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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There is much we still don’t know about the origin of our Sun. We know that stars form within large clouds of gas and dust known as stellar nurseries. These clouds collapse under their own weight to form hundreds of stars at once. But something has to trigger that collapse. The most popular view is that it is triggered by supernovae, which sends shock waves through the cloud. But a new model argues that the birth of our Sun might have been triggered by the more subtle process of a Wolf-Rayet star.

A Wolf-Rayet star is an old massive star on its way to becoming a supernova. They are distinguished by extremely strong stellar winds and their spectral lines tend to show they are rich in helium, but don’t contain much hydrogen. They are often observed within a nebula produced when the outer layers of the star are pushed outward.

Wolf-Rayet stars cast off outer layers gradually, rather than through a single explosion like supernovae, so it was thought that they couldn’t trigger star formation within a stellar nursery. But new computer simulations show that they might encourage stars to form within the layers of the surrounding nebula. The strong stellar winds of a Wolf-Rayet star can cause the surrounding nebula to compress into a more dense layer surrounding the star. Known as a Wolf-Rayet bubble, simulations show that it could be dense enough to trigger star formation.

This idea would seem to be supported by the composition of the early solar system. Meteorites from the early solar system have higher levels of aluminum-26 and lower levels of iron-60 than the surrounding interstellar medium. Wolf-Rayet stars produce lots of aluminum-26, which would tend to get caught within the bubble.

There are other possible explanations for these elemental abundances, so the Wolf-Rayet model isn’t conclusive. It does show, however, that there is more than one way to trigger star formation, and the origin of our Sun may have a richer history than we once thought.

Paper: Vikram V. Dwarkadas et al. Triggered Star Formation inside the Shell of a Wolf–Rayet Bubble as the Origin of the Solar System. ApJ 851, 147 (2017)

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Water is an essential ingredient for life on Earth. If we want to find life on another world, we look for the presence of liquid water. We know there is liquid water on other worlds, but the question remains whether our planetary sibling, Mars, has liquid water. 

We have long known that Mars was once a wet world. Isotope measurements show Mars was an ocean world in its youth. Rain fell from the sky, and rivers flowed. But today Mars is cold and dry. Ice can be found on its surface, but long gone are the Martian rivers and seas. With weak gravity and little magnetic field, the red planet simply couldn’t hold a rich atmosphere, and so the liquid water evaporated and froze.

But it’s possible that some water might exist beneath its surface, and ice in its soil might be able to liquify under the right conditions. There is evidence to support this idea, such as through recurring slope lineae. These dark lines along the sides of hills look similar to what you would see if you scooped your hand through wet sand. Rather than a rich flow of water, it is a damp seeping through the sand. Looking at the images, it certainly looks like a liquid flow, but astronomers know the risk of simply assuming something to be true based upon appearances. If it looks like liquid water it’s worth a further look, but appearances are not proof.

In 2015 there was a great deal of excitement about these lineae when astronomers found evidence of perchlorate salts within them. The salts could mix with water to created a brine, and this brine could remain liquid even at the temperature and pressure of Mars’ summer weather. In the winter of Mars it would freeze, and this would explain why the recurring slope lineae seem to appear seasonally in the summer.

But while liquid water is a good explanation, it isn’t the only possible explanation. This is why the discovery was met with cautious optimism. Now it seems it was good to be cautious. New observations from the Mars Reconnaissance Orbiter point to a dry answer. Careful analysis shows that the lineae occur along steeper slopes, but stop when the slope reaches a shallow enough angle of incline. This critical incline is consistent with the critical incline seen in flows of dust and sand. It’s similar making a hill out of sand. It’s easy to build a shallow hill by piling on ever more sand, but if you try to make a steeper hill the sand topples down to the shallow angle.

New observations of recurring slope lineae show they behave more like dust than water. Credit: NASA/JPL/University of Arizona/USGS

Liquid water doesn’t have such a critical angle. If the lineae were due to liquid water, you might see it stop at a similar distance from its source, but water can still flow down shallow hills. So it seems this is a dry flow of dust and sand rather than a flow of water brine. If that’s the case, it could mean that Mars is very dry indeed. Perhaps too dry to support life.

As with earlier evidence, we should still be a bit cautious. There might still be liquid water on Mars in some places. The question of liquid water on Mars is so important that we want to be careful about both the evidence and our conclusions. It’s also the best way to do science if you want to get it right.

