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Sunshine’s Crazy Sloppy Path to You

This here?

Drawing of an arrow pointing to a little yellow speck in a large black blot
Drawing by Robert Krulwich
Drawing by Robert Krulwich

This is a bit of sunshine. It’s made of pure energy. It has no mass—nothing you can hold, touch, or (accurately) draw. It’s called a photon. Some think of it as a little rat-a-tat of energy packets.

Drawing of a black blog with four little yellow spots in a straight line
Drawing by Robert Krulwich
Drawing by Robert Krulwich

But, being pure energy, it goes so fast (at the speed of light) that its true nature is hard to detect—unless it bumps into something. Here it is colliding with an atom, kicking electrons up into higher, more energetic orbits …

Drawing of star bursts in a line
Drawing by Robert Krulwich
Drawing by Robert Krulwich

… and then, an instant later, those electrons settle back, energy is released, and —whoosh!—our photons are off again. This is what photons do: They get passed from atom to atom, absorbed and spit out, absorbed and spit out. For those of you who like spice in your lives, be grateful you’re not a photon, especially when you remember that most photons are gathered in dense clumps of burning plasma called stars.

Stars are crammed so tight that atoms get crunched, their electrons stripped away to form vast, free-floating electron clouds. So if you’re a typical photon, you spend most of your time slamming into electron after electron. Fwwaaack! Fwwaack! Fwwack! Your energy is absorbed, then released. You may be able to fly at the speed of light, but because you’re stuck in the middle of the sun, when you finish with one electron, you get to swoosh less than 1/63rd of an inch before you’re absorbed again.

Drawing of a sunshine surrounded by pink and purple circles with the capital letter ''E'' on each of them
Drawing by Robert Krulwich

If there were ever a shy photon, one that didn’t like crowds, I imagine that it would yearn to escape the madcap crunch of electrons. I see it deep inside the sun, crawling closer and closer to the surface, electron by electron, until—with any luck—it gets scooped up by one of those giant solar flares and then flung …

Gif of a solar flare
Image courtesy of NASA
Image courtesy of NASA

… Whoosh! … across the quiet, empty highway of space, careening along at a crazy 670,616,629 miles per hour, free, free at last, like a happy racehorse.

This happens to real photons. Some do get free of their stars, do escape into space as solar radiation. And if they happen to crash into Earth, we call them “sunshine,” and when we go to beaches, lie down, and rub ourselves with lotions, we wait for them to bang into atoms and warm us up.

But consider this: We don’t appreciate how long it takes for sunshine to escape the sun. Every bit of sunshine warming your skin has a long history—wonderfully, fantastically, ridiculously long. Next time you’re at the beach looking up, think about this story.

Picture of sunlight filtering through a tree in Yellowstone
Photograph by Tom Murphy, National Geographic Creative
Photograph by Tom Murphy

Imagine A Photon

Let’s start at the very center of the sun, where it’s blazingly hot (10 million kelvins). Here, hydrogen atoms are slamming into each other so hard that their protons fuse and form helium, and with every crash, little bits of pure energy are released. Those are our heros, the photons.

In the crowded middle of the sun, a photon can only move a little way before bumping into another atom. We don’t really know how dense it is in there, but scientists figure our photon hero will zing between a tenth of a millimeter (four thousandths of an inch) to a centimeter (four-tenths of an inch) before its next crash. That’s a crazily small step considering that our photon has to travel 700,000 kilometers to get to the edge of the sun.

Drawing showing the sun and measuring it's radius of 700,000 km with an arrow
Drawing by Robert Krulwich

That’s almost twice the distance from the Earth to the moon. So how many steps is it to the surface?

The Nightmare Begins

You don’t want to know. Because here’s the nightmare: When a photon exits an atom, it can go in any direction. It can go up toward the sun’s surface. Or back to the sun’s center. Or sideways. Or any way. Its moves are totally random.

Mathematicians have a name for this: a drunkard’s walk. It describes a guy so drunk that every step he takes is totally arbitrary, and mathematicians have figured out how long it would take this guy, who’s totally blotto, to get from point A (lamppost number one) to point B (lamppost number two).

Drawing of a drunk man leaning on a light post with a straight arrow drawn to a nearby light post
Drawing by Robert Krulwich

The answer, writes Richard Gaughan for the blog Synonym, is “that if his starting point and ending point are separated by 10 steps, it will take him, on average, 100 [undirected] steps to get there—that’s 10 squared.” Ten times the steps required.

Drwaing of a drunk man leaning on a light pole with a crazy zig zagged path of arrows leading him to a nearby light post
Drawing by Robert Krulwich
Drawing by Robert Krulwich

The same goes for our photon. By the time it has zigged then un-zigged, zagged then un-zagged its way to the sun’s edge, it will have had billions, maybe trillions of collisions in every direction.

So to our big question: how long will that take?

