Ten light years away there is a star that could tell us about the origins of our solar system. Known as Epsilon Eridani, it is a bit smaller and cooler than our Sun, but similar in composition. It is also only about 500 million years old, giving us a view of what our own solar system may have been like in its youth. New work now finds the system is similar to our own. 

We’ve known for a while that Epsilon Eridani has a disk of debris surrounding it. It is in keeping with the idea that planetary systems form from such disks during a star’s youth. We understand the basic process of planetary formation pretty well, but where the details get fuzzy is how and when planets form. Do they form further from the star and migrate inward over time? Do large planets form first and dictate where other planets might form? Computer models can only take us so far. To make matters worse, we now know that our solar system is a bit unusual, so we can’t rely on it as a typical model. But Epsilon Eridani could help.

The system has at least one Jupiter-sized planet. This planet, known as Epsilon Eradani b, or AEgir, has about the same distance as Jupiter in our solar system. It also has an asteroid belt within AEgir’s orbit, just as we have one within Jupiter’s orbit. Far beyond AEgir’s orbit is a comet belt, similar to the Kuiper belt beyond Neptune. It’s hard to determine the details beyond that, since each region of debris within the system emits light at different wavelengths. In particular the long infrared wavelengths often emitted are largely absorbed by our atmosphere, making them impossible to observe from the ground.

This is where SOFIA comes in. SOFIA is a 2.5 meter telescope mounted in a Boeing 747. SOFIA can observe these long infrared wavelengths because it flies high above much of Earth’s atmosphere. New observations from SOFIA found Epsilon Eridani has two asteroid belts. In addition to the one within AEgir’s orbit, there is a narrow asteroid belt between AEgir and the comet belt. Thin belts of debris would tend to spread out over time, so it is likely that this belt is shepherded by one or two additional planets. The spacing of this new belt and the comet belt implies two planets, similar to Uranus and Neptune. Over time, the gravitational tug from these planets and AEgir would cause material from the outer belt to migrate toward the inner one. The inner asteroid belt is likely stable over time.

These new planets still need to be confirmed, and even the existence of AEgir can be debated. But the formation of gaps within the debris disk of Epsilon Eridani validates models for the formation of our solar system.

Paper: Kate Y. L. Su, et al. The Inner 25 au Debris Distribution in the epsilon Eri System. The Astronomical Journal, Volume 153, Number 5 (2017)

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Although our Sun is the only star in our solar system, that isn’t the case for every planetary system. It’s estimated that about than half of stars could be binaries, though the exact percentage is still hotly debated. What we do know is that binary stars are common. This has raised the question about how planets might form in binary systems. 

The usual view of planetary formation is that planets form within disks of material around a young star. While such protoplanetary disks are common around single stars, there has been debate about whether they are common around binary stars. While we have observed debris disks around young binary stars it hasn’t been clear whether such disks would be stable long enough for planets to form. After all, the gravitational interactions between two orbiting stars might make the surrounding region hostile to stable orbits.

So far all the exoplanets we’ve found around binary stars have been large, Jupiter-like planets. They would have formed in the outer icy regions of the system. But what about rocky, Earth-like worlds? Could they have formed closer to the stars where orbits might not be so stable? A newly observed binary system suggests that they could.

The system is called SDSS 1557. It consists of a white dwarf about the mass of our Sun, orbited by a large brown dwarf about 60 times more massive than Jupiter. Recently astronomers have observed a rocky asteroid belt surrounding the system.

Diagram of SDSS 1557 showing the debris ring around the two central stars. Credit: J. Farihi, et al.

This is important for two reasons. First, since the main star is a white dwarf, this is an old system. Our Sun will eventually become a white dwarf, but only after another 5 billion years when it reaches the end of its life. So it is possible that this asteroid belt has been stable for quite some time. Plenty of time for planets to form. Secondly, since the astroid belt is rocky and high in metals, it could form planets that are much more similar to Earth in size and composition.

None of this means that the system actually has rocky planets. In our own asteroid belt no planets formed because of the gravitational pull of Jupiter. A similar type of gravitational dance might have occurred near SDSS 1557. But what this discovery does show is that we can’t rule out the possibility that Earth-like planets might exist with two Suns.

Paper: J. Farihi, et al. A Circumbinary Debris Disk in a Polluted White Dwarf System. arXiv:1612.05259 [astro-ph.EP] (2017)

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On a broad level we understand how planets form quite well. A disk of gas and dust forms around a young star, and clumps within this disk gravitationally collapse into planets. But there are details of this model that still pose challenges. In particular, we don’t entirely understand how small pockets of dust can get large enough to become proto-planets. But new simulations of planet formation are beginning to solve this mystery. 

