[ALMA] Thanks to the extraordinary sharpness of ALMA’s images, a research team has discovered rings, grooves, and cavities in extremely compact disks surrounding young stars.
The observations show that structures associated with planet formation extend into regions comparable in size to the inner solar system. This suggests that many small disks may have appeared uniform simply because their details were too small to distinguish.
Some of the most striking images from the Atacama Large Millimeter/submillimeter Array (ALMA) show young stars surrounded by vast disks of gas and dust with complex patterns of rings and grooves. These structures can trap solid material and indicate the locations where planets begin to form.
However, until now the astronomical community has observed these spectacular structures primarily in large, bright disks. Much smaller disks tended to appear uniform and relatively lacking in detail, raising the question of whether they evolved differently or whether their structures were simply too small to observe.
ALMA images of the 11 compact planet-forming disks observed in the star-forming region of Lupus. With the highest angular resolution available with ALMA at a wavelength of approximately 0.9 millimeters, the team revealed previously hidden structures in several of these small disks. The lower right panel shows the orbits of Earth, Mars, Jupiter, and Saturn, along with the asteroid belt, for scale reference. The comparison illustrates how these observations allow us to explore planet formation at distances comparable to those of our own solar system. Each white bar represents two astronomical units; one astronomical unit is equal to the average distance between the Earth and the Sun. Credit: P. Curone et al. – ALMA (ESO/NAOJ/NRAO)
Now, a team led by Pietro Curone, from the Center for Astrophysics and Related Technologies (CATA) and the University of Chile, along with Gijs Mulders, from the Pontifical Catholic University of Chile; Nienke van der Marel, from the Leiden Observatory; and Laura Pérez, from the University of Chile, has managed to observe them in much greater detail.
With ALMA’s antennas in their most extended configuration, the team observed 11 extremely compact protoplanetary disks in the star-forming region of Lupus. The observations, made in ALMA’s Band 7 at a wavelength of approximately 0.9 millimeters, achieved an angular resolution of about 0.02 arcseconds. At the distance of Lupus, this corresponds to a scale of approximately two astronomical units, or twice the average distance between the Earth and the Sun. This is the highest angular resolution currently available with ALMA at this wavelength.
With that resolution, several disks that had appeared simple began to reveal hidden details. The team detected clear substructures within 10 astronomical units of the central star in four of the 11 disks and found hints of similar features in three others. In one of the systems, Sz 112, ALMA was able to distinguish a central cavity extending to a distance of just 1.6 astronomical units from the star.
“For a long time, smaller disks appeared much simpler than the large, spectacular disks that ALMA made famous. What we found is that at least some of that difference was an illusion caused by resolution,” explains Curone. “When we look in sufficient detail, rings, grooves, and cavities begin to appear, even in these tiny systems.”
The result suggests that compact disks may not constitute a fundamentally different planet-forming environment. Many could contain the same types of structures found in larger disks, but concentrated in regions only a few astronomical units across.
This is especially important because the regions explored in this study are much closer to the orbital scales of many known planetary systems. Previous high-resolution ALMA studies have primarily distinguished structures located at distances of several tens of astronomical units from their stars. However, many exoplanets are found much closer to their stars. Therefore, compact disks offer a way to investigate planet formation in regions that more closely match the locations where many planets eventually form.
“The central question we posed was whether small disks truly lacked structure or whether their details were simply beyond our resolution limit,” explains Mulders, co-author of the study. “To answer this, we compared the new observations of compact disks with previous ALMA studies of planet-forming disks.”
Comparison of ALMA images of HL Tau (left), a well-known example of a large, bright planet-forming disk, and the much smaller Sz 112 (center). The image of HL Tau has been adjusted to show how it would appear if it were at the same distance as Sz 112, allowing for a direct comparison of their physical sizes. The enlarged view of Sz 112 (right) reveals a central cavity on a scale comparable to that of Earth’s orbit. The study identifies Sz 112 as the smallest known transition disk: a planet-forming disk with an internal cavity. The scale bars represent 20 astronomical units in the left panel and 1 astronomical unit in the enlarged view. Credit: P. Curone et al. – ALMA (ESO/NAOJ/NRAO)
The comparison revealed that the detection of structures depended heavily on the level of detail with which each disk had been observed. Among the disks observed with sufficient resolution to reliably identify structures, substructures were detected in approximately 44% of the extremely compact disks with radii between 3 and 10 astronomical units, and in about 55% of those with radii between 10 and 30 astronomical units. In disks with radii greater than 30 astronomical units, the detection rate exceeded 90%.
The result suggests that the previously reported scarcity of rings and grooves in small disks was due, at least in part, to insufficient angular resolution, rather than their actual absence.
“This is a good example of how our understanding changes as resolution improves. A disk that appears uniform when observed from farther away can reveal a rich internal structure when we manage to distinguish details separated by just a few astronomical units,” says Pérez, co-author of the study.
These structures are important because the dust in a protoplanetary disk doesn’t simply stay where it originated. The solid particles tend to lose angular momentum and move inward, toward the star. Without some mechanism to slow this migration, millimeter-sized grains could disappear from the outer part of the disk in a much shorter time than the disk’s own lifetime.
The rings and grooves can indicate changes in the pressure of the gas that traps these particles. By slowing or stopping their movement inward, these pressure traps can maintain concentrations of solid material and create favorable conditions for planetary growth.
“Our new ALMA observations suggest that these traps can exist just a few astronomical units from young stars, even in extremely compact disks,” says van der Marel, co-author of the study. “One possible explanation for some of the newly detected grooves is that they were created by young planets.”
The team estimated what types of planets could produce the observed structures, assuming each groove was caused by a single planet. For structures located within five astronomical units of the star, the necessary masses fall within the range associated with super-Earths and mini-Neptunes. These potential planets would have combinations of mass and orbital distance that are difficult to explore with both current exoplanet surveys and disk observations focused on much larger orbital scales.
“If some of these spaces are the product of planets, we could be seeing the footprints of relatively low-mass planets while they are still forming, at orbital distances that have been very difficult to explore until now,” Curone adds.
The team cautions that rings, grooves, and cavities do not, in themselves, constitute direct detections of planets. Several physical mechanisms can generate structures in protoplanetary disks, and current observations do not completely rule out explanations that do not involve planets. The team finds 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 seemingly compact systems, Sz 73 and Sz 130, exhibit faint rings extending far beyond their bright central disks. Four other systems show possible hints of similar outer emission. This raises the possibility that some disks classified as very small contain more extensive material that has remained hidden by their faint emission.
Taken together, the results point to a view of protoplanetary disks as part of a continuum. Large and compact disks could share many of the same basic structures and physical processes, differing mainly in the scales at which these manifest themselves.
By distinguishing structures just a few astronomical units across, ALMA is beginning to connect observations of planet-forming disks with the configuration 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 point to the locations where the planets that will eventually emerge from them are forming.
Additional information
This research is presented in the article “Probing Planet Formation within 10 au: Dust Substructures in Extremely Compact Protoplanetary Disks“, (Open Access) by P. Curone and his team, published in The Astrophysical Journal Letters .
The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership between 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) of Taiwan; and by NINS, in cooperation with the Academia Sinica (AS) of Taiwan and the Korea Astronomy and Space Science Institute (KASI).
The construction and operation of ALMA 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 unified leadership and management for the construction, commissioning, and operation of ALMA.
Astrobiology, Astronomy, exoplanet,
