in M15) using a collection of radio telescope across the planet, and using parallax (the object’s apparent motion in the sky which is actually a reflection of the Earth’s motion around the Sun) found it was located about 7,000 light-years away—one-fifth the distance to M15! Clearly this was neither a distant galaxy nor a weird star in the cluster. That distance puts it firmly inside our own galaxy. The plot thickens.

Finally, another team of astronomers looked at observations from a variety of telescopes, hoping to nail down the identification of this thing. They found it in more radio observations from VLA, and interestingly Hubble Space Telescope observations show what looks to be a faint red source at that position. The kicker is that they also looked in data of M15 taken by the orbiting Chandra X-Ray Observatory, and found nothing; whatever it is, it’s too faint to be seen in even a 30-hour observation.

Hubble images of black hole
Hubble images of the object across the range of visible light (red to blue from left to right) show that it appears faintly in red wavelengths but not bluer ones.

Tetarenko et al., from the paper

All this put together points toward a very peculiar object: a black hole orbiting a tiny red dwarf star. If the black hole has a mass typical of such objects, about 10 times the mass of the Sun, the red dwarf is about 0.1 to 0.2 times the Sun’s mass (also typical). It’s likely the two orbit each other very closely, with an orbital period of roughly one to two hours. That’s close enough together that the star is grossly distorted by the black hole’s gravity, and material is flowing from it to the hole.

When material does this, it tends to form a disk around the black hole called an accretion disk. It piles up there before falling in to the black hole. The disk can be very hot, glowing brightly in X-rays, and also blast out a wind of subatomic particles. In this case though, the disk appears to be very weak and not as hot, which is why it’s not bright in X-rays even though its wind (or possibly other structures called jets) generates radio emission.

Other types of objects can be bright in radio and quiet in X-rays, but each one of these was systematically eliminated from consideration by the astronomers; for example a planetary nebula would show fuzziness in the Hubble images, but none is seen. In the end, the best candidate is indeed a low-mass binary black hole system.

Assuming this truly is as advertised, it’s the first accreting X-ray quiet black hole binary ever seen outside of a globular cluster. It’s also one of the lowest mass black hole binaries known. And that has some very cool implications.

This object was found essentially by accident in a radio observation of a globular cluster. But it’s not in the cluster; it’s much closer and just happens to lie near it in the sky—that’s fortunate, otherwise it may never have been discovered! Given that one of these was found in a very small region of the sky observed, that implies there are a lot of them scattered throughout the galaxy. Using some simple assumptions, the astronomers find that there may be anywhere from 25,000 to 150 million such low-mass black hole binaries in the Milky Way alone!

That’s a lot. It’s a wide range because of a lot of the uncertainties involved, but even on the low end, that’s three times more than predicted by looking at how stars are born and evolve over time (you need a low-mass star orbiting a high-mass one, so that the bigger one blows up and forms a black hole at the end of its life). And on the high end it’s thousands of times the number predicted.

How can the numbers be so far off? That’s a good question, and it’s not clear. Maybe this really was a very lucky observation, and just happened to spot one such object even though the odds were long. Maybe our stellar formation models are off. Either way, it shows us we need to search for more of these objects so we can figure out what’s going on. That’s not easy, as they’re usually found in X-ray surveys of the sky, and they’re X-ray quiet. Looking for the kind of radio emission these types of objects give off specifically will help, as will looking toward the galactic center for them, since they should be more common where there are more stars. A new generation of more sensitive X-ray telescopes coming soon will do well here, too.

I think this is all wonderful. It’s still possible to discover new objects by accident today and use them to learn important characteristics of the galaxy in which we live. And if it means more black holes to study, hooray! Getting a bigger sample of the weirdest objects in the Universe can only be a good thing for science and our understanding of the cosmos.

*Before you freak out, not to worry: Space is big, and even if there are zillions of these things out there, over the entire lifetime of the Universe the chance of any getting near enough to Earth to hurt us is essentially zero.

June 27 2016 9:00 AM

Comets Scoff at Gravity

On Earth, we are keenly aware of gravity. It shapes and modifies everything we do, including our architecture, our behavior, and the landscape around us.