The basic problem with the dust-to-planet model is as follows: as dust starts to clump together, fast moving dust particles would be more likely to smack into them and break the clumps apart. Even if the clumps do hold together, the drag created by particle collisions would cause the clump to spiral inward toward the star before it could get too large. It would seem that the disk of material around a young star is a bit hostile to dusty clumps.

While this seems to be an obvious problem, the interaction between gas and dust is quite complex. It turns out that a new model focusing on these interactions seems to solve the issue.

When dust clumps are small, the flow of any surrounding gas will tend to push them around. This is what can create the kind of drag that causes them to fall towards the star. But when the clumps become large enough they will tend to push the gas around instead. This is similar to the whoosh of air you feel as car speeds past you. As the dust clumps orbit the star, the reaction with surrounding gas creates a pressure region that actually encourages more dust to clump together. As a result, once dust clumps reach a critical size they would tend to grow quickly rather than being ripped apart.

The team that developed this model demonstrated that the process works for a variety of initial conditions, from disks that have less dust and are more diffuse to more dense disks with lots of dust. So the transition from small clumps to early planets is less problematic than we’ve thought.

Paper: J.-F. Gonzalez, et al. Self-induced dust traps: overcoming planet formation barriers. Mon Not R Astron Soc 467 (2): 1984-1996 (2017).

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Planets form out of a disk of material swirling around a young star. As clumps form in this protoplanetary disk, they collide an merge to become the planets we see today. The composition of those planets depends upon where they formed. Those forming closer to the star tend to be dry and rocky, while those forming farther from the star tend to be rich with water. This is because the heat of the star tends to drive away volatiles like water, producing what is known as a frost line or ice line. Beyond this distance it’s cold enough for ice to exist. Closer than the ice line and it’s too warm. At least that’s the idea. Actually observing the ice line of a young planetary system is a challenge. 

The ice line of V883 Orionis compared to our solar system. Credit: ALMA (ESO/NAOJ/NRAO)/L. Cieza

For a Sun-like star, the ice line is about three astronomical units from the star. That’s about the middle of the asteroid belt for our solar system. Imaging that line in a young system hundreds of light years away is difficult. But recently the star V883 Orionis has pushed its ice line much farther away. It’s only a bit more massive than our Sun, but as material from its protoplanetary disk has been consumed by the star it’s gotten much hotter. It’s currently about 400 times more luminous than our Sun. As a result, it’s ice line has been pushed back more than 40 astronomical units, which would put it beyond the orbit of Neptune in our solar system.

This is far enough out that the Atacama Large Millimeter/submillimeter Array (ALMA) is able to see the ice line directly. Not only does this validate the existence of ice lines in a planetary system, it also demonstrates how the ice line can shift significantly during the formation period of a solar system. Over time V883 Orionis will dim to a luminosity similar to the Sun’s, and it’s ice line will shrink accordingly. It’s an excellent example of the complexity of planetary formation.

Paper: Lucas A. Cieza, et al. Imaging the water snow-line during a protostellar outburst. Nature 535, 258–261 (2016)

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Traditionally the difference between comets and asteroids is that comets have tails and asteroids don’t. As we’ve studied comets and asteroids, however, we’ve found the aren’t so clearly divided. It is not simply a matter of comets being icy snowballs and asteroids being dry rocks. Nor is it simply a matter of having a tail. As we’ve seen, asteroids can form tails as they enter the inner solar system. Recently we’ve seen a comet that doesn’t have a tail, and it may hold important clues to the origin of our solar system.

The comet C/2014 S3 has been called a manx comet due to its lack of tail. It’s lack of tail indicates that it doesn’t have icy volatiles on its surface.  Based upon what little coma is has, it’s estimated it has about a millionth the surface volatiles of a typical comet, which makes it much more like an asteroid in composition.

Actual image of the manx comet (as opposed to the artist rendering above). Credit: University of Hawaii Institute for Astronomy

Given that it’s so dry and rocky, why call it a comet at all? It turns out its trajectory takes it from about twice the distance of Earth all the way out to the Oort cloud, so it has a distinctly comet-like trajectory. The fact that it has come from the Oort cloud with very little ice is actually quite interesting. If it had formed in the Oort cloud it should have plenty of ice and would definitely have a tail. So it’s most likely that this particular body formed in the inner solar system where most volatiles are boiled off, and then was thrown outward to the Oort cloud.

