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NGC-1559

A galaxy’s stars hold clues to its history. So, if we look at the stellar populations of several galaxies along with the properties of the galaxies themselves, we may be able to tease out trends in galaxy evolution.

The number of new stars formed versus the number of existing stars for different types of galaxies. Star-forming galaxies hang above the “main sequence”, with “starburst” galaxies lying at the extreme. [V. Buat]

Galaxies and the Stars that Make Them

To begin with, what stellar properties can you use to study galaxy evolution? Age is a key one. A stellar population with more old stars suggests that the galaxy formed most of its stars a long time ago, while a stellar population dominated by young stars points to more recent star formation.

The abundances of different elements are also insightful. Astronomers often consider the abundance of elements that aren’t hydrogen and helium relative to hydrogen (metallicity) and the abundance of alpha elements, like carbon and oxygen, relative to iron. The former property can tell us how many generations of stars the galaxy has hosted, and the latter can tell us how long the galaxy was forming stars.

As for galaxies themselves, the overall mass and size of a galaxy can be linked to the stars it holds. Another useful quantity is velocity dispersion, or how stars move within the galaxy. Recently, more subtle properties like the surface distribution of mass across a galaxy and its gravitational potential have been used as probes of galaxy evolution. In a new study, a group of researchers led by Tania Barone (The Australian National University/The University of Sydney) explored how these structural properties could be related to stellar properties.

Starting from the upper-left plot and moving clockwise: log of age versus gravitational potential, log of age versus surface mass density, metallicity versus surface mass density, and metallicity versus gravitational potential; best-fit relations are shown by the solid line. Galaxies plotted in yellow have a smaller radius than galaxies plotted in blue. The inset plots show the fit residuals, the difference between the measured and best-fit age/metallicity. If the slope of the residual distribution (given by the numbers in the inset) is small, it’s likely that the relation actually exists. Click to enlarge. [Adapted from Barone et al. 2020]

Tying Things Together

Barone and collaborators focused on star-forming galaxies in this study. While all galaxies could be said to be star-forming, a galaxy must be forming stars at a higher than average rate to be labeled a star-forming galaxy. This distinction is important since combining results from different types of galaxies could obscure any existing trends.

Age and elemental abundances can be calculated under different assumptions, one based on the brightness of the galaxy and the other based on the galaxy’s mass. However, Barone and collaborators found that under both of these assumptions, age appears to correlate with surface mass density and metallicity correlates with gravitational potential. Interestingly, both these relations were also seen in a similar study of early-type galaxies that formed when the universe was younger.

So what do these correlations say about galaxy evolution? The age–mass-density relation suggests that star formation is dictated by the availability and density of star-forming gas. The metallicity–gravitational-potential relation suggests that galaxies with lower potentials have a harder time keeping material from being whisked away by astronomical winds.

One way to explore these relations further — aside from observations — is using cosmological simulations. For now, this study is a great example of how we can assemble the clues we have to solve the puzzle of galaxy evolution!

Citation

“Gravitational Potential and Surface Density Drive Stellar Populations. II. Star-forming Galaxies,” Tania M. Barone et al 2020 ApJ 898 62. doi:10.3847/1538-4357/ab9951

coronal hole

coronal hole schematic

Coronal holes form where magnetic field lines open into space (B) instead of looping back to the solar surface (A). [Sebman81]

Want to be able to predict the flurries of high-energy particles that slam into Earth from the Sun, triggering dramatic geomagnetic storms? Then you’ll first need to track the holes in the Sun’s outer atmosphere. In a new study, scientists explore whether we can train computers to identify these patterns.

Tracking the Dark

The Sun’s outer atmosphere — its corona — is far from uniform. Both across the Sun’s face and over time, the corona varies in its density and corresponding appearance, looking brighter in some regions and forming large, dark coronal holes in others.

Coronal holes represent regions of low-density plasma containing magnetic field lines that open out into space rather than looping back down to the Sun’s surface. These areas of open field lines provide an excellent escape point for the solar wind, the high-energy particles that flow off of the Sun.

