The New Horizons Pluto flyby was an ambitious mission. At the time of launch, its destination was known only as a blurry distant body.  We knew some of its properties, such as its mass and rough surface coloring, but we weren’t even certain of its exact size. But the laws of gravity are extremely precise, so we could ensure New Horizons would reach its target. The mystery was what it would find. 

What New Horizons discovered surprised us all. Rather than a cold inert world, we found Pluto has a thin atmosphere, that it has icy mountains, and is likely thermally active. The mission showed us just how strange and wondrous the solar system could be beyond Neptune. It piqued our interest in similar distant bodies.

Hubble image that discovered 2014 MU69. Credit: NASA, ESA, SwRI, JHU/APL, and the New Horizons KBO Search Team

We know even less about other worlds. Their tremendous distance and small sizes make them difficult to study. We have discovered lots of objects, some with rings, some with moons, and some even larger than Pluto. But each of these are seen only as small blurry dots even with our best telescopes. A mission to any of these worlds would be as costly as New Horizons, and would be a difficult sell in our current economic environment.

So the New Horizons team looked for distant worlds their spacecraft might be able to reach. They settled on a small body known as 2014 MU₆₉. Discovered eight years after the launch of New Horizons, it is a small world only about 20 to 30 kilometers wide. From the blurry images we have, it appears to be a close or contact binary, similar to the comet 67P/Churyumov-Gerasimenko. It’s difficult to tell, because 2014 MU₆₉ is about a billion miles further away from the Sun than Pluto, or about 25% more distant.  We do, however, know the path of its orbit, and with the equations of Newton’s gravity the New Horizons team knows it can reach it.

The date of the flyby is now scheduled for 1 January, 2019. To save power, the spacecraft is currently in hibernation until June. By August it will have woken up and will start taking images of 2014 MU₆₉. These will help pinpoint the exact path for New Horizons, and ensure a close and safe flyby.

This extended mission is a huge challenge. We’ve never attempted such a distant flyby, and even less is known about 2014 MU₆₉ than was known about Pluto. But the rewards promise to be huge. For the first time we will observe a body in the outer solar system close-up. It promises to be just as surprising as Pluto, if not more.

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Ah, Pluto, for such a small and distant world you are able to stir controversy. 

In 2006 the International Astronomical Union (IAU) formally defined a planet as an object which 1) orbits the Sun, 2) is massive enough to be in hydrostatic equilibrium (basically that means it’s round), and 3) it has cleared the neighborhood around its orbit. Since Pluto didn’t satisfy the third criteria, it was tossed out of the official list of planets, much to the outcry of the general public. Pluto is still considered a dwarf planet, along with Ceres, Eris, Haumea, and Makemake, but that was small consolation. Then when New Horizons flew past Pluto in 2015, it found that Pluto was a rich geological world, with mountains and thin blue skies. The images captured by the probe showed that Pluto was very planet-like, and there were new calls to redefine Pluto as a planet.

Even some astronomers have issues with the current definition of a planet. To begin with, a planet must orbit the Sun, so the thousands of exoplanets orbiting other stars are not planets. That’s fine if you want to keep planets and exoplanets separate, but most people would figure that an Earth-like body orbiting any star would be a planet. Then there is the third criteria, the whole “cleared it’s neighborhood” business. If it weren’t for that, Pluto would still be a planet. The problem with this criteria is that the more distant a planet is, the more difficult it is to clear an orbit. Earth is a planet under the current definition, but if Earth were beyond Pluto in the Kuiper belt, it wouldn’t be a planet. That seems rather arbitrary.

So Alan Stern (principle investigator for the New Horizons mission) and others have proposed a new definition: A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters. In other words, if a body is basically round but too small to be a star, then it’s a planet.

Saturn’s moon Enceladus is under hydrostatic equilibrium. Credit: NASA/JPL-Caltech

This would broaden the definition of planet significantly. If this definition were adopted by the IAU Pluto would officially be a planet once once again. So would all the exoplanets we’ve discovered so far, and so would rogue planets that wander cold deep space all alone. Anything with a diameter larger than about 500 km, all the way up to bodies 15 times more massive than Jupiter would be considered planets.

But such a definition might too broad. Not only would Pluto be a planet, but so would its largest moon Charon. So would our Moon, making Earth a double planet. So would the largest moons of Jupiter and Saturn. The definition would shift our solar system from 8 planets to more than a hundred. Sure, Pluto would be a planet, but who cares at that point?

