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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With the New Horizons flyby of Pluto, we now know the small world has a rich geography with mountains and even a thin atmosphere. Pluto looks like it has all the makings of a planet, and yet astronomers still see fit to place it outside the realm of planets. While this is deeply frustrating to the general public, it’s clear that Pluto is very different from the 8 “classical” planets. It’s smaller than our Moon, and it’s orbit is more inclined and eccentric than the others. It also doesn’t meet the official criteria for what makes a planet.

The official definition for a planet comes from a 2006 resolution of the International Astronomical Union (IAU), which defines 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. Pluto satisfies the first two, but not the third. And that’s unfortunate, because it’s that third point seems the most arbitrary. After all, asteroids cross Earth’s orbit all the time, does that mean it hasn’t cleared its orbit? Not quite. Earth is much, much larger than anything else in its general orbit. The same can’t be said for Ceres in the asteroid belt, or Pluto among other trans-Neptunian objects. It’s pretty clear from the IAU standards that neither Pluto nor Ceres are planets.

Solar system bodies under the new planet criteria. The dotted line represents the age of our solar system. Credit: Jean Luc Margot

While the IAU standard is pretty clear for our solar system, it isn’t so clear for exoplanetary systems. If we were to find a Pluto-sized object orbiting another star, how would we know whether it has cleared its orbit? But a new paper being published in the Astronomical Journal presents a new way to define planets that agrees with the current definition and can be used for 99% of known exoplanets. The new definition is based on the fact that any planet-like object in a solar system takes time to clear other objects from its orbit. The time it takes depends upon its mass (larger objects clear regions faster) and how close the object is to its star (closer objects orbit the star more quickly and have more chances to clear a region). That means clearing an orbit depends upon three things: the mass of the object, its distance from the star, and the age of the stellar system. From those three parameters you can define a threshold where an object’s orbit is considered clear.

When the standard is applied to our solar system, Pluto and Ceres still don’t match the definition of a planet. They are far too small and too distant from the Sun. When we apply the same criteria to exoplanets where we know all three parameters, they all meet the standard of planet. That’s not too surprising, since we’re more likely to find large close exoplanets than more “iffy” ones like Ceres or Pluto.

What’s interesting about this new method is that it could be used as the only criterion for defining a planet. Anything large enough to clear its orbit within 10 billion years will be in hydrostatic equilibrium, for example. Things get even more interesting if you drop the criterion of needing to orbit the star. The Moon, for example, is just massive enough to meet the criterion on its own, so under this definition we could consider the Earth-Moon system to be a double planet. And even though moons of Jupiter and Saturn are larger than our Moon, they are farther from the Sun and therefore don’t meet the planet threshold.

So while the method won’t bring Pluto back into the planetary fold, it would tell us which objects orbiting other stars have all the makings of a planet.

Paper: Jean-Luc Margot. A Quantitative Criterion for Defining Planets. arXiv:1507.06300 [astro-ph.EP]

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Brown dwarfs are like the Pluto of stars. While they are large enough to produce heat like a star, they are not large enough to fuse hydrogen in their cores like our Sun and other stars. They typically have a mass between 20 and 70 Jupiters, and one of the central questions has been whether they are more planet-like or star-like. New research published in Nature points to a more planetary nature by discovering bright aurora on a brown dwarf. 

Aurora, commonly known as northern lights, occur when high energy charged particles strike the Earth’s upper atmosphere, causing it to glow. They occur largely at the polar regions because of an interaction between the charged particles and the Earth’s magnetic field. While they are common on Earth, they are also found on other planets like Jupiter that have a strong magnetic field. Stars, on the other hand, don’t have aurora.

