The entire universe in blog form

April 23 2014 12:00 PM

Hubble Drills Deep Into the Universe

What happens when you take a 2.4-meter telescope, launch it into space, and command it to stare at one spot in the sky for a solid 14 hours, taking data both in visible light (like our eyes see) and infrared?

This. This.

Hubble’s cross-section of the cosmos
Give me a moment to tell you why this image will destroy your brain. Click to nicely galactinate, or click here to hugely do so.

All photos by NASA/ ESA

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Can I get a “Yowza!” from the congregation? No? Maybe that’s because when I shrink this Hubble Space Telescope picture down to fit the blog you can’t really get a sense of what you’re seeing here. So click the picture to get the 1280 x 1280 image, or better yet, do yourself and your eyeballs a favor and take a poke at the huge 3900 x 3900 pixel version, because holy wow.

What you’re seeing here is a view of thousands of galaxies. Thousands. Sure, there are some stars in our own Milky Way punctuating this picture here and there but they are few, and just stomped flat by the number of whole galaxies you’re seeing. The stars can be distinguished from galaxies because they’re point sources; small dots. They also might have those lines going through them called diffraction spikes. Galaxies don’t usually get those because they’re fuzzier, spread out over many pixels. That suppresses the diffraction spikes.

So for example that bright point with pretty spikes you see toward the upper right is a star, probably a few thousand light years from Earth. That’s a long way to be sure, but even the nearest galaxies you can see in this image are hundreds of millions of light years away! Some are billions; the most distant object in this shot are at least 9 billion light years distant. That’s a million times farther away than any star in the picture.

When the light we see here left those galaxies, the Sun hadn’t yet formed. When the Earth itself was coalescing from countless specks of dust, that light still had half its journey here ahead of it.

So yeah. This stuff is far.

Hubble’s cross-section of the cosmos
CLASS B1608+656, a cluster of galaxies far, far away.

In fact you’re seeing galaxies at all different distances from Earth in this image, but the observation itself was taken to look at the cluster of galaxies in the center. Called CLASS B1608+656, it’s a clump of galaxies about five billion light years away. The mass of that cluster acts like a lens, bending space, magnifying objects behind it. This gravitational lens has distorted and amped up the brightness of a luminous galaxy located an additional several billion light years behind it, creating the weirdly shaped mess you see in the close-up above. Rings and arcs are common in such events.

But there’s so much more to this image; just scanning across it reveals an incredible variety and diversity of galaxies. Remember, too, you’re looking at objects as they existed eons ago; many are still growing, suffering collisions with other galaxies, giving them fantastic shapes. As an example, I’m fond of this little group near the top of the main image:

Hubble’s cross-section of the cosmos
A cosmic train wreck a million light years long.

I’m not precisely sure what to make of this. The bigger galaxies look to all be about the same distance from us, but that could be a coincidental alignment. Some of the galaxies are blue and clumpy looking, indicating they’re aggressively forming stars (hot, young, massive stars are preferentially blue), while some are quite red. The red ones may be very dusty, which reddens the light from stars, or they may be farther away, their light redshifted as it fights against the expansion of the Universe itself, losing energy along the way. It may be a mix of both. Unfortunately, this image was made using only two filters, so colors can be difficult to interpret, and don’t yield a lot of subtle information. The only way to know more about the galaxies would be to measure their distance, and I didn’t find anything in the literature about them.

That’s worth taking a moment to ponder, actually. These are entire galaxies, collections of tens of billions of stars, planets, dust, and gas clouds, each and every one a monstrous object on scales that dwarf our everyday experience … yet there are so many of each of them in this image alone we can’t possibly know their details. We can determine their coordinates on the sky, get a rough estimate of their distance, but there is simply no way to get a measure of them as individuals. They are too many. It’s like trying to get the life history of everyone who passes you on a busy New York City street corner. The task is too overwhelming.

And just in case I have not yet crushed your puny human mind, this image represents a tiny fraction of the entire sky; perhaps only one ten-millionth of it. That means there are hundreds of billions of galaxies like these scattered throughout the Universe.

