In all the cosmos there is only one planet known to harbor life. While Earth is special to us, there are countless similar worlds orbiting other stars. Since life arose early in Earth’s history, it seems likely that life could arise on other potentially habitable planets. But as we learn of both exoplanets and the history of life on Earth, we’ve found that things are a bit more complicated. 

In astronomy, “potentially habitable” simply means that the orbit of an exoplanet places it in a particular range of distance from its star. Close enough that its water wouldn’t deep freeze into a perpetual solid, and far enough away that its water doesn’t boil away leaving a dry husk of a world. Earth, as you would expect, lies within the habitable zone of the Sun.

But there are other things that make Earth so friendly to life. For one, our Sun is a stable main sequence star, so Earth has received a steady source of light and heat for billions of years. Earth also has a large moon, and the gravitational tug between the Earth and Moon creates tidal heating in Earth’s interior, producing volcanoes and other geologic activity that can bring rich material from the interior to Earth’s surface. In the outer solar system, the large moons of Jupiter experience similar tidal forces, warming their water to a liquid underneath their surface. Moons such as Europa might harbor life because of this tidal heating.

It turns out that our solar system is a bit unusual. Stars such as are much less common than smaller red dwarf stars. Most of the planets we’ve discovered orbit close to a red dwarf star. The Trappist-1 system, for example, has a least 7 Earth-sized worlds orbiting its star far more closely than Mercury orbits the Sun. Although Trappist-1 is about 90 times more massive than Jupiter, they are about the same size, and the planets orbit at a similar distance as the moons that orbit Jupiter. Since the orbits of these planets are not exactly circular, they experience tidal forces like the Jovian moons. So they could be geologically active in a life-friendly way.

Young red dwarfs can be rather hostile to life. They can produce large solar flares that can fry the atmosphere of a close planet, leaving them dry and arid. But Trappist-1 is an older, stable star, so its planets would have a steady stream of heat and light. In a recent paper, a team looked at the conditions for the Trappist planets, taking into account both the amount of heat they receive from their star, and the amount of tidal heating they generate. They found that planets d and e seem the most friendly for life, with moderate stellar heating and moderate tidal heating. They should be warm enough for liquid water, but cool enough to prevent a runaway greenhouse.

Of course the big question is whether these planets have ample water on their surface. That would depend critically on just how massive they are. While we have a good idea of their size, we aren’t as certain about their masses. So we’ll need more data to determine if life could survive on these nearby worlds.

Paper: A. C. Barr, et al. Interior structures and tidal heating in the TRAPPIST-1 planets. Astronomy & Astrophysics (2018)

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One of the biggest challenges in astronomy is observing the cold, dark dust surrounding a young star. These planetary disks, as they are known, are the birthplace of planets. Understanding them helps us understand how planetary systems form. But much of the gas and dust is so cold that they emit light mostly in the microwave range, which is difficult to detect. But with the construction of the Atacama Large Millimeter/submillimeter Array (ALMA) we can finally start to see details. 

A common feature of these planetary disks is their ringed pattern. The disks often have rings or arcs of thick dust separated by gaps. It has been thought that these gaps are caused by young planets, which tug on the gas and dust to make patterns in the disk, similar to the way Jupiter created gaps in the asteroid belt known as Kirkwood gaps. But new research finds that these ringed patterns might not be evidence of planets after all.

Computer simulations from a team at NASA show these gaps could be caused by ultraviolet light. When ultraviolet light strikes grains of dust, it can free electrons from the dust grains through the photoelectric effect. The free electrons then collide with surrounding gas, heating it up. As the gas heats and expands it tends to trap more dust grains. This reinforcing cycle is known as photoelectric instability. The computer simulations show that photoelectric instability can combine with other interactions to create the type of arcs and rings we see in young planetary disks.

This doesn’t mean there aren’t young planets orbiting these young stars, but rather that the presence of rings in a planetary disk doesn’t prove there are planets. Planetary formation is complex, and we will need to do further study to understand it.