One model of the early solar system, known as the Nice model, actually predicts the existence of such manx comets. As the planets began to form, they shifted their orbits dramatically, causing some of the material from the inner solar system to be thrown to the distant regions of our solar system. The timing of this would affect the amount of volatiles the material still has. If we can find similar manx comets, we should be able not only to confirm models of the early solar system, but might be able to fine tune the sequence of events.

Paper: Karen J. Meech, et al. Inner solar system material discovered in the Oort cloud. Science Advances Vol 2, No. 4 (2016) DOI: 10.1126/sciadv.1600038

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TW Hydrae is the closest T Tauri star, only about 180 light years away. T Tauri stars are young stars in the late stages of formation. This means any planetary system they have are also in the early stages, so they give us insight on just how planetary systems form. Studying these early planetary systems can prove difficult, since they consist largely of cold gas and dust, which can be challenging to observe. But recent observations from the Atacama Large Millimeter/submillimeter Array have given us the most detailed images of TW Hydrae yet, and they are kind of amazing. 

The TW Hydrae system has a gap at about 1 AU. Credit: S. Andrews, ALMA (ESO/NAOJ/NRAO)

The protoplanetary planetary disk of TW Hydrae happens to be face-on from our perspective, so ALMA has a very clear view of the disk’s structure. As with other young planetary systems, the disk has gaps indicative of early planet formation. With this observation and others, it is clear that young stars form planetary systems from the gas and dust of a protoplanetary disk. But this system is particularly interesting because it shows a gap at about 1 astronomical unit, which is the distance of the Earth from the Sun. It’s close to the resolution limit of AMLA, but there is a clear gap. So we now have evidence of Earth-distance planets forming in a solar system.

Paper: Sean M. Andrews, et al. Ringed Substructure and a Gap at 1 AU in the Nearest Protoplanetary Disk. The Astrophysical Journal Letters, Volume 820, Number 2 (2016)  arXiv:1603.09352 [astro-ph.EP]

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Earlier I noted that the presence of ammonia on Ceres was evidence that the dwarf planet formed further away from the Sun than it is now. The reason for that is that the material surrounding a young protostar varies significantly by distance. 

A young star surrounded by a protoplanetary disc.

The basic structure of a young solar system is that of a protostar surrounded by a protoplanetary disc. As the young star begins to shine, the surrounding disk is heated. Naturally, the material closer to the star gets warmer than the material farther away. In the close region, volatiles such as water and ammonia are broken apart or pushed outward by solar wind. As a result, material in an inner solar system will tend to be more dry and rocky, while the colder outer region will be more wet and icy. Dividing these two regions is a frost line or ice line. Closer than the frost line material is too warm for ice to form. Farther than the frost line ice can form more readily.

In the current solar system, the frost line is at about 5 AU, which is a bit closer than Jupiter, so currently all the rocky planets are inside the frost line, and all the gas giants are beyond the frost line. This would seem to imply that it’s the frost line that determines whether a rocky or gas planet will form. But we now know that things are more complicated than that. For one, the frost line in the early solar system was only about 3 AU, since the gas and dust of the protoplanetary disc absorbed light, keeping outer regions cooler. For another, we know that different planets form at different rates, and they can migrate during their formation. Large, Jupiter-like planets tend to form early on, and they tend to drift inward as they form. Our solar system with its inner rocky planets and outer gas planets seems to be more the exception than the rule.

It does seem that large gas planets will tend to originate beyond the frost line, and this may in fact mean that asteroid belts will tend to form in the region near the frost line. As larger planets form just beyond the frost line, their gravity would tend to disrupt a region in such a way that planets can’t form. This is how our own asteroid belt formed (rather than from an exploded planet as some have claimed).

The dynamics of planetary formation are complex, and there is still much we don’t understand, but we do know that the type of material making up a body shows the history of its formation. Presence of ammonia on Ceres shows that it must have spend time beyond the ice line. In that way it seems more connected to the outer region of the solar system.

Paper: Rebecca G. Martin, et al. Dead zones around young stellar objects: dependence on physical parameters. MNRAS 420 (4): 3139-3146 (2012)

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Here’s a fun question: how many planets were there in the solar system? You might be tempted to answer with “Nine until you astronomer jerks exiled Pluto!” but I’m not talking about a change in classification. I’m talking about the number of planets over the 5 billion year history of the solar system. We now have 8 classical planets, but did we once have more?

