Bad Astronomy
The entire universe in blog form

Oct. 22 2016 9:00 AM

Mars Lander Impact Site Seen from Orbit

So, half of the European Space Agency’s ExoMars mission went well the other day. The Trace Gas Orbiter is now circling the planet, and appears to be healthy and happy.

Unfortunately, the Schiaparelli lander didn’t do so well. Instead of gently touching down on the surface, an as-yet undetermined problem shut the landing rockets off while it was still two to four kilometers above the ground. It impacted at an estimated speed of 300 km/hr. That’s 180 mph. Oof.


The image above is from NASA’s Mars Reconnaissance Orbiter, and shows the impact site. The lander’s planned descent was very similar to that of NASA’s Curiosity rover. Its heat shield took the brunt of first slamming through the atmosphere and then was ejected. After that a parachute slowed it further, then it and the back shell attaching it to the lander were ejected. Finally, the lander itself would slow using rockets and drop down to the surface.

In the image near the bottom you can see a bright spot below the cluster of pits to the lower right; that’s likely to be the parachute and back shell. And near the top is the, um, rather large black spot. That smudge is 15 x 40 meters in size, or about half the size of an American football field. The lander’s rockets would disturb the surface as it came down, but not nearly that much. It’s far more likely it’s an impact site.

This animation shows it better:

From telemetry, it seems that the thrusters switched off far too early. That means the fuel tanks were nearly full of propellant, so it’s likely they exploded upon impact. That would explain the size of the impact disturbance. It’s not clear why it happened, but engineers are poring over the data and hopefully will figure it out soon.

This is pretty disappointing, but it’s important to note that that lander was a technology testbed, literally designed and built to test the tech needed to land a future rover or other equipment on Mars. For the most part, the lander was a success! Most of the hardware and software worked, but obviously a very important piece did not.

And more importantly, the Trace Gas Orbiter is doing well. This is an extremely sensitive observatory that will look for methane in the atmosphere of Mars. Methane is a simple carbon-based compound that breaks down easily in the atmosphere. It’s been detected in the air of Mars before but it’s very diffuse and difficult to nail down. Methane can be created both by geological activity (outgassing from the interior) and by biological action. Obviously, the question of life on Mars looms large, and if it exists and is anything like life here, it might make methane. Detecting the gas is a big clue.

TGO will not only map methane, it will also look for different isotopes of it. A carbon atom usually has six protons and six neutrons. An isotope of an atom has a different number of neutrons; for example carbon-13 has seven neutrons, not six. Biologically made methane usually has more C12 in it than C13, so mapping the various isotopes can give us clues about the origin of the gas as well.

So let me stress again that TGO is ticking along. And hopefully engineers at ESA will learn enough about what went wrong with Schiaparelli to make sure the next attempt will go a little bit more gently.

We humans have sent a lot of probes to Mars, and nearly half have failed in one way or another. Space exploration is hard. But it’s so very worth it. I hope the folks at ESA working on this remember this as they continue to strive to better understand the Universe around us.

Oct. 21 2016 9:00 AM

Twinkle Twinkle, Little Star, Who Wrote You? ’Twas Not Mozart.

“I never made a mistake in my life. I thought I did once, but I was wrong.” –Charles Schulz

Despite being a beloved internet personality, I have my flaws. Sometimes—rarely, of course—I make mistakes. I know. You’re shocked. Take a moment to recover, if you need one.


But it happens. If it’s small and doesn’t impact the writing in an article I fix it and move on, or (because TPTB at Slate are quite strict about such things) I issue a correction in the article itself. Sometimes the mistake is extensive, or worth diving into more fully, or as happens often is illustrative of an interesting issue, and in those instances I’ll write a separate follow-up article.

Sometimes it’s more of the “Oh. Huh!” variety. Such is the case here.

In my first book, Bad Astronomy, I offhandedly mention that the tune for “Twinkle, Twinkle, Little Star” was written by one Wolfgang Amadeus Mozart. Perhaps you’ve heard of him, and perhaps you’ve heard this claim as well.

However, it appears I and everyone else who repeats this little factoid are wrong. In an article at Woot, professional smart person Ken Jennings corrects this common misconception. It turns out the tune did not come from Mozart, but instead is actually a French folk song that dates from before him. I urge you to read Jennings’ article for details, as he mentions how Mozart got saddled with the credit.

Jennings also calls me out specifically for making the claim, especially and ironically in a book I wrote trying to correct misconceptions. Mea culpa. I remember writing that claim back in 2000 when I was drafting the book, and I even vaguely remember both thinking to myself that I should check its veracity, and actually doing so. However, I must have found some article that confirmed my own bias about the song’s authorship and went with it. Unfortunately, no one else in the editorial process caught it either, most likely simply assuming the old claim was correct.