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In our solar system Jupiter is the king of planets. It is 2.5 times more massive than the other planets combined. But it isn’t the most massive planet we know of. In the search for planets around other stars, we’ve found planets with masses up to 20 times that of Jupiter. All things being equal, we can imagine Jupiter-like planets having a fairly even distribution of sizes, but it turns out that’s not the case. 

A team of astronomers looked at the mass of hundreds of Jupiter-type planets, and found they tend to fall in two groups. One with masses of 1 – 4 Jupiters, and another with much larger masses. Large-mass Jupiters were found around all types of stars, but small-mass Jupiters were only seen around stars with a higher metallicity.

Planet mass vs metallicity for the analyzed stars. In the plot one can see the position of the two populations of giant planets. Credit: Santos et al. 2017

Stars are mostly hydrogen and helium. In astronomy other elements are referred to as “metals.” The metallicity of a star is a measure of how much of these metals a star has. The higher the metallicity, the more metals. Our Sun, for example, has a relatively high metallicity. The team found that metal rich stars like our Sun tend to have planets in the 1 – 4 Jupiter-mass range, while metal poor planets tend to have planets with 4 – 20 Jupiter masses.

The key to this mass difference could be in the way they form. There are two major ideas about how large planets can form. One model is the core accretion model, where a dense metal core forms first, and its gravity then attracts surrounding gas and dust to form a large planet. The other is the gravitational instability model, where gas and dust over a large area becomes gravitationally unstable and collapses under its own weight. It seems that the smaller Jupiters form via core accretion, which limits their mass, while larger Jupiters form by gravitational instability, which allows them to grow quickly.

It’s a bit early to conclude that formation mechanism is the cause of the two mass types. We’ll need to look at a larger sample of planetary systems to be sure. But this work does demonstrate the role of metallicity in planetary formation. Our solar system is just one example of a diverse range of planetary systems, and the formation of these systems was deeply dependent on the characteristics of their home stars.

Paper: N. C. Santos, et al. Observational evidence for two distinct giant planet populations. Astronomy & Astrophysics Vol. 603, A30, DOI: 10.1051/0004-6361/201730761 (2017)

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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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In science fiction we love seeing exotic worlds. One of the most famous is Luke Skywalker’s home planet of Tatooine, a desert world with two suns. While it makes for great science fiction, is it really possible for a habitable world to orbit two stars? 

In Star Wars, Tatooine’s two suns are similar in size, implying Tatooine orbits both stars.

It has generally been thought that the formation of planets in binary star systems would be problematic. If the two stars were relatively far apart, planets might form close to one star or the other. Such worlds would have two suns, but one sun would dominate the sky while the second, more distant sun would be smaller and dimmer. But in Star Wars Tatooine’s two suns are similar in size, implying Tatooine orbits a close binary system.

Planets like Tatooine had been thought to be unlikely, since the gravitational tugs of two stars would likely disrupt any planet formation in the region surrounding both stars. But a few years ago we began to find planets orbiting two stars, and computer models demonstrated that planets could form farther away from a binary system and migrate closer to the stars over time. So far the circumbinary planets we’ve discovered are large gas planets, but there is no reason an Earth-sized world couldn’t have a circumbinary orbit. But would such a world be habitable? Tatooine is portrayed as a barely habitable desert world. The idea being that the heat from two stars would make the planet dry and arid. But new research finds that this would not necessarily be the case.

A model of the temperatures for a Tatooine-like world. Credit: Max Popp & Siegfried Eggl.

A team looked at a binary system known as Kepler-35. The two stars of this system are fairly Sun-like G-class stars, and system has at least one circumbinary planet that orbits the stars once every 131 days. This planet is about 40 times the mass of Earth, and far too close to the stars to be habitable, but it demonstrates that an Earth-like world could orbit the system at a greater distance. So the team hypothesized an Earth-like world at a more comfortable distance, orbiting the stars once every 340 to 380 days. They then modeled the kind of temperature variations such a planet would have.