So gaze again at that image, one that drills a narrow but incredibly deep view through our cosmos, one that shows us both the awe-inducing grandeur and soul-squeezing immensity of it, and remember: The Universe is far, far larger than this still.

And yet here we are, pondering it. To those galaxies, we are the ones who are lost in the anonymous throng. Yet I would argue we are as important and interesting a piece of the Universe as any other we can imagine. We are part of it at the same time as we study it, and to me, that is part of what makes us great.

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April 23 2014 7:30 AM

Black Skies, Smiling at Me

This week, April 20–26, 2014, is International Dark Sky Week, a global effort to get people to appreciate the skies above them. Light pollution—excess light thrown into the sky by street lights, houses, buildings, and pretty much everything that makes light—reduces our ability to see the stars, sometimes dramatically. I lived in Chicago for a year, and on the clearest nights I could only see the very few brightest stars, struggling against the mighty orange glow of the city projected upward.

This light is wasted; it’s money thrown away, it’s low-efficiency, and in many cases the lights being used at night aren’t really doing a good job of illuminating the ground and making it safer.

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Astrophotographer Mark Gee (who has been featured on my blog before; see Related Posts below) made a short and lovely video to highlight the issue. His work is stunning and well worth your time.

Years ago, my wife and I took a weekend vacation to a campground in rural Virginia; it was a camp I attended a couple of times as a kid. The owners were family friends, and one weekend every year they had old friends over to spend time together and enjoy the gorgeous scenery.

We parked near the main house, and as we fussed inside the car getting our belongings together, another weekend-goer walked by us while he talked on his phone. We overhead one snippet of his conversation, where he said “… and you should see the sky at night here. It’s so dark, and there are thousands of stars!”

I turned to my wife and we smiled at each other. He had no idea an astronomer was sitting there and could hear him; that was a spontaneous exclamation from someone who simply had never seen a dark sky before, and was properly overwhelmed by it.

That night we all sat in a field and watched the brilliant stars sparkling in a field of velvety black. It truly was amazing, mesmerizing, surpassingly beautiful. The artwork of nature, displayed for all to see …

… who have the skies to see it. This experience is what light pollution is stealing from us, and this is why I support what the International Dark Sky Association is doing to raise awareness of it. They have a different theme every day this week talking about our brightening skies, and I encourage you to take a look to see what they’re doing about it. The sky belongs to all of us, and we all deserve a chance to see it.

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April 22 2014 7:30 AM

10 Planetary Facts for Earth Day 2014

Today is Earth Day, a worldwide celebration of our home planet. I know you’ll be hearing a lot of talk about the environment today, humanity’s global impact, and more … which is great, and I wholly support that conversation (obviously).

But as an astronomer and science dork, I do love me some fun factoids. So I gathered together/calculated a few to give you a better appreciation of our planet’s place in the Universe. You might want to check out the ones I posted last year, too. You can’t know too much about Earth.

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1) Earth Day is April 22 every year. On average (jumping over leap years and such), between subsequent Earth Days our planet moves about 940 million kilometers (580 million miles), the circumference of its orbit around the Sun. That means it’s faster—way faster—than a speeding bullet: about 30 km/sec (18 miles/sec)! Typical rifle bullets travel 1-2 km/sec, so the Earth outpaces them handily.

[UPDATE (Apr. 22 at 18:00 UTC): I'm getting some comments about this, mostly referring to how I measure that speed. Implicit in my words is that I meant relative to the Sun, so that's how quickly the Earth would move around the Sun. In general, the speed of the Earth (or anything) depends on what you're measuring its speed against. Since I'm on Earth, it's not moving at all relative to me, and if you're in a distant galaxy we're moving away at a large fraction (or even faster than) the speed of light. All motion is relative... you can quote me on that!]

2) It’s not a small world after all. The surface area of the Earth is about 510 million square kilometers, or 197 million square miles. It’s not a perfect sphere (see listing No. 4 here), but if it were, using the surface area to find the Earth’s diameter would give you a ball 12,742 km (7,900 miles) across.

Venus with goatee
In hindsight this should've been obvious.

Photo by NASA & Tatiana Yurchenko/Shutterstock & Phil Plait

3) The Earth is the biggest of the terrestrial (rocky, as opposed to Jupiter-like gas giant) planets in our solar system.