Comets, on the other hand, think of gravity as more of an afterthought.

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The image above is from the Rosetta spacecraft, still traveling along with the comet 67P/Churyumov-Gerasimenko as it orbits the Sun. Both were about 474 million kilometers from the Sun and 449 million kilometers from Earth when this shot was taken.

Rosetta was considerably closer to the comet than that, of course. It was just a hair under 30 kilometers from 67P when it snapped this shot, and the resolution on the full-size image is a stunning 0.5 meters per pixel: about 18 inches!

jagged peaks
A closer look at those peaks.

ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

I love the jagged peaks surrounding the flatter plain, towering shards 100 meters high that clearly don’t have to deal with a whole lot of forces like gravity, wind, and the like (another angle on this same region can be seen here). You can see rocks resting comfortably at all sorts of weird angles, partially due to friction with the surface being more than enough to overcome any sliding due to the weak gravity, but also because “down” becomes a complicated topic if you’re standing there. The comet has two big lobes connected by a thin neck, and what you call “down” changes rapidly and strongly with position.

In general, if you were standing on the surface, you’d feel gravity something around 0.0001 times that of Earth. I’d weigh about a quarter ounce, as much a sip of water. A solid jump would fling you away from the comet forever.

The reason behind this is simple: Comets aren’t terribly big—67P is roughly four kilometers across and has a mass considerably less than a typical Rocky Mountain. This makes the force of gravity on the comet pretty weak, barely enough to hold it together.

context shot
Some context on the image; the arrow indicates the flattish plain in the region of the comet named Seth, though the peaks around it are harder to see from this angle. A very large pit (labeled Seth_01 here) can be seen in the lower left corner of the main image, partially cut off.

ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Even though conditions on the comet seemingly defy the forces we deal with on the surface of our much larger planet, the comet is actually incredibly fragile. It’s made of rock and ice, and when it gets near the Sun on its 6.4 year orbit that ice turns into a gas. This can dislodge rocks, shift surface features, and generally erode things away (and also create the iconic tail of a comet). In a relatively short time (millions of years? Less?) the comet will lose its structural integrity and fall apart. We’ve seen comets do this.

Such will be the fate of 67P as well. But in the meantime, there’s much to learn! Rosetta is still observing the comet, still sending back valuable data, and still helping us understand these bizarre frozen remnants of the solar system’s past.

My thanks to Emily Lakdawalla who helped me find the location of this shot on the comet’s surface. Follow her on Twitter!

June 26 2016 9:00 AM

A Sunday Morning Brain Teaser For You

Hey, I know just you want on a Sunday morning! A physics problem!

If it helps, think of it as a brain teaser. Like many such, it’s deceptively simple, but when you start to think about it a lot of concepts collide and it’s not as obvious as you might suppose at first glance.

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Here’s my friend Dianna Cowern, aka Physics Girl, with the brain-stumper. I suggest you do as she recommends, and stop the video to think about it before she gives the answer.

Did you get it right? I will admit it: I did. For the right reason, too, which I almost had to laugh at; many times when I think about a physics problem like this I can argue it both ways depending on what angle I take on the problem. In those cases I know I’ve forgotten something in my deducings.

In this case, I knew that rock is denser than water (as long as it’s not pumice, I suppose), so in the boat the rock was displacing its weight, but in the water it was displacing its volume. If rock is denser than water, its weight in water is larger than its volume in rock, so it displaces less water when it’s actually in the water, and the level goes down.

That kind of reasoning can be hard to follow if you’re not used to doing it. One slip-up loses the chain of logic. I was actually thinking that as I was going through the steps, and a part of my brain whispered to me, “Do an extreme case; it’ll be easier.” Then one minute later Dianna suggested just that, and I did laugh out loud.

Extreme cases may not solve the exact problem you’re facing, but they really help with getting through the logic, because there’s an intuition you get living in the real world about how some physics works. You really do! For example, you may know that if you throw a ball at about a 45° angle it’ll go farther than if you throw it at a higher or lower angle. If you do the physics you find the equations are symmetric around that angle, meaning that really is the best angle to throw a ball for distance.