They found that a water-rich world in such an orbit could be habitable. Because of the two stars, the planets orbit wouldn’t be as circular as Earth’s orbit, which would mean a wider variation of seasonal temperatures. But with plenty of water in the air the temperature swings would be moderated. The increased light from the two stars would also mean less cloud cover, which would mean large land regions would be quite arid, similar to desert regions on Earth.

So Tatooine-like worlds are at least possible. Whether such a world actually exists waits to be seen, but it’s good to note that the search for Earth-like worlds should probably include looking at binary star systems.

Paper: Max Popp & Siegfried Eggl. Climate variations on Earth-like circumbinary planets. Nature Communications 8, Article number: 14957 doi:10.1038/ncomms14957 (2017)

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We’ve found thousands of exoplanets in recent years, but the holy grail of planet hunting is to find a truly Earth-like world. A planet similar in size to Earth, with a rich atmosphere and a surface temperature capable of sustaining liquid water. 

We tend to imagine such a world as looking very like Earth, with a bright yellow Sun and a nitrogen-rich atmosphere. But as we’ve discovered more exoplanets, we’ve learned that it is far more likely for a planet to orbit a small, red-dwarf star. Red dwarfs make up 75% of the stars in our galaxy, and most of them have planets and even planetary systems. In order for such worlds to be potentially habitable, planets of red dwarfs must be very close to their star. Much closer than Earth is to the Sun. But red dwarfs are known to have periods of violent solar flares and x-ray emissions, and this could strip close planets of any atmosphere, particularly for worlds the size of Earth or smaller. But new observations of the exoplanet GJ 1132b find that Earth-sized worlds can maintain an atmosphere, even when they are close to their red dwarf sun.

A size comparison of Earth and GJ 1132b. Credit: Wikipedia

GJ 1132b is a “super-Earth” about 30% larger than Earth and about 60% more massive. This means it is likely a rocky planet like Earth rather than a small gas planet like Neptune. It is in close orbit with its red dwarf star, making a round trip in only 1.6 days. This means it is far too hot to be potentially habitable. The planet gets 16 times more light energy than Earth, so its surface temperature is likely similar to that of Venus. But given its similar size to Earth and the extreme closeness to its star, GJ 1132b is an excellent test of whether Earth-sized planets can maintain an atmosphere.

An illustration of the transit method for finding exoplanets. Credit: NASA Ames

GJ 1132b can’t be observed directly. It was discovered by the transit method, where the apparent brightness of a star is seen to decrease slightly as a planet passes in front of the star. The larger the dip in brightness, the larger the planet. After GJ 1132b was confirmed as an exoplanet in 2015, a team of astronomers began to observe its transits at various wavelengths, from the visible into the infrared.  They found that at one particular infrared range (the Z-band) the planet blocked more of its star’s light than at other wavelengths. In other words, at Z-band wavelengths, the planet appeared to be lightly larger. If the planet lacked an atmosphere, then we would expect the planet to block the same amount of light at all wavelengths, giving it the same apparent size. But if it has an atmosphere, the atmosphere would block light only at certain wavelengths such as infrared, just as our own atmosphere does. Given the size of GJ 1132b and the particular wavelengths its atmosphere blocks, it is likely a thick atmosphere rich in water vapor and methane.

This is an exciting discovery. We’ve discovered atmospheres around a super-Earth before, but never one so close in mass to Earth, and never one so close to its star. This discovery demonstrates that at least some red dwarf planets can maintain a rich atmosphere, and that would be true of potentially habitable planets that are a bit further from the star. We haven’t found the holy grail of exoplanets yet, but we now know such a planet is possible.

Paper: John Southworth, et al. Detection of the atmosphere of the 1.6 Earth mass exoplanet GJ 1132b.  arXiv:1612.02425 [astro-ph.EP] (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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There are seven planets orbiting a small dwarf star known as TRAPPIST-1. In 2016, three planets were discovered around the star, and today four more were announced. All of these worlds are roughly the size of Earth, and three are potentially habitable. On its own, TRAPPIST-1 would be easy to overlook. It’s a dim, 18th magnitude star, 40 light years away in the constellation of Aquarius.  So what led astronomers to look for planets around this unassuming star, and how did they find them? 