4) Venus, the closest planet in the solar system to Earth’s size, has a diameter of 12,104 km (7,504 miles), 95 percent the width of Earth. It has about 82 percent the mass of Earth, too, making it our twin. However, its thick atmosphere is mostly carbon dioxide, it rains sulfuric acid, the air pressure on its surface is 90 times Earth’s, and the average temperature is a lead-melting 460 C (860 F). So it’s more like our evil twin.

5) Nearly 2,000 confirmed exoplanets (worlds orbiting other stars) have been found so far. The smallest, Kepler-37b, is barely bigger than our own Moon! Another, KOI-314c, has the same mass as Earth but is so hot it’s puffy, with a huge atmosphere. The planet that has the best chance of being most like Earth is Kepler-186f, which has 1.1 times the Earth’s diameter and is the right distance from its star to have liquid water. We really don’t know what it’s like beyond that, though. It might be more like Venus, or Mars.

6) We’ve only just started looking for other planets, though. There may be billions of Earth-size planets in our galaxy alone.

south of Earth
The vast majority of the Southern Hemisphere is water. This is more obvious when you look down from over the South Pole.

Photo by Google Earth

7) Coming back home again, Earth has something no other planet we know of has: a lot of water on the surface. It’s about 71 percent water by area, dominated by the Pacific Ocean, which covers a staggering 155.6 million square km (60 million square miles) of the surface. That’s nearly a third of the planet.

8) Most of the Southern Hemisphere (by a long shot) is covered in water: About 80 percent of the planet’s area south of the equator is water. North of the equator it’s about 60 percent.

9) Water exists naturally in all three physical states on Earth’s surface: solid (ice), liquid, and gas (water vapor). Due to a peculiar property of water—called its triple point—it can even exist in all three states at the same location and time. So now when things go wrong, you can say, “Well it could be worse: It could be raining and snowing and steaming!”

10) The more we look, both in our neighborhood and in deep space, the more we find that our Earth is one-of-a-kind. Even if we do eventually spot those billions of other planets similar in size to Earth, it’s unlikely they will be just like ours, with our exact balance of chemistry, temperature, and life. Heck, in the distant past the Earth didn’t look like it does now, and it’s the same planet.

What this tells us is that what we have now is precious, unique, and vital to us and ours. We don’t have an emergency backup, a summer home, a spot we can retreat to. The Earth is all we’ve got. We need to treat it that way.

April 21 2014 11:06 AM

Slate Plus You

I’ve been writing for Slate for about a year and a half now, and one thing I like about the magazine is that it’s all online; there’s no print version. That means there’s not a lot of baggage leftover from print media (which a lot of bloggers—including me sometimes—call “old media”). The folks behind the scenes have been working the ‘Net a long time and know their way around.

That means they’re also willing to try new things, and we’re rolling one out now that I think looks pretty interesting: Slate Plus. This is premium content, added material, including podcast extras, interviews, early access to Slate events, behind-the-scenes stuff, and more.

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Our fearless leader, David Plotz, explains what this is all about, but in a nutshell Slate Plus is a membership program; monthly access is $5, and yearly is $50. Mind you, this is not a paywall; Slate will still be free, and you need pay nothing for the usual yammering from me and all the other writers here you’ve been seeing all along. But Slate Plus has more stuff and more access. There’s a free trial for two weeks you can try if you take a look now.

If you do want to sign up, I ask you use this link to become a member of Slate Plus. Why? Well, for one thing, if you do you'll get a $5 credit, essentially a month for free. So yay!

Also, we’re running a contest among Slate staff and contributors to see who can gather the most sign-ups, with a tidy (if modest) monetary award going to the top hunter/gatherers. I have no clue if I will place in the top tier or not, but if I do, I will donate that money to DonorsChoose.org, which is like a KickStarter for school science funding. Teachers in undersupported schools put together projects for their kids and ask for donations from the public. This is a great organization doing a lot of good for students all over the U.S., so I’ll be happy to send some filthy lucre their way if enough of my droogs here sign up for Slate Plus.