Ah well. If you buy my book and are saddled with existential conflict due to this error, I suggest you take a pen, cross out the offending portion, and smile knowingly as you have gained more knowledge, a noble goal.

And if you’re the kind of person who delights when someone in my position makes an error, I  suggest you buy somewhere between 10 to 20 copies of the book and distribute them among your friends, so that you may all gather and bask in my factual wrong turn.

Of course, none of this affects another book, Astronomically Correct Twinkle Twinkle, written by my friends Henry Reich and Zach Weinersmith, which is quite delightful. Ironically, I suppose, I fact checked their book, and came up empty of errors. But then, they’re both smarter than I am, and combined they possess fierce intellect.

Now, for penance, I’m going to go listen to “The 1812 Overture,” which I hear was written by Francis Bacon.

Oct. 20 2016 9:00 AM

The Lone Icy Volcano of Ceres

It’s hard to pick the one weirdest thing about Ceres. It’s the largest object in the asteroid belt between Mars and Jupiter. Most of the billions of chunks of debris in the belt really are leftover rubble from the early solar system, but some grew larger by the power of their own gravity, drawing more material in.

As it happened, Ceres was well on its way to becoming a planet before it ran out of material to feed on. Bigger than an asteroid, but smaller than a planet, scientists call it a protoplanet.


Out past Mars, the early solar system had lots of ice, and Ceres trapped this material as well. Its surface is dotted with white spots, what are now thought to be salt deposits left over as briny water leaked to the protoplanet’s surface and sublimated away.

The surface is heavily cratered, of course, and hilly. But one features stands out … and I do mean literally.

Ahuna Mons is the lone true mountain on Ceres, and it’s far and away the tallest: It towers an amazing 4 kilometers over the surface, and is 18 km across at its base. Nothing else on Ceres even comes close to it.

But Ceres doesn’t have tectonics like Earth does. So what could possibly create such a huge feature?

Ice. And lots of it.

Ahuna Mons
Ahuna Mons from above shows it to be a flat-topped mountain.


We know salty water is oozing up to the surface because we see those shiny spots. Most of the spots are small, indicating low flow. But apparently that’s not the case for Ahuna Mons. New research just released analyzed data from the Dawn spacecraft, which has been orbiting Ceres since March 2015.

There must have been a lot of salty water under the surface at the location of Ahuna Mons, and it kept pushing its way up and out. Mixing with the rock and dust of Ceres, it piled up over time, forming the mountain out of muddy brine.

On Earth, when material flows out of the interior and forms a mountain, we call that mountain a volcano. But on Ceres, the material isn’t lava, it’s ice. So, Ahuna Mons isn’t just a volcano. It’s a cryovolcano. We know of several objects in the solar system with cryovolcanism, including moons of the outer planets, and even Pluto.

And this gives us another piece of the puzzle that is Ceres. Clearly, ice plays a role locally in the surface of Ceres. However, another bit of research shows that its role is less important over long stretches of terrain.

While the surface of Ceres is loaded with craters, it seems to have a deficit of truly big craters; several are expected to have formed from impacts, but only a couple exist (and even those are faint and hard to trace). It looks like the crust of Ceres is weak on the large scale, so that material in big craters can flow around and erase them, but still relatively stiff on local scales, allowing smaller craters to exist.

All of these data are clues to how Ceres formed, how it changed over time, and why it came to look and behave the way it does now. Astronomers and planetary scientists are like sleuths, mulling over the evidence to figure out whodunit. And how.

We’ve only been studying Ceres up close for a year and a half, and look how amazing it is! I wonder what else we’ll find there as we hold our magnifying glass to it.

Oct. 19 2016 9:00 AM

How Big Is Proxima Centauri’s Planet?

Earlier this year, astronomers announced amazing news: The closest star to the Sun, called Proxima Centauri, has a planet! Not only that, but the planet’s size is roughly the same as Earth’s, and at its distance from Proxima it gets about the same amount of heat as the Earth does from the Sun.

In other words, we’re looking at a potentially Earth-like planet.


Potentially. The problem is that we don’t really know how big the planet is; and I mean that literally. Its diameter is unknown, and we need that to get any kind of understanding of what the planet may be like.

Why? Because if the planet (called Proxima Centauri b, or just Proxima b) is small and massive that means it’s dense, and so is will probably be composed of mostly metals and rock. If it’s large and massive it’ll be far less dense, and could be rocky with lots of water. Somewhere in between means it could have the same overall composition as Earth. These are three very different outcomes, and they all depend on the planet’s size. My friend Ian O’Neill has a good article outlining this.

The only way we know to measure the physical size of a planet is if it transits its parent star; that is, the orbit of the star is edge-on as seen from Earth, so we see the planet pass in front of the star once per orbit. When it does that the star’s light drops a bit, and the amount it drops depends on the size of the planet: A big planet blocks more light than a small one. If we know the size of the star (and we generally can determine that) then the amount the light dips tells you the size of the planet.