TRAPPIST-1 is known as an ultra-cool dwarf star. It has only 8% the mass of our Sun, or about 84 times the mass of Jupiter. If it were much smaller it wouldn’t have enough mass to fuse hydrogen in its core, and would instead be a brown dwarf. Although it’s about 80 times more massive than Jupiter, it isn’t much larger than Jupiter. That’s because the star much more dense than a planet due to its gravitational weight. Although it is a star, it’s size makes it somewhat Jupiter-like.

Artist’s depiction of an ultra-cool dwarf star like TRAPPIST-1 (left) with brown dwarfs of 65 and 30 Jupiter masses (center) and Jupiter (left). Credit: NASA/IPAC/R. Hurt (SSC)

In some ways Jupiter can be seen as a miniature planetary system.  Eight of its moons have nearly circular orbits, and four of them are the size of our Moon or larger. Other gas planets in our solar system have large moons in nearly circular orbits, so some astronomers speculated that a small star like TRAPPIST-1 might have Earth-sized planets orbiting close to the star. Although ultra-cool dwarf stars are dim and cool, a close planet might be warm enough to support liquid water on its surface. And unlike larger dwarf stars,  ultra-cool dwarfs might lack large solar flares that would threaten the habitability of close planets.

So a team of astronomers started looking for evidence of planets around small dwarf stars. Using the TRAPPIST–South (TRAnsiting Planets and PlanetesImals Small Telescope–South) telescope at ESO’s La Silla Observatory in Chile, they observed the changing brightness of stars like TRAPPIST-1, hoping to catch a planet as it passed in front of the star. This is known as the transit method of planetary detection, since the planet transits in front of the star from our vantage point.

Animation showing the transit of a planet. Credit: NASA/Hubble.

TRAPPIST-1 is so small and distant that we can’t see a planet transit directly, such as when Venus has transited the Sun. Even viewed with a telescope like TRAPPIST–South, the star looks like a point of light. But when a planet passes in front of the star, it blocks some of its light. We see this as a slight dimming of the star. It’s a simple method in principle, but in practice can be quite complicated. Transiting planets aren’t the only thing than can cause a star to dim. So can things such as starspots and other solar activity. There have been cases where what initially looked like a planet turned out to be a false positive.

This plot shows the varying brightness of TRAPPIST-1 during an unusual triple transit event. Credit: ESO/M. Gillon et al.

To make sure a dip in brightness is actually a planet, astronomers need to make multiple observations to ensure they follow a regular pattern. The more transits they observe, the more confident they can be that it really is a planet. That’s part of the reason only the three closest planets were announced last year, and the outer four planets were announced today.

Although the only evidence for these planets is the dimming of TRAPPIST-1, there is still quite a bit we know about them. To begin with, the size of a planet determines the amount of dimming during a transit. Larger planets block more of the star, and there for the dip in brightness is larger. Knowing the size of TRAPPIST-1, the team could measure the transit dimming due to each planet to determine its size. This is how we know all seven planets are roughly the size of Earth. Some a bit larger, and some a bit smaller.

Artist’s illustrations showing the size of the TRAPPIST-1 planets compared to those of our solar system. Credit: NASA

We also have a good idea of their orbits. The time between transits tells us how long each planet takes to orbit the star, known as its orbital period. The orbital period of a planet depends upon the mass of its star and the planet’s distance from the star. Since we know the mass of TRAPPIST-1 reasonably well, we can calculate the distance of each planet. By measuring the length of each transit we can also get an idea of each planet’s speed. Since these speeds agree with the speed of a circular orbit, we know the orbits of these planets are fairly circular, just like planets in our solar system.

It’s an amazing system, but could it be a habitable system? That’s a completely different question, and one that must be explored more cautiously. Three of these planets are at a potentially habitable distance, meaning that theoretically liquid water could exist on their surface.  While we know the size of these planets is similar to Earth, we don’t know their mass. We also have no data on whether they have atmospheres, or if they are wet or dry. All of this affects their potential habitability. But even without this information we can make some educated guesses, given what we know about other planets and planetary systems. It’s a fascinating topic, and one you can read all about in a post by my colleague and astrophysicist Ethan Siegel.