Extreme cases exploit that knowledge you’ve gained through just existing in a Universe bound by physical laws, giving you an answer that either makes sense or is absurd, allowing you to get a better grip on things. I use them all the time when trying to figure things like this boat puzzle out, and they really do help. Just remember they’re extreme cases and may not represent the actual answer you want. Apply them carefully, and remember they’re a tool, not a solution themselves.

If you liked this problem and the video, Dianna makes lots of them and they’re really good. This one about mirrors is great and generates a lot of interesting discussion (lots of people say the answer is obvious, but it clearly isn’t to a lot of people). I also like this one about hurricanes and soap bubbles.

You can find more on her website, Physics Girl, and on her YouTube channel.

Tip o’ Archimedes’ bathtub to Mental Floss via Jennifer Ouellette.

June 25 2016 11:20 AM

The Stars at Night Are Big and Bright … Deep in the Heart of the Milky Way.

Sometimes you just need to look at pretty stars.

That image was taken by the Hubble Space Telescope back in 2009 (but just released recently), using the Advanced Camera for Surveys. It shows a region of the sky very near the center of the Milky Way galaxy, where stars are packed pretty closely together—think of them as city lights, and you see more when you look downtown.

Interestingly, the stars are displayed pretty close to their natural colors. Hubble cameras are equipped with a wide variety of filters that let through light of not just various colors, but also various bandpasses; that is, the range of colors. A narrow bandpass means you’re seeing a very thin slice of colors (say, centered in red), whereas a wide passband lets through light at a bunch of different colors. These filters have various uses; looking at gas clouds, for example, is usually better using narrow bandpasses to isolate the light emitted by specific elements.

June 24 2016 9:00 AM

Neptune Just Got a Little Dark

So Thursday I wrote about Pluto possibly having a liquid water ocean under its surface, which is pretty amazing. But other worlds in the solar system have stuff going on too, y’know.

Like Neptune. It has a new dark spot.

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Neptune is what we call an ice giant; bigger than rocky planets like Earth and Mars, but smaller than Jupiter and Saturn. It’s not literally made of frozen stuff; it’s called an ice giant because planetary scientists tend to call things like methane, ammonia, and water ices when dealing with outer worlds.

Neptune orbits pretty far from the Sun, about 4.5 billion kilometers out. That makes it pretty cold, and you might not expect the atmosphere to have much action. For a long time telescopic observations of it didn’t show much (it’s so far away that even though it’s nearly four times wider than Earth, it’s not terribly big in telescopes), but in 1989 the Voyager 2 probe flew past it, revealing a gorgeous deep blue world with a banded atmosphere, and, very surprisingly, a huge dark spot, which was somehow named the Great Dark Spot.

Since then our ‘scopes have gotten better and more of these spots have been found. We now know they’re anticyclones—high pressure systems—in Neptune’s troposphere, the deeper layer of atmosphere under its stratosphere. Since they’re high-pressure systems, we may be peering deeper into Neptune’s atmosphere when we see them.

color view of Neptune
When full color filters are used, the brighter, whiter clouds appear.

NASA, ESA, and M.H. Wong and J. Tollefson (UC Berkeley)

Dark spots on Neptune tend to be associated with bright clouds around their rims, which may be from methane clouds condensing as air blows around and above the dark spots. This happens on Earth …  well, with water instead of methane. Moist air rising up can condense to form clouds; we see this on the windward sides of mountains as the air lifts up to go over the obstacle. In this case, they’re called orographic clouds. With Neptune, the methane freezes, crystallizes, and becomes bright white to form the lovely thin white clouds around the dark spots.

Dark spots come and go. The Great Dark Spot had disappeared by the time Hubble looked for it in 1994, but other ones had appeared in 1995. This new one found is the first one seen in well over a decade. They can last for many years, because they spin in the same direction Neptune does. That sets up a stable feedback system that helps keep the spot rotating (in that case it’s called a vortex). We see this on Jupiter, Saturn, and possibly other places on Neptune, too.