We’ve known for quite a while that the surface temperatures are rather cool. The most massive brown dwarfs can have temperatures about half that of the Sun, while the smallest brown dwarfs can have surface temperatures no warmer than an oven. But whether their atmospheres are more like that of stars or planets has been an unanswered question.  In this new work, the team noticed a brown dwarf that emitted bursts of strong radio energy about once every 2.8 hours. This pulsar-like behavior could be caused by charged particles interacting with the dwarf’s strong magnetic field, or it could be due to interactions with its atmosphere. To find out the team observed the object in the visible spectrum. What they found was that the spectrum matched that of hydrogen that has been struck by charged particles. In other words, these bursts are due to very bright aurora.

This is the first case of aurora being observed on an object outside our solar system. Combined with other research that shows brown dwarfs can have clouds, it’s clear that the atmospheres of brown dwarfs are more planet-like than star-like.

Paper: G. Hallinan et al. Magnetospherically driven optical and radio aurorae at the end of the stellar main sequence. Nature. Vol. 523, p. 568. doi: 10.1038/nature14619 (2015)

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When Johannes Kepler proposed a new model of the solar system in the early 1600s, it was a revolutionary idea. The model addressed many of the problems with earlier circular-orbit models, and greatly simplified the calculation of planetary motions. Still, the model was so radical that it wasn’t fully accepted until Newton was able to derive the model from his law of universal gravitation. What made Kepler’s model so powerful is that is required only three simple rules, which we now call Kepler’s laws.

Conic sections and their relation to orbits.

The first law is that planetary bodies move in ellipses with the Sun at one focus. Kepler was only concerned with planets, but when Newton derived the law he found that it could be more generally stated that solar system bodies followed conic sections. A conic section can be found by taking a circular cone and slicing it at different angles. Slice it horizontally and you get a circle. Slice at a slight angle and you get an ellipse. Slicing ever steeper you eventually get a parabola, and then a hyperbola. Planets move in nearly circular ellipses, while comets can move in parabolas (or nearly parabolic ellipses) and even hyperbolas.

Demonstration of Kepler’s second law. Credit: Wikipedia

While the first law determines the shape of an orbit, the second law described how planets move along their orbit. Expressed geometrically, it states that a line from the Sun to a planet will sweep out an area at a constant rate. This means that when a planet is farther from the Sun (and thus a longer line) it moves more slowly, and when close to the Sun it moves more quickly. While this seems like an awkward rule, it was actually quite useful for astronomers at the time who used geometry to calculate planetary motion. We now know that this law is an expression of a quantity known as angular momentum, and simply states that the angular momentum of an orbiting body is constant.

A plot showing Kepler’s third law relation.

The first two laws are sufficient to calculate the motion of planets, so it was another decade before Kepler came up with his third law. This one relates the orbital motions of different planets to each other, and a kind of unifying principle of planetary motion. What it says is that if you square the orbital period of a planet (in Earth years) and cube the semi-major axis of its orbit (in astronomical units) then you will find that they will equal each other. All planets follow this relation, so it is a basic principle of orbital motion. We now know that they just happen to be equal to each other because of the choice of units. More generally, the two quantities are proportional to each other, and the constant of proportionality depends upon the mass of the central body (in this case the Sun). The fact that all planets follow the same constant is due to the universal law of gravity.

The interesting thing about Kepler’s laws is that by the time they were confirmed by Newtonian gravity, it was also shown that they were only an ideal approximation. In the real solar system, gravitational interactions between planets cause them to deviate slightly from Kepler’s laws. In the end, Kepler’s laws are only a model, and not a true physical law. But despite its problems, it is remarkably accurate for such a simple model. Even though we’ve moved on with Newton’s gravity, and later Einstein’s general relativity, we still use Kepler’s laws as a basic calculation. It’s good enough to be used to calculate orbits of exoplanets, the masses of binary stars, and even the mass of the black hole in the center of our galaxy. It’s a great example of how simple models are extremely useful, even when they aren’t perfect.