Personally, I think the proposed definition is too general. You could argue that Pluto should be a planet (I disagree), but Saturn’s small moon Enceladus shouldn’t be a planet. Even the common definition of a moon is that it orbits a larger body. In science fiction we have no problem with moons being large and Earth-like, even habitable. They are still moons. I think there is also something to be said for some kind of orbit-clearing condition. It is quite likely that there are thousands of objects larger than Enceladus in the outer edge of our solar system, and they aren’t the same as closer objects like Mars or Mercury.

All of this is worth discussing.  As we learn more about both our own solar system and others our definition of what a planet actually is will have to adapt. Whether we end up with hundreds of planets or only eight will depend on what we want the word “planet” to mean.

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Pluto is emitting x-rays, and we don’t know why. 

X-rays aren’t something we’d expect from Pluto, since the planet has no clear way of generating them. It’s a small, cold world with little magnetic field. Some solar x-rays might scatter off Pluto in our direction, but the level of x-rays is higher than what could be produced by scattering. So what gives?

The most likely explanation is that the x-rays are produced through an interaction between the solar wind and Pluto’s atmosphere.  As the New Horizon’s flyby found, Pluto’s atmosphere is actually quite stable, so interactions with the solar wind could produce x-rays. Similar interactions between the solar wind and the comas of comets have been seen to produce x-rays. But the level of x-rays from Pluto is even higher than we’d expect from such an interaction, so that isn’t the whole story.

While these x-rays are a mystery, it’s important to keep in mind that the amount of data is actually quite small. There’s enough data from the Chandra spacecraft to know that it’s not a random fluke, but it’s difficult to get much detail from such a small sample. What this study does show is that Pluto does produce x-rays, and perhaps we should give it a closer look.

Paper: C.M. Lisse, et al. The puzzling detection of x-rays from Pluto by Chandra. Icarus. (2016) DOI: 10.1016/j.icarus.2016.07.008

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Pluto has blue skies and water on its surface, but don’t think about a vacation there quite yet.

Pluto’s blue sky as seen above is really more of a haze. On Earth the sky is blue due to light scattering off molecules such as nitrogen in the atmosphere, an effect known as Rayleigh scattering. Since blue light scatters more strongly than red, our sky takes on a bluish color. Pluto doesn’t have a thick atmosphere, but the atmosphere it has is filled with soot-like particulates known as tholins. These particles also scatter light, but by a different effect known as Mie scattering. While Rayleigh scattering tends to occur in all directions, Mie scattering varies with scattering angle. Longer wavelengths (reds) tend to scatter more uniformly, while shorter wavelengths (blues) tend to scatter at slight angles. When Pluto’s atmosphere is backlit, as in the image above, it’s mostly blue that we see.

Regions of exposed ice are false-colored blue.

While there’s water on Pluto, it’s frozen as ice. When earlier images of Pluto showed large mountains on the world, we knew that they were most likely water ice, since other ices likely to be found on Pluto aren’t stiff enough to support mountains. But simple images weren’t enough to rule out other possibilities, such as some kind of rock. Infrared spectroscopy from the Linear Etalon Imaging Spectral Array (LEISA) found regions where water ice is exposed on the surface, including the mountain region. What’s interesting is that most of the surface is not exposed water ice, which means it is covered with a layer of other materials. The fact that Pluto is covered with other “volatiles” is more evidence that Pluto is a dynamic world.

Of course these new observations haven’t told us what we didn’t already suspect. We figured tholins would make Pluto’s sky look blue, and we suspected Pluto’s mountains were ice. But with science a suspicion isn’t enough. As data from New Horizons continues to trickle in, we’ll continue to confirm suspicions and we’ll likely get a few more surprises.

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Now that New Horizons has zipped past Pluto, it’s main mission is to gradually transmit back all the data it gathered during the flyby. We’ve already learned some amazing things from the mission, and we’re certain to learn more as the rest of the data reaches us. But not content with just one flyby, the New Horizons team has another target in their sights.

The target they’ve chosen is known as 2014 MU69. It’s about 35 km across, and is part of the Kuiper belt, which is an icy outer region of objects similar to the asteroid belt between Jupiter and Mars. The encounter will give us an opportunity to see a Kuiper belt object up close, so we should learn a lot form a type of solar system body we currently know very little about.

If all goes as planned, New Horizons should make its flyby of MU69 in January 2019.

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There are mountains on Pluto, and that actually tells us quite a bit about the dwarf planet.

We’re just beginning to get some low resolution images from New Horizon’s flyby of Pluto, but already the image tell an interesting story of the dwarf planet. For one, Pluto has jagged mountains on its surface. Most of the surface is covered with frozen methane and nitrogen, but neither of these solids are strong enough to form mountains. However water ice is strong enough at Pluto’s temperature, and that’s likely what these mountains are made of. That Pluto probably has a “bedrock” of water ice.