Why is this important? Well, to be frank, atmospheres are complicated. Planets spin, and warm up, and have different stuff in their atmospheres, and sometimes warming or cooling changes the layering and condensation and evaporation rates, and it’s a mess. Understanding them in terms of their physics is really hard.

In some ways the outer planets are simpler than Earth: They’re mostly air. But there are other complications, like gases in abundance we don’t have here (like hydrogen and helium). Also, while the major source of Earth’s heat is the Sun, the outer planets don’t get nearly as much sunlight. Plus, they still have lots of warmth leftover from their formations (yes, they’re still cooling after 4.56 billion years), and for Neptune that’s its major source of heat. So it’s warmed from the inside out, the opposite of Earth.

All these worlds are test cases for our understanding of how atmospheres behave, including our own. Plus, of course, just understanding things is good. Neptune is a huge, massive, complex world, and worthy of our attention just because it exists and is near enough for us to study it.

June 23 2016 9:00 AM

Does Pluto Harbor an Ocean Under All That Ice?

Before the New Horizons space probe zipped past Pluto in July 2015, we weren’t sure what to expect. A lot was known about Pluto in general—given its density, it was likely a mixture of ice and rock, for example—but very little about was known about, say, the surface structure.

Then the little spacecraft flew by the little world, and our knowledge exploded. The close-up pictures were amazing, both beautiful to behold and tantalizing for the brain. There were lots of surprises, of course, including how diverse the surface was: There are frozen plains of nitrogen, mountains of water ice, dark and bright spots, huge fields of pits on the surface, and much more.

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Scattered around the surface are another type of feature: huge cracks, some hundreds of kilometers long and many kilometers deep. They’re pretty interesting, and their presence has led one group of planetary scientists to make an astonishing claim: There may be a liquid water ocean under the surface of Pluto!

If this is true, it’s a very big deal. Pluto is small and extremely cold, so the last thing you’d expect is liquid water anywhere within 3 billion kilometers of it. How can this be?

First, the evidence. The cracks are called extensional tectonic features, meaning you get them when the surface extends, expands. Imagine covering a balloon with mud. Let the mud dry, then inflate the balloon a little bit. What happens? As the balloon expands, it pushes the mud from underneath. Mud isn’t stretchy like rubber, so instead of smoothly expanding it cracks, allowing the pressure underneath to be relieved.

That may be happening on Pluto. The solid crust (mostly water and nitrogen ice) is feeling pressure from underneath, expanding, and cracking. But what could be causing the expansion? Liquid water. And lots of it.

Pluto is mostly ice and rock. However, there is likely to be some amount of radioactive elements in its core. Our own Earth has them, and the heat generated by the decay of these elements is a major source of our planet’s internal heat, even 4.56 billion years after it formed.

Pluto interior
Pluto's layers may include a crust of ice, a liquid ocean under that, and a large core of mostly rock with just enough radioactive material to keep it warm.

Jcpag2012/NASA/Pat Rawlings/Phil Plait

Studies show that even a small amount of such material (including uranium, thorium, and potassium) could produce enough heat inside Pluto to melt some of the water ice. I’ll admit this surprised me; my gut reaction is that Pluto is so small that it would lose heat faster than the radioactive materials could generate it.

The new research just published looks into that. They show that the rocky material inside Pluto insulates it and keeps that heat from leaking away too quickly. The rock acts like a blanket, keeping the heat inside Pluto, and over time it could be enough to not only melt a substantial amount of ice, but keep it liquid even to today.

Well, mostly liquid. Pluto is pretty cold, and some of that water, especially closer to the surface, could start to freeze. When liquid water turns into ice, it expands, and it’s that expansion that’s proposed to cause the crust of Pluto to expand with it, creating the cracks.

How weird is that? Tiny, frigid Pluto, long thought to be a frozen ball of ice, may yet have some spring in its step.

And there’s more. The new research shows that if the ice shell covering Pluto is thick enough—deeper than about 260 km—then the water under the surface could form a strange kind of ice called ice II. It’s still made up of water molecules like regular ice, but under higher pressures (caused by the thicker shell) the molecules realign themselves, forming a different crystal structure than normal ice.