Knowing this, some astronomers observed Proxima using the Microwave and Oscillations of Stars Telescope (or MOST), a ‘scope with a 15 centimeter mirror that orbits the Earth. MOST is tiny; it only has a mass of 53 kg (that’s less than the mass of a typical adult human) and has a relatively small mirror, but being in space means it’s very stable. It doesn’t have to worry about peering through Earth’s atmosphere, which is wiggly and wavy. That means even a small ‘scope can make very precise measurements of a star’s brightness.

They observed Proxima for more than 40 days, looking for any tell-tale dip in its brightness. And they found one! Better yet, it was about what you’d expect from a planet the size implied by Proxima b’s mass. But the problem is the data are a bit noisy, and the transit inconclusive. It’s possible, even likely, it’s just a statistical fluctuation in the data, and so in the paper they say the odds are actually against the transit being real.

Proxima cen b dip
That dip is just what you'd expect from a planet ... but also from random fluctuations in the data. Drat.

Kipping et al.

That’s disappointing. But not really unexpected; if the planet is roughly the size of Earth, the orbit has to be almost exactly edge-on for us to see a transit. Even a small tilt means it would appear to pass above or below the star from our viewpoint. The chance of a transit was only expected to be about 1.5 percent anyway.

Helpfully, David Kipping from the Cool Worlds group—and also the lead author on the Proxima transit paper—has made a video explaining this, too:

By the way, that tilt affects the mass we find for the planet, too. The planet’s existence was found because as it orbits Proxima, it pulls on the star with its gravity. While the planet makes a big circle around the star, the star also makes a smaller circle. We call that reflexive motion. The more massive the planet, the bigger the reflexive motion of the star.

As a planet orbits its star, the star moves, too.
As a planet orbits its star, the star moves, too.

llustration by NASA/Spaceplace

In the research that found Proxima b, they found the likely mass of the planet is about 1.27 times Earth’s. But that depends on the tilt of the orbit! If the orbit is edge-on we see the reflexive motion maximized; if we see the orbit face-on we won’t see any reflexive motion at all. So the mass we find is the minimum for the planet; it could be larger if the orbit is tilted.

The thing is, at around twice the mass of the Earth, a planet starts to look more like Neptune than our fair world. It has enough gravity to accumulate a thick atmosphere, and would not be Earth-like as we think of it. So knowing the exact mass is important. And we don’t know it.

But there’s hope. As the paper authors point out, observations in the infrared, light outside the range our eyes can see, would help. Proxima is several hundred times brighter in the infrared, making the observations easier. Also, like many red dwarfs, Proxima is a flare star, blasting out huge stellar storms due to its magnetic activity. That makes observing it very difficult; it keeps changing its brightness, and that interferes with planet transit hunting. But in the infrared flares are much dimmer, again making it easier to observe there.

Mind you, the odds of Proxima b actually having an orbit that allows it to transit are long. But not zero! I think it’s worth a big infrared telescope’s time to look for a transit. This is, after all, the closest known planet in the Universe outside our solar system.

I think it’s a good idea to get to know our neighbors.

Oct. 18 2016 9:00 AM

A Spiral Galaxy Defying the Cosmic Flow

I’d like to introduce you to an interesting galaxy today. The reason it’s interesting is because it’s surprising, and in a way that caught me off guard.

It’s called M98 (or NGC 4192; every object in the sky is in multiple catalogs and has multiple handles), and it’s a spiral galaxy much like the Milky Way. It’s located about 50 million light-years away, which isn’t exactly close on a cosmic scale but isn’t all that far away either. If I had to make an analogy, it’s like it’s in the next town over.


We see M98 at a pretty low angle, so it appears nearly edge-on to us; spiral galaxies are pretty flat, and can have wildly different appearances depending on our viewing angle. Still, the spiral pattern is obvious enough, and you can see bright blue regions where stars are being born; those trace the arms. There is also lots of patchy dust along the arms; molecules of silica and aluminum as well as complex carbon-based molecules that are more like soot than anything else.

I like the central region of the galaxy; it’s bright but from this angle is cut in half by a dust lane, distorting the apparent shape of the usually elliptical hub.

All in all, it’s quite lovely, and that shot by the New Technology Telescope really shows it off.

But in that way it’s like a zillion other spirals. So what makes this one special?

Unlike nearly every single other galaxy in the Universe, this one isn’t moving away from us. It’s moving toward us.

There’s no danger of a collision! At its speed of 150 km/sec, it would take a hundred billion years to get here, so don’t wait up. Also, it’s probably not heading directly at us, because it’s part of the Virgo Cluster, a grouping of about  thousand galaxies bound by their own gravity. It’s the closest true cluster to us, and our own small Local Group of a couple dozen galaxies is like a small town near a bigger one. M98 is part of the Virgo Cluster, so it’s in orbit around the cluster center. We’re way outside the cluster, so it can’t hit us.