For this reason, identifying the locations, sizes, and timescales for formation and evolution of coronal holes is critical for predicting solar wind behavior and forecasting space weather. In addition, tracking coronal holes over solar cycles can shed light on how magnetic fields drive solar activity and may even help us to better predict upcoming solar activity.

A Full-Surface View

synoptic map process

The process of making a synoptic map from daily solar disk images. Click to enlarge. [NSO]

So how do we identify and track coronal holes? It’s surprisingly difficult to map out holes in the Sun — especially large ones like these! Coronal holes often don’t fit within a single image of the Sun’s disk, as viewed from Earth; instead, we have to wait for the Sun to rotate through its month-long period and take successive images as it turns.

To get a comprehensive view of the entire solar surface, we transform these individual disk images into a shared coordinate system, and then we stack and combine the daily images within each rotation period, creating what’s known as a synoptic map for each rotation.

If we can then identify the coronal holes in a uniform way within each of these synoptic maps, we’ll have a useful data set for studying how coronal holes vary across the Sun’s surface and over time! In a new publication, a team of scientists led by Egor Illarionov (Moscow State University, Russia) has found a way to do this that uses computers to do the heavy lifting.

synoptic maps and CH boundaries

Overlaid synoptic maps and algorithm-identified coronal hole boundaries (green outlines) for three different solar rotation periods: two during solar minima (a and c) and one during a solar maximum (b). [Illarionov et al. 2020]

Training the Machine

Illarionov and collaborators use a training set of daily solar disk images to teach a convolutional neural network — a type of machine learning algorithm that’s especially good at analyzing images — how to identify coronal holes. The strength of a convolutional neural network is that it works equally well analyzing any image shape; the authors can therefore use this same algorithm, once trained, to accurately identify coronal holes in the synoptic solar maps.

Using this approach, Illarionov and collaborators construct a coronal hole catalog spanning 2010–2020. They conduct preliminary analysis of the evolution of coronal holes between solar minimum and solar maximum, and they compare these maps to maps of magnetic flux across the same times.

Looking to dive in, yourself? The authors make their code and the resulting coronal hole maps publicly available. With this dataset on hand, we can hope to soon learn more from these holes in the Sun.

Citation

“Machine-learning Approach to Identification of Coronal Holes in Solar Disk Images and Synoptic Maps,” Egor Illarionov et al 2020 ApJ 903 115. doi:10.3847/1538-4357/abb94d

gas-giant transit

What’s going on around the hot Jupiter exoplanet HAT-P-41b? This planet’s atmosphere is harboring a mystery, recently revealed by observations that span infrared through ultraviolet (UV) light.

Expanding the Spectrum

transmission spectroscopy

As a star’s light filters through a planet’s atmosphere on its way to Earth, the atmosphere absorbs certain wavelengths depending on its composition. [European Southern Observatory]

At the latest count, we’ve discovered nearly 10,000 confirmed and candidate exoplanets. One way to study these distant worlds is using transmission spectroscopy: we can explore a host star’s spectrum as its planet passes in front of it. The light that filters through the planet’s atmosphere is imprinted with spectral signatures that we can then analyze to learn about the physics and chemistry at work in the planet’s atmosphere.

Ideally, we’d gather spectroscopic observations of planet transits across a broad range of wavelengths, because each region of the electromagnetic spectrum provides an additional constraint for atmospheric models. But so far we’ve probed fewer than 20 exoplanets in the UV, a regime that can reveal critical details of atmospheric physics.

In a new study, a team led by Nikole Lewis (Cornell University) adds one more planet to this collection, the hot Jupiter HAT-P-41b — but what these scientists found was unexpected.

HAT-P-41b's transmission spectrum

HAT-P-41b’s transmission spectrum from 0.2 to 5.0 μm (red/blue/gray data points) plotted along with several atmospheric models (green, teal, and purple lines). [Lewis et al. 2020]

Unexpected Absorption

Lewis and collaborators conducted one of the most comprehensive explorations of an exoplanet atmosphere yet, combining high-precision Hubble and Spitzer observations to construct a transmission spectrum spanning UV through infrared wavelengths. They then use multiple different approaches to conduct detailed atmospheric modeling for HAT-P-41b from their observations.