So give it a shot! I’ll note that we’re still trying to figure out what I can do to contribute to Slate Plus as well. Maybe you should tell me what you’d like to see! An interview every now and again, a light-hearted debate with another Slatester, a video tour of my ridiculously entropic home office? Leave a comment and let us know!

And, as always, thanks for reading my blog. With or without Slate Plus, it’ll be right here where you left it, don’t worry.

April 21 2014 7:30 AM

A Dragon Chases the Space Station

Dragon and ISS
A Dragon chases down a huge denizen of space. Click to embiggen.

Photo by Bill Longo, used by permission

I post a lot of images taken with some pretty nice astronomical gear, from “amateur” telescopes up to multibillion dollar space observatories.

But it doesn’t always take a lot of fancy equipment to get great shots. Sometimes it just takes a camera, a tripod, and a little foreknowledge.

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Armed with just this, astronomer Bill Longo took the image above from his observatory outside Toronto. It’s a stack of eight 30-second exposures for a total of four minutes using a Canon T3 camera and a 6.5mm lens.

It shows the night sky facing west, with the bright winter stars of Auriga and Gemini setting, with the amazingly bright planet Jupiter punctuating the twins’ belly. And that bright streak seemingly bisecting Jupiter? Why, that’s just the International Space Station moving across the sky, its 100-meter length reflecting sunlight down to Earth.

And if you look very carefully, just under the ISS trail is a much fainter one: That’s the SpaceX Dragon capsule chasing down the station. This picture was taken on April 19, 2014, just hours before the private spaceship met up with ISS and was successfully grappled to its berthing point. Longo provides an annotated version of the picture that’s helpful:

annotated version of above photo

Photo by Bill Longo, used by permission

Imagine! Six astronauts from around the world are in that bright streak, and were about to grapple an American spacecraft using a robotic arm built in Canada, so they could bring aboard a new round of scientific experiments, food, and supplies for their stay on board a working space station that’s been in orbit around our fair world for more than 5600 days. And it’s bright enough to not only be seen from the ground, but easily seen, and photographed with equipment you can pick up at a local store.

You can keep your flying cars and hoverboards. We live in the future, now, and all you have to do to prove it is go outside look up.

Tip o’ the lens cap to Bill Longo for sending me his photo.

April 20 2014 7:30 AM

Happy Easter SUN Day!

Today is Easter Sunday, a Christian holiday. It actually has a tie-in with astronomy, since it’s celebrated on the first Sunday after the first full Moon after the March equinox. The equinox was March 20, and the full Moon was Monday (during the lunar eclipse, not so coincidentally), so here we are.

As is the case with most holidays in America, there’s a secular component to it as well. In this case, the Easter Bunny (which originated in Germany, incidentally) started out with religious connotations, but now resembles more of a lagomorphic Santa Claus.

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Whatever the origins, and whatever your beliefs, I think we can all agree that it’s fun to bite the ears off a chocolate bunny first (and/or perform hideous experiments on Peeps). So with an eye toward the astronomy and a tongue firmly in cheek, I present to you a picture of the Sun taken by astrophotographer Göran Strand:

© Fotograf Göran Strand
Wascally, isn't it? Click to lepusenate.

Photo by Göran Strand

What you’re seeing there is the edge of the Sun to the upper right, and a detached prominence to the lower left. A prominence is a huge blast of solar gas blown off the Sun by intense magnetic fields; usually these are connected to the Sun’s surface but sometimes become detached. It’s almost as if it…

[takes off sunglasses]

… hopped off the Sun.

Also, to give you an idea of its size, Strand included a picture of the Earth … the shape of which he changed to match the theme.

Or maybe Strand is a Heinlein fan. After all, it was he who once wrote, “The Earth is just too small and fragile a basket for the human race to keep all its eggs in it.”

That’s a fine sentiment for any season.

Note: In the interest of full disclosure, Strand took this photo on Sept. 4, 2013. But how could I resist running it today?

April 19 2014 7:30 AM

That’s No Moon … Well, Actually, Yeah It Is

lunar eclipse laser
Now witness the firepower of this fully armed and operational lunar ranging station! Click to midichlorianate.

Photo by Dan Long, used by permission

So there’s a picture you don’t see every day. Clearly, Vader’s forces were not at all happy about the lunar eclipse. 