Paper: Colin M. Dundas, et al. Granular flows at recurring slope lineae on Mars indicate a limited role for liquid water. Nature Geoscience doi:10.1038/s41561-017-0012-5 (2017)

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There are countless opinions about how to cook the best turkey. Some suggest slow roasting it for hours, while others prefer cooking at high heat. Some even dare to deep fry it in peanut oil. Of course all of these cooking suggestions are Earth-based, which is a bit limiting. Suppose you wanted to have a Thanksgiving dinner elsewhere in the solar system, such as Venus?

While Mars is a perennial favorite for human exploration, Venus is a strong second. It is similar in size to Earth, and has a thick atmosphere. Plus, at an altitude of about 50 kilometers above its surface, the temperature and atmospheric pressure is similar to that of Earth. So in many ways it would be much more welcoming to Earth explorers than Mars. Since Venus has the highest surface temperature of any planet in the solar system, it also would provide the opportunity to cook our Thanksgiving turkeys with natural heat.

Unfortunately, we couldn’t simply roast our turkey on the Venusian surface. With a surface temperature of 460 ^oC ( 860 ^oF) and an atmospheric pressure more than 90 times that of Earth at sea level, our turkey would be blackened to a crisp while the stuffing is still cold. Not even a foil tent can prevent that tragedy. So we would have to be a bit more creative.

Atmospheric temperature of Venus as a function of depth. Credit: Wikipedia

Given that a crewed mission to Venus is already a huge engineering challenge, it’s safe to assume that any colonists living in Venus’ upper atmosphere could send airships deeper into the atmosphere with relative ease. As you move closer to the Surface of Venus, the temperature and atmospheric pressure increases, so it would really be a matter of getting your turkey to a particular atmospheric depth for a certain amount of time. For example, Alton Brown suggests the high-temperature method for turkey, roasting it at about 500 ^oF  (260 ^oC) for about 2.5 hours. So you would just need to put your turkey into a probe and send it to a height of about 25 kilometers for a couple hours. While you’re at it, why not put all your food in different probes. Your pumpkin pie would need to go to an altitude of 35 km for about 40 minutes. Send your corn bread to about 35 km for 30 minutes, and so on.

If you timed your probes right, all your dishes could be cooked and piping hot at the same time. All you’d need to do is pour a glass of wine and brace yourself for the inevitable political arguments over dinner. Such as whether those colonists on Europa should be allowed to form an autonomous government.

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From the depths of space it came. Speeding towards Earth, it passed within 60 lunar distances before it was discovered by the Pan-STARRS telescope. It’s a small asteroid or comet, like many that pass near Earth, but this one began its journey in another star system. 

We know this not because it has unusual characteristics, but simply from the path of its orbit. Planets, asteroids and comets follow elliptical paths around the Sun, we can determine their orbital paths by measuring their speed and distance from the Sun over a period of time. Since their orbits are determined by gravity, we can use Newton’s gravitational law to figure out its path knowing its speed and position.

Because of the gravitational pull of the Sun, an object moves faster when it is closer to the Sun and slower when it is farther away. At its farthest distance from the Sun, an object doesn’t have enough speed to overcome the Sun’s gravitational pull, so its path curves toward the Sun to make another orbit. By measuring the speed of this particular object, named A/2017 U1, we can calculate how far it could travel from the Sun before turning back. What we found was that it’s moving so fast it will escape the Sun’s gravity. It will leave our solar system never to return.

This can sometimes happen to a comet or asteroid that formed in our solar system. An asteroid can make a close approach to a large planet like Jupiter, giving it a boost of speed that sends it hurtling out of the solar system. We use this effect with spacecraft such as Voyager and New Horizons, which are now on their way toward interstellar space. But that couldn’t have happened to A/2017 U1. The path of its orbit is almost perpendicular to the orbits of the planets, and it hasn’t passed near any of the large planets in our solar system. So it must have been moving fast when it entered our solar system.

We’ve long thought that interstellar comets and asteroids were possible. We’ve known that with the right gravitational boost comets and asteroids can leave our solar system on rare occasions, and it stands to reason that comets and asteroids in other star systems can occasionally leave theirs. Given time, some of them will eventually visit other stars. But this is the first confirmed example of an interstellar visitor.

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Beyond the orbit of Pluto lies a small world known as Haumea. Named after Hawaiian goddess of fertility, it is one of the five dwarf planets in our solar system. It is about half the size of Pluto, but it shares something in common with the gas planet Saturn: an icy ring. 