Paper: M. Gillon et al. Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1. Nature (2017)

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A potentially habitable, Earth-like world has been discovered around the nearest star from the Sun, Proxima Centauri. It sounds pretty cool when you say it like that. While it is a great discovery, there are reasons to be cautious. 

To begin with, this particular planet, known as Proxima B, was discovered using the Dopper method rather than the transit method. This means it was discovered by looking at how the light from Proxima Centauri shifts due to the motion of its surface. While such Doppler effects can be due to the gravitational influence of an orbiting planet, it can also be caused by things like solar flares. Sometimes this leads to a false positive, so we really need more data to be sure it’s really a planet. Assuming the data holds up, calling the planet “Earth-like” is a bit optimistic. Since the Doppler method only measured stellar motion towards us and away from us (radial motion) it only gives the minimum mass of a planet. If the planet’s orbit is highly tilted relative to us, then its mass will be larger. The quoted size of 1.3 Earth masses is actually the low end, and it’s more likely to have a mass of 2 – 3 Earths, making it more of a super-Earth planet.

The planet is also in Proxima Centauri’s habitable zone, which again means less than you might think. The “habitable zone” of a star is just a simple calculation of an distance where liquid water could exist on a planetary surface. Actual surface temperature depends not only on distance, but also atmospheric density and composition. One need only look at the variation between Venus, Earth, and Mars to see how much that matters. Since Proxima Centauri is a red dwarf, there are other challenges to habitability. For one, any planet in a red dwarf’s habitable zone is likely to be tidally locked, with one side under constant noon while the other side is in constant night. Such planets are likely to experience constant extremes of temperature, rather than a reasonable Earth-like variation. Then there is the fact that red dwarfs like Proxima Centauri are quite active, with large solar flares and bursts of x-rays that could fry a close planet like Proxima B.

All that said, there is some reason to be optimistic. Computer simulations of the planet’s orbit indicate that it likely formed farther away from Proxima Centauri, and therefore is likely to have plenty of water. It’s mass is large enough that it could have a strong magnetic field like Earth due to an iron core and geologic activity, and this could protect the planet’s surface from solar flares and x-rays. Calling the planet potentially habitable is not too much of a stretch. There is a chance that if its mass is on the low end and conditions are favorable it could look somewhat like Earth.

But the real draw for Proxima B is that it is only 4 light years away. That’s still about 180,000 times the average distance from Earth to Mars, but it is close enough that we can imagine sending a space probe to Proxima B. The proposed Project Longshot mission envisioned just such a mission that would take about 100 years. It’s a long mission, but humans have undertaken century-long projects before. Just as the call of the Moon led to the Apollo mission, and Mars inspires near-future missions, Proxima B inspires a mission to the nearest star.

That’s the power of a planet like Proxima B. It allows us to hear the call of the wild.

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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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A new planet has been discovered orbiting a star 1,200 light years away. While exoplanets are discovered all the time these days, this one was discovered by direct observation. You can see it in the image as the small brown dot to the upper left of the bluish central star. It’s kind of amazing that we can see it directly, but it’s also kind of amazing that it exists at all. 

The new planet orbits a star known as CVSO 30. It has a mass of about 5 Jupiters, and orbits the star at a distance of about 660 AU, with an orbital period of about 27,000 years. It’s large size and distance is the reason we can observe it directly. CVSO 30 is known to have at least one other planet, which orbits the star about every 10 hours.

What’s interesting is that CVSO 30 is a T-Tauri star, and still in the process of forming. It’s unlikely that such a large planet could have formed in its current location, so it is probably the product of a close interaction with another planetary body. It likely formed much closer to the star, and then was thrown into it’s current wide orbit. It’s a good example of just how dynamic early solar systems can be.

Paper: T. O. B. Schmidt, et al. Direct Imaging discovery of a second planet candidate around the possibly transiting planet host CVSO 30. arXiv:1605.05315 [astro-ph.EP] (2016)

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