Here’s the fun bit. The Universe, as you may know, is expanding. One way to think of it is that space itself is getting bigger, and as it does galaxies are swept along with it. Galaxies aren’t really moving away from each other, they’re just floating along with the local flow.

But in many ways it’s like they really are moving away. One way is that their light is redshifted; the wavelength of the light they emit is stretched (it’s very similar to the Doppler effect that makes a motorcycle go EEEEEEeoowwwwwww as it passes you, changing the pitch of the noise). Practically every galaxy in the Universe shows this redshift, and in fact that’s how all this was discovered in the first place. The farther away a galaxy is, the more it’s light is shifted.

But not every galaxy shows it. Close by galaxies have much lower redshifts, and if the galaxy itself is moving rapidly through space (and not just with it), that local velocity will get added to or subtracted from the recession velocity.

One example of this is the monstrous Andromeda galaxy, which is headed toward us at high speed. We actually will collide with it, though not for quite some time (like, 4 billion years). But it shows a distinct blueshift in its light; it’s moving around faster than space is expanding.

M98 is doing the same thing. That surprised me when I saw it in a catalog; it’s far enough away that the Universal expansion should make it recede from us at about 1,000 km/sec.

But then I saw it was in the Virgo Cluster, and I understood. The massive gravity of all those galaxies means they orbit the center at a decent clip, so some galaxies are redshifted more than average as they head away from us, in the part of their orbit taking them to the other side of the cluster. Some have lower velocities because they’re headed toward us in their orbits.

But M98 is still unusual because it can completely overcome the recession of the cluster, and actually be physically headed toward us. That’s almost certainly because it’s recently interacted with another galaxy in the cluster; when galaxies pass each other one can be flung away at high speed, something like a slingshot effect. M98 may very well have done this, and that’s why it’s blueshifted, not redshifted.

As you look to more distant clusters this gets rare or nonexistent, because at that distance the cosmic expansion dominates, and it doesn’t matter how fast the galaxy is moving: It can’t overcome that recession. All galaxies past a certain distance are redshifted, which is yet another reason (among many, many others) that we know the Universe actually is expanding.

That’s pretty cool. I like surprises when I’m reading up on lovely astronomical objects; that means I’ve learned something. M98 is headed toward us, a rare blueshifted galaxy. Huh. That just adds to its beauty and intrigue to me.

It’s a really beautiful Universe, and it’s also a really interesting one. I’d say that’s its best quality.

Oct. 17 2016 9:00 AM

NASA Sets Sight on Mars. I Just Hope Its Aim Is True.

Last week, President Obama reaffirmed that NASA will put humans on Mars “by the 2030s.” In an editorial on the CNN website, he wrote about an initiative that will help enable it: deep-space crew habitats designed by private companies for the long-duration mission. NASA is pitching in $65 million over the next couple of years for the companies to create prototype modules on the ground that can be used to develop the understanding needed to do the real thing.

My response to this is: OK. Sounds cool.


I know, that’s a bit tepid. But I’ll admit I’m conflicted about all this.

First, let’s clear up a small misconception I’m seeing here and there: NASA has been talking about putting people on Mars for years now, and we’ve known for a while their goal has been sometime in the 2030s. The president’s announcement isn’t really about the date; it’s just letting people know that important steps are being taken.

I’m actually pretty happy about this intermediate step. There are a lot of moving parts to a Mars mission, and a critical one is keeping your astronauts alive and happy during the monthslong voyage. Going to the Moon only takes three days, so a small capsule might be a bit cramped but won’t drive the passengers stir crazy.

On a trip to Mars, which’ll take six months or more, you need something roomier. These habitats have to be big enough to give the crew some elbow room, some space (if you will), but also not be too big to get into space in the first place. One of the companies in the running, Bigelow Aerospace, is looking to make inflatable habitats, for example. That might sound odd, but a lightweight and strong material could be packed into a small space for launch, inflated upon reaching orbit, and provide good protection against radiation and small meteoroids. They’re testing one version on the space station right now.

I love this idea; it’s innovative and could very well be the best way to protect people in space for months at a time. It might be how we go to Mars. But the other companies (like my local Colorado’s Sierra Nevada Corp.) will be working on this as well.

But still … going to Mars relies on the Space Launch System, or SLS, rocket and the Orion crew capsule, and as I’ve written many times, I’m not a fan. They are incredibly expensive, won’t fly often, and have a huge political albatross tied around their necks.

SLS in flight
Don't get me wrong: As this artwork shows, SLS will be an amazing thing to see fly.


On the other hand, as my space-writing colleague Eric Berger points out, SLS and Orion are almost certainly inevitable. They have baggage, yes, but they’re being built, right now, and should have a first launch test in 2018 (I’m still skeptical on that, but we’ll see; I’m happy to be proven wrong).