Ice II is denser than regular ice, denser even than liquid water. If it forms, the ocean under the surface would shrink, contracting to a smaller volume. If that happened, you’d expect to see compressional features on the surface of Pluto, like thrust faults.

However, none was found, so it’s unlikely ice II ever formed. That suggests the water under the surface is still liquid, even today. Incidentally, the cracks on Pluto are fresh-looking, with few craters marring them. This indicates relative youth, though it’s hard to know what that means on Pluto. Millions of years? Tens of millions?

Still, this is an intriguing idea. I expect there will be some back-and-forth on this, as the surface of Pluto is examined more closely. For example, there’s a mountain on Pluto that looks like it was pushed up, and then began to subside. If that’s the case, what does that tell us about the thickness of the crust there, and the forces underneath? Also, there are cracks on Pluto’s surface that aren’t linear so much as radial, as if whatever pressure under the surface were greater there than average. Maybe there’s material welling up there, pushing up on the surface.

And of course, there’s another thought. A source of heat, liquid water, and complex chemicals are three ingredients needed for life. Now have a care here: I am in no way saying there’s life under Pluto’s surface! My point is more subtle than that: We wonder if life exists out in the Universe, and the key to that is how commonly the necessary conditions arise. What we’re seeing on Pluto—and on Enceladus, and Europa, and Titan—is that these conditions at least in part appear to pop up quite a bit just in our solar system alone. Even in almost literally the last place you’d expect.

Oh Pluto, will you ever stop surprising us?

I certainly hope not.

June 22 2016 9:00 AM

On the Ball of the Midnight Sun

Imagine you are on a large sphere.

Got it? Now imagine that sphere is spinning. As it does, two poles are defined: You can call them what you like, but “top” and “bottom” work. Halfway around the sphere from the top one toward the bottom is the sphere’s midsection. You can call it what you like, but “waist” works.

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Now imagine there’s a source of light some distance off. Imagine too this source of light is carefully placed such that the axis of rotation of the sphere is perpendicular to the direction to the light. In other words, the sphere spins vertically. The light then illuminates half the sphere, but as the sphere spins every point on its surface will eventually get lit during one rotation.

If you stand on a pole, let’s say the top one, you spin in place, once per sphere rotation. To you, the source of illumination spins around the sky, neither rising nor setting. It just goes around. The same is true if you’re on the bottom pole, too.

If you’re on the waist, in one spin you make a big circle, the size of the circumference of the sphere. For half a spin the source of illumination is visible, but for the other half it’s behind the sphere and you can’t see it. The light rises, goes directly overhead, and sets again after half a rotation, then rises again a half rotation later. If you’re anywhere else on the sphere except a pole (and if the sphere is very large compared to you), the Sun will rise and set but won’t go overhead.

But now let’s tilt the sphere. It can go any way, but let’s say the top pole it tipped toward the light. If you stand on the waist, the light still rises and sets, but instead of going overhead it makes a lower arc in the sky, never quite reaching the overhead point. How much it misses depends on how much the sphere is tilted.

tilted Earth
Maybe you see where I'm going with this.

Phil Plait

If you stand on the bottom pole, tipped away from the light, you’re still spinning in place, but you’re facing away from the light. The light is always blocked by the sphere, so you never see it. It’s always dark.

If you stand on the top pole, you’re still spinning in place, but now the light appears to go up and down over one spin. It doesn’t get blocked by the sphere, but it does get low, close to where the sphere blocks the sky (let’s call that the “horizon”). If you move away from the pole, down toward the waist by just the right amount, then the light will just kiss the horizon as it dips down before moving back up again. The distance you have to be from the pole for it to do this depends on the amount the sphere is tipped.

Now. If you’re on the top pole, what would that look like?

That wonderful time-lapse video was taken by astrophotographer Göran Strand just a little more than a day ago. He was standing on a big sphere—the Earth. The Earth is tilted, by about 24°. It goes around the light source (the Sun), but the tilt stays the same, so at one point in that journey the top (north) pole is tipped as much toward the Sun as it can be. We call that point in its orbit the solstice.

