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29 November 2016

It Turns Out the Sun Is More Chill Than We Previously Thought

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: The Mount Wilson Observatory S-index of the Sun
Authors: Ricky Egeland, Willie Soon, Sallie Baliunas, Jeffrey C. Hall, Alexei A. Pevtsov, and Luca Bertello
First Author’s Institutions: High Altitude Observatory; Montana State University
Status: Accepted to ApJ

One of the few certainties that we have as humans is that the Sun always comes and goes, always in intervals of 24 hours, and will continue to do so for the next 5 billion years or so. Owing to this familiar cycle, we sometimes take for granted that our host star is not itself completely stable. It is active, and this activity shapes the evolution the solar system and life on Earth. Because we use the Sun as a reference for other stars, it is thus crucial that we measure its activity as accurately and precisely as possible.

Blame Magnetic Fields

Mount Wilson Observatory’s 100-inch telescope is one of the telescopes that was used to make solar observations for the HK project. [Ken Spencer]

Stars are big balls of hot gas with lots of moving parts. The ones that are similar to the Sun (i.e., solar-type stars) have large convective atmospheres, which act just like boiling water inside a cooking pot. The convective circulation of plasma generates magnetic fields, and the stellar rotation, in turn, makes field lines wrap around the star, creating a stellar dynamo. When the magnetic field lines concentrate, they produce dark spots in the stellar surface and spectacular mass ejections; the activity of a star is measured by the strength of these episodes.

One precise and accurate way of assessing stellar activity uses the features in stellar spectrum known as the Ca II H & K lines. Their strength can be easily measured with spectrometers, and are then translated into a ratio called the S-index (the higher it is, the more active the star). The most famous survey of stellar activity is the HK Project at Mount Wilson Observatory (MWO), which consisted of assessing the S-index of many stars in the sky and ended up becoming the standard calibration for current studies on activity. The problem is that not all observations are carried out with the same instrument, and hence systematic errors start to become a serious problem.

Context Is Key

In order to understand the role of activity in the physics of stellar and planetary evolution, it is important to place the Sun, the one star we know best, in the same context as the others. In today’s paper, the authors aim to precisely and accurately measure its activity using spectroscopic observations of the Moon — which reflects sunlight — obtained with the same instrument employed at the MWO.

Sun-like stars have activity cycles with periods of the order of a few years. The solar cycle has an 11-year period, encompassing a minimum and a maximum. The authors directly measured the minimum, maximum and mean S-index of the Solar Cycle 23 (1996–2007, although the MWO data goes only up to 2003), and found that they were significantly lower than previous estimates of the same cycle. This result shows that, when using different instruments, systematic errors plague the measurements of activity — but the good news is that now we can correct them by applying a better calibration with results from today’s paper.

The solar activity during Cycle 23. The MWO data are the red dots, and the red curve is a fit to the data. Most of the activity cycle measured by previous studies (dashed curves) are visibly shifted to higher values. [Egeland et al. 2016]

Well, now that we have dealt with accuracy, what about precision? As it turns out, there is no lack of solar activity data in the literature, which are now correctly calibrated. The authors used them to constrain the solar activity minimum, maximum and mean values within less than 1% for all indices.

Composite time series of the S-index of the Sun, using various data sets (colored symbols), which are now correctly calibrated. The colored curves are fits to the data. [Egeland et al. 2016]

Effects on Future Studies

So, the Sun is slightly less active than we previously measured, and this impacts our understanding of solar-type stars the most. By correctly placing our star in the context of others, we can better assess how common it is, which helps us answer questions about the conditions necessary for life to emerge and how it evolves along with the star. These results also rectify some inconsistencies previously observed in the activity of solar-type stars, and again reminds us of a critical aspect of science: systematics matter. The article itself will serve as a guide on measuring stellar activity, paving the way towards better practices in the field.

Bonus if you read to the end:

The Sun from September to November 2016. Larger GIF available here.[ESA/NASA/SOHO]

About the author, Leonardo dos Santos:

Leonardo is a graduate student of Astronomy at University of São Paulo, Brazil, and a visiting student at University of Chicago. His current research consists of studying physical and chemical properties of stars similar to the Sun, hunting for exoplanets and software development for astronomical instrumentation. When not sitting in front of a computer, he enjoys doing night sky photography, cooking, riding his mountain bike and failing miserably at pretty much all these tasks.

22 November 2016

The True Colours of a Blue Straggler

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Formation and Evolution of Blue Stragglers in 47 Tucanae
Authors: J. Parada, H. Richer, J. Heyl, J. Kalirai, R. Goldsbury
First Author’s Institution: Department of Physics & Astronomy, University of British Columbia, Canada
Status: Published in ApJ

Just by looking at a star’s colour, astronomers can learn a few things about its properties. The most direct one is its temperature: red stars like Betelgeuse are cool, blue stars like Rigel are hot. Another property we can estimate by eye is the apparent magnitude, a concept idealised by the Greek Hipparchus almost 2000 years ago. He ranked stars from brightest to faintest in a scale of one to six, one denoting the twenty brightest stars he could see, and six denoting those he could barely spot with the naked eye. This scheme was adapted into a logarithmic scale by N. R. Pogson in 1856, and is still in use nowadays. The apparent magnitude depends on the star’s luminosity and distance, so if we know the latter, we can estimate the former using the measured magnitude. Knowing the luminosity and the temperature we’ve estimated from the star’s colour, we can place it in a Hertzprung-Russell (HR) diagram and infer other properties, like evolutionary stage, mass, and radius. Unfortunately, distance is a very hard thing to measure accurately in astronomy. However, there’s a way to overcome that: studying stars in clusters. In a star cluster, all stars are basically at the same distance from us, so we can ignore the effect of the distance and compare the apparent magnitude of these stars directly. We can then build a colour-magnitude diagram (CMD), the observational version of the HR diagram. Lots of our knowledge about stellar evolution came from comparing theoretical evolutionary models to this kind of diagram.

Blue stragglers in the open star cluster NGC 3766. [ESO]

With the development of high resolution imaging, CMDs revealed the presence of some unexpected stellar populations. A prominent population is blue straggler stars (BSS). These stars appear as a blue extension of the main sequence (MS) in globular clusters, right above the so-called turnoff point, where stars are about to leave the main sequence. They are unexpected because, given the cluster’s age and metallicity, stars with their properties should already have evolved off the main sequence. Blue stragglers are still there though, like they were too lazy to leave the MS. How they are formed and where they go when they finally evolve are two questions still up for debate (bite1, bite2). The authors of today’s paper studied the blue stragglers in the cluster 47 Tucanae with the aim of shedding some light on the subject.