49 Trillion Trillion Collisions

Well we can figure this out. If we assume the sun is dense with electrons, and each little “step” is a tenth of a millimeter, to go straight from the center of the sun to the edge will take 7 trillion steps.

Drawing showing a zig zagged path out from the center of the sun, creating a tangled mess until the path reaches the edge of the sun and moves in a straight line
Drawing by Robert Krulwich
Drawing by Robert Krulwich

But because our photon is a drunk, its true path will take the square of 7 trillion steps, which works out to 49 trillion trillion collisions before it reaches the surface. Even moving at the speed of light, that will take, more than half a million years.

Half A Million Years!

That’s a long time to wait to become sunshine.

Or …

On the other hand, if we assume a slightly emptier sun, with the traveling distance between electrons a bigger whole centimeter, that works out to fewer steps (only 490 billion trillion) to the sun’s surface—and that in turn works out to journey that lasts roughly 5,000 years.

5,000 years?

But even 5,000 years is a long time. A photon created 5,000 years ago landing on Earth today started its virtual journey around the time the great pyramids were being constructed in Egypt.

If you choose to think about sunshine this way (and I realize photons, being massless, aren’t individuals, and strictly speaking, they can’t be characters in a drama), but if you let the poet in you dance a little, you can go to the beach this weekend, feel the sunshine on your face, and think about how long it took for that to happen.

And whether you’re getting a hit of warmth that’s half a million or 5,000 years old, remember this: Roughly eight minutes ago those photons were still part of the sun, still banging their way through dense throngs of electrons. But once they got flung into space, they raced across the cosmos at the speed of light, wind in their photon hair, and eight or so minutes later, they banged into Earth, and after bouncing through our atmosphere, they settled on you.

Yes, it took a ridiculously long time for photons to get to the edge of the sun, but that last leap to you?

It was short. Crazily, joyously short.

Thanks to Aatish Bhatia for trying to help me with the physics. If I made mistakes, they’re mine, not his, but like a good friend, he tried valiantly to keep me out of trouble. For those of you want a denser look at the math, I recommend “Ask the Space Scientist”, from NASA, where Dr. Sten Odenwald concludes: “…it takes a LONG time for light to leave the sun’s interior.” But here’s a couple of paragraphs…

“The interior of the sun is a seething plasma with a central density of over 100 grams/cc. The atoms, mostly hydrogen, are fully stripped of electrons so that the particle density is 10^26 protons per cubic centimeter. That means that the typical distance between protons or electrons is about (10^26)^1/3 = 2 x 10^-9 centimeters. The actual ‘mean free path’ for radiation is closer to 1 centimeter after electromagnetic effects are included. Light travels this distance in about 3 x 10^-11 seconds. Very approximately, this means that to travel the radius of the Sun, a photon will have to take (696,000 kilometers/1 centimeter)^2 = 5 x 10^21 steps. This will take, 5×10^21 x 3 x10^-11 = 1.5 x 10^11 seconds or since there are 3.1 x 10^7 seconds in a year, you get about 4,000 years.

Some textbooks refer to ‘hundreds of thousands of years’ or even ‘several million years’ depending on what is assumed for the mean free patch. Also, the interior of the sun is not at constant density so that the steps taken in the outer half of the sun are much larger than in the deep interior where the densities are highest. Note that if you estimate a value for the mean free path that is a factor of three smaller than 1 centimeter, the time increases a factor of 10!”

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The Close of Cosmos, and Golden Voices in the Stars

Two golden records, each carrying the sights and sounds of planet Earth, are hurtling toward the stars at thousands of miles per hour. Borne into the sky by the twin Voyager spacecraft, these interstellar time capsules are coded in the key of science. They are Earth’s emissaries to the cosmos, chosen to represent our planet if ever the disks, each the size of a large dinner plate, are clutched by alien hands.

As Cosmos: A Spacetime Odyssey reminded us during its final episode, “No other objects touched by human hands have ever ventured this far from home.” Indeed, the two Voyager spacecraft are sailing ever farther from home, exploring the solar system’s frontier like the maritime surveyors of yesteryear.

Except these explorers are never coming back.

For the next decade or so, the spacecraft will continue to send messages full of data to their home planet. But then, eventually, their power supplies will dwindle and those messages will fade. As the silent spacecraft sail through an even quieter cosmic sea, their primary mission will no longer be to gather data. It will be to ferry the murmurs of Earth, inscribed onto those golden records, through the darkness.

The committee tasked with selecting the Golden Record’s playlist, chaired by Carl Sagan, had about six months to figure out what to put on there. Among others, Beethoven, Peruvian panpipes, the roar of a rocket launch, photos of Jane Goodall and the Great Wall of China, and greetings spoken in 55 languages made it aboard.

My mom, Amahl, is one of those Voyager voices. She’s speaking her native Arabic, and her message is simple: “Greetings to our friends in the stars. We wish that we will meet you someday.”

