ALMA Reveals Hidden Planet-Forming Structures in the Smallest Disks

ALMA images of the 11 compact planet-forming disks observed in the Lupus star-forming region. Using the highest angular resolution available with ALMA at a wavelength of about 0.9 millimeters, astronomers revealed previously hidden structures in several of these small disks. The lower-right panel shows the orbits of Earth, Mars, Jupiter, and Saturn, together with the asteroid belt, for scale. The comparison illustrates how these observations probe planet formation at distances comparable to those within our own Solar System. Each white scale bar represents two astronomical units; one astronomical unit is the average distance between Earth and the Sun. Credit: P. Curone et al. – ALMA (ESO/NAOJ/NRAO)

A comparison of ALMA images of HL Tau (left), a famous example of a large, bright planet-forming disk, and the much smaller Sz 112 (center). HL Tau has been rescaled to appear as it would at the distance of Sz 112, allowing a direct comparison of their physical sizes. The enlarged view of Sz 112 (right) reveals a central cavity on a scale comparable to Earth’s orbit. The study identifies Sz 112 as the smallest known transition disk—a planet-forming disk with an inner cavity. The scale bars represent 20 astronomical units in the left panel and one astronomical unit in the enlarged view. Credit: P. Curone et al. – ALMA (ESO/NAOJ/NRAO)

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Using ALMA’s exceptionally sharp vision, astronomers have uncovered rings, gaps, and cavities in extremely compact disks around young stars. The observations show that planet-formation structures extend into regions comparable in size to the inner Solar System, suggesting that many small disks may have looked smooth simply because their details were too small to resolve.

Some of the most striking images from the Atacama Large Millimeter/submillimeter Array (ALMA) show young stars surrounded by broad gas and dust disks divided into intricate rings and gaps. These structures can trap solid material and may mark places where planets are beginning to form.

But astronomers have mostly seen those spectacular structures in large, bright disks. Much smaller disks have often appeared smooth and relatively featureless, raising the question of whether they evolve differently — or whether their structures have simply been too small to see.

A team led by Pietro Curone of the CATA (Center for Astrophysics and Associated Technologies) and Universidad de Chile, with Gijs Mulders of Pontificia Universidad Católica de Chile, Nienke van der Marel of Leiden Observatory, and Laura Pérez of Universidad de Chile, has now taken a much closer look.

Using ALMA in its most extended antenna configuration, the astronomers observed 11 extremely compact protoplanetary disks in the Lupus star-forming region. The observations, made in ALMA Band 7 at a wavelength of about 0.9 millimeters, reached an angular resolution of roughly 0.02 arcseconds. At Lupus's distance, that corresponds to scales of about two astronomical units, or twice the average distance between Earth and the Sun. This is the highest angular resolution currently available with ALMA at this wavelength.

At that resolution, several disks that had appeared simple began to reveal hidden detail. The team detected clear substructures within 10 astronomical units of the central star in four of the 11 disks and found indications of similar features in another three. In one system, Sz 112, ALMA resolved a central cavity extending to only 1.6 astronomical units from the star.

“For a long time, the smallest disks looked much simpler than the large, spectacular disks that ALMA made famous. What we find is that at least part of that difference was an illusion caused by resolution,” explains Curone. “When we zoom in far enough, rings, gaps and cavities begin to appear, even in these tiny systems.”

The result suggests that compact disks may not represent a fundamentally different kind of planet-forming environment. Instead, many may contain the same kinds of structures found in larger disks but compressed into regions only a few astronomical units across.

That is particularly important because the regions probed in this study are much closer to the orbital scales of many known planetary systems. Previous high-resolution ALMA surveys have largely resolved structures at distances of several to tens of astronomical units from their stars. Yet many exoplanets are found much closer in. Compact disks therefore offer a way to investigate planet formation in regions that more closely overlap with the places where many planets ultimately reside.

“The central question for us was whether small disks truly lack structure, or whether their features were simply beyond our resolution limit,” explains co-author Mulders. “To answer that, we compared the new observations of compact disks with previous ALMA studies of planet-forming disks.”

The comparison revealed a strong dependence on how well a disk had been resolved. Among disks observed with enough resolution to reliably reveal structure, substructures were detected in about 44% of extremely compact disks with radii between 3 and 10 astronomical units and in about 55% of disks between 10 and 30 astronomical units. For disks larger than 30 astronomical units, the detection rate rises above 90%.

The result suggests that the previously reported scarcity of rings and gaps in small disks was driven, at least in part, by insufficient angular resolution rather than by their genuine absence.