Blue stragglers: what we know so far

The only way to explain the blue stragglers being brighter and bluer than the turnoff is if they went through some rejuvenating process. There are different ways to do that, but two main conditions must be followed: i) one MS star must be involved, and ii) one of the stars involved gained mass to become rejuvenated. This mass gain gives the star more fuel, extending its main sequence lifetime. There are two possible ways for this to occur: mass transfer or merger. The authors considered three different scenarios: i) direct collision of stars, ii) evolution of primordial binaries, and iii) hierarchical evolution of triple systems. The resulting blue straggler can have different characteristics depending on the mechanism that caused it. The populations we observe are a combination of all these mechanisms, but one of them can prevail over the others depending on the environment.

Combining theory and observations: what 47 Tucanae can teach us

In order to try to identify the origins and possible evolution of the blue stragglers in 47 Tucanae, the authors first separated the stars into different stellar populations. First, based on the stars’ location on the CMD, the cluster was split into two blue straggler populations, bright (bBSS) and faint (fBSS), each containing half of the blue straggler sample, main sequence (MS), binary main sequence (MSBn), and red giant branch (RGB). This is illustrated in Fig. 1. The authors then estimated the cumulative distribution of each of these populations along the radius of the cluster, also shown in Fig. 1. The first thing to notice is that the bBSS and the fBSS samples are actually different: the bBSS is more concentrated in the inner regions than the fBSS is. Moreover, the bBSS cumulative radial distribution does not resemble any of the other populations, making it difficult to link its formation to any specific group. The fBSS, on the other hand, show a distribution very similar to the MSBn, pointing to a binary origin for the fBSS. The estimated masses of these two populations are also similar, with the fBSS being slightly smaller, as would be expected for a final product of binary evolution. The mass of the bBSS is in contrast higher, suggesting an origin involving triple or multiple stellar systems.

Figure 1: The left panel shows a colour magnitude diagram using Hubble's filters F255W and F336W. The stellar populations identified are indicated. On the right panel, the cumulative distribution for each population is shown. The numbers of stars in each panel are slightly different because a completeness correction was done on the right panel, taking into account stars that aren't detected because of limitations in the method.

Figure 1: The left panel shows a CMD using Hubble’s filters F255W and F336W. The stellar populations identified are indicated. On the right panel, the cumulative distribution for each population is shown. The numbers of stars in each panel are slightly different because a completeness correction was done on the right panel, taking into account stars that aren’t detected because of limitations in the method.

Next the authors relied on evolutionary models calculated with the code MESA (Modules for Experiments in Stellar Astrophysics) to identify the region occupied by evolved blue straggler stars (see Fig. 2). Calculating the cumulative radial distribution, they noted that what they call evolved blue stragglers (eBSS) follow a similar distribution to the blue stragglers, suggesting they are indeed linked. Moreover, they found an excess of stars in the RGB and the horizontal branch (HB) when compared to the expected number considering only single evolution. According to their estimates, this excess can be explained by stars evolving from blue stragglers into these regions. So it appears that the blue stragglers have a post-MS evolution comparable to that of a normal star of the same mass. There’s still some disagreement between the lifetimes the authors estimated and others found in the literature, indicating a more detailed study of individual blue straggler properties is in order to better constrain these values. Future studies using high quality spectra may help with that.

Figure 2: The left panel shows again the cluster’s CMD, now with some MESA models overplotted. The sequences have initial masses of f 0.9, 1.1, 1.4, and 1.8 solar masses, from bottom to top. The right panel shows the cumulative distribution for the samples selected on the right panel. There’s again a completeness correction that makes the number of stars slightly different in the plots.

In short, the authors verified that different mechanisms leading to blue stragglers can in fact be identified within a cluster. Interactions in multiple systems seem to dominate in the central regions, while binary evolution seems to be the dominant mechanism in the cluster outskirts. The former leads to more massive, brighter objects, while fainter blue stragglers are explained by the latter. The evolution of the blue stragglers seems to be similar to simple MS stars with same mass, which makes it easier to model their evolution. However, this still has some discrepancies with other results, so more detailed studies, focused on individual objects, are needed. The blue stragglers have not shown all their true colours yet.

About the author, Ingrid Pelisoli:

I am a second year PhD student at Universidade Federal do Rio Grande do Sul, in Brazil. I study white dwarf stars and (try to) use what we learn about them to understand more about the structure and evolution of our Galaxy. When I am not sciencing, I like to binge-watch sci-fi and fantasy series, eat pizza, and drink beer.

15 November 2016

Investigating One of the Strongest Starbursts in the Universe

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: SPT0346-52: Negligible AGN Activity in a Compact, Hyper-starburst Galaxy at z = 5.7
Authors: J. Ma, et al.
First Author’s Institution: University of Florida
Status: Accepted to ApJ

In recent years, the South Pole Telescope has discovered an intriguing population of gravitationally lensed dusty star-forming galaxies. These galaxies, some of the most distant and faint in Cosmos, are able to be studied because their light is magnified by a foreground gravitational lens. Using this phenomenon, astronomers can learn more about the stars, gas and dust — not the kind of dust under your bed that you should probably vacuum up some day, but instead the type that blocks, absorbs and scatters light — in the very distant universe.

One galaxy in particular drew the researchers’ attention as it stood out in the 2500 square degree (that’s about 6% of the sky; for comparison: the full Moon subtends about 0.2 square degrees) survey undertaken by the South Pole Telescope: its name is SPT-S J034640-5204.9, though we shall call it SPT0346-52.

A 20″x20″ cutout of SPT-3462-52 showing HST (grey), Spitzer (blue contours), and ALMA observations (red contours). [Ma et al. 2016]

Galaxy SPT0346-52

This galaxy, the main protagonist of today’s astrobite paper, is located at a redshift of z=5.656. That means that, since its light can only travel at a finite speed, we are observing it at a time when the universe itself was a mere 1 billion years old. This makes SPT0346-52 part of the first population of galaxies to have formed in the universe, and one of the most distant of the type of galaxy known as “dusty star-forming galaxies” to have been observed thus far. Because of this it has been studied intensely using a wide range of wavelengths to probe its physical properties: so far it has been observed with HST, Spitzer, Herschel, ALMA, APEX and the VLT!