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Today marks the Winter Solstice for those of us in the northern hemisphere, and Summer Solstice for those in the southern hemisphere. While the solstice is often assumed to be the shortest (or longest) day of the year, though the complexity of planetary motion means that isn’t quite the case. It does, however, mark the lowest (or highest) transit of the Sun across the sky, and it is what’s often used to mark the change of season. But of course that only applies to Earth. What about the other planets?

Mercury

Because Mercury’s orbit is rather elliptical, the path of the Sun through Mercury’s sky is rather complex. Mercury rotates at a relatively constant rate, but its orbital motion speeds up and slows down due to its orbital eccentricity. This means that the motion of the Sun is generally East to West, as it is on Earth, but there are times when the Sun can be seen to stop, move West to East for a while, then stop and return to its general East-West motion. It occurs about every 58 days, so that would like mark “high summer” on Mercury.

Venus

The axial tilt of Venus is only about 3 degrees, compared to 23 degrees for Earth, so the seasonal variation of Venus is smaller. Venus also has a very thick atmosphere, so there isn’t much of a temperature variation with the seasons. However due to Venus’ smaller orbit, each formal season would last only about 56 days instead of Earth’s roughly 90-day seasons.

Mars

Mars has an axial tilt similar to Earths, and also has a more elliptical orbit. Combined with a thinner atmosphere, this means that martian seasons are actually more extreme than Earth’s. Because of this, the ice caps of Mars are structurally very different. The northern ice cap is water ice, and in winter is covered with a layer of carbon dioxide. The southern ice cap has a permanent layer of carbon dioxide about 8 meters thick. For this reason it was once thought that the northern ice cap was water ice and the southern carbon dioxide.

Jupiter

Jupiter has a small axial tilt, so again seasonal variation is relatively small. Seasons do, however, last about 3 years.

Saturn

With a similar axial tilt as Earth, there is some seasonal variation to Saturn’s atmosphere. It’s thought that the rings could also play a role, given that they block varying amounts of sunlight at different times of the year. Because the orbits of Saturn’s moons are typically along the equatorial plane of Saturn, Titan’s seasonal variation depends more on the tilt of Saturn than its own axial tilt. This is something we’re only beginning to study.

Uranus

The axial tilt of Uranus is about 82 degrees, which means Uranus is basically a world on its side. This means that for 20 years one pole of the planet is bathed in sunlight while the other is in constant shadow. Winter and Summer are therefore quite extreme in terms of sunlight. But Uranus is so distant from Earth that this is less of a factor than the planet’s own internal heat.

Neptune

Given Neptune’s distance from the Sun, it was long thought that seasons would be non-existent on the planet. But we’ve now seen that Neptune’s atmosphere does seem to go through seasonal variations. Since Neptune’s seasons last 40 years, it will be a while before we’ve observed a complete seasonal variation of the planet.

Pluto

What about Pluto? We don’t know yet. There’s some indication that it’s elliptical orbit causes variation in the amount of atmosphere the planet has, but we’ll have to wait for New Horizons to make its flyby before we know more.

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This week the Rosetta probe will release Philae to land on the comet 67P/Churyumov-Gerasimenko. It will be the first soft landing on a comet, but not the first probe to land on another world. Besides our homeworld, humans have walked on the surface of one other body and landed probes on five. If successful Philae will mark the 6th solar system object we’ve landed upon.

The Moon was the first celestial body we landed on. The Soviet Union’s Luna 2 successfully impacted the lunar surface in 1959, and by 1966 Luna 9 made the first soft landing on the Moon. By 1969 the United States successfully landed Neil Armstrong and Buzz Aldrin as the first humans to walk upon another world. It was soon followed by five other manned missions. Since then there have been lots of lander and orbiter missions to the Moon, the most recent being China’s Chang 3 rover mission.

The first successful landing on Mars was achieved with the U.S. Viking 1 mission in 1975. It was soon followed by Viking 2. These missions were the first efforts to discover life on another planet. In 1996 Pathfinder marked the first rover on Mars. Today the Curiosity rover continues that tradition. Europe, Russia and India have all had successful mission to the Red Planet.