The basic Nice model. Credit: Gomes, et al.

There are actually some hints that our solar system may have had five gas planets in its youth rather than the current four. We know, for example, that the orbit of a planet can migrate as it forms, and this is particularly true for larger gas planets. We also know that the planets in our solar system didn’t form in their current location. Just how our solar system came to have large outer gas planets is still a matter of some debate. There are two main models that have been proposed. The Grand Tack model proposes that Jupiter migrated toward the Sun as it formed, then entered a gravitational resonance with Saturn that drove the planets outward. An alternative is the Nice model, which posits that Jupiter was roughly at its current distance when it entered a resonance with Saturn. The resulting resonance drove Neptune (initially closer than Uranus) to the outer edge of the solar system, pushed Uranus and Saturn outward.

The Nice model seems to be a better fit, and can explain why the inner and outer solar system are so radically different. However computer simulations of the Nice model imply it is unlikely with only 4 gas planets. However if you add a fifth gas planet into the mix, then the four current planets could reach their present orbits at the cost of ejecting the fifth planet.

That might seem like a tweak model just to make the Nice model work, but the idea has a testable prediction. In the models, either Jupiter or Saturn must have a close enough encounter with the fifth planet to eject it. Neither Uranus or Neptune would be large enough. But such a close encounter would effect the orbits of the moons of Jupiter or Saturn. So this raises an interesting question: could the current moons of Jupiter or Saturn have survived such an encounter given their orbits?

A new paper in the Astrophysical Journal addresses this question. Using computer models, the team looked at the gravitational effect of a fifth planet interaction on Callisto (the outermost of Jupiter’s large Galilean moons) and Iapetus (the outermost large moon of Saturn). If neither of these moons could survive an interaction with the fifth planet, then the model isn’t well supported. What the team found is that Iapetus had only a 1% chance of being in its current orbit after ejection of a planet by Saturn, but Callisto had a 42% change of being in its current orbit if the fifth planet was ejected by Jupiter. The latter is well within the realm of possibility.

So its at least possible that there was an extra gas planet in the early solar system. And if there was once a fifth gas planet, it was almost certainly ejected by Jupiter.

Paper: Ryan Cloutier, et al. Could Jupiter or Saturn Have Ejected a Fifth Giant Planet?  The Astrophysical Journal, Volume 813, Number 1 (2015)

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One of the reasons radio observatories such as ALMA and others are so useful is that different wavelengths of radio emissions let us tune in on different sizes of dust grains in the universe. That’s because dust grains tend to emit radio signals with wavelengths around their own size. This fact allows us to study the types of dust being formed in early planetary systems.

Map of dust grains around DG Tauri. Credit: J. Greaves, et al.

For example, recent observations from ALMA found millimeter-sized dust grains orbiting a brown dwarf star known as Rho Ophiuchi 102. This is somewhat surprising, since it would imply that clouds around small brown dwarfs are similar to dust clouds around larger stars. It also suggests that brown dwarfs may form rocky planets. Another team using the e-Merlin array found somewhat larger grains around the star DG Tauri. Despite the limited resolution, it is clear that a dust belt has formed around the young star.

What these observations show is that protoplanetary disks are not only common around stars, but that dust seems to be present early on. It’s one more clue pointing to the idea that planets are the norm rather than the exception for stars in our universe.

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There are five small planets orbiting a star known as Kepler-444. These planets are all smaller than Earth, and they are all very close to their parent star, with “years” lasting less than 10 days. None of this is really a big deal given the vast number of exoplanets we’ve discovered, but what is unusual is the age of the star, which is estimated to be about 11 billion years old.

The traditional view of planet formation is that rocky Earth-like planets wouldn’t form around early stars. The reason for this is that elements other than hydrogen and helium are formed within stars, so only after some of the first stars died and exploded would things like iron, carbon and silicon be available for rocky worlds. But we now know that model is a bit too simplistic.

Observed transits of the five planets. Credit: Campante, et al.

One of the ways we determine the age of a star is by its metallicity. That is, the amount of “metals” (anything but hydrogen and helium) a star contains. That’s because the less metal a star has, the older it is likely to be. Our Sun is only about 5 billion years old, for example, and has a relatively high metallicity. We’ve seen some correlation between the metallicity of a star and the type of planets that might form, specifically that higher metallicity stars are more likely to have large Jupiter-like planets. Kepler-444, by contrast, is a metal-poor star with a low metallicity. It isn’t the type of star we’d expect to have a planetary system, and yet it clearly does. Given the size of these planets, they are likely to be rocky worlds as well.