On the third hand, though, both SpaceX and Blue Origin have plans to build very large and powerful rockets. SpaceX has had a series of unfortunate events lately, and the Heavy is behind schedule, but I’d lay odds it’ll be flight-tested before the SLS is upright. Blue Origin is taking slower, more methodical steps (its slogan is gradatim ferociter, “step by step, ferociously”), but it also may very well have a huge booster ready to go by the time SLS takes humans into orbit.

I understand NASA’s desire to have a rocket independent of those—and certainly there’s considerable pressure from Congress on them to build one as well—but I still wonder if it makes more sense to focus instead on private rockets than government ones. And the process through which Orion and SLS came to be leaves a bad taste in my mouth.

That may be why my enthusiasm has been somewhat restrained for NASA’s Mars plans. Sure, as Berger states, it may be time to learn to love SLS, as long as NASA is committed to use it wisely. NASA does seem to understand it needs to fund commercial crew heavily as well, so perhaps in time the right balance will be found. If they find a way to not have to spend the $3-4 billion a year currently used for the space station, that’ll free up a lot of funds for other ventures. A NASA official has talked publicly about handing it over to private ventures as early as 2024, the date when the current budget for ISS operations ends, and has already talked about opening up the station to commercial company use.

But there is more. Another issue I have is that NASA hasn’t really been as forthcoming as I’d like on details of how it plans to go to Mars, and what it plans to do after that first mission. I certainly don’t want a “flag and footprints” mission, framed like the Apollo space race. That doesn’t support sustainability, and if we go to Mars, by damn we should go to stay. What I’m hearing does sound more like a long-term plan for many missions, so that’s good. But some more details would be nice.

I’ll note that in his op-ed Obama mentioned the Asteroid Redirect Mission, which will cost a lot of money and has somewhat nebulous goals. I was for it initially—moving a seven meter asteroid into orbit around the Earth or Moon sounds amazing, and will be useful scientifically—but I cooled on it after I found out it’ll cost well over a billion bucks … and that’s only to retrieve it. To send astronauts to it and study it will cost billions more. I’m not convinced it’s worth that kind of money; smaller, robotic missions make more sense to me at this point. I sometimes wonder if ARM was invented just to give SLS and Orion something to do.

And surrounding all of this is the issue of funding. President Obama can make these statements, but Congress has to approve them. I don’t think it’s too much to note that things are about to change in our government. Certainly Obama will be leaving office, and perhaps Hillary Clinton will take over. While she’s stated she plans to sustain SLS, I really don’t know how firm her commitment is to Mars. As for Trump, if he’s president the least of my worries is NASA. Not so incidentally, the face of Congress may change in November as well, and who knows what it will look like in the late 2020s? Incidentally, for an excellent review of this, once again Eric Berger is your guide.

Look: I am all for going to Mars (and, better yet, going to the Moon first). I’d love to see a human boot print on Mars, especially sometime in the next couple of decades. It’s possible Elon Musk and SpaceX will do it first, though his Interplanetary Transport System is big on flash and short on some important details (at least details we’ve seen). He may very well get there first, and I will applaud if he does. As I’ve said before, I wouldn’t bet against him.

But, to be honest, I like to see NASA being the innovator in situations like these. Let them pave the way, and make it easier for others to follow. I’m glad they have the Red Planet in their sights!  I just hope the path they’ve plotted is one that’ll get us there.

Oct. 14 2016 9:00 AM

The Sky Is Filled With Galaxies

A new paper just published in the prestigious Astrophysical Journal makes a stunning claim: There are 10 times as many galaxies in the Universe as we previously thought. At least. The total number comes in at about 2 trillion of them.

Two. Trillion. Galaxies.


Now, let me be clear. This doesn’t meant the Universe is 10 times bigger than we thought, or there are 10 times as many stars. I’ll explain—I mean, duh, it’s what I do—but to cut to the chase, what they found is that there are lots of teeny, faint galaxies very far away that have gone undetected. So instead of being in a smaller number of big galaxies, stars are divvied up into a bigger number of smaller ones.

What the astronomers did was look at extremely deep images of the Universe taken in surveys, for example the Hubble Ultra Deep Field. Hubble stared at a single point in the sky for nearly 1 million seconds—that’s more than 11 straight days—just seeing what it could see. The result is an image that is staggering in both its beauty and profundity. By counting the galaxies seen in the image, and then extrapolating to the whole sky, you can calculate that there are roughly 100 billion galaxies in the observable Universe.

That’s a lot of galaxies. But wait. It turns out that in this case, there really is more.

Surveys like the UDF are limited. Galaxies that are very faint are hard to see. We know there are small, faint galaxies in the Universe; there are lots of them close to us. Even then many of them are barely detectable because they have so few stars. Remove them to a distance of a few billion light-years and they’re faint. Even Hubble can’t see ‘em.