Strand was just the right distance from the North Pole, so that the path the Sun made in one rotation (one day) just barely stays above the point where the Earth blocks the sky (the horizon). He saw it make a big circle in the sky, taking one spin (one day) to go around.

Someone much farther away from the pole saw the Sun rise and set over the course of that day, but not Strand. Whereas it was the middle of the night for that person farther away, Strand never saw the Sun leave the sky.

And that’s why this phenomenon is called “the Midnight Sun.” Strand took that photo from just inside the Arctic Circle (24° south of the north pole), in Gällivare, Sweden, at just about midnight local time. Gällivare is at a latitude of 67°N, or 90 – 67 = 23° from the pole. Anyone there or closer to the pole never saw the Sun set that day.

And anyone inside the Antarctic Circle, 24° away from the south pole, never saw the Sun rise that day. It’s always night there right now, and if you’re on the pole you won’t see the Sun rise until September.*

Think of it! You are on a tilted sphere that is madly spinning as it whirls through space around a distant light source, creating day and night and seasons and climate and weather, which over billions of years has impacted geology and sculpted life to adapt to the day/night cycle, the annual cycle, the change in light versus distance from the poles. We set our calendars to these cycles, our clocks, our very lives to these cycles caused by our rotating, oblique planet.

Don’t think science affects your life? Think again. It affects everything.

* Right now, two members of an NSF team there are very ill, and a rescue mission has been mounted to bring them back north for medical attention. It is always dark there, and very, very cold, so this is nothing short of a heroic effort. The plane landed there on Tuesday and has begun the arduous journey back.

June 21 2016 9:15 AM

Two Baby Alien Worlds Show Us How to Cook a Planet

Two new exoplanets have been discovered, and they’re important milestones in our understanding of how alien solar systems behave. That’s because both are very young, both are massive, and both orbit their stars very close in, closer than Mercury orbits the Sun.

First, a quick intro to the problem: When astronomers first started finding planets around other stars in the mid-1990s, they were surprised to find that many were as big and massive as Jupiter but so close to their parent stars that they had orbital periods measured in days. Given that Mercury, the innermost planet in our solar system, is small and takes three months to circle the Sun, these “hot Jupiters” were pretty shocking. How did they get there?

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Models of planetary formation show that it’s very unlikely a big planet can form so close to the star. Most likely, they form farther out and somehow migrate in toward the star. One way would be to interact with the disc of debris circling the star from which the planet formed. As it plows into this material, it can drop down to the star. It’s unclear if this happens very early in the life of the planet (like, within a million years or so after it forms) or much later, like right before the debris disk is blown away by the star millions of years later.

Another way would be for it to gravitationally interact with other planets in the system, which can affect their orbits. This method takes a while, though, and is slower in general than the debris disk method. The problem: Most planets found are old, a billion years or more, so it’s hard to know just when the hot Jupiter moved.

These two new planets change that.

One is called K2-33b and was found by the Kepler spacecraft, the one that has discovered so many alien worlds. The star, K2-33, is about 460 light-years away and is a cool red dwarf, only about 0.6 times the mass of the Sun and shining at a feeble 15 percent as bright*. It’s part of a loose cluster of stars called the Scorpius-Centaurus OB Association, which is known to be young. The colors of the star also indicate youth (it’s still very hot from its formation, so it’s bluer than you’d expect), and the smoking gun is the presence of lithium in the star’s atmosphere; that element gets destroyed by a star very quickly, so seeing any at all means the star is very young. All together, this points to an age of no more than 20 million years, and most likely closer to 11 million years.

Our own Sun is 4.56 billion years old, so this star is basically a wee bairn.

K2-33b, the planet (note the “b”) is roughly 60,000 kilometers across, a bit less than five times Earth’s diameter. It has no more than the mass of Jupiter, and likely has less—it’s more like a super-Neptune or a mini-Jupiter. It orbits the star every 5.4 days, so it’s close.