An image of Haumea with its two moons Hi’iaka and Naumaka. Credit: CalTech, Mike Brown et al.

Haumea is difficult to study because of it’s small size and great distance. From initial observations we couldn’t even pin down its size with accuracy. This is because Haumea spins very quickly on its axis. A day on the small world would only be 4 hours long. Because of its rapid rotation, the world is flattened into a somewhat egg-like shape known as an ellipsoid.

But in January of this year, Haumea passed in front of a dim star as seen from Earth, which allowed astronomers to use the occultation of the star to determine the size and shape of Haumea more precisely. A team of nine observatories observed light from the star as the dwarf planet passed in front of it. Since each observatory viewed the star from a different point of view, they saw different parts of the planet block the star. This allowed the team to confirm Haumea’s spheroidal shape.

But the team also found that the star briefly dipped in brightness a bit before and a bit after Haumea crossed its path. This is consistent with a ring around the small world. Based upon observations, the ring has a radius of about 2,300 kilometers, and is about 70 kilometers wide. It’s not nearly as grand as Saturn’s rings, but it makes Haumea only the second small world known to have a ring system.

Paper: J. L. Ortiz, et al. The size, shape, density and ring of the dwarf planet Haumea from a stellar occultation. Nature 550, 219–223 (2017)

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There are eight known planets in our solar system. Beyond the orbit of Neptune, there are small icy worlds like Pluto and Eris, but none that rival the planets in size. But could there be a planet lurking in the depths of our solar system? 

It’s possible that there is a planet in the outer solar system, but it is so far away and so cold that it would be difficult to see. As we continue to make high resolution sky surveys, and as new telescopes such as the James Webb Space Telescope are launched, we will be able to find such planets in time. If any exist. But there is another way to look for these planets, and that’s by the gravitational tug they exert on other bodies.

Neptune was discovered in this way. After the discovery of Uranus, it was noticed that its motion didn’t quite agree with the predictions of Newtonian gravity. But the deflection of its orbit could be solved if it was caused by the pull of an undiscovered planet. In the mid 1800s John Couch Adams and Urbain Le Verrier calculated the position of such a planet independently. Soon afterward Johanne Galle discovered Neptune using Le Verrier’s predictions. This method has also failed, such as when strange motions of Mercury predicted a planet that doesn’t exist. It was later found that Mercury’s oddness is due to the effects of general relativity.

Seeing the gravitational tug from a much more distant world is much more difficult. The distances are greater and the gravitational influences much weaker. However there are hundreds of known objects beyond Neptune (or Trans-Neptunian Objects) and together they give us a statistical tool to look for other planets. In recent years there has been some evidence to support the existence of a super-Earth planet in the outer solar system. This was supported by the fact that the orbits of the most distant Trans-Neptunian Objects (TNOs) seemed to be clustered together. But the most recent survey of outer worlds clearly demonstrates that this clustering was likely due to detection bias, and the distribution of TNOs doesn’t support the existence of a super-Earth planet 9. Ethan Siegel has recently about this, and you should really check it out.

No if there’s no planet 9, is that it for solar system bodies? Maybe not. While the latest survey of TNOs rules out a super-Earth, it leaves open the possibility of a smaller planet, sometimes called Planet 10. The planets of our solar system have orbits that are all roughly along the same plane, known as the invariable plane. Since the solar system as a whole is gravitationally isolated, we would expect the orbits of TNOs to have a similar orientation. Orbits would vary a bit, but the average orientation should be near the invariable plane. But this latest survey found that the orbital orientations of distant TNOs are tilted about 8 degrees from the invariable plane. For a single orbit that isn’t unusual, but it is odd for lots of orbits to have this orientation. This kind of shift can occur due to a gravitational tug from a larger body, through what is known as the Kozai mechanism. Based on the data, this planet would be at least the size of Mars.

As it stands the evidence for planet 10 is relatively weak. There’s a small chance that the odd orbits of TNOs is merely random chance. But this work demonstrates that we are closing in on a final answer to the number of planets in our solar system. If they are out there, we will find them in the next several years, and if not we’ll confirm they don’t exist.

Paper: Kathryn Volk and Renu Malhotra. The curiously warped mean plane of the Kuiper belt.  arXiv:1704.02444 [astro-ph.EP] (2017)

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Mars is a cold, arid world with a thin atmosphere, but that wasn’t always so. In its youth Mars was wet with rivers and oceans. Presumably that also means young Mars had rainfall. But how much did it rain on Mars? New research proves not only did it rain on Mars, the rains were strong enough to shape the planet’s surface. 