Another thing you’ll notice is that these mountains are not just large, they are also jagged. They haven’t been worn down, nor are they pummeled by craters. This would indicate that they are fairly young, perhaps less than 100 million years. On the billion-year scales of the solar system, that’s quite recent, and that would imply Pluto is geologically active. That’s surprising, because Pluto doesn’t experience tidal forces, which is what often keeps the icy moons of the outer planets active. What could be driving Pluto’s activity isn’t clear.

So already it’s clear that Pluto is full of surprises.

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For decades Pluto was simply a faint dot on a photographic plate. When it was discovered in 1930 by Clyde Tombaugh, it was indistinguishable from a faint 13th magnitude star except for its slowly changing position over the course of days. We called it a planet, but we really had no idea what type of solar system body it was.

From photographic plates we could make a reasonable determination of its orbit, and we found that it was rather different from the other planets. It was much more elliptical, and its orbital plane was tilted relative to most of the other planets. From a faint photographic point we could only estimate its size. Some thought it might be a large and dark planet, while others small and bright. There just wasn’t enough information to tell.

The small bulge indicates the presence of a moon.

It wasn’t until 1978 that we discovered Pluto had a moon. It was then that James Christy noticed a small bulge on a magnified image of Pluto that would appear and disappear. We called the moon Charon, but again we knew little about it. With better imagery, the orbits of Pluto and Charon gave us an indication of their mass. We could finally start determining the characteristics of Pluto. We found that it has a density less than half that of Earth, and that its surface is mostly nitrogen ice. It’s composition was more like a comet than a rocky planet.

By the 1990s we began to discover other Pluto-like objects in the outer solar system, but Pluto remained our favorite. Beloved perhaps for what we didn’t know about the world rather than what we did. So in 2006 we launched a probe toward the small world. To reach Pluto in a reasonable time, it would need to move quickly. It was the fastest spacecraft we’ve ever launched, and over the next 9 years it would need to travel nearly 5 billion kilometers away from Earth. It also had to arrive at just the right position at just the right time. It posed quite a challenge given that when New Horizons was launched we still didn’t know Pluto’s size with great precision.  Yet as New Horizons makes its closest approach today, it is within 70 kilometers and 72 seconds of its predicted path. That level of precision over billions of kilometers is nothing short of amazing.

The media will be flooded with news of Pluto today, and rightfully so. In less than a century we have turned a faint point of light into a beautiful reddish world. And this is only the beginning of what we will learn from New Horizons in the coming months. After decades of planning, we have traveled to what was once our most distant planet.

Well done, humanity. Well done.

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New Horizons is closing in on Pluto, and that means we’re finally getting some detailed images of the small world. With the appearance of new surface features comes a new batch of questions.

The latest comes from a new color images of Pluto and Charon. The images were created by combining high resolution images from the Long-Range Reconnaissance Imager (LoRRI) with color data from RALPH. What’s clear is that there is a chain of features along the equator of Pluto. Whether these are features similar to the equatorial region of Iapetus or some type of cryovolcanic activity is still not clear. It’s also clear from the image that Pluto and Charon are quite different in both color and brightness. Given that the two bodies are thought to have formed from a collision with another body, it will be interesting to see how they can be so different.

The best Hubble image of Pluto and Charon.

We’re still in the speculation stage, since the data is so new, but the exciting thing about these images is that Pluto is no longer a spot a few pixels wide. It’s a world with clear features and with mysteries to be revealed. We’re finally exploring everyone’s favorite little world, and that’s a huge scientific achievement.

New Horizons will make its closest approach on July 14, so we can expect a flurry of images leading up to that time. Of  course it will take months for the all the data to be transmitted back to Earth, so long after the flyby we’ll Pluto is going to be rising in the news.

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As the New Horizons spacecraft approaches Pluto, there’s a great deal of anticipation about just what we’ll learn about the dwarf planet. One of the key areas of interest is Pluto’s moon system. We know that Pluto has at least five moons, with Charon being the largest by far, and four smaller moons Styx, Nix, Hydra and Kerberos. From Hubble observations, we know that Hydra and Nix seem to have oblong shapes. We know this from their varying brightness over time, which suggests that their rotation means that sometimes a wide side faces us (making it appear brighter) and other times a narrow side faces us (appearing dimmer). There’s a small chance that this variation in brightness could be due to a radically different albedo on different sides of the moons, but that isn’t likely to be the case. In fact recent research on Nix would further points to its elongated shape.

Hubble image of Pluto’s moons.