The result? The authors show that HAT-P-41b can’t be neatly described by the models we usually fit to hot Jupiters. To fit the full range of observations, there must be a molecular species that readily absorbs near-UV light present in unexpectedly large quantities in this hot Jupiter’s atmosphere. Lewis and collaborators argue that the most likely candidate is H- (the hydrogen anion, or a hydrogen atom with an extra, easily-ejected electron), although other candidates include CrH, AlO, and VO.

A New Chemistry

What does this mean? Based on the temperature measured for HAT-P-41b (a toasty ~1,700 K), the chemistry we typically assume for hot Jupiter atmospheres falls several orders of magnitude short of producing enough H- to explain observations. Instead, the authors posit that there must be some new chemistry, not yet taken into account, that produces the excess H- abundance — possibly driven by the intense ultraviolet radiation from this planet’s F-type host star.

JWST

An artist’s illustration of the upcoming James Webb Space Telescope. [NASA/JWST]

This striking discovery has broad implications. If correct, it likely means that other hot Jupiters also contain large amounts of H- (or an equivalent near-UV absorber) in their atmospheres. In turn, this would imply that our understanding of hot Jupiter atmospheres needs to be substantially reworked.

If nothing else, the Lewis and collaborators’ results demonstrate the importance of broad-spectrum observations of planetary atmospheres, as well as the benefit of using multiple reduction and interpretation techniques to analyze the observations. This careful approach will become even more valuable with the upcoming launch of the James Webb Space Telescope and the new era of exoplanet atmosphere exploration it will usher in!

Citation

“Into the UV: The Atmosphere of the Hot Jupiter HAT-P-41b Revealed,” N. K. Lewis et al 2020 ApJL 902 L19. doi:10.3847/2041-8213/abb77f

neutron star merger

When a pair of neutron stars collide, they emit a fireworks show. Could some of the low-energy light produced in these mergers be detectable years later? A team of scientists thinks so — and they’re pretty sure they’ve found an example.

A Rainbow of Signals

In addition to gravitational waves, a slew of electromagnetic radiation is produced in the merger of two neutron stars, spanning the spectrum from gamma rays to radio waves.

In 2017, the now-famous neutron star collision GW170817 gave us a first look at this expected emission: it revealed a short gamma-ray burst, infrared and optical light from ejecta in a kilonova, and relatively short-lived X-ray and radio afterglows caused by high-speed outflows.

But there’s one expected type of emission that was missing from GW170817, and it’s never before been spotted in any neutron star collision: radio flaring.

NS merger schematic

Illustration of radio emission from a neutron star merger. During the merger, some neutron star matter is flung outward. This ejecta interacts with the interstellar gas, producing a years-long radio flare. [Lee et al. 2020]

Radio Secrets Revealed

Models of neutron star mergers predict that when ejecta are flung out from the stellar collision, they’ll expand into space, eventually running into the surrounding medium of interstellar gas and dust. The subsequent interaction of the ejecta with the interstellar medium should produce radio flaring.

The emission from these radio flares is expected to be quite long-lived — lasting for years or even decades — which means we could hope to find these signals long after the time of the explosion that produced them. But radio flares are also likely to be relatively faint, so we could only expect to spot flares from nearby collisions (within ~650 million light-years). Additionally, only mergers that occur in environments with dense surrounding gas and dust will light up brightly enough for us to spot.

With these constraints, it’s perhaps not surprising that we haven’t found any radio flares marking past mergers yet. But a team of scientists led by Kyung-hwan Lee (University of Florida) recently waded through decades of radio data from the Very Large Array — and in a recent publication, they’re announcing that one transient may be the first identified radio flare from a stellar collision.

A Decades-Old Collision

radio light curves

Observational data for FIRST J1419+3940 and best-fit radio light curves at three different frequencies. The data are better fit by the neutron-star-merger model (solid lines) than the long-gamma-ray-burst model (dashed lines). [Lee et al. 2020]

FIRST J141918.9+394036 is a radio transient located in a dwarf galaxy 280 million light-years away. Lee and collaborators compile survey data on this source spanning 23 years and evaluate possible explanations for the radio emission.