To study Martian rain, two geologists used the same rainfall models used for Earth. These are well studied, and are proven to work well. But there are differences between Earth and early Mars. One big difference is that gravity on Mars is about a third that on Earth, meaning that water droplets would fall more slowly and strike the surface with less energy. Then there is the fact that the atmosphere of Mars has changed significantly. Young Mars had an atmosphere four times thicker than modern Earth.

Because of these factors, there wasn’t much rain on Mars despite plenty of liquid water. Instead water vapor would tend to coalesce into small droplets to form a thick fog. This fog could make the surface of mars wet, but wouldn’t alter the terrain much. As the atmosphere of Mars thinned to a pressure similar to Earth’s, larger water droplets could form. Given the low gravity, could merge into quite large rain drops. On Earth large raindrops tend to break apart as they fall faster, which limits their size to about 6 millimeters in diameter. Falling at a slower speed, Martian water droplets could grow to about 7.5 millimeters. Torrential rains with large rain droplets created surface runoff that cut paths on the surface of Mars. These “gully washers” can be seen today, such as in the image above.

Paper: Robert A. Craddock and Ralph D. Lorenz. The changing nature of rainfall during the early history of Mars. Icarus, volume 293, (September 2017).

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Io is a violent world. Tortured by the gravitational forces of Jupiter, it erupts with sulphur and lava. Our own planet has volcanoes and lava flows, but it pales in comparison to Io. 

From Earth, astronomers can observe changes in the brightness of Io’s surface, but we had to send probes such as Voyager to Jupiter to see active and erupting volcanoes on Io. This proved that Io is the most geologically active body in the solar system. We can infer much from Io by comparing it to volcanic activity seen on Earth, but there are significant differences. Earth is a water-rich planet with a significant atmosphere, while Io is dry with little atmosphere. This raises questions about how things like lava flows behave on the small moon.

On the surface of Io, Loki Patera is a hot spot that brightens and dims every 400 – 600 days. The most popular explanation is that Loki Patera is a lava lake more than a million times larger than any on Earth. But even high resolution images of Io from the Galileo mission have failed to confirm this idea.

Recently, astronomers used a fortunate celestial event to solve this mystery. Io is the closest Galilean moon to Jupiter. Periodically the next closest Galilean moon, Europa, passes in front of Io as seen from Earth (known as a transit). As it does so, Io’s surface is gradually blocked and revealed by icy Europa. This lets astronomers create a map of Io, particularly Loki Patera. By observing Io during a transit of Europa, they found that the brightness and temperature of the region increased steadily from one end to the other.

Map of Loki Patera showing the variation in brightness and temperature. Credit: K. de Kleer, et al.

This is consistent with the behavior of a lava lake that is overturning. That is, cold lava at the surface of the lake sinks, churning hotter lava to the top. The astronomers also confirmed an island in the center of Loki Patera that has been there since Voyager photographed the region in 1979. This again supports the lava lake model. All it took to verify a lava lake on Io was a fortunate transit by an icy world.

Paper: K. de Kleer, et al. Multi-phase volcanic resurfacing at Loki Patera on Io. Nature 545, 199–202 doi:10.1038/nature22339 (2017)

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A prominent feature of Jupiter’s surface is the Great Red Spot, which has been observed continuously since the 1830s. Jupiters red spot was easy to discover because of its prominent coloring. Now a new spot has been discovered that can only be seen in the infrared. 

Dubbed the “Great Cold Spot,” it exists not in the thick clouds of Jupiter, but in its thin upper atmosphere, a region driven by Jupiter’s strong magnetic field.  Jupiter magnetic field causes ionized particles to stream toward its magnetic poles, creating aurora similar to those we see on Earth. When ionized particles strike Jupiter’s upper atmosphere, it causes that region of the upper atmosphere to heat up. It’s generally been thought that the temperature of the upper atmosphere was rather static, with its temperature cooling as you move farther from the polar regions. But detailed infrared images show Jupiter’s upper atmosphere is complex and dynamic.