From ALMA observations it could be determined that the foreground lensing galaxy (a “mere” 8 billion light-years from Earth) was magnifying the light from SPT0346-52 by a factor of about 5.6, and that the intrinsic infrared luminosity of SPT0346-52 was a factor of 1000 higher than that of our nearest neighbour, the Andromeda galaxy. The star formation rate of SPT0346-52 was calculated to be about 4500 solar masses per year — compare that to the Milky Way’s “meagre” rate of 1 solar mass per year! In addition, given that with a radius of about 0.6 kpc it’s about 25 times smaller than our own galaxy, this indicates that SPT0346-52 has one of the highest star formation rate densities of any known galaxy!

While much has been learned about this fascinating galaxy from previous multi-wavelength observations, one intriguing question remains:

What causes its high luminosity?

Does SPT0346-52’s high infrared luminosity arise solely from intense star formation, or does it harbour an obscured active galactic nucleus (AGN)? This is the question explored in today’s paper.

Several indicators appear to favour the presence of an AGN: the dust temperature in SPT0346-52 is found to be about 52 K, making it warmer that that of typical dusty star-forming galaxies. The luminosity ratio of some of its emission lines is consistent with a quasar (the most energetic and distant type of active galactic nucleus), and it shows strong H2O emission lines which are similar to those found in distant quasars. However, SPT0346-52 does not show any indications of an AGN in deep optical spectroscopy.

In order to shed more light on this mystery, the authors of today’s paper undertook two further observations, probing the extremes of the electromagnetic spectrum. They observed SPT0346-52 in X-rays using the Chandra satellite. A clear detection of X-ray photons would be indicative of the signature of an accreting black hole powering an AGN, while a purely star-formation-driven galaxy would emit a lot less X-ray flux. Additionally, the authors undertook radio observations with the Australian Telescope Compact Array (ATCA). Similarly to X-rays, radio waves can be used to distinguish star-forming galaxies from radio-loud active galactic nuclei.

Spectral Energy Distribution of SPT0346-52

The best-fitting spectral energy distribution for the galaxy SPT0346-52. It uses input flux measurements from a range of wavelengths in order to find the best-fitting spectrum given a range of model spectra. Since a galaxy’s spectrum changes with age, mass, dust content and other factors, the best-fitting model can tell us a lot about the physical properties within the galaxy. In this best-fitting model the authors found that AGN likely contribute less than 5% to the total infrared luminosity of this galaxy. [Ma et al. 2016]

Results and What They Can Tell Us

In almost 14 hours of X-ray observations, just over 3 photons were detected, and over one hour of radio observations yielded no detection at the source position…

However, together with the previous multi-wavelength observations, it was possible to use the upper limits from the X-ray and radio part of the spectrum to constrain the shape of the spectrum of SPT0346-52. This indicated that a pure starburst spectrum of the galaxy is the best fit to the observations. Similarly, the upper limits of the ATCA radio observations are consistent with a star-forming galaxy, and would have to be a factor of at least 2 higher for a radio-loud AGN.

One of the Most Extreme Starbursting Galaxies in the Universe

Also, while the presence of an AGN cannot be completely ruled out, the authors conclude that star-formation — and not AGN activity — is the dominant driver behind the far-infrared emission observed. The authors plotted SPT0346-52 on a diagram showing the distribution of X-ray luminosity vs far-infrared luminosity for both star-forming and AGN-dominated galaxies (see Fig. 3). This shows that SPT0346-52 lies left of the dividing line between star-formation and AGN, making it likely that it is indeed powered by very high star formation rates.

One question, however, remains:

Fig. 3: This plot shows the positions of a range of galaxies on a far-infrared vs. X-ray luminosity diagram. SPT0346-52 is the red arrow, while star-forming galaxies are marked as blue S, and AGN as yellow A. Using the dividing lines between AGN and star-formation driven luminosities indicated on the plot suggests that SPT0346-52 does not contain an AGN. Hence it appears as though SPT0346-52’s extreme brightness is powered by star-formation.

What Could Power Such High Star Formation Rates?

The authors offer several possible explanations for the finding that SPT0346-52 has one of the highest star formation rate densities (a measure of how many stars form each year in a given volume) in the universe.

They suggest that SPT0346-52 may have an especially high star formation efficiency; probably higher than about 40%. This means that SPT0346-52 converts 40% of its available hydrogen gas into stars; a more typical values for galaxies like the Milky Way is around 5%. Additionally, the gas fraction — the amount of gas available for this star formation within the galaxy — is also exceptionally high at above 40%. The authors suggest that it might be possible to explain the very high star formation rate in SPT0346-52 with such a high gas fraction, even without a very high star formation efficiency.

Hence, despite the many observations of SPT0346-52, several mysteries still remain unsolved. The authors suggest that this would make SPT0346-52 an ideal candidate for follow-up observations with the JWST, which is due to launch in 2018…

About the author, Steph Greis:

I’m a third (out of four) year PhD student at the University of Warwick, UK, where I study local analogues to redshift z~5 Lyman break galaxies (LBGs) which are some of the earliest galaxies in the Universe. When I’m not thinking about galaxies far, far away, I enjoy reading, cooking, and geocaching.

8 November 2016

Modest Chaos in the Early Solar System

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: The Asteroid Belt as a Relic From a Chaotic Early Solar System
Authors: Andre Izidoro, Sean N. Raymond, Arnaud Pierens, Alessandro Morbidelli, Othon C. Winter, David Nesvorny
First Author’s Institution: Laboratoire d’astrophysique de Bordeaux, Pessac, France
Status: Accepted to ApJ

The puzzling architecture of the Solar system has long been a headache for planetary dynamicists. We can sort of divide its structure (cover image, not to scale) into several zones. First, the terrestrial planets Mercury, Venus, Earth and Mars, which are divided from the gas and ice giant planets by the asteroid belt. Beyond the ice giants there is the Kuiper belt, which spans out to very large distances from the Sun. One longstanding conundrum in this ordering is the relatively small mass of Mars and existence of the asteroid belt in between Mars and Jupiter. It shouldn’t be there, and Mars should be way bigger. In fact, Mars is only 10% of the mass of Earth and therefore seems to have never accreted enough material to become a fully fledged planet. In planet formation, Mars-sized objects are usually termed “planetary embryos”, as we think this is the intermediate stage of a planet’s growth.

Sailing adolescent Jupiter to blame?

To explain the apparent dip in the mass-distance relation of our solar system (increasing mass with planetary distance from the Sun until Jupiter, and then decreasing again), planetary scientists developed a model deeply rooted in dynamical principles: the “Grand Tack”. The idea of the model is that Jupiter, directly after its formation out of the early Solar nebula at ~3.5 AU (AU = distance Sun–Earth), migrated toward the Sun, became “tacked” on at ~1.5 AU and then migrated outward again to its current positon at 5.2 AU. By doing so it depleted the mass concentrated at the locations of nowadays Mars and the asteroid belt, and thus Mars was never able to grow bigger. It also successfully explains some other issues, such as the inclinations and excitations of asteroids and the transition from water-poor to water-rich asteroids in the middle of the belt.