While this source could potentially be explained as a long-gamma-ray-burst afterglow — light from an off-axis jet produced by a collapsing star — the radio data aren’t fit best by this picture. Instead, the authors show via models that this transient’s light curve is best described as the decay of a radio flare, just as predicted from a neutron star merger. This means that FIRST J141918.9+394036 likely marks a decades-old collision of two stars.

Within a few years, further observations of FIRST J141918.9+394036 will allow us to better distinguish between models and confirm its nature. And as we find more signals like this one, we can use these observations to further understand the origin and physics of neutron star mergers — potentially illuminating everything from the formation channel of binaries to the equation of state for neutron stars.

Citation

“FIRST J1419+3940 as the First Observed Radio Flare from a Neutron Star Merger,” K. H. Lee et al 2020 ApJL 902 L23. doi:10.3847/2041-8213/abbb8a

Einstein Cross

Seeing quadruple? In a rare phenomenon, some distant objects can appear as four copies arranged in an “Einstein cross”. A new study has found two more of these unusual sights — with an unexpected twist.

Searching for Rare Crosses

Gravitational lensing — the bending of light by the gravity of massive astronomical objects — can do some pretty strange things. One of lensing’s more striking creations is the Einstein cross, a configuration of four images of a distant, compact source created by the gravitational pull of a foreground object (which is usually visible in the center of the four images).

strong lensing

Diagram illustrating how light from a distant source can be bent by a foreground object to produce four identical images of the source. Click to enlarge. [NASA/ESA/D. Player (STScI)]

The canonical example of this phenomenon is the Einstein Cross, a gravitationally lensed object called QSO 2237+0305, seen in the cover image above. In this case, as with the majority of known Einstein crosses, the background source is a distant quasar — the small and incredibly bright nucleus of an active galaxy. But other sources can be lensed into Einstein crosses as well, under the right circumstances.

In a new study led by Nicola Napolitano (Sun Yat-sen University Zhuhai Campus, China), a team of scientists presents confirmation of two new Einstein crosses discovered within the 1,000 square degrees imaged in the Kilo-Degree Survey using the Very Large Telescope (VLT) in Chile. Einstein crosses are unusual enough to begin with, but these two discoveries are especially rare: the lensed sources are not quasars. Instead, they’re entire galaxies.

Galaxies, but Bite-Size

When a distant galaxy is lensed by a foreground object, it’s commonly smeared out into an an arc or a ring; this is because galaxies are large, extended objects. But if a galaxy is compact enough, the entire galaxy can be lensed into quadruple images instead of a smeared-out ring. Such is the case with Napolitano and collaborators’ new discoveries, KIDS J232940-340922 and KIDS J122456+005048: they’re both quadruply lensed compact galaxies known as post-blue nuggets.

KiDS Einstein Crosses

Detection and confirmation of the Einstein crosses, KIDS J232940-34092 (top two rows) and KIDS J122456+005048 (bottom two rows), via the KiDS survey and the MUSE integral field spectrograph on the VLT. Click to enlarge. [Napolitano et al. 2020]

What’s a blue nugget? This adorable categorization applies to a type of galaxy found only in the early universe. Blue nuggets are extremely small, quite massive, and undergoing a violent burst of star formation that produces lots of large, bright, blue stars.

Blue nuggets are thought to be suddenly quenched — their star formation is cut off — early in their evolution. As their star population then evolves, these post-blue nuggets then transition into red nuggets, compact collections of red stars that are theorized to become the cores of today’s large elliptical galaxies.

A Bright Future

By spectroscopically following up their Einstein cross discoveries, Napolitano and collaborators show that the two sources are both very compact, massive galaxies with low specific star formation rates. Their properties are consistent with post-blue nuggets currently undergoing quenching — which means that these Einstein crosses are excellent sources to study to learn about galaxy evolution.

The authors predict that, with future observatories like the Vera Rubin Observatory, Euclid, or the China Space Station Telescope, we may be able to find many thousands of Einstein crosses like these. There’s a lot to learn ahead!