By our best estimates, Nix is shaped roughly like an American football, and is about 57 km on its long side and 27 km along its short side. Because of this irregular shape, the gravitational pull of Pluto and Charon exert a twisting force (torque) on the small moon. If Nix simply orbited a single mass, then this torque effect would tend to stabilize the moon, and perhaps even cause it to tidally lock similar to the way our Moon is tidally locked to Earth. But because Nix is pulled by two large bodies orbiting each other, the torque is irregular. Recent computer simulations show that Nix would therefore have a chaotic motion where its rotation doesn’t follow a regular pattern. As a result, the apparent brightness of Nix should vary in a chaotic way.

This is what Hubble observations find. If Nix were roughly spherical and its brightness variations were simply due to varying albedo, we would expect them to follow a regular pattern. So it would seem that Nix is truly oblong. Of course to know for sure, we’d like to get some direct images of the small moon, which we’ll hopefully get during the Pluto flyby.

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NASA has released the first color image of Pluto and its moon Charon from the New Horizons spacecraft. While it may not look like much, it’s only going to get better from now until the spacecraft’s closest approach in July. 

Although the image above is touted as a color image, it’s actually a composite image that approximates Pluto’s actual colors. Like other spacecraft, the CCD cameras on New Horizons only detect brightness within a particular spectral range. These are then combined to create a simulated color image. For this mission there are four detectors, for blue, red, near infrared, and a narrow band filter specifically for methane spectra.

The best Hubble image of Pluto. Credit: HST

While a color image of Pluto and Charon is impressive, it’s an indication that New Horizons is now close enough to get better images than we’ve had before. In addition to the imagery, there are several other detectors on the probe, measuring things like solar wind, dust particles, atmospheric spectra and radio telemetry. Since the spacecraft will make a fast flyby of Pluto, data will need to be gathered quickly.

But for now we can enjoy the distant dwarf planet in living color.

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New Horizons is a small spacecraft on its way to Pluto. It will make its closest approach next Summer. To get to Pluto in a reasonable time, the spacecraft is heading there at high speed. This means it will zip past Pluto and head out into the Kuiper belt. While Pluto is a worthy goal, it would be nice if New Horizons could observe other objects in the outer solar system. But given the high speed of the spacecraft, and the low mass of Pluto, there isn’t a good way to use the planet’s gravity to change direction towards a particular Kuiper belt object (KBO). Basically, New Horizons is on a straight trajectory out of the solar system. So instead astronomers have been searching for KBOs that are along the path of New Horizons, and they’ve found some candidates.

Credit: Alex Parker, SwRI

The three possibilities are KBOs known as PT1, PT2 and PT3. In this case PT stands for “potential target.” Of these PT1 is in best alignment with the trajectory of New Horizons, and thus would require the smallest delta-v adjustment. What’s exciting about this is that we haven’t had a good look at any Kuiper belt objects, given their large distance and small size. The closest we’ve got are moons such as Neptune’s Triton, which we think is a captured KBO.

If successful, New Horizons will make its flyby of PT1 in January of 2019.

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Just how big is Pluto? The answer is we aren’t entirely sure. We know the diameter is between 2,300 and 2,400 kilometers, but it’s hard to pin down beyond that.

The reason for the uncertainty has to due with the fact that Pluto has an atmosphere. Although we can determine Pluto’s mass by observing the orbit of Pluto’s moon Charon, determining the planet’s size is most precisely done by watching the planet transit in front of a distant star. By observing the transit from different positions on Earth, we can determine Pluto’s size. But the thin atmosphere of Pluto means that the observed transit is a bit fuzzy. Instead of seeing the distant star wink out as Pluto occults it, we see it fade out as the atmosphere occults it. So the calculated size of Pluto is approximate. Just to be clear, Pluto’s atmosphere is extremely thin. It’s surface pressure is about 300 times thinner than the atmosphere of Mars, but it is enough to blur the diameter measures of the planet.

Recently, however, a paper in Icarus has used Pluto’s atmosphere to more accurately determine its size. The paper itself focuses on levels of methane in Pluto’s atmosphere, and how it varies with position and time. But knowing the distribution of methane and its variation with pressure, the team could also calculate the thickness of Pluto’s atmosphere. From this, they could calculate its diameter. The result was 2360 km, give or take about 20 km.

That is likely the best measurement we’ll have until New Horizons makes a flyby of Pluto next summer. For now we’ll just have to settle for an approximate Pluto.

Paper: Lellouch, Emmanuel, et al. Exploring the spatial, temporal, and vertical distribution of methane in Pluto’s atmosphere. Icarus doi:10.1016/j.icarus.2014.03.027 (2014)

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