“This is a good example of how the picture changes when we gain resolution. A disk that looks smooth from farther away can show rich internal structure once we can separate features only a few astronomical units apart”, says Pérez, a co-author of the study.

The structures matter because dust in a protoplanetary disk does not simply remain where it was born. Solid particles tend to lose angular momentum and drift inward toward the star. Without some mechanism to slow that migration, millimeter-sized grains could disappear from the outer disk much faster than the lifetime of the disk itself.

Rings and gaps can signal changes in gas pressure that trap those particles. By slowing or stopping their inward drift, these pressure traps can preserve concentrations of solid material and create favorable locations for planet growth.

“Our new ALMA observations suggest that such traps can exist only a few astronomical units from young stars, even in extremely compact disks,” says co-author of the study van der Marel. “One possible explanation for some of the newly detected gaps is that young planets created them.”

The researchers estimated which kinds of planets could produce the observed structures if a single planet caused each gap. For structures within five astronomical units, the required masses fall within the range associated with super-Earths and mini-Neptunes. These possible planets would occupy a region of planet mass and orbital distance that is difficult to probe with both current exoplanet surveys and disk observations focused on much larger orbital scales.

“If planets are responsible for some of these gaps, we may be seeing the signatures of relatively low-mass planets while they are still forming, at orbital distances that have been very difficult to access before,” adds Curone.

The team cautions that rings, gaps and cavities are not themselves direct detections of planets. Several physical mechanisms can create structure in protoplanetary disks, and the current observations do not completely rule out non-planetary explanations. The authors find that a planetary origin is consistent with the structures detected in several disks, but further observations are needed to determine what produces them.

The observations also revealed another surprise. Two of the apparently compact systems, Sz 73 and Sz 130, have faint rings extending well beyond their bright central disks. Four additional systems show tentative signs of similar outer emission. This raises the possibility that some disks classified as very small may have more extended material that has remained hidden because it is faint.

Together, the results point toward a more continuous picture of protoplanetary disks. Large and compact disks may share many of the same basic structures and physical processes, with the main difference being the scales on which they occur.

By resolving features only a few astronomical units across, ALMA is beginning to bridge the gap between observations of planet-forming disks and the architectures of mature planetary systems. Future high-resolution observations of more compact disks, at different wavelengths and using tracers of both dust and gas, will help determine how common these hidden structures are and whether they truly mark the formation sites of the planets that will eventually emerge from them.

Source: ALMA (Atacama Large Millimeter/submillimeter Array)Press Releases

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Additional Information

This research appears in “Probing Planet Formation within 10 au: Dust Substructures in Extremely Compact Protoplanetary Disks2” by P. Curone et al. in the Astrophysical Journal Letters.

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Southern Observatory (ESO), the U.S. National Science Foundation (NSF), and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science and Technology Council (NSTC) in Taiwan, and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning, and operation of ALMA.

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Invisible Colors: Why Astronomers Use Different Radio Bands

A 21-cm view of the Pinwheel Galaxy (M33). The rainbow of colors is due to the rotation of the galaxy, which Doppler-shifts the radio light. Credit: NRAO/AUI/NSF

Radio light comes in a rainbow of colors. We see these colors with radio bands, and each band has a story to tell about the universe.

Radio astronomers view the universe in several ranges of wavelengths we call bands. The Very Large Array (VLA) uses wavelengths ranging from 4 meters to less than a centimeter. The Atacama Large Millimeter/submillimeter Array (ALMA) uses radio bands ranging from a couple of centimeters to a third of a millimeter. But why do radio telescopes use such a wide range of wavelengths? The answer lies in the many ways that objects emit radio light, and how this light interacts with the gas and dust of interstellar space.

Long radio wavelengths, such as those seen by the VLA’s Band 4, are typically produced by ionized gas. It lets us see where hot plasma is located in our galaxy. These long wavelengths are also useful because most neutral gas is transparent at these wavelengths. This means very little of this light is absorbed as it travels through space. Shorter wavelengths of light are often emitted by particular atoms or molecules. One of the most important of these is the 21-centimeter line, which is emitted by neutral hydrogen. This wavelength is one of the best ways to observe the distribution of matter in a galaxy since hydrogen is by far the most abundant element in the universe.

Wavelengths in the 10-cm to 20-cm range are particularly good for radio sky surveys, such as the VLA Sky Survey (VLASS). Radio galaxies are particularly bright in this range as are the jets emitted by supermassive black holes. By scanning the sky at these wavelengths, VLASS has captured images of nearly 10 million radio sources.