Citation

“Discovery of Two Einstein Crosses from Massive Post-blue Nugget Galaxies at z > 1 in KiDS,” N. R. Napolitano et al 2020 ApJL 904 L31. doi:10.3847/2041-8213/abc95b

ESO VLTI Image of Antares

As stars evolve from the red giant branch to the red clump, they accumulate lithium on their surfaces. How does this accumulation happen?

The Red Giant Branch… 

Stars with less than eight solar masses, or low mass stars, live fairly placid lives. A low mass star would start off burning hydrogen in its core like all main sequence stars. Once the core hydrogen has been exhausted, the star resorts to burning hydrogen in a shell surrounding its passive core, which is now mostly helium. This stage of life is called the red giant branch (RGB) stage.

While the helium core may not be burning any material, that doesn’t mean it’s not doing anything! The sheer mass of the core means that it collapses in on itself to the point that the only thing holding it up against gravity is something called electron degeneracy — you can’t fit more than one electron in a space meant for only one electron.

Qualitiative HR Diagram Showing the Horizontal Branch

A qualitative stellar evolution track going all the way to the horizontal branch. A low-mass star would begin its life somewhere on the main sequence line before moving up the red giant branch, undergoing helium flashes, and moving on to the horizontal branch. Click to enlarge. [Richard Pogge]

…and the Red Clump

This standoff doesn’t last forever though, as conditions become ideal for helium to ignite and start core burning again. This helium ignition is called a helium flash. The star is now at the horizontal branch stage, where it continues to burn hydrogen in a shell around the helium-burning core.

Multiple helium flashes can occur as the star transitions to the horizontal branch, but the first is the strongest. Cooler horizontal branch stars appear red and tend to cluster in a particular region in brightness–temperature space, aptly called the red clump (RC).

Observations of RGB and RC stars have found that RC stars have more lithium on their surface than RGB stars do. This suggests some enriching process — a process that results in more heavy elements being present in a region — occurs between the RGB and the RC stages. To investigate what could be behind this enriching, Josiah Schwab (University of California, Santa Cruz) used stellar evolution models combined with our knowledge of how material moves in stars.

Mixing Things Up

Evolving lithium abundance in different stellar models

Luminosity versus lithium abundance for evolving stellar models with two different starting masses: 0.9 solar mass (blue) and 1.2 solar masses (orange). The plot shows both standard models with no mixing (solid lines) and models that assume mixing from the helium flash (dashed lines). The stars indicate where the red clump stars occur in the mixing model. The gray ellipse surrounding the stars shows the expected location of red clump stars based on observations. [Schwab 2020]

One way to create lithium in a star is to start with helium. Deep within the star, helium can fuse into beryllium. When the beryllium is transported to cooler regions closer to the star’s surface, it can experience electron capture — when the nucleus of an atom absorbs one of the electrons orbiting it — and become lithium.

Schwab suggested the first helium flash that happens between the RGB and RC stages can trigger internal waves that mix material in the star. In some stars, this mixing would deplete lithium, but with simulations Schwab showed that the opposite happens as stars transition from the RGB to the RC, enhancing the amount of lithium present at the star’s surface.

An observational check for this flash-induced mixing would be to determine lithium abundances for stars that are just beginning to evolve from the RGB to the RC, since the first helium flash occurs right at the start of this transition. More detailed stellar models will also be useful, but for now it seems the core of this mystery is solved!

Citation:

“A Helium-flash-induced Mixing Event Can Explain the Lithium Abundances of Red Clump Stars,” Josiah Schwab 2020 ApJL 901 L18. doi:10.3847/2041-8213/abb45f

stellar binary mass transfer

The nearby star Regulus — the heart of the constellation Leo — has long been known to be in a binary. But though the bright, main-sequence star is easy to spot, we’ve yet to detect Regulus’s companion! A recent study now presents what may be a first look at this mysterious object.

A Future Entwined

If you’re a star in a close binary system, your fate is not your own. Instead, your future is heavily dependent on how you interact with your companion — especially as you both age.