1975 also marked the first landing on Venus with the Soviet landers Venera 9 and 10. There have been several flyby missions and a few orbital missions since then, but the Veneras mark the only successful landings on Venus. Given the extreme temperatures and pressures of the planet’s surface, that’s not entirely unexpected.

In January of 2005 the Huygens probe entered the atmosphere of Saturn’s moon Titan. It landed on the moon’s surface about two and a half hours later, sending images and data back to the Cassini orbiter, which relayed the data to Earth. Though much colder, the surface and weather of Titan is somewhat similar to Earth’s, with lakes and seasonal changes. They are just lakes and clouds of methane and ammonia rather than water. Titan marks the most distant world we’ve ever landed upon.

Later that same year Japan’s Hayabusa probe landed on the asteroid 25143 Itokawa. The goal of the mission was to gather samples of the asteroid for return to Earth. The sample mission failed, but dust captured by the probe was successfully returned to Earth in 2010.

Now this week we will at a comet to this list. It’s pretty amazing when you think about it. We humans are not bound to our homeworld. We’ve begun to step out into space, and our efforts are getting better and more common.

Score one for the humans.

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A rogue planet is a planet-sized object that doesn’t orbit a star. Instead these objects move through the galaxy just as stars do. They’ve long been thought to exist, but their small size and low temperatures made them difficult to observe.

Then in 2011, a team looked at data from the Optical Gravitational Lensing Experiment (OGLE), which observed 50 million stars in our galaxy looking for momentary brightenings of a star due to an effect known as microlensing. If a massive object passes in front of our line of sight to a star, the light from the star is gravitationally lensed around the object, and this microlensing effect can make the star appear brighter.

The team observed over 400 microlensing events, out of which 10 were due to Jupiter-mass planets not orbiting a star. From these observations, the team estimated that there are likely two rogue planets for every star in the galaxy. That would mean there are 200 to 800 billion rogue planets in our galaxy alone.

Since these rogue objects don’t orbit a star, you might wonder why they would be called “planets” in the first place. After all, asteroids are moon-sized objects that orbit the Sun like planets, and we don’t call them “rogue moons”. The reason they are often referred to as planets is that they are generally thought to have once been planets orbiting a star, but were then ejected from the stellar system due to a close encounter with either another planet in the system or another star. The asteroids, in contrast, formed in orbit around the Sun, and were not once moons around a particular planet, which is why they would not be considered rogue moons.

But new evidence suggests that not all rogue planets begin their lives orbiting a star. In a recent paper in Astronomy and Astrophysics, observations of the Rosette Nebula found dust clouds smaller than the orbit of Neptune, with masses of about 10 Jupiters. These can be seen in the image above.

Small clouds such as these, known as globulettes, have been observed before. But these particular globulettes have two properties which make them very interesting. Observations of their radio emissions find that they are unusually dense. This means that despite their small size, they are likely to gravitationally collapse into planet-mass objects. The second is that they are moving exceptionally fast away from the nebular region, about 80,000 kilometers per hour (50,000 mph). At this speed they will escape the gravitational pull of the nebula to become freely roaming objects. This is the first evidence that rogue planets can be born free.

It would seem then that referring to these rogue objects as “planets” might be a bit unfounded. Some have proposed calling them Interstellar Planetary-Mass Objects, while the International Astronomical Union (IAU) has proposed calling them sub-brown dwarfs. But the term “rogue planet” will likely be used for quite a while.

Of course the term rogue planet also raises images of a planet racing through our solar system, either colliding with Earth or sending it into a dangerous orbit. This idea was popularized by the 1951 movie When Worlds Collide, based on the book of the same name. In reality such an event is extraordinarily unlikely. The odds of a dangerous close approach is essentially zero, and even if a rogue planet passed close to our solar system the planets would be hardly affected.