The low metallicity of Kepler-444 would imply it is an older star, but astroseismology (the stellar version of helioseismology) gives an age of about 11.2 billion years, give or take a billion. That means it formed when the universe was only about 2 – 3 billion years old. So it seems that planetary systems could form early on, even when metals were fairly rare.

It would seem then that planets have been around almost as long as there have been stars.

Paper: Campante T. L. et al. An ancient extrasolar system with five sub-Earth-size planets. ApJ 799 170. (2015)

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Every now and then in astronomy we’ll get an image that lets us actually see phenomena we have previously just deduced from other observations. The image above is one of them. It was taken by the Atacama Large Millimeter/submillimeter Array (ALMA), and shows an exoplanetary system in the process of forming. This isn’t an artistic rendering, it’s an actual image.

We’ve known for a while that planetary systems form along with a star. Known as the nebular model, the basic idea is that as a star forms within a large nebula (known as a stellar nursery) the surrounding gas and dust form an accretion disk around the star. Over time, protoplanets form within these disks, eventually clearing the system and becoming a planetary system such as our own solar system. We have a lot of evidence to support this model. We’ve observed stars forming within nebulae, and we have computer simulations showing how stars form an are cast out of a stellar nursery.  We’ve observed protoplanetary disks around young stars, and we have computer simulations showing how protoplanets begin forming within these disks. We’ve also found lots and lots of exoplanets. So we’ve known that out in the universe there are young stars where protoplanets are actively forming, we just haven’t observed them directly. Until now.

This particular image is of a star known as  HL Tauri. It is a young T-tauri type star about 450 light years away. The image was taken at wavelengths on the order of a millimeter, which is particularly good at penetrating the nebular material surrounding the young star. Because ALMA is an array of telescopes spread across 15 kilometers, it can capture images with a higher resolution than Hubble.

The ALMA telescope array. Credit: ALMA (ESO/NAOJ/NRAO), C. Padilla

It’s the detail of this image that is astounding. Not only can you clearly see the disk, you can actually see gaps in the disk. These gaps are due to protoplanets either clearing the region of their orbit, or creating resonances within the disk to produce gaps, similar to the way Jupiter produces Kirkwood gaps in the asteroid belt. This image is crystal clear evidence of protoplanet formation, just as the nebular model predicts.

ALMA is still in its early stages. In a way, it is one of ALMA’s baby pictures as it gets up to speed. It also happens to be a baby picture of a whole new solar system.

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With the growing number of discovered exoplanets, we’ve come to realize that planets and solar systems are very diverse. For ages it was suspected that planets orbited other stars. It was also generally thought that they would be similar to our own, with small, rocky, inner planets and large, gaseous outer worlds. Now we know that Jupiter-type planets are often quite close to their sun. We know that planets don’t just form around stars like our Sun, but also around red dwarfs, and sometimes even alone without a sun. We’ve also found planets that orbit two stars.

Although we’ve found planets in binary star systems, it isn’t clear how they could have formed. Before computer simulations, it was thought that planetary orbits in a binary star system would be too unstable to exist for long. We now know that there can be regions around a star in a binary system that where planets can have quite stable orbits. But the other question has been whether such planets could form in the first place. In a binary system, the two stars would tend to capture most of the material as the solar system formed, which would leave little left over for planets to form.

Images of the debris ring of GG Tau-A. Credit: Anne Dutrey, et al.

But a paper this week in Nature seems to show that planets can, in fact, form in a binary system. The work is based on data gathered at the ALMA radio telescope array, and shows a binary star system in the process of forming planets. The system is known as GG Tau-A, and images show it has two debris regions. The first is a large debris ring around the binary system itself, while the second is a debris region around the primary star. This by itself isn’t too surprising. The outer ring is the remnant of the binary system’s formation, and the inner region has yet to be swept clean by the two stars.

But the team also found evidence of material from the outer ring being swept to the inner region. Thus, while inner region material might be captured by the stars, it is replenished by material from the outer ring. This means the inner region could contain material long enough for planets to form.

The question remains as to whether this transfer-effect is common in binary systems, or if this is just an unusual fluke. But if it is common we could have an explanation for binary solar systems. Either way, it is one more example of just how diverse planetary systems can be.

Paper: Anne Dutrey, et al. Possible planet formation in the young, low-mass, multiple stellar system GG Tau A. Nature 514, 600–602 (2014)

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