Leo

Regulus is the brightest star in the constellation Leo; it’s visible as the star in the lower right corner of the constellation. The bright object below Leo in this photo is Jupiter. [Till Credner]

Such is likely the case for Regulus, a star just ~80 light-years away. Once thought to be a single star, this bright, blue-white, rapidly rotating main-sequence star has since been shown to wobble — it dances in a 40-day orbit with a too-dim-to-spot partner.

May I Have This Dance?

The dance of an interacting, evolving binary is complex. As the more massive star in the pair ages into the red giant stage, it grows larger in size. Eventually, its radius becomes comparable to the separation between it and its less massive, main-sequence partner. At this point, mass and angular momentum begins siphoning off of the massive star onto its main-sequence partner.

As mass and angular momentum continue to transfer, the orbit of the binary first shrinks, and then expands again. At the end of the transfer, the main-sequence star is now bright and rapidly rotating, and the originally massive star has become nothing more than a faint, stripped-down stellar core.

Evidence suggests that this stellar dance is exactly what has happened with Regulus and its undetected companion— but an observational detection of its dim partner would cement this picture. Now, in a study led by Douglas Gies (Georgia State University), a team of scientists may finally have come up with this proof.

Missing Partner Found

Regulus spectrum

Mean, normalized spectrum of Regulus (generated from 786 CFHT/EsPaDOnS and TBL/NARVAL spectra). Below it are three model spectra used for cross-correlation analysis. [Gies et al. 2020]

Gies and collaborators used the high resolution and high signal-to-noise ratio of the CFHT/EsPaDOnS and TBL/NARVAL spectrographs in Hawaii and France to search for the spectral features of Regulus’s dim companion. Though this object is too faint to detect in individual spectra, the authors combine the information from a large set of spectra, use clever cross-correlation analysis to tease out the signature of the weak signal.

Their analysis paid off: Gies and collaborators’ work revealed weak spectral features that have all the properties expected for the spectral signature of the stripped-down companion of Regulus.

The authors find that this surviving core is tiny — just 0.31 solar mass, compared to the Regulus’s 3.7 solar masses — and scalding hot, at perhaps 20,000 K (35,500 °F)! This stripped core will eventually cool and become a white dwarf star.

Regulus and its faint partner confirm our understanding of how stars in a close binary influence each others’ fate. What’s more, the properties of this pair suggest that there may be many, many cases of faint white dwarfs and their progenitors orbiting around bright, rapidly rotating stars.

Citation

“Spectroscopic Detection of the Pre-White Dwarf Companion of Regulus,” Douglas R. Gies et al 2020 ApJ 902 25. doi:10.3847/1538-4357/abb372

solar system

What thoughts keep you awake at night? If it’s questions about how our solar system is going to end … wow, you really focus on the big picture! But some scientists have wondered the same thing, and they’ve got an answer for you: part of it will be swallowed, and the rest is probably going to disintegrate.

After the Sun Grows Old

Studying the likely fate of our solar system is “one of the oldest pursuits of astrophysics, tracing back to Newton himself,” according to the opening of a recent publication led by Jon Zink (UC Los Angeles). Though the tradition is long, this field is complicated: solving for the dynamical interactions between many bodies is a notoriously difficult problem.

red giant Sun

As the Sun evolves, it will become a red giant star, growing in size until it has engulfed the inner planets. [Roen Kelly]

What’s more, it’s not just the dynamics of unchanging objects that need to be taken into account. The Sun will evolve dramatically as it ages off the main sequence, ballooning up to a size that engulfs the orbits of Mercury, Venus, and Earth and losing nearly half of its mass over the next 7 billion years.

The outer planets will survive this evolution, but they won’t escape unscathed: since the gravitational pull of the Sun’s mass is what governs the planets’ orbits, our Sun’s weight loss will cause the outer planets to drift even farther out, weakening their tether to our solar system.

What happens next? Zink and collaborators play out the scenario using a series of N-body numerical simulations.