One day, late in the spring of 1977, Linda Salzman Sagan (then Carl’s wife) had asked my mother for help. She wanted to know if Mom would lend her voice to the Voyager record. Linda was searching for fluent speakers of the many languages to be featured on the record, preferably people living near Ithaca, New York. Mom, who was already helping gather photos for the record and who worked on Cornell University’s Ithaca campus, was ideal.

So she carefully composed a greeting expressing the desire to make contact with the beings that might listen to her voice. She practiced for days. Mom says she was most concerned about speaking in her clearest, most rhythmic Arabic. “I was speaking on behalf of all the millions of people who speak Arabic,” she says. “And I wanted to do it justice.”

Mom was nervous when she recorded her 6-second greeting. But then the task was over, her voice immortalized. “It was pretty organized. You walk in, do your greeting, and off you go,” she recalls. It was early June.

In August, she and my dad went to Cape Canaveral, Florida, where they watched the launch of Voyager 2 on August 20, 1977. Voyager 1 launched 16 days later. And now, 37 years later, the spacecraft are still speeding toward that undiscovered country.

Mom and Dad watch the Voyager 2 launch, along with my brother Paul and TK (left).
Mom and Dad watch the Voyager 2 launch, along with my brother Paul and Bill Howard (left), then at NSF. (Amahl Drake)

As Mom watched this final episode of Cosmos, and the part describing Voyager, she says she was struck by a tremendous emotional punch. “Of all the people who are inhabiting this planet, only a few of us got to have our voices on [the record]. And even though it may or may not ever be intercepted, just the idea of it representing human beings and floating through space…,” she says. “That’s quite humbling.”

I like knowing that my mom’s voice, as it sounded in the spring of 1977, is boldly going where no spacecraft has gone before. The golden records, carrying traces of our planet into the cosmos from which it came, will live for another billion years. Hers and the other Voyager voices, plus the songs and sounds and images on the record, could very well be the longest-lived traces of Earth, the longest-lived pieces of evidence proving that humanity once existed.

mom voyage
Voyager 2 heads for the stars on August 20, 1977. (Amahl Drake)
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Venus Is More Than Just a Cautionary Tale

Venus shines more brightly in Earth’s sky than any of our stars, but it gets surprisingly little attention nowadays (unless it’s being used as an example of a planetary What Not To Do).

Not that long ago, though, humans seemed rather interested in hurling tons of hardware at our sister planet, which is roughly the same size as Earth. From the 1960s through the early 1980s, Venus was the target of 19 Soviet space missions. The Venera program launched 16 probes at the cloudy world and managed to grab several photos from the planet’s parched surface. NASA has sent a half-dozen spacecraft to the planet – the most recent being Magellan, which orbited Venus between 1989 and 1994, and mapped 98 percent of the planet’s surface.

Venera 13 Lander image of the surface of Venus. The probe survived on the surface for two hours, in 1982. Soviet Planetary Exploration Program/NASA)
Venera 13 Lander image of the surface of Venus. The probe survived on the surface for two hours, in 1982. (Soviet Planetary Exploration Program/NASA)

It took nearly a decade for the next spacecraft to arrive at our nearest neighbor. Since 2006, the European Space Agency’s intrepid Venus Express spacecraft has been exploring the sulfuric planet. The mission has revealed much about the mysterious world that hides beneath its cloudy shroud. But later this year, when the spacecraft plunges through the 250-kilometer thick clouds and falls to the planet’s surface, Venus will again be alone. At least until next year, when the Japanese space probe Akatsuki is supposed to arrive. Our sister planet was once rather Earthy in many respects, so Earthy that it’s possible life may have gained a feeble foothold early on. Billions of years ago, scientists say, Venus had oceans, and a pleasant climate. But those oceans evaporated and the planet fell prey to a runaway greenhouse effect. The once-friendly, fertile world was transformed into a roasting, hellish home.

Anyway. In addition to its role as a warning to others, Venus offers a wealth of fun and crazy facts. Here are a few Venusian wonders.

1. Venus endlessly traces the same series of five shapes in Earth’s sky. The Mayans used these shapes, and Venus, as a basis for their cosmology and calendar.

2. Venus will always be a morning or evening “star.” It will never be a mid-day star, as Mars and Jupiter can be, because it orbits in between the sun and Earth.

3. The surface of Venus, at roughly 840 degrees F, is the hottest place in the entire solar system (aside from the sun, of course). It’s even hotter than Mercury.

4. Difference in temperature between day and night on Venus: 0 degrees.

5. Difference in temperature between the planet’s equator and poles: 0 degrees.

6. Reason for #4 and #5: That enormously thick, carbon dioxide atmosphere redistributes heat very efficiently. In other words, if you’re on Venus and you need to cool off, your best bet is to go up. Roughly 30 miles up, where the pressure and temperature finally relax and become something Earth-like (see #8).