Given its youth, that strongly indicates that massive planets can migrate rapidly toward their star right after they’re born, perhaps even while they’re still forming.

K2-33b orbital scale
When I say close, I mean close: K2-33b orbits its star a lot closer even than Mercury does the Sun.

NASA/JPL-Caltech

But wait! There’s more! We have the other planet, too.

Before I even get started, let me say that this second planet has not yet been confirmed, but given the circumstances it seems very likely it’s real. I’d be happier with more independent evidence, but the authors indicate the probability of it actually existing is very high.

This one orbits the star V830 Tau, which has a mass almost the same as the Sun, but is twice the radius and about 20 percent more luminous. The planet orbits the star every five days at a distance of just 8.5 million kilometers—that’s very close indeed. Even better, the age of the star appears to be only about 2 million years, which is even younger than K2-33!

So, in both cases, we have a massive planet, which is orbiting very close to its star and is very young. That’s very exciting! They show that planetary migration can happen almost immediately, even as the planet and star are still settling down after their formation.

Mind you, this doesn’t mean planets don’t move later in life. In fact, we have evidence that planetary interactions do happen (some planets orbit their stars backward, in the opposite sense of the star’s rotation, which can happen if the planet has undergone a game of celestial billiards with other planets).

So what we now seem to find is that there’s more than one way to cook a Jupiter, and it can happen right away or it can take a while.

This is amazing. I’ll remind you that until 1995 we didn’t even know if other planets existed around other stars like the Sun. It’s only been 20 years, and we’ve learned so much about them! We have thousands to study, and they come in so many flavors: Big, small, hot, cold, puffy, compact … we see planets like the ones in our solar system, and ones that are entirely alien.

And we’re still just getting started here. We know of something like 3,000 planets right now, but there are likely hundreds of billions in our galaxy alone! With a sample size that vast, even something very unlikely is bound to happen. What other strange new worlds await us?

* Correction, June 21, 2016: I originally wrote that K2-33 has a diameter of 0.6 times the Sun's, but I meant the mass. Weirdly, it's actually about the same size as the Sun, because it's still so hot from formation that it's inflated a bit. It'll shrink as it ages.

June 20 2016 11:01 AM

Happy Full Moon Solstice!

On Monday at 22:34 UTC (6:34 pm Eastern U.S. time), the Sun will reach its highest declination in the sky, its farthest point north for the year. That is the moment of the June solstice.

This means that, in the Northern hemisphere, we have the longest day of the year, and the shortest night. If you live your life standing on your head in the Southern hemisphere, it means you have the shortest day, and the longest night.

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I enjoy writing about the solstices and equinoctes* when they happen, so you can read all about how and why they occur in past articles. I’ll note that Monday is not the date of the earliest sunrise and latest sunset though; that has to do with the Earth’s orbit being slightly elliptical, so I’ll make a special point of linking to this article last year where I explain why that happens. I’ll also note that some people call this the first day of summer (or winter for those in the south), but I disagree; I tend to think of it as actually the midpoint. You can read about that to your brain’s delight as well.

Earth tilt animation
The Earth's axis stays pointing in one direction in space as it orbits the Sun, so sometimes we're tipped toward the Sun, sometimes away. This is why we have seasons and solstices.

Tfr000/Wikipedia, Creative Commons License

No, instead of spending time on that here, I’d prefer to point out something rather special that led to me wandering down a rabbit hole Sunday night as I researched it: Not only is the solstice Monday, but the Moon is full on Monday as well. That moment occurred at 11:02 UTC (07:02 Eastern; I’ll note it’ll look full all day and probably even Tuesday as well).

A full Moon on the same day as the June solstice (or the December one, for that matter) is relatively rare. As I thought about this Sunday night, I wondered just how rare it was. My first thought was that it probably happens once every 30 years or so, since the full Moon can occur on any day, and there are 30 in June.

But then I realized it’s not that simple. Sometimes in astronomy two cycles can beat together in unusual ways, throwing off what you might expect. So I dug into it. I found a list of solstice full Moon dates on the Farmer’s Almanac website, and perusing the numbers it appears that we get a full Moon on the June solstice roughly every 19 years or so … or multiples thereof.