Figure 2: Chaos in the giant planets's orbits and their long-term stability. During the gas disk phase the giant planets migrated inward toward the Sun and are eventually locked in so-called mean-motion resonances. After the gas dissipation Saturn and Jupiter orbit in chaotic, but stable orbits. (Fig. 8 from the paper)

Chaos in the giant planets’ orbits and their long-term stability. During the gas-disk phase the giant planets migrated inward toward the Sun and are eventually locked in so-called mean-motion resonances. After the gas dissipation, Saturn and Jupiter orbit in chaotic but stable orbits. [Izidoro et al. 2016]

A more “primordial” origin

Alternatively, the mass-depletion at Mars’ location could have very early roots, even before (or during) the formation of Jupiter, caused by microphysics in the disk, as was explained in this Astrobite about the pile-up and evolution of dust. Here, the initial small dust grains in the disks did not accumulate everywhere in the disk and start to form planets. Instead, they end up only in specific regions and thus planet formation is concentrated in some narrow zones. In our case, this could be in two locations: one in the inner region, where the terrestrial planets reside nowadays, and one in the outer region, possibly around the region of Jupiter (more details given in this recent review paper). Such a scenario however poses another problem. The Grand Tack scenario from above easily explains the current relatively high inclinations and excitations of the asteroids in the belt: Jupiter drops by and gives everyone a huge gravitational swing due to its enormous mass. In the other scenario this isn’t the case — so we would nominally expect the asteroids to remain in relatively calm and low-excitation orbits.

But Andé Izidoro and collaborators think differently. They propose that instead of migrating (like in the Grand Tack model), Jupiter and Saturn were on chaotic but stable orbits. This scenario is illustrated in the figure above, which shows how the gas and ice giants evolved over time in such a configuration. Essentially what happens is that all the planets roughly formed at approximately where they are today. However, as our early solar system was subject to dynamic evolution (we possibly lost some planetary embryos early on), the orbits of Jupiter and Saturn moved continously and chaotically through the disk (though not nearly as dramatically as in the Grand Tack scenario). Thus, Jupiter came close to the asteroid belt again and its gravity excited the asteroids. You’ll probably ask yourself now where the great difference lies. This is highlighted in the figure below, where the deviations between the Grand Tack and the “chaotic motion” pictures become readily apparent.

In the Grand Tack model, Jupiter is formed within a sea of primordial planetesimals/asteroids (which are divided into S- and C-types: water-poor and water-rich). It migrates inwards and then outwards, and scatters some of the outer asteroids to their current locations. In the “low-mass asteroid belt”/”chaotic motion” model the S- and C-type deviation and the mass-depletion at around Mars’ current location is primordial. Jupiter and Saturn form later and evolve in chaotic but stable orbits, which scatters some of the outer asteroids inside. [Morbidelli & Raymond, 2016, arXiv:1610.07202]

What now?

Instead of one we have now two models, both of which can give us an answer to why our solar system’s architecture looks like it does today. Fortunately, the two models make different predictions about how the dynamics evolved in the disks. In the coming years we may thus be able to distinguish between them by testing their differences, and we may be able to get deeper insights into how our home in the universe evolved into how it looks today.

About the author, Tim Lichtenberg:

I am a graduate student at ETH Zurich in planetary astrophysics. By combining astro- and geophysical numerical approaches I try to understand the influence of star-forming environments on the formation of terrestrial planets. Occasionally, when I am not at the bus swinging back and forth between the institutes, I enjoy doing sports or become involved in science outreach projects.

1 November 2016

A Long Time Ago Around a Star Far, Far Away…

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: OGLE-2012-BLG-0950Lb: The First Planet Mass Measurement from Only Microlens Parallax and Lens Flux
Authors: N. Koshimoto, A. Udalski, J.P. Beaulieu et al.
First Author’s Institution: Department of Earth and Space Sciences, Osaka University, Japan
Status: Accepted in AJ

In today’s paper, we’re looking at a planet discovery. The planet in question is called OGLE-2012-BLG-0950Lb. That’s a bit of a mouthful, so for the purposes of this article I’m going to nickname it Oggy.

Oggy is an unusual planet. It’s intrinsically fairly normal: a planet roughly the size of Neptune, orbiting an M-dwarf star. But Oggy is 3 kiloparsecs, or 9,800 light years, away, making it one of the most distant planets ever discovered. That’s because it was discovered via the microlensing technique.

A Quick Microlensing Primer:

Microlensing is a bit of a forgotten relative of observational exoplanet science — we hear about transit and radial velocity planets all the time, and direct imaging is intuitively simple and makes pretty pictures, so it gets lots of press time too. The microlensing technique, meanwhile, has been quietly churning out planet detections since 2004. And yet, if you asked your average exoplanet scientist how microlensing works, they’d probably panic and mutter something about general relativity before running away as soon as possible.

Microlensing works like this: your system consists of a lens star, which magnifies light from a source star. The source star is moving relative to the lens, and the closer it gets to the lens, the more the light rays are bent, and the more the lens magnifies the source. One could plot contours of equal magnification — they would be circles, traced around the lens star, as I’ve tried to demonstrate in Figure 1. The closer to the lens, the more a source would be magnified. The background source will move across the diagram following the red arrow, and as the flux increases and then decreases a flux peak is observed.

Figure 1: On the left, the blue contours show regions of equal magnification — increasing towards the centre where the star is positioned. On the right is the sort of flux curve you’d observe as the source passes behind the lens, following the red arrow. [Astrobites]

Now, if the lens star also has a planet, this can warp the light rays even more. In fact, the interference between the two gravitational sources can even create places where the magnification of the source is infinite. Alternatively, depending on the system geometry, it can reduce the magnification of the lens for a very short time. As the source moves across the sky, there’s an added peak or trough, on top of the peak from the lens magnifying the source. Figure 2 shows a schematic of this setup.

Figure 2: Three lens diagrams for the three possible scenarios. Left: a source is magnified by a lens star. Centre and right: the lens star now hosts a planet, which also warps the light: either enhancing or reducing the lens effect for a short period of time, and creating an additional bump or dip on top of the lensing curve. [Astrobites]

Confused yet?