7. But you wouldn’t want to be on Venus, because you’d die. The pressure on the planet’s surface is 90 times that of Earth (so, roughly the equivalent of being beneath 8,000 feet of water).

8. That doesn’t mean the planet is necessarily lifeless. It’s possible that organisms could live in those acid clouds, which contain water, energy, and nutrients.

9. Also, Venus might be actively volcanic. Scientists are still working on sorting this out, but evidence from the surface suggests geologically recent volcanic eruptions (within the last million years).

10. Greenhouse gases create efficient incubators. Venus’ thick shroud reflects roughly 80 percent of the sunlight that hits the planet. This means that the small amount of sunlight that does get through is enough to superheat the planet – thanks to those gases and that massive atmosphere.

Thank you to astrobiologist David Grinspoon, author of Venus Revealed, for helping me with this post!

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An Alien Origin for Life on Earth

This is the second of three blog posts associated with this week’s episode of Cosmos: A Spacetime Odyssey, which addresses life in the universe. Read the first one here.

Whether the universe is filled with alien beings who wish upon stars, struggle to understand the subatomic realm and argue over who’s paying for dinner is not yet known. We’re looking for them.

But in the search for life, there’s another fundamental question that has gone unanswered for millennia. Step one in the development of any civilization is life itself. How, exactly, does life get going?

“The essential message of life has been copied and recopied for more than three billion years,” says Neil deGrasse Tyson, on this week’s episode of Cosmos: A Spacetime Odyssey. “But where did that message come from?”

Even on Earth, the origin of that on-switch is murky. We don’t know how a pile of organic molecules, their atoms arranged in intricate rings and bridges, gained the ability to survive and replicate, to wall themselves off from a young Earth’s iron-rich seas and oxygen-free air.

Some ideas suggest that life’s first gasp came from shallow ponds, warmed by a sun still in its childhood; others point to bubbling hot springs, clays, ice, or to warm, energy-rich vents erupting from the deep ocean floor.

For decades, scientists have tried to replicate the planet’s primordial recipe for life. They’ve mixed salty brews, spiced them with metals and smelly gases, and jolted the mixes with electricity, or sunlight, or heat, then reset the timer and started all over with a new handful of ingredients and instructions. These experiments have taught us a lot. Among other things, we’ve learned that amino acids are sort of easy to make from scratch, that complex metabolic pathways can emerge from a seemingly random mix of ingredients, and that single-stranded, ribonucleic enzymes can replicate themselves indefinitely.

But none of these experiments have produced the secret sauce that sparked the first single-celled organisms. Each time, when the oven timer chirped, there was no life.

Enter: Another theory that’s been simmering for years (millennia, even). What if, it asks, instead of being baked from scratch on Earth, life came from the stars?

“If life can withstand the hardships of space, and endure for millennia, then it could ride the natural interplanetary transit system from world to world,” Tyson says. “What this means is that life doesn’t have to start over again.”

Illustration of the 1833 Leonid Meteor shower. Could meteorites have carried life to Earth? (Edmund Weiss/Wikimedia)
Illustration of the 1833 Leonid Meteor shower. Could meteorites have carried life to Earth? (Edmund Weiss/Wikimedia)

Called panspermia, the theory suggests that organisms hitchhiking from one world to another can spread the organic seeds of life throughout the cosmos. Launched into space aboard blasted out bits of planetary debris, these space-faring life-forms could, upon arrival at an alien planet, survive and thrive – perhaps evolving into spiders, sharks (or spidersharks?), dandelions and elephants.

Obviously, no one knows whether panspermia actually happens. For years, the idea failed to gain strong scientific traction. But recent pieces of circumstantial evidence suggest that in some environments, such as the inner solar system, versions of panspermia aren’t so farfetched.

For starters, fragments of other planets have made their way to Earth. We have pieces of Mars and Mercury (maybe), and (probably) Venus on our planet. Pieces of Earth have undoubtedly made their way to our neighbors. This exchange of crusty planetary material, if it harbored the right kind of hardy organism, could conceivably transfer life from one world to the next, says astronomer Caleb Scharf of Columbia University.

“I’d say that a plausible, but entirely unproven, mechanism exists for the transfer of viable organisms,” Scharf says.

There are creatures on Earth that would probably consider an interplanetary trip a worthy challenge. Take tardigrades, for example, the tiny, tough invertebrates that have survived 10 days in space. Lichens have survived the same freezing vacuum for more than two weeks. Some microbes, like Deinococcus radiodurans, are especially tolerant of the levels of radiation they’d likely encounter during a trip to Mars. And organisms frozen for centuries beneath the ice in Antarctica have been revived in labs.

“We have no reason to believe that some microbes can’t survive interplanetary journeys inside of meteorites,” says astrobiologist David Grinspoon of the U.S. Library of Congress.