A Solar System No More

The authors’ simulations explore what happens to our outer planets after the Sun consumes the inner planets, loses half its mass, and begins its new life as a white dwarf. Zink and collaborators show how the giant planets will migrate outward in response to the Sun’s mass loss, forming a stable configuration in which Jupiter and Saturn settle into a 5:2 mean motion resonance — Jupiter will orbit five times for every two orbits of Saturn.

solar system ejection

This plot shows when each outer planet is ejected from the solar system in the authors’ 10 simulations (represented by different colors). Click to enlarge. [Zink et al. 2020]

But our solar system doesn’t exist in isolation; there are other stars in the galaxy, and one passes near to us roughly every 20 million years. Zink and collaborators include the effects of these other stars in their simulations. They demonstrate that within about 30 billion years, stellar flybys will have perturbed our outer planets enough that the stable configuration will turn chaotic, rapidly launching the majority of the giant planets out of the solar system.

The last planet standing will stick around for a while longer. But within 100 billion years, even this final remaining planet will also be destabilized by stellar flybys and kicked out of the solar system. After their eviction, the giant planets will independently roam the galaxy, joining the population of free-floating planets without hosts.

Our fate, then, is bleak: the combination of solar mass loss and stellar flybys will lead to the complete dissolution of the solar system, according to these simulations. The good news? This fate is many billions of years in the future — so you needn’t lose sleep over it.

Citation

“The Great Inequality and the Dynamical Disintegration of the Outer Solar System,” Jon K. Zink et al 2020 AJ 160 232. doi:10.3847/1538-3881/abb8de

rogue planet

Scientists have long believed that there may be billions to trillions of rogue planets drifting through our galaxy, unattached to any host star. A recent study has now identified one such candidate — potentially the first terrestrial-mass world we’ve spotted on the run.

Severing Attachments

rogue planet

Artist’s impression of a free-floating, Earth-like planet. [Christine Pulliam (CfA)]

We’ve discovered more than 4,000 exoplanets in the last three decades, spanning a dramatic range of masses, sizes, temperatures, compositions, orbital properties, and more. The vast majority of them, however, share one feature: they all orbit a star.

While this may seem like normal behavior — after all, we’re rather attached to our own star, here on Earth — planetary formation models predict that there should be a large population of free-floating planets in our galaxy. According to the models, these typically sub-Earth-mass planets get kicked out from their parent systems through interactions with other bodies (usually bullying gas giants).

How can we observationally confirm this picture? Without the beacon of a host star’s light, free-floating planets are challenging to detect — but they’re discoverable via a method called gravitational microlensing.

Gravitational microlensing illustration

Gravitational microlensing is a powerful tool for detecting exoplanets. This illustration shows the bending of light from a background source by a planetary system in the foreground. [NASA Exoplanet Exploration]

The Lens Is the Thing

When light from a background source passes by a massive body on its way to us, the intervening object acts as a gravitational lens, bending the light.

In the case of microlensing, the intervening lens object is small — a stellar- or planetary-mass object — so the lensing doesn’t produce a resolvable ring of light like in strong lensing. Instead, we see a brief brightening of the background source as the lens passes in front of it. From the shape of the light curve, we can then infer lens and source properties.

Roughly 100 planets have been discovered in microlensing events so far — but in most of these cases, the lensing mass is actually a combination of a planet and its host star. Only a handful of objects have been found so far that might be free-floating planets, and they’ve all been of relatively large mass.

That is, until now.

Short Blip, Small Planet

OGLE-2016-BLG-1928

The 23-yr long OGLE light curve of the microlensing event OGLE-2016-BLG-1928 (top) reveals a single brightening. A closeup of the magnified part of the light curve (bottom) shows the structure of the event and its best-fit model. [Mróz et al. 2020]

A recent study led by Przemek Mróz (California Institute of Technology) presents a new discovery gleaned from data from two gravitational lensing telescopes: the shortest-timescale microlensing event seen yet, OGLE-2016-BLG-1928.

The event was located in high-cadence survey fields, so though the brightening timescale was just 41.5 minutes, the Optical Gravitational Lensing Experiment (OGLE) and the Korea Microlensing Telescope Network (KMTN) managed to capture a joint total of 15 magnified data points. By modeling the light curve, the authors establish that OGLE-2016-BLG-1928 is either a free-floating planet, or its host is located at least 8 au away from it. 

Assuming that the planet is located in the galactic disk (which the authors deem likely based on their data), it’s estimated to weigh ~0.3 Earth mass, or roughly 3 times the mass of Mars.