Applying this to Oggy:

During the summer of 2012, two different microlensing surveys — one called OGLE and one called MOA — both independently identified a flux ramp-up around the same star. A source was just beginning its journey behind a lens. They monitored the event, and I’ve shown their data in Figure 3. This matches the situation in Figure 2: there’s a wide peak as the source passes behind the star, and a teensy dip at day 6149, where the planet briefly dims the source — a zoom-in of this dip is shown in the bottom panel.

Figure 3: The microlensing event where planet Oggy was identified. Top: the whole event as the source star passes behind the lens. Bottom left: a close up of the effect of the planet on the observed flux. Bottom right: a close up of the final dimming of the event as the source passes out from behind the star. [Koshimoto et al. 2016]

All of these data can be fitted to gain an understanding of the lens and planet system, and work out what’s actually been seen by the team. However, there are a lot of free parameters in the system, and only a few free parameters for your curve, so the fit is degenerate. In other words, the curve isn’t enough to constrain all the parameters, and there are multiple different good fits. There are various ways to solve this problem: for example, the length of time that the source is magnified might tell you about the source diameter (since it will be magnified for slightly longer if it is slightly bigger). However, in the case of Oggy, the source size is consistent with zero — so the errors are too big for the magnification duration to give us any information.

However, the total passage of the star is a fairly long event, with the star being magnified for a total of ~120 days. This means that the orbital motion of the Earth during the microlensing event is actually detectable! The models that include the so-called ‘microlens parallax’ are shown in Figure 3, and they fit better than those that ignore the Earth’s motion. For completeness, the authors also use a third model, one they call the xallarap model, where the orbital motion of the lens is considered (kind of like a backwards parallax). However, this is found to be a less good fit than the parallax model.

By determining the microlens parallax, and with some additional Keck telescope observations that confirm the flux of the lens star, the authors calculate a mass for the planet and for the star. It’s 35 times the mass of the Earth, orbiting an M-dwarf host star, with a projected distance of three times the Earth–Sun distance. Oggy is the first planet for which a mass has been determined with just the parallax and the lens flux.

So, why should you care?

Microlensing is exciting as it’s the only method that can find these extremely far away planets. This allows us to figure out whether our part of the galaxy is representative: does the galactic bulge have as many planets as we do out here in a spiral arm? Are the planets in the galactic bulge generally bigger or smaller than ours? For now, microlensing is the only technique that can answer this question.

Since several microlensing surveys are now being carried out with simultaneous ground and space observations (using the Spitzer and Kepler telescopes), there will be more and more planets identified where the microlens parallax can be very precisely determined — so the method used here will be valuable in calculating masses for these future discoveries. Oggy is a great benchmark, proving that these missions will give valuable scientific output.

About the author, Elisabeth Matthews:

I’m currently a second year PhD student at the University of Exeter, in the south of England, where I’m aiming to detect and characterise extra-solar planets and debris disks via direct imaging. So far this has meant lots of detecting background stars that happen to be near bright, nearby stars and no detecting of actual planets — but hopefully my luck will change soon!

25 October 2016

Evolution in the Stellar Nursery

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: The JCMT Gould Belt Survey: Evidence for Dust Grain Evolution in Perseus Star-forming Clumps
Authors: Michael Chun-Yuan Chen, J. Di Francesco, D. Johnstone, et al.
First Author’s Institution: University of Victoria
Status: Published in ApJ

Deep in the cold and dusty corners of the universe, baby stars are formed. When a molecular cloud (aka an interstellar dust bunny) made out of gas and small dust particles reaches a critical density, it begins to form protostars as the gas in the cloud gravitationally collapses. Scientists can study these stellar nurseries to understand how gas is transformed into giant stars. Unfortunately, molecular hydrogen, which composes ~99% of a molecular cloud, is hard to measure at the low temperatures at which clouds form. Instead, the dust in the clouds is studied, as it behaves in a manner similar to the gas.

Recently, Mike Chen and his collaboration investigated several stellar nurseries in one molecular cloud, Perseus, in order to map the temperature and other parameters that they use to understand the evolution of dust grains in the regions. They look primarily at the regions’ temperatures and a parameter called the dust emissivity spectral index, or beta. This index is critical to estimating the mass and temperature of the star-forming structure and is dependent on the properties of the dust grains, like size and composition.

Using the JCMT sub-millimetre telescope, the group derived maps for temperature, dust emissivity spectral index and optical depth (i.e. how well you can see through it). Plotting known stars and young stellar objects on the map, they were able to see relations between these objects and the dust grain properties. The results from the temperature maps showed distinct warm regions within the clouds — though by warm we only mean about 15 K (-430°F). Most of the areas that had above average temperatures also had a young stellar object, which is likely responsible for the region’s extra warmth. However, not all regions containing young stellar objects were warm. It could be that these regions contain particularly infant objects that haven’t yet had the time to warm their dust blankets.

Temperature maps for two dense regions in the Perseus molecular cloud. Red indicates higher temperatures and blue, colder. Circles, triangles and stars indicate young stellar objects and stars. [Chen et al. 2016]

Examination of the dust emissivity spectral index maps showed significant differences between star-forming clumps in the Perseus molecular cloud. Furthermore, the group noticed smooth structure on small scales which indicates that the dust emissivity spectral index is affected by its local environment. Different clumps in one cloud showed different distributions of the index. This shows that the dust grains are probably growing significantly as the clump itself evolves.

Dust emissivity maps for two dense regions in the Perseus molecular cloud. Yellow indicates regions with higher emissivity index and blue, lower. Circles, triangles and stars indicate young stellar objects and stars. [Chen et al. 2016]

When comparing the dust emissivity spectral index and temperature maps, Chen and his collaborators noticed the colder temperature regions tended to have higher beta indices. This could indicate an evolution of the dust grains. As the temperature drops, ice can begin to grow on the dust grains, giving them a larger size and thus a higher beta index; in warmer regions where stars are forming, this ice sublimates and the index is lower. In other words, the beta index decreases when the grain size increases. A grain with a ice mantle, however, can have a higher beta index compared to a bare grain of the same size. However, it’s a complicated picture and another theory suggests that instead large grains are produced in the densest gas near protostars, which are typically warm. While this work begins to probe the origin of dust grains with low dust emissivity indices, further observations are necessary to see the full picture of grain evolution.

About the author, Mara Johnson-Groh:

Mara is working on her master’s at the University of Victoria, Canada. In a nutshell, her research is taking pretty pictures of the universe. She spends half of her time with the Gemini Planet Imager Exoplanet Survey team trying to directly image new exoplanets, and the other half looking for galaxies responsible for damped Lyman alpha systems in the images of quasars. When she’s not looking at scientifically pretty pictures, she’s capturing earthly beauty with digital photography or trying to stay grounded with rock climbing.