But, he says, spending two weeks in space and living to tell the tale is different from crash-landing after a decades-long interplanetary voyage and setting up shop in a new world. It isn’t enough to simply arrive – organisms have to thrive.

“We tend to separate the possibility of exchange of viable organisms between planetary bodies and the possibility that they can ‘seed’ a world,” Scharf says. “It’s just not clear that even the hardiest Earth microbes, dumped supersonically onto Mars (for example), are going to get a foothold. The Martian surface is nasty for terrestrial biology.”

Interplanetary panspermia as a dispersal mechanism seems fairly plausible, then, if unproven. Is it possible that young Earth, Venus, and Mars traded life-forms for a few hundred million years? (See Scharf’s treatment of a panspermic paradox here.)

Cosmos also introduced the idea of interstellar panspermia, which magnifies all the challenges associated with the inner solar system’s planets playing meteorite ping-pong. In other words, it’s a bit trickier to transport life from stellar system to stellar system. Distances are much greater, and the time it would take for a meteorite bearing life-forms to arrive on another world is substantially longer. To some scientists, it seems unlikely that such a thing is possible.

“Eventually, cosmic radiation would shred the genetic material beyond any ability to self-repair,” Grinspoon says.

On the other hand, he notes, stars are born in clusters. And at the age when young stars are busy assembling their planetary systems, the distances between them are much smaller. Our solar system would have exchanged material with these systems, Grinspoon says. Perhaps it’s during this period of stellar infancy that stars stud their sister systems with seedlings.

And there’s a third version of panspermia waiting in the wings, one proposed by Francis Crick and Leslie Orgel in 1973: Directed panspermia, or the idea that intelligent beings intentionally send life to other worlds.

“It now seems unlikely that extraterrestrial living organisms could have reached the earth either as spores driven by the radiation pressure from another star or as living organisms imbedded in a meteorite,” they wrote. “As an alternative to these nineteenth-century mechanisms, we have considered Directed Panspermia, the theory that organisms were deliberately transmitted to the earth by intelligent beings on another planet.”

But the pair concludes that there’s feeble evidence supporting the deliberate seeding of Earth with alien life, and similarly feeble evidence supporting the abiotic emergence of life on Earth. “Both theories should be followed up,” they wrote.

The idea that faraway, intelligent beings – or perhaps the bored teenagers of the species – might be intentionally hurling life at planets is truly science fiction. But it’s a universe of infinite possibilities, right?

“If you imagine that intelligent, technological life exists on other worlds – which I do imagine,” Grinspoon says, “Then what would lead you to conclude that nobody anywhere in the galaxy has ever tried such a stunt?”

Looking toward the stars for our origins seems, perhaps, like the kind of explanation one ought to turn to when all other attempts to flick life’s on-switch have failed. It’s more than plausible that the building blocks of life – amino acids, nucleobases, sugars – were delivered to Earth by asteroids or comets. We know that asteroids in the solar system are carrying complex organic molecules. And not only that, complex organic molecules have been spotted wafting through interstellar space.

Put more simply, the galaxy is littered with the building blocks of life. But it could also be littered with life itself. And maybe, from across the interstellar sea, some of those organisms came to Earth, crawled out of their space rocks and flourished on their new cosmic shores.

NASA researchers have found the building blocks of DNA in a meteorite. (NASA's Goddard Space Flight Center/Chris Smith)
NASA researchers have found the building blocks of DNA in a meteorite. (NASA’s Goddard Space Flight Center/Chris Smith)


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Celebrate Hubble’s 24th With Giant Galaxies, Dying Stars and Cosmic Chaos

Twenty-four years and two days ago, on a Tuesday morning, the space shuttle Discovery hitched a ride to low Earth orbit from Cape Canaveral, Florida. Aboard the shuttle? NASA’s newest eye in the sky, the Hubble Space Telescope, an instrument capable of peering deep into the cosmos and capturing the universe’s inhabitants in exquisite detail. It had taken decades of design and planning to get the telescope ready for work. The next day, on April 25, astronauts delivered the telescope to space.

Then, scientists eagerly waited for Hubble to start revealing cosmic secrets.

But a flaw in the telescope’s primary mirror meant the images weren’t sharp. Observing incredibly faint objects, such as very distant galaxies, wasn’t possible. It would be three years before the first of five servicing missions let astronauts correct the defect and upgrade Hubble’s vision to what it should have been.

Since then, though, the Hubble space telescope has continually delighted Earthlings with its breathtaking views of stars, galaxies, and our planetary neighbors. Its impact on science has been no less important. Among other discoveries, Hubble helped scientists determine that the universe is expanding at an accelerating rate. This discovery, which happened in the late 1990s, is something we still can’t fully explain.