The astronomers who did this research had an interesting problem. If these galaxies are too faint to see, how do you count them?

The answer is two-fold. One is to look out as far as we can to see all the galaxies we can, and then add up all the galaxies we can see in a given volume of space. By carefully observing these galaxies, we can lump them into bins according to size. So, you find at a given distance there are so-and-so many galaxies with a mass of 10 billion times the mass of the Sun (the mass of the Sun is referred to as a “solar mass”, and it’s a handy unit) in a given volume. In that same volume there are more galaxies with a billion solar masses, and fewer with 10 billion.

Those numbers change with distance. When we look at galaxies really far away, we see them as they were when they were younger, because it takes a long time for their light to reach us. Galaxies really got started forming a few hundred million years after the Universe itself formed, but most were small. Over time they merged together to form bigger galaxies like ours (the Milky Way). So you need to carefully count up all the galaxies in a given volume of space at a certain distance from us, and then do that again for a region of space farther away, and so on.

At the same time, these faint galaxies are easier to see close to us, and harder farther away. So to get an idea of the number when we can’t actually see them, the researchers looked at individual galaxies nearby and figured out what kinds of stars they’re expected to have in them. Most have a few really bright, massive stars, and lots of smaller, fainter ones. The ratio tells you how bright a given galaxy is.

They calculated this for all kinds of galaxies, right down to really small ones with about a million times the mass of the Sun. Galaxies don’t get much smaller than this; objects with lower masses are more like clusters of stars in bigger galaxies, not galaxies themselves.

They can then combine these two pieces of information: How many faint galaxies there are near us and how bright they are, with how many galaxies of a given mass are in a volume of space. When they did that, they could extrapolate to figure out how many really faint galaxies there are at the most distant reaches of space, up to a distance of more than 13 billion light-years. When the light from those galaxies left on their journey to us, the Universe itself was only about 650 million years old!

And that’s how they found that there are at least 2 trillion galaxies in the Universe.

Hubble’s colourful view of the Universe
A small section of the Hubble ultraviolet portion of the Ultra Deep Field. That bright thing is a star in our own galaxy. Pretty much everything else in this shot is a whole galaxy, very very far away.

NASA,ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI

Mind you, just because we don’t see 90 percent of the galaxies in the Universe doesn’t mean this explains dark matter or anything like that. We know that’s not made of any kind of normal matter like the stuff that makes up stars, planets, you, and me. These unseen galaxies are extremely far away, and made of stars and gas and dust just like galaxies here are. It’s just that they’re faint.

And it doesn’t mean the Universe has 10 times more mass than we thought. The mass is the same, it’s just distributed differently than we thought. It’s like knowing there are 1 million people in a city, and finding out they live in 100,000 buildings when you thought they were only in 10,000. There are more buildings, but not more people.

As the authors themselves say:

The total number of galaxies in the universe is an interesting scientific question, although it may not reveal anything fundamental about the cosmology or underlying physics of the universe.

So yeah, this is cool, but not necessarily critical knowledge.

But it does say some pretty interesting things. It means that we should expect to find a helluva lot more galaxies when we take deeper surveys, perhaps with observatories like the James Webb Space Telescope, which should launch in 2018. And it lends a lot of support to the idea that small galaxies formed first in the Universe and grew large as they ate each other. We kinda knew this already, but it’s nice to see independent evidence of it.

And yes, even I have to admit that in the end it’s still just cool. Two trillion galaxies is a lot.

A whole lot. A Universe worth.

Oh, there is another thing, and this one is the coolest of them all. Because we now know how many galaxies there are, how they’re distributed throughout the Universe, and roughly how big they are physically, it’s possible to calculate how much of the sky is covered in galaxies. Think of it this way: If you’re in a very thinly populated copse of trees, you can look around and see things outside the copse; buildings and such in the distance. But if you’re deep within a forest of trees, everywhere you look you see trees.

So the researchers did this, and they found an astonishing thing: Given all the numbers they calculated, it looks very much that every single part of the sky is covered at least in part by a galaxy!

Do you see what this means? No matter where you look—up, down, left, right—and no matter how much you magnify the view through a telescope, at some point wherever you’re looking there’s a galaxy. It might be close by, or more likely crushingly far away, but it’s out there.

The sky is literally covered in galaxies.

How about that? It gives me a chill just to write that. Wow.

But when I read things like this—once the scientific wonder sinks in—I am always struck by a much deeper and far more wonderful notion: That we can know these things! We look up and we think about what we see, and we use math and science and engineering and we count the very essence of the Universe itself.

We are a part of the Universe, we are driven to understand it and ourselves, and that makes us mighty.

Oct. 13 2016 9:00 AM

Earthrise, From the Moon

If you go outside over the next few nights shortly after sunset and look east, you’ll see the waxing gibbous Moon rising. In the twilight you can see quite a few details on its surface with your naked eye. For example, most obvious are dark “seas”—actually called maria, gigantic impact sites that filled with dark basaltic lava.