Light with wavelengths of a centimeter or two is often emitted through a process known as synchrotron radiation . When electrons speed through a strong magnetic field, the magnetic field forces them to move in tight spirals along the magnetic field lines. Because of this, they emit radio light. Synchrotron radiation is particularly useful at mapping the magnetic fields near black holes. Another process that emits light in this range is known as a maser or microwave laser. We’re most familiar with simple laser pointers that emit coherent red light, but in interstellar space pockets of water can emit coherent light with a wavelength of 1.3 centimeters. Since these water masers emit a very specific wavelength of light, they can be used to measure the rate at which the universe expands.

Black hole-powered radio galaxies discovered by VLASS
Credit: NRAO/AUI/NSF

Radio wavelengths on the order of a millimeter are particularly useful for studying cold gas and dust. Dust grains in interstellar space emit light with wavelengths on the order of their size, and since much of this dust is about a millimeter in size, that’s the wavelength where they emit the most light. These short wavelengths can be difficult to observe, in part because our atmosphere absorbs much of the light at these wavelengths. But they are also vitally important for the study of young planetary systems. ALMA has been able to capture disks of gas and dust around young stars and has even seen how gaps form within these disks as young planets begin to form. It is revolutionizing our understanding of how exoplanets form.

ALMA Observatory image of the young star HL Tau and its protoplanetary disk. One of the best images ever of planet formation, this image reveals multiple rings and gaps that herald the presence of emerging planets as they sweep their orbits clear of dust and gas. Credit: ALMA(ESO/NAOJ/NRAO); C. Brogan, B. Saxton (NRAO/AUI/NSF)

But perhaps one of the more interesting radio bands is ALMA’s Band 6, which captures light with wavelengths from 1.1 – 1.4 mm. It has been used to study how red giant stars generate heat, and the distribution of molecules in planetary nebulae. But it was also used to create one of the most powerful radio images of recent years, that of the supermassive black hole in the heart of galaxy M87. Band 6 receivers were used on radio telescopes across the world as part of the Event Horizon Telescope (EHT), and the data they gathered was combined to create the first direct image of a black hole.

Radio light is invisible to our eyes, so it’s easy to think of all radio light as the same. But radio is filled with colors, just as the colors of visible light we can see, and radio astronomy is at its most powerful when we use all the colors of its rainbow.

Source:National Radio Astronomy Observatory (NRAO)/News

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UA Astronomers Track the Birth of a 'Super-Earth'

This is an artist’s impression of a young star surrounded by a protoplanetary disk in which planets (not shown to scale) are forming. 

Illustration: ESO/L. Calçada

The protoplanetary disk around HL Tau, a million-year-old sunlike star located approximately 450 light-years from Earth in the constellation of Taurus, dwarfs our solar system (right). Taken by the ALMA array, this image reveals a series of concentric and bright rings, separated by gaps — features astronomers have struggled to explain until now. Credit: ALMA (ESO/NAOJ/NRAO)

The protoplanetary disk around HL Tau, a million-year-old sunlike star located approximately 450 light-years from Earth in the constellation of Taurus, dwarfs our solar system (right). Taken by the ALMA array, this image reveals a series of concentric and bright rings, separated by gaps — features astronomers have struggled to explain until now. (Credit: ALMA (ESO/NAOJ/NRAO))

Ruobing Dong, currently the Bart J. Bok Fellow at the UA's Steward Observatory, is interested in exoplanets and, in particular, how they form from protoplanetary disks surrounding newly born stars.

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"Synthetic observations" simulating nascent planetary systems could help explain a puzzle that has vexed astronomers for a long time.

A new model giving rise to young planetary systems offers a fresh solution to a puzzle that has vexed astronomers ever since new detection technologies and planet-hunting missions such as NASA's Kepler space telescope have revealed thousands of planets orbiting other stars: While the majority of these exoplanets fall into a category called super-Earths — bodies with a mass somewhere between Earth and Neptune — most of the features observed in nascent planetary systems were thought to require much more massive planets, rivaling or dwarfing Jupiter, the gas giant in our solar system.

In other words, the observed features of many planetary systems in their early stages of formation did not seem to match the type of exoplanets that make up the bulk of the planetary population in our galaxy.

"We propose a scenario that was previously deemed impossible: how a super-Earth can carve out multiple gaps in disks," says Ruobing Dong, the Bart J. Bok postdoctoral fellow at the University of Arizona's Steward Observatory and lead author on the study, soon to be published in the Astrophysical Journal. "For the first time, we can reconcile the mysterious disk features we observe and the population of planets most commonly found in our galaxy."