Every day, tales of life and death in the universe are told through faraway supernovas, galactic collisions and clusters, and violent stellar nurseries. These stories are often accompanied by profoundly beautiful images. Some of these, like the million-second-long exposure that produced the Hubble Ultra-Deep Field, need to be viewed full-size for the appropriate amounts of cosmic oomph. Others, like the Pillars of Creation, have become extremely well-known — looking at these photos can be like seeing the smiling face of an old friend.

Here, in honor of Hubble’s 24th launchiversary, are 25 images that might be slightly less familiar…and I’ve added one to grow on, just for good measure.

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Milky Way Has 4 Billion Years to Live — But Our Sun Will Survive

Four billion years from now, our galaxy, the Milky Way, will collide with our large spiraled neighbor, Andromeda.

The galaxies as we know them will not survive.

In fact, our solar system is going to outlive our galaxy. At that point, the sun will not yet be a red giant star – but it will have grown bright enough to roast Earth’s surface. Any life forms still there, though, will be treated to some pretty spectacular cosmic choreography.

Currently, Andromeda and the Milky Way are about 2.5 million light-years apart. Fueled by gravity, the two galaxies are hurtling toward one another at 402,000 kilometers per hour. But even at that speed, they won’t meet for another four billion years. Then, the two galaxies will collide head-on and fly through one another, leaving gassy, starry tendrils in their wakes. For eons, the pair will continue to come together and fly apart, scrambling stars and redrawing constellations until eventually, after a billion or so years have passed, the two galaxies merge.

Then, the solar system will have a new cosmic address: A giant elliptical galaxy, formed by the collision and merger of the Milky Way and Andromeda.

This isn’t a chapter ripped from science fiction – it’s a real, scientific prediction. That science can forecast such events was the focus of the third episode of Cosmos: A Spacetime Odyssey. That Newton could describe the orbits of planets, and Halley the return of his eponymous comet, and contemporary astronomers, the end of the Milky Way – this gift of foresight is really a mathematical understanding of the physical laws that govern the movements of celestial bodies.

“Using nothing more than Newton’s laws of gravitation, we astronomers can confidently predict that several billion years from now, our home galaxy, the Milky Way, will merge with our neighboring galaxy, Andromeda,” host Neil DeGrasse Tyson says. “Because the distances between the stars are so great compared to their sizes, few if any stars in either galaxy will actually collide. Any life on the worlds of that far-off future should be safe, but they will be treated to an amazing, billion-year long light show.”

The galactic collision that closes out the third Cosmos episode follows the sequence in the animation below, which is based on a 2006 simulation by astrophysicist Brant Robertson*.

Video: NASA/Vimeo

Now, how on Earth do we know this is going to happen?

The story starts in the early 1900s, when astronomer Vesto Slipher measured the radial velocity of Andromeda — in other words, he calculated the speed at which the galaxy was moving toward or away from Earth. Slipher did this by looking for a telltale stretching or compression in the light from Andromeda arriving at Earth: Light from objects that are moving away from us is slightly stretched, or red-shifted. Light from objects moving toward us is blue-shifted, or compressed.

The result was a little bit surprising.

“We may conclude that the Andromeda Nebula is approaching the solar system with a velocity of about 300 kilometers per second,” Slipher wrote in the Lowell Observatory Bulletin in 1913 (Andromeda was called a nebula back then because astronomers didn’t realize it wasn’t part of the Milky Way; Slipher’s calculation strongly suggested that idea needed rejiggering).

So Andromeda was zooming toward us – that much at least seemed clear. Whether its arrival would mean the end of the Milky Way was still uncertain. For decades, scientists had no way of knowing whether Andromeda and the Milky Way would collide head-on, or if they would slip past one another like star-filled vessels in the cosmic night.

Turns out, it’s relatively easy to measure the velocity of faraway objects moving toward or away from us, but much more difficult to determine their sideways motion (something astronomers call “proper motion”). The farther away something is, the harder it is to measure its sideways motion, which doesn’t produce those telltale stretched or compressed wavelengths that astronomers can work with. Instead, astronomers rely on detailed observations of an object’s position relative to background stars – a small and subtle shift that without superior telescopes can take centuries to become apparent.

Artist's conception shows the future Earth, whose oceans have boiled away due to the Sun's increasing heat, exiled to the outskirts of the new merged galaxy that astronomers have dubbed "Milkomeda."  (David A. Aguilar, CfA)
Artist’s conception of the future Earth, whose oceans have boiled away due to the Sun’s increasing heat, exiled to the outskirts of the new merged galaxy that astronomers have dubbed “Milkomeda.” (David A. Aguilar, CfA)
Around 2007, Harvard University astrophysicist Avi Loeb decided to revisit the question of Andromeda’s impending arrival. “Most theorists are interested in reproducing systems from our past that are observed now, and are reluctant to make predictions that will be tested only billions of years from now,” Loeb says. “The rationale was unclear to me; I am curious about the future as much as I am about the past.”