But looking through Earth’s atmosphere limits our view of the Moon, causing the view to boil and waver. Even our largest telescopes can’t see features much smaller than 100 meters or so across.


And that’s why we send probes there: to see our nearest neighbor even nearer, and plumb its secrets from up close. In September 2007 the Japanese space agency JAXA sent the SELENE probe to the Moon. Nicknamed Kaguya, it took high-resolution images as it circled the Moon in a polar orbit. Twice a year, the orbit of the spacecraft lined up in such a way that it could see the Earth itself rising above the horizon. It took images so rapidly that a movie could be made from them. JAXA just released a huge set of that data, which contained enough images to make such a movie.

So photographer Nicolaus Wegner did just that. The result? Sheer, gorgeous art.

Holy. WOW.

Wegner touched up the imagery a bit, speeding up the frame rate and doing some color balancing. But for the most part what you’re seeing here is what the spacecraft itself saw as it orbited the Moon.

And it’s so beautiful! The lunar surface is so rugged and barren, yet capable of astonishment. There’s so much of it! I didn’t recognize any features until the Tsiolkovsky crater swung in to view at the 1:14 mark, its floor covered in dark basalt like the maria.

But for all the beauty, it’s the Earth that captures our imagination in these views. It’s far brighter than the Moon, reflecting four or more times as much sunlight as the Moon’s surface. And the color … while the Moon is dull and gray, the Earth positively radiates blue and white. My heart aches seeing how familiar it looks over that unfamiliar rocky moonscape.

… but then the end of the video shows the crescent Earth setting and rising, with the glare of the Sun swamping the scene, flooding it with light. That thin crescent may be home, but its phase reminds us that it’s a planet, a world floating in space just like countless others.

There a great many benefits to exploring space, have no doubt. But perhaps the most important, the one reason we must do it, is that it maintains our perspective. It forces us to see our planet and ourselves from literally a different angle, and shows us just how small we are, and how great we can be.

Oct. 12 2016 9:00 AM

S Marks the Spot of a Planet Being Born Around Another Star

About 450 light-years from the Sun, in the constellation of Ophiuchus (yes, that Ophiuchus), lies a young star. If it were off by itself somewhere it would be an unremarkable star, a red dwarf roughly half the mass of the Sun. The galaxy is lousy with them.

But it’s not by itself. It’s sitting in a vast dense cloud of gas and dust sometimes called the Ophiuchus star-forming region. Stars are being born in this huge nebula, and Elias 2-27, as the star is called, is one of them. Stars form as pieces of the cloud collapse under their own gravity (generally instigated by some sort of event that disturbs the cloud like a nearby supernova or a collision with another cloud).


A clump of material shrinks, and flattens into a disk (I describe this in Crash Course Astronomy: Intro to the Solar System; start at time 5:36). The star forms in the center, and out from the center clumps of material form, stick together, grow, and become planets. Sometimes the gravity of a planet can attract enough material from around it that it carves a gap in the disk, similar to the gaps in Saturn’s rings.

Theoretically, a planet forming can cause spiral patterns in the disk, like the spiral arms of a galaxy. One has never been directly seen in a protoplanetary disk, but there have been hints.

That’s changed now. Using the Atacama Large Millimeter/submillimeter Array, or ALMA, which observes light with much longer wavelengths than our human eyes can see, just such a spiral has finally been detected directly. Elias 2-27 has a beauty, too.

context of the star and spiral
Just a little context for you. The Ophiuchus region is pretty amazing. Click to embiggen.


The star itself is buried by dust in the center, too enshrouded to be seen (actually, the dust in the Ophiuchus cloud blocks all the visible light coming from the star and the disk; the submillimeter wavelength light can penetrate that junk and be picked up by ALMA). Just outside that is a classical flat disk, seen as the yellowish ellipse in the center (it’s probably close to circular in real shape, but we see it at an angle). Outside that is a dark gap, which is exactly what you’d expect from a forming planet. The planet itself isn’t visible.

The spiral starts just outside the gap, and the arms extend to about 10 billion kilometers from the star (Neptune orbits the Sun about five billion kilometers out, for reference). This sort of structure is called a density wave pattern; it’s not really a physical structure, it’s regions where the gravity of the disk sets up conditions that particles are denser there. So the spiral arms don’t wind up!

It’s more like a traffic jam. Cars can enter the jam from behind, stay it a while, then move out the front. The jam persists, even as the individual cars move in and out of it; I describe this as well in Crash Course Astronomy: The Milky Way (start at 4:58).

This is just what you’d expect from a planet in that gap; the gravity disturbs the disk, and the density wave pattern arises naturally from there. Given that the arms start there, I’d bet cold cash there really is a planet or other massive object in or very near that gap.