How exactly planets form is still an open question with a number of outstanding problems, according to Dong.

"Kepler has found thousands of planets, but those are all very old, orbiting around stars a few billion years old, like our sun," he explains. "You could say we are looking at the senior citizens of our galaxy, but we don't know how they were born."

To find answers, astronomers turn to the places where new planets are currently forming: protoplanetary disks — in a sense, baby sisters of our solar system.

Such disks form when a vast cloud of interstellar gas and dust condenses under the effect of gravity before collapsing into a swirling disk. At the center of the protoplanetary disk shines a young star, only a few million years old. As microscopic dust particles coalesce to sand grains, and sand grains stick together to form pebbles, and pebbles pile up to become asteroids and ultimately planets, a planetary system much like our solar system is born.

"These disks are very short-lived," Dong explains. "Over time the material dissipates, but we don't know exactly how that happens. What we do know is that we see disks around stars that are 1 million years old, but we don't see them around stars that are 10 million years old."

In the most likely scenario, much of the disk's material gets accreted onto the star, some is blown away by stellar radiation and the rest goes into forming planets.

Although protoplanetary disks have been observed in relative proximity to the Earth, it is still extremely difficult to make out any planets that may be forming within. Rather, researchers have relied on features such as gaps and rings to infer the presence of planets.

"Among the explanations for these rings and gaps, those involving planets certainly are the most exciting and drawing the most attention," says co-author Shengtai Li, a research scientist at Los Alamos National Laboratory in Los Alamos, New Mexico. "As the planet orbits around the star, the argument goes, it may clear a path along its orbit, resulting in the gap we see."

Except that reality is a bit more complicated, as evidenced by two of the most prominent observations of protoplanetary disks, which were made with ALMA, the Atacama Large Millimeter/submillimeter Array in Chile. ALMA is an assembly of radio antennas between 7 and 12 meters in diameter and numbering 66 of them once completed. The images of HL Tau and TW Hydra, obtained in 2014 and 2016, respectively, have revealed the finest details so far in any protoplanetary disk, and they show some features that are difficult, if not impossible, to explain with current models of planetary formation, Dong says.

"Among the gaps in HL Tau and TW Hya revealed by ALMA, two pairs of them are extremely narrow and very close to each other," he explains. "In conventional theory, it is difficult for a planet to open such gaps in a disk. They can never be this narrow and this close to each other for reasons of the physics involved."

In the case of HL Tau and TW Hya, one would have to invoke two planets whose orbits hug each other very closely — a scenario that would not be stable over time and therefore is unlikely.

While previous models could explain large, single gaps believed to be indicative of planets clearing debris and dust in their path, they failed to account for the more intricate features revealed by the ALMA observations. The model created by Dong and his co-authors results in what the team calls synthetic observations — simulations that look exactly like what ALMA would see on the sky. Dong's team accomplished this by tweaking the parameters going into the simulation of the evolving protoplanetary disk, such as assuming a low viscosity and adding the dust to the mix. Most previous simulations were based on higher disk viscosity and accounted only for the disk's gaseous component.

"The viscosity in protoplanetary disks may be driven by turbulence and other physical effects," Li says. "It's a somewhat mysterious quantity — we know it's there, but we don't know its origin or how large its value is, so we think our assumptions are reasonable, considering that they result in the pattern that has actually been observed on the sky."

Even more important, the synthetic observations emerged from the simulations without the necessity to invoke gas giants the size of Jupiter or larger.

"One super-Earth turned out to be sufficient to create the multiple rings and multiple, narrow gaps we see in the actual observations," Dong says.

As future research uncovers more of the inner workings of protoplanetary disks, Dong and his team will refine their simulations with new data. For now, their synthetic observations offer an intriguing scenario that provides a missing link between the features observed in many planetary infants and their grown-up counterparts.

The study, "Multiple Disk Gaps and Rings Generated by a Single Super-Earth," by Ruobing Dong, Shentai Li, Eugene Chiang and Hui Li, will be published on July 13 in the Astrophysical Journal.

This simulation of a lone super-Earth in a protoplanetary disk takes into account the effects of dust in addition to gas, resulting in a much more realistic picture. After 2,000 orbits, narrow gaps and multiple ring features emerge, just like those seen in actual observations such as the ones by ALMA. Credit: Shengtai Li and Ruobing Dong

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Source: UA (University of Arizona)/News

VLA Shows Earliest Stages of Planet Formation

Combined ALMA/VLA image of HL Tau

Credit: Carrasco-Gonzalez, et al.; Bill Saxton, NRAO/AUI/NSF

ALMA image of HL Tau at left; VLA image, showing clump of dust, at right

Credit: Carrasco-Gonzalez, et al.; Bill Saxton, NRAO/AUI/NSF.