So how do our prospects look for finding more of these free-floating low-mass planets and verifying the expectation that they’re plentiful? Certainly, this OGLE detection proves it’s possible — and with the power of upcoming observatories like the Nancy Grace Roman Space Telescope, odds are good that we’ll be able to spot more of these drifting terrestrial worlds.

Citation

“A Terrestrial-mass Rogue Planet Candidate Detected in the Shortest-timescale Microlensing Event,” Przemek Mróz et al 2020 ApJL 903 L11. doi:10.3847/2041-8213/abbfad

solar corona

Sinuous, undulating waves in the Earth’s atmosphere play a large role in driving the weather patterns on our planet. A new study now describes how similar motion can govern the behavior of the Sun — and what we stand to learn from it.

Seeing the Future

Rossby waves jet stream

Visualization of Rossby waves in the Earth’s northern hemisphere jet stream. See the end of the article for a video. [NASA/Goddard Space Flight Center Scientific Visualization Studio]

When you plan a sunny picnic outing for the weekend, you can thank Carl-Gustav Rossby for his role in enabling the weather forecasts you’re now able to check.

In 1939, Rossby first identified large-scale waves in the Earth’s atmosphere. These slow meanders of high-altitude winds are visible as long, persistent undulations in the jet stream that carry cells of warmer or cooler air to different regions of the planet.

Through this transport, Rossby waves are critical in driving the day-to-day weather patterns that we experience at middle and higher latitudes on our planet’s surface. Our understanding of the hydrodynamics of Rossby waves is, consequently, one of the things that enables us to make (approximate) weather predictions on timescales of roughly 14 days.

Waves Far and Wide

But Rossby waves aren’t specific to Earth’s atmosphere; they can arise naturally within any fluids that exhibit differential rotation. Scientists have studied some cases of Rossby waves in detail — like those in the Earth’s oceans, or in Jupiter’s atmosphere. But less is known about the role of Rossby waves on an even larger rotating body: the Sun.

Are the motions of the Sun’s atmosphere governed by these same waves? And if so, can we figure out how to model them similarly to how we model Rossby waves on Earth, thereby unlocking a key to making solar weather predictions on 14-rotation (that’s around a year, given the Sun’s rotation period!) timescales?

solar rossby waves

Observations of Rossby waves on the Sun. [Scott W. McIntosh, NCAR/HAO]

But What About Magnetic Fields?

The answer to the first question is yes: signs of Rossby waves have already been observed on the Sun, in the form of persistent, global velocity patterns that evolve on timescales longer than a solar rotation period, but shorter than a solar cycle.

The answer to the second question, however, is less clear. Why? Because there’s a complicating factor: unlike Earth’s lower atmosphere, the Sun is strongly magnetized. A new study led by Mausumi Dikpati (National Center for Atmospheric Research) now walks us through the basic physics involved in Rossby wave development in the Sun, and discusses how the Sun’s magnetic fields influence those waves.

Sketching a Wavy Picture

MHD Rossby Waves

The propagation of the two classes of magnetic Rossby waves: retrograde waves (left) and prograde waves (right). [Dikpati et al. 2020]

Using a simple model, Dikpati and collaborators show that different waves form in the hydrodrynamic and magnetohydrodynamic cases. When magnetic fields are present, two different classes of waves develop that propagate in opposite directions relative to the mean atmospheric flow. The authors also demonstrate what we should expect for fluid particle trajectories within these waves — which is important for understanding observations.

The basic physics described here is a first step that now needs to be expanded to include more complex interactions. But this starting point demonstrates that Rossby waves likely play an important role in organizing the motions of the Sun’s atmosphere. And once we’ve developed more detailed models of this process, perhaps we’ll be able to check our phones for the solar weather forecast for the year!

Bonus

Check out this wildly awesome NASA-produced simulation showing the development of Rossby waves in the Earth’s northern jet stream.

Citation

“Physics of Magnetohydrodynamic Rossby Waves in the Sun,” Mausumi Dikpati et al 2020 ApJ 896 141. doi:10.3847/1538-4357/ab8b63

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