11 October 2016

Why Are Pulsar Planets Rare?

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Why Are Pulsar Planets Rare?
Authors: Rebecca G. Martin, Mario Livio, and Divya Palaniswamy
First Author’s Institution: University of Nevada Las Vegas
Status: Accepted to ApJ

Pulsar planets were the first type of planet ever discovered beyond the solar system, and this discovery shocked the astronomical world. These were not the planets we expected: solar system-like planets around a Sun-like star. Instead, these planets orbited a pulsar, a rapidly rotating neutron star (the extremely dense core of a massive star that exploded as a supernova). However, since their initial discovery in 1992, only five such pulsar planets have been found, making them quite rare. Fewer than 1% of pulsars have been found to host planets. In this paper, the authors explore how these planets may have formed as a way to explain the rarity of pulsar planets.

Figure 1: The mass and semi-major axis of pulsar companions in colored circles (blue for confirmed planets, red for main-sequence stars, black for low-mass stars and brown dwarfs, green for neutron stars, purple for heavy white dwarfs, and yellow for low-mass white dwarfs). The violet asterisks are our eight solar-system planets, the Moon, and the asteroid-belt dwarf planet Ceres, for reference. The black lines are the detection limits for fast millisecond pulsars (bottom line) and more normal pulsars (top line). [Martin et al. 2016]

Formation scenarios

Planets that survive the supernova: The most obvious formation scenario is that the planets formed simultaneously with the original star just like our own solar system. However, many astronomers believe that stars above three solar masses (3x the mass of the Sun) can’t form planets, and stars that supernova into neutron stars are at least eight solar masses. Even if they could form, the planet would have to avoid being eaten when the star swells up into a red supergiant and then stick around after the supernova explosion removes most of the mass from the system, an unlikely scenario.
Supernova fallback disk: After the supernova, some of the material falls back into a disk where, just like with a protoplanetary disk, it might form planets. However, this “fallback disk” is expected to have little angular momentum, which means the material likely doesn’t have enough rotational speed to avoid falling back directly onto the neutron star. (For example, a rocket shot vertically up would fall back down to Earth. It needs “sideways” velocity to get into orbit and avoid hitting the Earth.)
Destruction of a companion star: A low-mass companion star orbiting a neutron star loses mass through evaporation — which, if strong enough, can entirely destroy the star. The star’s debris can then form a disk orbiting the neutron star with a mass about 10% the mass of Earth.
Evaporation of a companion: An alternative outcome of evaporation from the intense pulsar radiation is that the companion star just loses so much mass that it is reduced down to planetary size.

Artist’s illustration of a binary system in which the left star is exploding as a supernova. [ESA/Justyn R. Maund (University of Cambridge)]

The authors determine that pulsar planets are likely formed only when there is a low-mass companion star to the neutron star. Almost every star with enough mass to become a supernova is born with a companion star, but only 10% of these companions have low enough mass to make pulsar planets a realistic possibility. Of these, only about 10% are able to survive on a gravitationally bound orbit after the massive star goes supernova. This means that only ~1% of neutron-star progenitors (stars that eventually become neutron stars) even have the potential to form pulsar planets.

In the case of a star being disrupted and forming a disk, the disk receives intense radiation from the pulsar that heats and helps evaporate the disk. If the surface density of a disk is very large, dead zones are formed in the disk where material is able to build up to form planets. Disks with a lower surface density are not able to effectively shield the disk to prevent its evaporation and therefore cannot form any planets. Only when the companion star is disrupted into a massive enough disk is there a real possibility of a planet then forming.

Results

Only under a very specific set of circumstances can planets form around pulsars. They require a companion star with a low mass, which only about 10% of neutron star progenitors have. Of these, only 10% can then survive the supernova explosion. Of the survivors, some may be evaporated into a planetary mass, while others may be disrupted by the pulsar. In the case of disruption, the subsequent disk then needs to be dense enough to withstand the intense pulsar radiation long enough to create stars.

Out of the five pulsar planets known, the authors believe that the three planets in the PSR 1257+12 system were formed from the disk of a disrupted star, the planet orbiting PSR J1719-1438 is the core of an evaporated white dwarf, and the planet around PSR B1620-26 was captured along with its white dwarf, with the planet now orbiting both of them as a circumbinary planet.

About the author, Joseph Schmitt:

I’m a 5th year graduate student at Yale University. My main research is on the discovery, characterization, and statistics of exoplanets. I’m also one of the science leads on the citizen science project Planet Hunters, a website where the general public can join the search for exoplanets.

4 October 2016

Gravity-Darkened Seasons on Planets

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Gravity-Darkened Seasons: Insolation Around Rapid Rotators
Authors: John P. Ahlers
First Author’s Institution: University of Idaho
Status: Accepted to ApJ

On Earth, our seasons come about due to Earth’s tilted rotational axis relative to its orbital plane (and not due to changes in distance from the Sun, as it is commonly mistaken!) Essentially, this is due to the varying amounts of radiation that Earth receives from the Sun in each hemisphere. But what would happen if the Sun were to radiate at different temperatures across its surface?

It’s hard to imagine such a scenario, but a phenomenon known as gravity darkening causes rapidly spinning stars to have non-uniform surface temperatures due to their non-spherical shape. As a star spins, its equator bulges outwards as a result of centrifugal forces (specifically, into an oblate spheroid). Since a star is made of gas, this has interesting implications for its temperature. If its equator is bulging outwards, the gas at the equator experiences a lower surface gravity (being slightly further away from the star’s center) a lower density and temperature. The equator of a spinning star is thus considered to be “gravitationally darkened”. The gas at the star’s poles on the other hand, has a slightly higher density and temperature (“gravitational brightening”) since it is closer to the center of the star relative to the gas at the equatorial bulge. Thus, there is a temperature gradient between the poles and equator of a rapidly rotating star.

While this is an interesting phenomenon in itself, the author of today’s paper introduces a new twist: what if there’s a planet orbiting such a star, and what implication does this gravity darkening have on a planet’s seasonal temperature variations? Compared to Earth. exoplanets have potentially more complex factors governing its surface temperature variations. For example, if a planet’s orbit is inclined relative to the star’s equator (see Figure 1), it can preferentially receive radiation from different parts of its star during the course of its orbit.

Fig 1: All the parameters describing a planet’s orbit. In this paper, the author mainly focuses on the inclination i, which is the angle of a planet’s orbital plane relative to the star’s equator. [Wikipedia]

The author claims that this effect can cause a planet’s surface temperature to vary as much as 15% (Figure 2). This essentially doubles the number of seasonal temperature variations a planet can experience over the course of an orbit. However, the author does not attempt to model the complex heat transfer that occurs on the planet’s surface due to the atmosphere and winds.