Loeb and then post-doc T.J. Cox simulated the impending collision and merger of Andromeda and the Milky Way using estimates of Andromeda’s proper motion. Their results showed a better than decent chance of the two galaxies smashing into one another, and a pretty good possibility of the solar system being punted to the outskirts of the resulting elliptical galaxy, which Loeb named “Milkomeda.”

In 2012, a team of astronomers based at the Space Telescope Science Institute re-did the collision calculations, this time using direct measurements of Andromeda’s proper motion. After all those years, the team was able to get those measurements with the Hubble space telescope – and an observing campaign that used years of data, beginning with images snapped in 2002.

“We compared images taken at different times with the Hubble Space Telescope, and measured how much the Andromeda stars have moved relative to the fuzzy galaxies in the distant background,” says astronomer Sangmo Tony Sohn. “This gives us a sense of how fast the Andromeda stars moved across the sky.”

The team concluded that Andromeda’s proper motion was tiny – and that a head-on collision was pretty much inevitable. That might sound a little bit traumatic, but it’s not all that unusual for galaxies to merge. The Hubble space telescope has captured some glorious images of faraway mergers and collisions, and astronomer Halton Arp included a number of galactic interactions in his “Atlas of Peculiar Galaxies,” published in 1966. They’re all really pretty.

The good news is that, as Tyson says, stars are so far apart that even though galaxies are colliding, the probabilities of stellar collisions are small. So the sun and its planets will likely survive the birth of Milkomeda, though Earth will no longer be able to call the Milky Way home. And we’ll no longer live in a spiral galaxy: Milkomeda will be elliptical in shape, and it’ll probably look pretty red, which you can see toward the end of the 2012 team’s animation, and in the animation above.

So there’s no doubt this merger is going to be a spectacle – and there’s a good chance that the Triangulum, a smaller, nearby galaxy, will get sucked into the fray. I, for one, am disappointed that I won’t be able to watch this great cosmic light show. For now, the best I can do is enjoy the sequence of illustrations below.

Present day; 2 billion years from now; 3.75 billion years; 3.85 billion years; 3.9 billion years; 4 billion years; 5.1 billion years from now; and 7 billion years from now, when the galaxies have formed a huge elliptical galaxy. (NASA/ESA/Z. Levay and R. van der Marel, STScI/T. Hallas and A. Mellinger
Present day; 2 billion years from now; 3.75 billion years; 3.85 billion years; 3.9 billion years; 4 billion years; 5.1 billion years from now; and 7 billion years from now, when the galaxies have formed a huge elliptical galaxy.

*9:45pm PDT, 3/24: This post has been updated to attribute the embedded animation to astrophysicist Brant Robertson, now at the University of Arizona, and his colleagues. NASA recently redid the animation.

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A Guide to Lonely Planets in the Galaxy

Rogue planets are homeless worlds. They have neither sunrises nor sunsets, because unlike the planets we’re more familiar with, these lonely worlds aren’t tethered to a star. Instead, they travel in solitary arcs around the Milky Way’s core.

Earlier this week, Cosmos: A Spacetime Odyssey, introduced many of its viewers to the concept of these lonely planets.

“The galaxy has billions of them, adrift in perpetual night. They’re orphans, cast away from their mother stars during the chaotic birth of their native solar systems,” Neil DeGrasse Tyson says, as a planet emerges from the darkness. “Rogue planets are molten at the core, but frozen at the surface. There may be oceans of liquid water in the zone between those extremes. Who knows what might be swimming there?”

In the days that followed the show’s premiere, social media sites lit up with questions from viewers wondering what, exactly, these rogue worlds are — and could there really be billions of them, as Tyson said?

(The answer is yes. Probably.)

For decades, astronomers hypothesized that free-floating planets existed. But scientists needed a way to find them. The two most well-known ways of finding exoplanets rely on telltale signals coming from the planets’ stars – either wobbles caused by the gentle tugs of an orbiting planet’s gravity, or the slight dimming produced when a planet passes between Earth and its star.

So how do you find planets that have no stars?

For now, the best methods include looking for a young rogue’s heat in the infrared, and a technique called gravitational microlensing that works well for older, cooler planets, says astronomer David Bennett of the University of Notre Dame. Microlensing takes advantage of gravity’s ability to bend and mess with light. If a massive object – say, a rogue planet – passes between a star and Earth, the planet can act as a lens, curving and tweaking the star’s light as seen from Earth. In general, the more massive the planet, the more affected the light.

So far, using either method, we can’t easily detect starless planets that are smaller than a Jupiter, or at least 300 times the mass of Earth.