New images of a young star made with the Karl G. Jansky Very Large Array (VLA) reveal what scientists think may be the very earliest stages in the formation of planets. The scientists used the VLA to see unprecedented detail of the inner portion of a dusty disk surrounding the star, some 450 light-years from Earth.

The star and its disk were studied in 2014 with the Atacama Large Millimeter/submillimeter Array (ALMA), which produced what astronomers then called the best image ever of planet formation in progress. The ALMA image showed gaps in the disk, presumably caused by planet-like bodies sweeping out the dust along their orbits. This image, showing in real life what theorists had proposed for years, was surprising, however, because the star, called HL Tau, is only about a million years old -- very young by stellar standards.

The ALMA image showed details of the system in the outer portions of the disk, but in the inner portions of the disk, nearest to the young star, the thicker dust is opaque to the short radio wavelengths received by ALMA. To study this region, astronomers turned to the VLA, which receives longer wavelengths. Their VLA images show that region better than any previous studies.

The new VLA images revealed a distinct clump of dust in the inner region of the disk. The clump, the scientists said, contains roughly 3 to 8 times the mass of the Earth.

"We believe this clump of dust represents the earliest stage in the formation of protoplanets, and this is the first time we've seen that stage," said Thomas Henning, of the Max Planck Institute for Astronomy (MPIA).

"This is an important discovery, because we have not yet been able to observe most stages in the process of planet formation," said Carlos Carrasco-Gonzalez from the Institute of Radio Astronomy and Astrophysics (IRyA) of the National Autonomous University of Mexico (UNAM).  "This is quite different from the case of star formation, where, in different objects, we have seen stars in different stages of their life cycle. With planets, we haven't been so fortunate, so getting a look at this very early stage in planet formation is extremely valuable," he added.

Analysis of the VLA data indicates that the inner region of the disk contains grains as large as one centimeter in diameter. This region, the scientists said, is presumably where Earth-like planets would form, as clumps of dust grow by pulling in material from their surroundings. Eventually, the clumps would gather enough mass to form solid bodies that would continue to grow into planets.

The VLA observations, made in 2014 and 2015, received radio waves with a wavelength of 7 millimeters.

The earlier ALMA observations of HL Tau were made at a wavelength of 1 millimeter. The VLA images showed a similar level of detail as the ALMA images.

"These VLA observations are the most sensitive and show the most detail of any yet made of HL Tau's disk at these longer wavelengths," said Claire Chandler, of the National Radio Astronomy Observatory (NRAO). "The VLA's ability to produce such high-quality images in this region is very important to advancing our understanding of these initial stages of planet formation," Chandler added.

The VLA study of HL Tau was an international collaboration, involving the UNAM, the MPIA, the NRAO, and the Spanish Consejo Superior de Investigaciones Cientificas (CSIC). The project leaders were Carlos Carrasco Gonzalez (UNAM) and Thomas Henning (MPIA). The scientists are reporting their findings in the Astrophysical Journal Letters.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

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Dave Finley, Public Information Officer
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Source: National Radio Astronomy Observatory (NRAO) 

A glowing jet from a young star

 HH 151 

Credit: ESA/Hubble & NASA

Acknowledgement: Gilles Chapdelaine

This image shows an object known as HH 151, a bright jet of glowing material trailed by an intricate, orange-hued plume of gas and dust. It is located some 460 light-years away in the constellation of Taurus (The Bull), near to the young, tumultuous star HL Tau.

In the first few hundred thousand years of life, new stars like HL Tau pull in material that falls towards them from the surrounding space. This material forms a hot disc that swirls around the coalescing body, launching narrow streams of material from its poles. These jets are shot out at speeds of several hundred kilometres per second and collide violently with nearby clumps of dust and gas, creating wispy, billowing structures known as Herbig-Haro objects — like HH 151 seen in the image above.

Such objects are very common in star-forming regions. They are short-lived, and their motion and evolution can actually be seen over very short timescales, on the order of years. They quickly race away from the newly-forming star that emitted them, colliding with new clumps of material and glowing brightly before fading away.

A version of this image was entered into the Hidden Treasures image processing competition by Gilles Chapdelaine.

Source:  ESA/Hubble