Fig. 2: Some examples of seasonal temperature changes of a planet for various orbital parameters. The top left figure shows the orientation of the planet’s tilt (precession angle, color-coded to match the plots), and the times corresponding to one orbit around the host star. In each subplot, the author shows the flux a planet would receive for different orbital inclinations (i.e. the angle i in Fig. 1). [Ahlers 2016]

Not only that, but there is also some variation in the type of radiation that a planet receives during the course of its orbit. Since the poles of rotating star are at a higher temperature, it will radiate relatively more ultraviolet (UV) radiation compared to the equatorial regions. The author claims that a planet orbiting in a highly inclined orbit will alternate receiving radiation preferentially from a star’s poles or equator, causing the amount of UV radiation to vary as much as 80%. High levels of UV radiation can cause a planet’s atmosphere to evaporate, as well as other complex photochemical reactions (such as those responsible for the hazy atmosphere on Saturn’s moon Titan).

As we discover new exoplanets over the course of the coming years, we will likely find examples of planets potentially experiencing these gravitationally darkened seasons. This will have interesting implications on how we view the habitability of these other worlds.

About the author, Anson Lam:

I am a graduate student at UCLA, where I am working with Steve Furlanetto on models of galaxy clustering and their applications to the reionization era. My main interests involve high redshift cosmology, dark matter, and structure formation.
Previously, I was an undergraduate at Caltech, where I did my BS in astrophysics. When I’m not doing astronomy, I enjoy engaging in some linear combination of swimming/biking/running.

27 September 2016

Shooting for the Stars (and the Damage of Doing So)

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: The Interaction of Relativistic Spacecrafts with the Interstellar Medium
Authors: Thiem Hoang, A. Lazarian, Blakesley Burkhart, and Abraham Loeb
First Author’s Institution: Canadian Institute for Theoretical Astrophysics, University of Toronto
Status: Submitted to ApJ

On the Doorstep of the Solar System…

Artist’s impression of the planet Proxima b, orbiting in the habitable zone of the red dwarf star Proxima Centauri, the closest star to the solar system. [ESO/M. Kornmesser]

On its voyage to Pluto, the New Horizons probe broke the speed record for a spacecraft (and anything humans have ever created, for that matter), traveling at a blistering speed 40 times faster than a bullet. However, even at these Earth-shattering speeds, it would take New Horizons about 75,000 years to reach the distance of Proxima Centauri, the nearest star to our Sun. And now that we know this star has a potentially habitable Earth-like planet orbiting it, tens of thousands of years is just too long to wait.

Thankfully, physicist Steven Hawking, adventure capitalist Yuri Milner, and Facebook CEO Mark Zuckerberg concocted a better idea. They put their great minds (and loads of money) together to propose the Breakthrough Starshot initiative, a plan to send a fleet of centimeter-sized spacecraft to the nearest star system. These spacecraft would be accelerated to a significant fraction of the speed of light by the force of radiation pressure from high-powered, Earth-based lasers and light sail technology. Breakthrough Starshot claims that the technology will be developed to accelerate these spacecrafts to 20% of the speed of light, which means it would only take a quick 20 years to travel the 4 light-years to Proxima Centauri. However, every great idea has complications. Today we’ll confront one of the biggest ones: all that stuff that is between Earth and Proxima Centauri.

Another One Bites the Space Dust

On average, the interstellar medium (ISM) has only 1 atom inside of every cubic centimeter of space. However, a 1-square-centimeter spacecraft would still run about into two million trillion ISM atoms on its way to Proxima Centauri. Coincidently, this is about the number of atoms contained in a single grain of salt. Though getting hit with a grain of salt’s worth of atoms over the course of the journey doesn’t seem all too detrimental, keep in mind that the spacecraft are charging at these atoms at 20% of the speed of light, so that the spacecraft sees the atoms incoming at 60 million meters per second! These tiny atomic bullets can still alter the material structure of the spacecraft by creating microscopic holes and heating up the spacecraft material by imparting their kinetic energy to atoms of the spacecraft. If the bullet is bigger (say, grains of interstellar dust, which are typically made up of a few molecules and are about 1000 times larger in diameter than atoms), then the kinetic energy of the impact is also bigger. Hoang et al. analyze the effects that the gas and dust on the way the Proxima Centauri would have on the Starshot spacecraft.

Explosive evaporation of spacecraft material, in the frame of reference of the spacecraft. Figure from today's article.

Explosive evaporation of spacecraft material, in the frame of reference of the spacecraft. [Hoang et al. 2016]

By studying the effects of gas and dust bombardment on quartz and graphite, the authors gauged which particles would be the most detrimental to the spacecraft, how they would affect the spacecraft, and what protective measures could possibly be taken to reduce damage. First they analyzed the effect of interstellar gas. Though hydrogen and helium make up most of the material in the ISM, they found that the heavier and rarer atoms (such as oxygen) would have a more notable effect on the trip to Proxima Centauri. These atoms could produce tiny holes in the spacecraft, called damage tracks, that would penetrate up to a tenth of a millimeter deep.

Dust, though less plentiful than gas in the ISM, was found to be more harmful to the spacecraft over the course of the trip. Collision by a normal interstellar dust grain could provide enough energy to evaporate material at the impact site — known as explosive evaporation. Furthermore, the atoms at the impact site become ionized, and the energetic electrons can then transfer their kinetic energy to nearby atoms on the spacecraft, raising the heat of this material. Over the course of the journey to Proxima, the impact of such dust could result in a half-millimeter layer of the spacecraft to be completely eroded, which is larger than it sounds, since these are only centimeter-scale spacecraft. Subsequent melting from these dust collisions could result in damage another couple millimeters deeper into the material. The figure below shows some of the main results of the study, plotting the damage by dust and gas on material moving through the ISM at different fractions of the speed of light.

The thickness of surface damaged by dust and gas bombardment for quartz (left) and graphite (right). The x-axis plots the column density (the amount of material along a given line of sight between an observer and an object), with the grey band indicating the column density expected towards Proxima Centauri. Though evaporation by dust is material-independent, graphite is a better conductor, which lessens melting by dust and track formation by gas. [Hoang et al. 2016]

What if the spacecraft was unfortunate enough to encounter an abnormally large dust grain? The authors found that grains larger than the width of a silk fiber would completely destroy the gram-scale spacecraft (for reference, the average interstellar dust molecule is about 1000 times smaller than this). However, since such large grains are quite rare, they found this concern to be negligible. Based on the quantity of these abnormally large grains in the ISM, the chance that one of these spacecraft would encounter such a grain on its journey to Proxima Centauri is 10^-50, which is so incredibly unlikely that I can’t even think of a real-world analogy.