While this structure has been expected to exist in these star- and planet-forming disks, none has ever been seen before, and this shot is so clear! It’s wonderful. ALMA has been a powerful tool for astronomers, peering deep inside clouds, at galaxies, at material surrounding black holes, at objects in the distant solar system, at dying stars (yeah, click that; the image is amazing), and at quite a few forming stars.

The equipment available to astronomers today is nothing short of stunning. We are looking farther and better into the Universe than we ever have before, and what we’re finding is remarkable. Understand: When we look at objects like Elias 2-27, we’re seeing what the Sun may have looked like 4.6 billion years ago!

It’s easy to think of astronomy as looking outward, away from ourselves. But it’s the opposite; the more we look out, the more we see in.

Oct. 11 2016 9:00 AM

Time’s Arrow Explained by Minute Physics

Why does time flow from the past to the future?

That’s an extraordinarily deceptively simple question. It seems so, well, straightforward. But when you start to really investigate it, you wind up going down a rabbit hole of twisty, complicated physics.


When I first started reading about this, I was surprised to learn that it’s tied to entropy. That’s a concept in physics that has a lot of different ways to think about it, but the most common colloquially is to say it’s the degree of disorder in a system. The pieces in a completed jigsaw puzzle are highly ordered, but those same pieces when you first open the box are highly disordered. So the latter has higher entropy.

What does this have to do with time? My friend Sean Carroll—a cosmologist who spends his time thinking about, um, time—and Henry Reich, who draws Minute Physics, collaborated on a series of videos explaining this. As I write this article the first two are out, and they’re intriguing. Here’s the first one:

I can’t wait to see the rest! They’ll be out soon. In the meantime, Sean has books on this topic: From Eternity to Here, which is excellent, and The Big Picture, which I am currently reading right now. It’s also very, very good.

I’ve always struggled with the concept of things like entropy, time, and Boltzmann Brains. Talking with Sean has helped, but reading his books and watching those videos will go a long way, too. It’s amazing to me, as he explains in the video, that the second law of thermodynamics is the only (or one of the only) basic macroscopic physics equations that has time in it explicitly as moving from past to future. Why? Entropy.

It’s like dealing out a hand of five playing cards. There are roughly 2.6 million different hands you could get this way. But only a handful of them have what we would think of as value. A straight, for example, or a flush. If you get 2 3 4 5 6 of hearts, that’s a straight flush, and is extremely ordered. That means it has very low entropy.

Another hand, like 3 6 8 J K, with different suits, is not ordered at all. It has high entropy. Those high entropy, unordered hands are far more common than low entropy, ordered hands. That’s why we value the latter. The odds of getting a straight flush in five card stud are about 1 in 72,000, but 50 percent of the time you won’t even get a pair.

So if you shuffle the cards and deal them, you are far more likely to get a disordered high-entropy hand than an ordered, low-entropy one. That’s why they call it gambling.

Minute physics
Curse you, second law of thermodynamics!

Henry Reich

Another way to look at it is to imagine a container full of gas. On the molecular level, the molecules of gas are distributed more or less randomly. If you swap two molecules with each other the gas looks pretty much the same, and that’s also true if you move a molecule a little bit, say, to the left or right. The number of different ways you could swap or move molecules without changing the nature of the gas is immense, which means it’s incredibly high entropy. If you cool the gas so much it becomes a liquid, or a solid, there are far fewer states each molecule can occupy, so the entropy, the state of disorder, is lower.

If you sit around and watch that gas for a bazillion years, chances are it will always look pretty much the same, even as the molecules move around. The chance of them suddenly liquifying, or all moving to the left side of the container, is extremely small. High entropy states are hugely more likely than low ones. And if you find yourself in a low-entropy state, after a moment the odds are you’ll be back in some high-entropy one.

That’s what Sean and Henry are outlining in those videos. We’re in a relatively low entropy Universe right now. We see it expanding, we see stars dying, we know eventually matter and energy will be more randomly distributed. We’re moving toward a higher entropy Universe, which strongly implies that it was lower entropy in the past. That’s the Big Bang.

Eventually, the Universe will become so disordered that entropy will be maximized. At that point, time has little or no meaning. After all, if you move stuff around and it looks exactly the same, how do you measure time? Time is a measure of the change in events. If everything is the same, time has nothing to measure.

Mind you, I’m still exploring all these concepts, and I’m no expert. But maybe, after watching those videos and reading this, you’ll get a taste of just how deep this runs. It goes straight to the heart of our most basic philosophies, of some of the biggest questions we can ask. Why is there something rather than nothing? Why is that something—the Universe itself—the way it is, and not some different way? Why does time exist, and why does it flow into the future? Why don’t we remember the future and predict the past?

These are heady questions, and I’m thankful that there are people like Sean trying to figure them out, and people like Henry helping the rest of us come along for the ride. It’ll be interesting to see where this goes. Or where it went. Either way.