I know, it really does look like the Moon was shooting out a laser at a passing ship, but that’s an illusion: In fact, that laser is hitting the Moon, and it was sent from Earth.

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While you and I were busy watching the total lunar eclipse on Monday, a bunch of astronomers were zapping it with high-powered lasers. They do this every now and again to find out exactly how far away the Moon is (and to provide yet another test of relativity).

Apollo astronauts left a series of retroreflectors there, devices that are designed to reflect light back in exactly the same path it came in. If you shoot a retroreflector with a laser, the beam will come back directly at you. Over the course of 800,000 kilometers (500,000 miles) to the Moon and back the beam spreads out a lot, so a telescope is used to collect the photons from the laser.

Since we know the speed of light very accurately, the time it takes for the beam to hit the Moon and come back tells us its distance. Think of it this way: If you are traveling at exactly 100 kilometers per hour, and you drive for exactly one hour, you know you drove 100 kilometers. Same thing here, but the car is a photon, the speed is the speed of light, and the distance is a wee bit more then you’d go for a family outing.

In this case, the retroreflector was left by the Apollo 15 team. I knew this right away! How? Because this:

Apollo landing sites
Close-up of the laser shot (left) compared to a map of the Moon showing the Apollo 15 landing site (right; Apollo 17 is also listed).

Photo by Dan Long; Soerfm/wikipedia

Apollo 15 landed on the very eastern edge of Mare Imbrium (the large dark circle; actually a lava plain), near the border with Mare Serenitatis. As you can see, the beam converges right at the Apollo 15 spot. I know it looks like the beam is coming from there, but that’s perspective for you! The beam appears to get smaller with distance, and your eye can’t tell if it’s getting smaller as it heads away, or getting bigger as it comes closer.

Astronomers have been measuring the Moon’s distance for many years, and it’s from that we’ve learned the Moon is moving away from the Earth by about 4 centimeters per year due to the complicated interaction with Earth’s gravity. But today I learned something about this: During a full Moon, the amount of light reflected back from the Moon drops. This was a mystery for quite some time, but it turns out that’s due to the way the mirrors there are set up; the Sun shines down them and heats them up, and they lose their efficiency at reflecting light back to us. During an eclipse, though, the Earth blocks the Sun during the full Moon, so the mirrors don’t heat up. They reflect light back to us just fine, proving that solar heating was the problem.

It’s funny to think that while I was collecting photons to make photos and video of the Moon, astronomers a thousand kilometers south of me were sending photons to the Moon … and getting them back.

Tip o’ the X-Wing to Dan Long for letting me use his photo, and to APOD, where I first saw it. I'll note this is at least the fourth time I've used a variation of this headline, and it's funny every flippin' time.

Correction, April 20, 2014: I had originally attributed the photo incorrectly to Tom Murphy, who works on the ranging experiment.

April 18 2014 9:07 PM

Incredible: SpaceX Dragon Capsule Video Taken From the Ground!

Update, April 19 at 15:00 UTC: Several people on Twitter have pointed out that what is seen in this video is most likely not the capsule and deployed arrays, but actually the upper stage of the Falcon 9 rocket and the ejected solar panel covers. That does seem likely to me given the scale of the objects, so I have sent Legault an email asking him about it, and I will update this post again when I know more.

Correction, April 19 at 15:15 UTC: Aha! Yes, I do now think what we are seeing here is the upper stage of the Falcon 9, the Dragon capsule, and the two solar panel covers (used to protect the Dragon's panels during launch, and which are ejected before the panels are deployed). What I somehow missed at the end of the video are the silhouettes of trees in the distance. That gives a sense of scale to the video; the Dragon capsule itself plus the solar panels would be far smaller than seen here. Also, around 30 seconds in you can see the stars of the Big Dipper in the background, showing the field of view of this video is far larger than I had originally supposed.

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So to be clear, this is NOT just the capsule and solar panels, it is a larger set of objects that are physically separated in space by quite a bit, making it much easier to photograph them. This is still a cool video, but it isn't the technical achievement I first assumed. I have struck through the original text below and added the correct info.