ESO/P. Delorme
Free-floating planet CFBDSIR 2149-0403 is the blue spot marked by crosshairs in the center of the infrared image. Click to enlarge. ESO/P. Delorme
This image captured by the SOFI instrument on ESO’s New Technology Telescope at the La Silla Observatory shows the free-floating planet CFBDSIR J214947.2-040308.9 in infrared light. This object, which appears as a faint blue dot at the centre of the picture and is marked with a cross, is the closest such object to the Solar System. It does not orbit a star and hence does not shine by reflected light; the faint glow it emits can only be detected in infrared light. The object appears blueish in this near-infrared view because much of the light at longer infrared wavelengths is absorbed by methane and other molecules in the planet's atmosphere. In visible light the object is so cool that it would only shine dimly with a deep red colour when seen close-up.

Anyway, early observational hints of these untethered worlds turned up in the late 1990s, when a team of Japanese astronomers found evidence for warm, planetary mass objects in the Chamaeleon cluster, about 500 light-years away. Other teams soon reported more rogue candidates, in a cluster near the star sigma-Orionis, in the Orion nebula, in the Taurus star-forming region. More recently, in 2012, astronomers described a hot (700 degrees Celsius) homeless planet, clunkily named CFBDSIR2149-0403, just 100 light-years away.

Evidence for the “billions” parts of Tyson’s statement arrived in 2011. A microlensing study published in Nature suggested the Milky Way contains at least 400 billion star-less worlds, that the lonely planets are more common than stars like our sun. Data from two microlensing consortiums, known by the acronyms OGLE and MOA, pointed toward 10 possible free-floating planets, spotted over a two-year-long survey aimed toward the Milky Way’s galactic bulge.

Based on comparisons between the surveys’ detection efficiency, the probability of microlensing events, and the expected amount of lensing caused by planets and stars, the team concluded that these planetary lenses were everywhere. “There are statistical uncertainties in the analysis,” says Bennett, a member of the MOA consortium. “Four hundred billion planets is probably a good lower limit.”

But not everyone is convinced. Despite careful work by the authors, it’s still possible the objects detected are just very far from their stars, that they’re brown dwarfs (a type of low-mass pseudo-star that failed to ignite nuclear burning in its core), or that the galactic population estimates are off.

Since 2011, though, MOA has been hard at work analyzing larger data sets and refining estimates for how many free-floating planets populate the Milky Way. So far, Bennett says, new estimates appear to support the original finding that these rogue planets are really common. And, there are hints that we may soon be able to find smaller, untethered planets about the mass of Neptune – much too small to be mistaken for a failed star.

Southwest Research Institute
Artist’s conception of the solar system’s lost giant planet. Southwest Research Institute

Now, about those chaotic early years. Astronomers suspect that many free-floating planets are wandering through interstellar space because they’ve been kicked out of their home stellar systems. This process tends to happen early in a system’s history, says astrophysicist Greg Laughlin of the University of California, Santa Cruz. As planets in young systems settle into their orbits, their gravitational jostling can end up sending a sibling or two into space.

It’s possible that something like this happened in our solar system. Theories describing the early solar system don’t really work unless a fifth giant planet – another Uranus or Neptune – were present at the start (one of the problems with these models is that Earth sometimes ends up running into Venus, which we know didn’t happen). Later, as the planets begin to migrate, that fifth giant is kicked out of the solar system and sent flying into space.

Where it is now is anyone’s guess. “The damn thing could be half-way across the galaxy, for all we know,” says Konstantin Batygin, a post-doc at the Harvard-Smithsonian Center for Astrophysics.

Ok. What about the second part of Tyson’s quote? Could these worlds really have molten cores and subsurface oceans?

Perhaps surprisingly, the answer is yes. This part of the narrative echoes a paper published in 1999 by Caltech planetary scientist David Stevenson, who considered how Earth-mass planets cast from their solar systems might fare in outer space. Stevenson suggests that if these planets retained a hydrogen atmosphere, they could stay warm enough to have liquid water on their surface. A subsurface ocean could be present even without an atmosphere. And, larger planets are generally warmer than smaller planets, says Stevenson, who calculated that a cast-off Jupiter would only cool by about 15 Kelvin at its surface.

Let’s hope there are critters — preferably plesiosaurs or laser sharks — swimming in those rogue, subsurface seas.

 ESO/L. Calçada/P. Delorme/R. Saito/VVV Consortium
Artist’s representation of rogue planet CFBDSIR2149-0403. ESO/L. Calçada/P. Delorme/R. Saito/VVV Consortium
This artist’s impression shows the free-floating planet CFBDSIR J214947.2-040308.9. This is the closest such object to the Solar System. It does not orbit a star and hence does not shine by reflected light; the faint glow it emits can only be detected in infrared light. Here we see an artist’s impression of an infrared view of the object with an image of the central parts of the Milky Way from the VISTA infrared survey telescope in the background. The object appears blueish in this near-infrared view because much of the light at longer infrared wavelengths is absorbed by methane and other molecules in the planet's atmosphere. In visible light the object is so cool that it would only shine dimly with a deep red colour when seen close-up.