Nineteen years? That sounded familiar. It took me a few minutes, but then it clicked: That’s the Metonic cycle! Let me explain.

The Moon goes through a full phase of cycles (from full to new and full again) in about 29.53 days. That all by itself is interesting, and I talk about that in the episode of Crash Course Astronomy on the phases of the Moon:

One Earth year is, on average, about 365.24 days long. But there’s a funny coincidence here: 19 years is 6,939.56 days, and that is almost a perfect multiple of 29.53! Nineteen years is almost exactly 235 lunar phase cycles. That means that when you have a full Moon on a given date, 19 years later it’ll be on that same date once again. That’s what’s called the Metonic cycle. This fact has been known for about 2,500 years, which is pretty amazing.

But looking at the Farmer’s Almanac, you see it doesn’t seem to happen every 19 years. Why not?

This is where I really started to dig deep. I looked at leap years, and fractional leftovers between the lunar phase month (called the synodic month) and the Earth’s year, and on and on. That can account for some of the reason the full Moon doesn’t always fall on the same calendar date every 19 years.

Then I realized something: time zones.

Astronomical sites list the times of the solstices and full Moons in Coordinated Universal Time (UTC, similar to Greenwich time). That makes it easy for everyone, since you can just look up how far off your time zone is from that (for example, right now the East Coast of the U.S. is on Eastern Daylight Time, UTC – 4 hours).

But that can mess up the full Moon June solstice cycle. Why? Because the exact moment of the solstice changes year to year, and can even occur on different days! It can be on June 20, 21, or sometimes even 22. If the solstice occurs on June 21 at 23:59, and the full Moon two minutes later, technically they’re on different days!

Worse, that’s UTC. In the U.S., where it’s four to seven hours earlier than UTC, both would occur on the same calendar day. So it’s possible (and even likely) that one place in the world would see a full Moon on the same calendar day as the solstice, and another part of the world wouldn’t. What a mess!

Look at Monday’s solstice: It occurs at 22:34 UTC. For someone a couple of time zones east of the U.K., that means it happens on June 21. For them, they don’t get a full Moon on the day of the solstice. The exact moments of the solstice and full Moon are independent of time on Earth (the solstice occurs at the same moment for everyone on the planet, for example), but because we bin time up into days, that can throw off the days on which we say those events happened. Weird.

Going back to the Farmer’s Almanac, you may notice that while the solstice full Moon doesn’t happen every 19 years, it does appear to have a cycle of multiples of 19. For example, there was one in 1796, then the next in 1834, a gap of 38 years, 2 x 19. Other such gaps can be found. The gaps happen because the full Moon missed the calendar day of the solstice by some hours. Not only that, but that table is for the U.S. East Coast, so it doesn’t work for the whole world.

The root of this problem is using calendar days, which are arbitrary to some extent. There’s an overall 19-year cycle, but because of time zones it can get thrown off. A better way to do this would be to ask, “How often does a full Moon occur within a day of the solstice?,” or better yet within 12 hours before to 12 hours after the moment of the solstice.

In that case, I’d expect the 19-year Metonic cycle to be more obvious. However, looking that up using calendars for the full Moon (like this one) and the solstices (like this one) is difficult and tedious. The best way would be to run the calculations specifically looking for that, which I thought of too late to ask anyone to do for Monday’s events. I’ll leave that as an exercise for the reader.

Anyway, my point is … well, I guess I don’t have a point except that numbers are fun to play with and the cycles in the sky are neither always obvious nor simple to grasp.

But they’re there, and if a full Moon falling on the solstice is interesting enough to people that they go outside and take a look for themselves, then I’m all for it.

So happy full Moon June solstice! Enjoy it, because the next one won’t be for a while—June 21, 2062, in fact … if you use UTC.

*Equinoctes is the actual plural for equinox. Some dictionaries say that’s a bit old-fashioned, and equinoxes is now used, which is fine by me. Languages change over time. But I rather like the way equinoctes sounds, and I like using it. So I do.

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