Thierry Legault is a gifted astrophotographer renowned for his footage of the International Space Station and (once upon a time) the Space Shuttle that he takes through his telescope. His ability to capture these rapidly moving objects is nothing short of spectacular, and I always think there's no way he can top what he's already done.

Then I see this: Video of the SpaceX Dragon capsule in orbit a mere 25 minutes after launch!

Incredible. This is seriously amazing work; being able to spot the capsule, get it framed up, and then to track it by hand as it glides over France less than a half hour after launch? Holy wow. You can actually just make out the capsule itself, and the extended solar panels on either side (those power the Dragon for the two or so days it takes for it to catch up to the space station). To give you a sense of scale, the capsule is 7.2 meters (24 feet) long and 3.7 meters (12 feet) wide. The panels are about 16.5 meters (55 feet) across. It was roughly 370 km (230 miles) above the Earth's surface when he shot this.

What you're seeing here is wide-angle footage, showing the upper stage of the Falcon 9, the Dragon capsule, and the ejected solar panel covers moving along together in orbit around the Earth. This was taken a few minutes after the capsule separated from the rocket upper stage, so all the individual things you see here were still near each other in space. Over the next two days the capsule itself will "catch up" the space station and be grappled on Sunday.

So yeah Legault is the best. I've written about his work, many, many times; go read it. Trust me here: You'll be astonished at what people can do when they're at the top of their game.

Congrats to Thierry for this achievement, and to SpaceX for another successful launch!

April 18 2014 10:08 AM

SpaceX Launch Update: Next Attempt Is Today at 19:25 UTC

Update, April 18 at 19:40 UTC: The Falcon 9 lifted off on time at 19:25 UTC, and the launch went perfectly. Just under 11 minutes later the Dragon capsule was deployed into orbit. The solar arrays were deployed, and all looks good. Congrats to SpaceX! Also, if you have clear skies over the next two days, you can look for the capsule catching up to the ISS yourself. I suggest using Heavens-Above (you need to enter your location) to get pass predictions.

Quick update: SpaceX is go today at 19:25 UTC (3:25 p.m. EDT) to launch a Falcon 9 rocket to the International Space Station. The Dragon capsule atop the rocket is loaded with supplies and experiments for the astronauts orbiting the Earth. If it launches on schedule, it will rendezvous with ISS and be grappled on Sunday, April 20 at 11:14 UTC.

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A launch attempt on April 14 was aborted due to a helium leak in a valve that is involved with the first stage separation after launch. The valve has been replaced. The big problem now is weather; it’s not looking great. If the launch is delayed again, the next attempt will be Saturday, April 19 at 19:02 UTC (for an ISS meetup and grapple on April 22).

SpaceX Falcon 9
SpaceX's Falcon 9 sits on the launchpad waiting for the go signal. This was taken shortly before the April 14 abort.

Photo by SpaceX

You can read more about this mission in my first blog post about the launch. You can watch the launch live on the SpaceX webcast or on the NASA Ustream channel.

April 17 2014 2:09 PM

An Astronomical Discovery: Earth-Size Planet Found in Its Star’s Habitable Zone

Kepler-186f
Artwork depicting the newly discovered Earth-size planet Kepler-186f and its sister planets. Click to embiggen, and you really want to; this is a pretty stunning piece of artwork.

Drawing by NASA Ames/SETI Institute/JPL-CalTech

I have some cautiously exciting exoplanets news: Astronomers have announced the discovery of a planet that is very nearly the same size as Earth and orbiting its star in the habitable zone—that is, at the right distance from its star to have liquid water on its surface. We don’t know how Earth-like it is, but this shows that we’re edging closer and closer to finding another Earth, and this one is the best bet we’ve found so far.

The planet is called Kepler-186f and was discovered using the Kepler Space Telescope, which was designed to look for planets orbiting other stars. Kepler exploits what’s called the transit method: It stares at 150,000 stars all the time, looking for dips in the amount of light received from every star. The idea is that if a star has planets, and if we see the orbits edge-on, then every time the planet passes between us and its parent star it’ll block an teensy bit of light (usually far less than 1 percent).