The Great Cold Spot (indicated by the white arrow) it seen near the hot region of Jupiter’s magnetic pole. Credit: Tom Stallard/ESO

Using data from NASA’s InfraRed Telescope Facility, a team analyzed the thermal properties of Jupiter’s upper atmosphere near the polar region. They found a cold region similar in size to the Great Red Spot. With data gathered over 15 years, the team could see the cold spot evolve and change, growing and shrinking over time. This indicates that the feature is persistent and dynamic, similar to the Great Red Spot. Thus the upper atmosphere has a complex weather system similar to that of Jupiter’s lower atmosphere.

It remains to be seen whether there are other long-lasting features of Jupiter’s upper atmosphere, but for now we can say that parts of it can be pretty cool.

Paper: Tom S. Stallard, et al. The Great Cold Spot in Jupiter’s upper atmosphere. Geophysical Research Letters. doi: 10.1002/2016GL071956 (2017)

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Beyond the orbit of Neptune lay thousands of icy bodies. These distant worlds are known as Trans-Neptunian Objects (TNOs), and so far we have only been able to study a few of them. Trans-Neptunian Objects are difficult to study because they are dim and cold. They reflect hardly any light, and emit light due to their temperature at far infrared wavelengths. But the Atacama Large Millimeter/submillimeter Array (ALMA) has both the sensitivity and precision to study these distant worlds. 

The faint, millimeter-wavelength glow of DeeDee, as seen by ALMA. Credit: ALMA (ESO/NAOJ/NRAO)

Recently ALMA trained its antennas at 2014 UZ224, popularly known as DeeDee. The ALMA observatory has a resolution as high as the Hubble space telescope, but observes at millimeter wavelengths, just on the edge between the infrared and microwave range. This makes it perfectly suited to observe objects such as the cold dust around distant stars. It can also study TNOs such as DeeDee. Measuring the wavelengths at which DeeDee emits most of its light, the ALMA team found the dwarf planet’s temperature is about 30 Kelvin, similar to other distant dwarf planets such as Eris. The study also found DeeDee is about 635 kilometers in diameter. That makes it large enough for its own gravity to hold it in a roughly spherical shape, which is one of the criteria for a dwarf planet.

We’ve only just begun to study the outer edge of our solar system, and there are plenty of worlds out there to explore. There may even be a super-Earth. Studies like this demonstrate that we finally have the ability to learn about the most distant bodies in our solar system.

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Ever since Pluto was reclassified as a dwarf planet, there have been eight known planets in our solar system. We have found countless smaller bodies, and even several Pluto-like bodies beyond the orbit of Neptune, but nothing that would meet the criteria of being a planet. But recently there has been indirect evidence of a Uranus-sized planet lurking on the outer edge of our solar system, one that just might be lurking in the astronomical data we’ve gathered. 

The evidence for a ninth planet comes from the orbits of the six most distant known objects in the solar system. These are icy, Pluto-like objects well beyond the orbit of Neptune. If they were truly at the edge of our solar system, one would expect their orbits to be rather randomly distributed. But instead their orbits are bunched together a bit, which implies they interact with another large body. Based on orbital statistics, the estimated mass of this hypothetical body is about 10 times that of Earth, which would make it a bit smaller than Uranus.

A comparison of Planet Nine to Earth (left) and Neptune (right). Credit: Wikipedia

But indirect evidence of this ninth planet isn’t enough. To prove it exists we would have to observe it. The hypothetical orbit of planet nine puts its average distance about 700 times further than the Earth from the Sun (astronomical units, AU), but its orbit would be highly eccentric, so it would likely come as close as 250 AU. If this planet is currently in the close part of its orbit, then it may have already been observed. Large sky surveys such as WISE and Sky Survey could detect a Uranus sized planet out to about 300 AU. Planet nine could be lurking in their data, waiting to be found.

The problem is that identifying an object like planet nine is hard. It would be a faint object, and its motion across the sky would be rather small. So Zooniverse started a project where the general public could help categorize millions of objects in the in the Sky Survey data. Over just 3 days, more than 60,000 people identified about 5 million objects. Out of the data they found four objects that weren’t previously known and could satisfy the conditions for planet nine. The next step is to make new observations of these objects to see if they are asteroids, dwarf planets, or the hypothetical planet nine.

The chance that any of these candidate objects actually is planet nine is quite small. But this project is an amazing demonstration of the power of crowd sourced science. Without the general public, this project would have taken nearly four years. Although this particular data set has been analyzed, the good news is that other projects are also looking for the hypothetical planet. If you want to participate, check out Zooniverse’s Backyard Worlds, which is looking for planet nine and brown dwarfs.

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