So while some planets go rogue, and others are born rogue, they aren’t anything to be feared.

And none of them are named Nibiru…

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Orbital dynamics, that is, the motion of planets and stars about each other, is deeply dependent on computational modeling. The basic motion of one planet or star about another (the so-called two body problem) is fairly simple, and can be summarized by Kepler’s laws of planetary motion, the motion of multiple planets and stars is extraordinarily complex. In fact while the two-body problem is almost trivial to solve, the three-body problem has no exact general solution. As soon as you have three or more masses in your system, the motion can be highly chaotic.

There are, however, some broad patterns we see in many-body systems, such as resonances and the formation of gaps in the asteroid belt. There are also interesting effects that have some surprising consequences. One of these effects is known as the Kozai mechanism.

Within orbital dynamics there are certain properties such as energy and angular momentum that are conserved (or approximately conserved). This means mathematically that certain properties of a system are constants. The Kozai mechanism deals with a quantity related to angular momentum known as Lz. This quantity is a constant, and it relates how elliptical an object’s orbit is (its eccentricity) to the orientation or tilt of its orbit (its inclination).

Credit: Gravity Simulator

You can get an idea of how these are related by imagining a flexible hula hoop. If you hold the hoop in front of you, aligned vertical to the ground, then the hoop will look like a circle to you. If you tilt the hoop at an angle to the ground, then it will look like a squashed oval to you. Now, if you wanted to move the hula hoop vertical again, but keep it looking like an oval, then as you tilt it vertical you have to squash the hoop itself. To keep the apparent shape constant, if you change the angle, you also have to change how much the hoop is squashed. The Lz property is similar, in that the eccentricity and inclination can change in relation to each other, but the “apparent shape” (Lz) stays constant.

The Kozai mechanism does just that. Basically it is a resonance between a smaller object and a larger object that causes the smaller object’s orbit to change its inclination to become more aligned with the larger object, and doing so at the cost if increasing the eccentricity of its orbit. The effect is a little hard to visualize, but you can see an example in the image above. The central dot is a star, and the orbits of two planets are shown. Outside of view is a second star orbiting at a different angle from the planets. You can see how the orbit of the outer planet is gradually shifted due to the second star.

One of the ways the Kozai mechanism comes into play is between the orbits of Neptune and Pluto. Gradually Pluto’s orbit is being tilted toward the orbital plane of Neptune. As a result, the eccentricity of Pluto’s orbit increases. This can explain why Pluto’s orbit is so elliptical, while most of the other planets are not. Another consequence of this mechanism is that the point of Pluto’s closest approach to the sun (its periapsis) is shifted relative to Neptune’s, which is part of the reason why Pluto will never collide with Neptune even though it crosses Neptune’s orbit.

A similar resonance should occur with other minor planets similar to Pluto (the Plutoids). This means that their orbits should be clustered rather than randomly distributed. We already see that with other minor planets such as Makemake.

All due to an interesting orbital resonance.

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In 1801 a new planet was discovered in our solar system.  Just twenty years earlier the planet Uranus was discovered beyond the orbit of Saturn, and was the first planet discovered since the dawn of civilization.  The location of Uranus agreed with a (now defunct) model of planetary distances known as the Titus-Bode law, which had correctly predicted the distances of the known planets.  But the Titus-Bode law also predicted the existence of a planet between Mars and Jupiter, which had not been seen until the discovery of this new planet.  The 1801 planet had a distance within 1% of the prediction of Titus-Bode, and it was named after the Roman goddess of agriculture, Ceres.

Until the mid-1800s, Ceres was considered to be a planetary body.  In less than a decade after the discovery of Ceres, three more planets were discovered between Mars and Jupiter, and given the names Pallas, Juno and Vesta.  In 1846, Neptune was discovered beyond Uranus, raising the total number of planets in our solar system to 12.  Within ten years of Neptune’s discovery, dozens of new planets were discovered between Mars and Jupiter:  Astreae, Hebe, Iris, Flora, Metis, Hygiea, and the list continued to grow.