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This method is extremely powerful—about a thousand planets have been found this way in Kepler data! In fact, most of the planets found this way have been from Kepler. Another cool thing is that if you know how big the star is (and we generally do) then you can also determine the size of the planet by how much light it blocks.

Kepler-186f is one of the big success stories. It’s part of a mini-solar system, a five-planet system orbiting a red dwarf: a smaller, cooler star than the Sun. The other four planets (Kepler-186b-e) are all very roughly Earth-size, but orbit far closer to the star, ranging from 5.1 million kilometers (3.2 million miles) to 16.5 million kilometers (10 million miles)—for comparison, Mercury orbits the Sun at a distance of about 50 million kilometers (31 million miles), so this really is a solar system shrunk down. But even though the star is cooler than the Sun, these planets are close enough to it to be pretty hot; even the farthest of the four previously known would be hot enough to boil water on its surface (assuming it has a surface).

Kepler-186f and Earth
Worlds alike: a comparison of the sizes of Earth and Kepler-186f.

Drawing by NASA Ames/SETI Institute/JPL-CalTech

186f is different, though: It orbits farther out, about 53 million kilometers (33 million miles) from the star, where temperatures are more clement. Making some basic assumptions, it lies near the outside edge of the star’s “habitable zone,” where liquid water can easily exist on the surface of a planet. We know of several dozen planets like that in the galaxy so far, but what makes 186f special is its size: it’s only about 1.1 times the size of Earth! Together, these make it potentially the most Earth-like planet we’ve yet found.

I say potentially because honestly we don’t know all that much about it besides its size and distance from its star (and its year—it takes 130 days to orbit the star once). The next things we’d need to know about it are the mass, what its atmosphere is like, and the surface temperature. The gravity of the planet depends on its mass, and in many ways the atmosphere depends on the gravity. Unfortunately, we don’t know either, and we’re unlikely to. The techniques used to find planet masses aren’t up to the task for this planet—the star is too dim to get reliable data. The same is true for any air the planet might have as well. And without that, we don’t really know its surface temperature.

So we don’t know if this planet is like Earth, or more like Venus (with an incredibly thick, poisonous atmosphere that keeps the surface ridiculously hot), or like Mars (with very little air, making it cold). It could be a barren rock, or a fecund water world, or made entirely of Styrofoam peanuts, or some weird thing we haven’t even imagined yet.

Still, our models of how planets form are getting better, and we’re getting a handle on how they behave. According to what we know, it’s most likely that Kepler-186f is a rocky planet like the Earth, with a similar surface gravity. That in turn implies it could have water. But again, we just don’t know, and anything beyond this is speculation; there are a lot of factors in making a planet habitable. As a random one, it may take a magnetic field to make a planet livable. Ours protects us from the constant stream of subatomic particles the Sun emits, which, over several billion years, would have eroded away Earth’s atmosphere. That may be what happened to Mars.

To be fair, I’ll note that there is one planet found before that’s roughly the size of Earth (though bigger than Kepler-186f) and in its star’s habitable zone, but in that case the planet orbits at a distance where it receives about as much heat and light from its star as Venus does from the Sun … and look where that got Venus. Kepler-186f is therefore more likely to be Earth-like than that other planet, though again we can’t be sure with the information we have now.

Still, this is exciting news—after all, one of the main mission objectives of Kepler was to do just this: find an Earth-sized planet in its star’s habitable zone. So, my congrats to the team of astronomers involved: It’s very nice indeed to see a spacecraft achieve its goal! And there’s still lots and lots of data to go through from Kepler. There could easily be many, many more such worlds hidden in the blips of starlight Kepler has returned to Earth.

We’re pretty sure there are billions—billions—of Earth-sized planets in the galaxy. We now know of four that are in their star’s habitable zone (if you include Venus and the other planet I mentioned)… and we also know that some worlds are outside the strict definition of the HZ and yet still have liquid water (Jupiter’s moon Europa and Saturn’s moon Enceladus).

We’ve only just started looking. Who knows what else is out there?

Tip o’ the Lyot stop to Stephen Kane (one of the astronomers who found this planet, for his gracious help with info; go check out his website, Habitable Zone Gallery) and to the lead author Elisa Quintana.

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