Interstellar Dust Buster

So what can we do about this dusty problem? Hoang et al. propose multiple means of protection, such as deflecting the incoming dust grains with an electric field or scattering them off the path of the spacecraft with the radiation pressure of little lasers. However, the best approaches seem to be the simplest. Adding a thin layer of highly-conducting material, such as graphite or beryllium, to the front of the spacecraft would prevent the track formation from gas bombardment. Though it would add weight to the spacecraft, if this layer is a few millimeters thick it would also protect the sensitive components of the spacecraft from explosive cratering and melting by dust. The authors stress that geometrical considerations should also be taken into account. If the spacecraft are needle-like, then they have a smaller cross-sectional area for gas and dust to impact.

Though something important to consider, the hindrance of gas and dust on the way to Proxima Centauri is not a deal-breaker for the Starshot initiative. The biggest test will be testing our patience, because though 25 years is a blink of the eye for our universe, I can’t say it is the same for me.

About the author, Michael Zevin:

I am a graduate student studying physics and astronomy at Northwestern University. I am part of the LIGO Scientific Collaboration and work with Dr. Vicky Kalogera studying gravitational wave astrophysics, particularly parameter estimation of gravitational wave sources for the Advanced LIGO era. I received my B.S. from the University of Illinois in astronomy, physics, and music. Outside of school I enjoy teaching science at Chicago’s Adler Planetarium and Kids Science Labs, playing music around the Windy City, and looking up.

20 September 2016

An Ocean of Magma

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the new partnership between the AAS and astrobites, we will be reposting astrobites content here at AAS Nova once a week. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org!

Title: Predictions of the Atmospheric Composition of GJ 1132b
Authors: L. Schaefer, R. D. Wordsworth, Z. Berta-Thompson, and D. Sasselov
First Author’s Institution: Harvard-Smithsonian Center for Astrophysics
Status: Accepted for publication in ApJ

Pictured above is Mustafar, a world covered in red-hot magma (or lava, if you prefer). Mustafar is from Star Wars (and, I’m afraid, from the prequels), but there are several non-fictional planets that might be real-universe “lava worlds”, due to their intense heat. The Earth, and the other rocky planets of our solar system, are believed to have formed with temperatures high enough for their surfaces to be molten. In today’s article, we take a look at how this “ocean of magma” might affect the way that a planet’s atmosphere evolves.

The article’s authors studied the evolution of planets with water-dominated atmospheres — in particular, the rate of loss of the water’s constituent hydrogen and oxygen parts (the two are not necessarily lost at the same rate). This loss happens by several mechanisms. Of most interest to us today is a so-called “thermal escape mechanism”, in which a particle with enough energy can travel fast enough to break free of the planet’s gravity and escape into space. In this way, hydrogen will be lost faster than oxygen, because its lower mass means it needs less energy to escape. The energy in this case comes from radiation from the planet’s host star, particularly in the ultraviolet (UV) part of the electromagnetic spectrum.

Indeed, much of the previous work in this area has found that hydrogen disappears from the planet’s atmosphere while some amount of oxygen is left behind, forming O₂. This could be bad news for hopes that O₂ might be used as a sign of life on other worlds. However, it also might not be the whole picture. The same models predict that Venus, in our own solar system, should have this residual atmosphere of O₂, but we see no sign of it.

Where does the magma come in?

One possible solution to this puzzle might involve interactions between the planet’s atmosphere and its surface, particularly when the surface is molten, as water can dissolve in molten rock. Today’s authors attempted to include these interactions with the planet’s surface in their models of a planet’s atmosphere. The surface in their models begins as an ocean of magma, then slowly cools to solid rock. The team then attempted to model a water atmosphere, allowing H and O to both escape into space and dissolve into the surface.

Figure 1: The solidification time of the magma ocean depended on the amount of water in the atmosphere — the more water there was, the slower the ocean solidified, because of the greenhouse effect. The ocean also lasted longer in models with less radiation from the star (magenta) than in models with more radiation (blue). For models in which more than 10% of the planet’s mass was water, (the top of the lines here), the magma lasted for the entire length of the calculations, a total of 5 billion years! [Schaefer et al. 2016]

The team found that the length of time it takes the ocean to solidify depends strongly on the amount of water they give the atmosphere at the start of their models — water is a greenhouse gas, and so the more water there is in the planet’s atmosphere, the longer it takes the planet to cool. They found that the atmospheric composition at the end of their models depended on the atmospheric composition at the start, on the amount of UV light hitting the planet, and on the composition of the magma ocean.

Figure 2, above: the temperature of the planet’s mantle and surface evolving over time, for models with more stellar radiation (blue) and less (magenta). Below: The fraction of the total amount of water contained in the magma ocean, the atmosphere and the solid mantle over time. The magma ocean’s fraction drops off as the ocean solidifies, and almost all of the atmospheric fraction is lost to space; however, the fraction trapped in the solid mantle increases as the ocean solidifies, and remains after the rest is lost. [Schaefer et al. 2016]

Artist’s illustration of the rocky exoplanet GJ 1132b, which orbits a red dwarf star and may host an atmosphere. [Dana Berry]

The team applied their models to GJ 1132b, a recently discovered exoplanet about the size of the Earth that orbits 65x closer to its star than we do to ours. GJ 1132b is a planet whose atmosphere should not be too difficult to study, so today’s authors hope to use future measurements of its atmospheric composition to test the predictions of their models. Their predictions for GJ 1132b depend on their starting conditions: if they set models going with a large initial amount of water — more than 5% of the planet’s mass — they can produce atmospheres with a large amount of oxygen, steam or both (with oxygen atmospheres more likely if the planet receives a large amount of UV radiation). Due to the greenhouse effect, a steam atmosphere implies that the planet’s surface is probably still molten. However, most of their starting conditions — any model with less than 5% water — result in an atmosphere with only a thin shell of O₂ and no steam.

Observations of GJ 1132b should be able to measure its atmospheric composition and test the team’s predictions. Until then, their model is applicable not only to plenty of other exoplanets, but also to Venus, where they expect the magma ocean to have cooled much faster due to Venus’ greater distance from the Sun.

About the author, Matthew Green:

I am a first year PhD student at the University of Warwick. I work with white dwarf binary systems, and in particular with AM CVn-type binaries. In my spare time I enjoy writing of all kinds, as well as playing music, board games and rock climbing. For more things written by me, take a look at my website.

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