While Uranus and Neptune were similar to the historical planets, these new planets were very different.  They all had roughly the same orbital distance (between Mars and Jupiter).  They were also significantly smaller than the other planets, even much smaller than our moon. Rather, they were more like Ceres, itself a small, rocky body.  It soon became clear that referring to all of these bodies as planets wasn’t very accurate.  So they were put into a new category of Sun-orbiting objects: asteroids.  Thus, Ceres lost its planetary status, demoted to King of the asteroids.  The number of planets in our solar system was thereby reduced to eight.

In 1930 a new planet was discovered, and given the name Pluto.  While Pluto was a small world (smaller than our Moon), it seemed alone beyond the orbit of Neptune.  But in the 1990s more objects were discovered beyond the orbit of Neptune.  By the mid-2000s, trans-Neptunian objects of a size similar to Pluto were discovered, and in 2005 one larger than Pluto was found.  It was named Eris, and for a while enjoyed status as the tenth planet.

But by then it was clear that Pluto was not alone.  Rather there were lots of objects that, like Pluto were small, icy and beyond the orbit of Neptune.  Again it was clear that referring to all these bodies as planets wasn’t an accurate description.  So in 2006 the International Astronomical Union formalized the definition of what constituted a planet.  They had to orbit the Sun, they had to be massive enough to be roughly spherical, and they had to be distinct among objects of their distance.  Pluto and Eris did not satisfy the last condition, being similar to other trans-Neptunian objects.  Like Ceres  before them, they lost their status as planets.

But the IAU also defined a secondary category for objects that satisfied the first two, but not the third.  They were given the name dwarf planets.  Under this definition, Pluto and Eris were categorized as dwarf planets.  Ceres was also promoted to dwarf planet status, as were two other trans-Neptunian objects, Haumea and Makemake.  So currently our solar system has 8 planets, 5 dwarf planets, and countless other small solar system bodies.

As our understanding of the solar system has grown, we have had to refine our naive concept of what makes a planet.  If you mourn Pluto’s exclusion from the solar planets, you should also mourn Ceres, who suffered her loss a century earlier.

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Observations from the ALMA telescope array has made some interesting observations of carbon monoxide in the vicinity of the star Beta Pictoris.  The results have been published recently in Science, and it tells an interesting story about comets and planetary formation.

Beta Pictoris is a young star with a protoplanetary disk.  It is only about 63 light years away, so it provides us with a good opportunity to study the formation of a solar system.  In this new paper, large quantities of carbon monoxide have been observed within the protoplanetary disk.  The reason this is interesting is that carbon monoxide is unstable when exposed to sunlight, so any carbon monoxide around Beta Pictoris would typically break down within a century.  Since the disk contains large quantities of CO, there must be a replenishing source, and that is likely cometary fragments.  Given the quantity of carbon monoxide, it is estimated that you’d need a cometary collision about once every five minutes.  That would imply a dense cluster of cometary bodies.

An extrapolated top-down view of CO around Beta Pictoris.
Credit: ALMA (ESO/NAOJ/NRAO)/W. Dent et al.

Further support of this idea comes from the distribution of the carbon monoxide around the star.  From our vantage point, the protoplanetary disk of Beta Pictoris is edge on.  This can make it difficult to determine the distribution of material within the disk.  But since carbon monoxide has a clear line spectrum, the Doppler shift of that spectrum can be used to determine the motion of CO within the disk, and hence its distribution.  What we see is at least one dense clump, possibly two, within the disk, as you can see in the figure.

Such a concentration is evidence of a planetary body within the disk.  This planet could cause cometary fragments to cluster in the region of the planet, resulting in a high rate of collisions.

So it seems that the disk of Beta Pictoris is far more active than originally thought.

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