For a long time, astronomers looked at small protoplanetary disks around young stars and saw only smooth, almost empty rings of dust and gas. Large, bright disks had long amazed with their complex patterns — rings, gaps and cavities in which, it was believed, planets are born. But compact systems seemed too simple. Perhaps they even evolve differently? Or are their details simply too small to make out?
The answer came from the Chilean Atacama Desert. The ALMA telescope network, working in its most extended antenna configuration, turned its gaze on 11 of the smallest disks in the Lupus star-forming region. The observations were made at a wavelength of about 0,9 millimetres. The angular resolution reached 0,02 arcseconds — roughly two astronomical units at the distance of the Lupus region. Two distances from the Earth to the Sun. It is precisely at such scales, where the Earth, Mars and the asteroid belt orbit in our Solar System, that the action unfolds.
In four disks, the scientists clearly saw inner structures: rings, gaps and cavities within ten astronomical units of the star. In three more systems, hints of similar features turned up. The system Sz 112 is especially impressive. In it, ALMA resolved a central cavity with a radius of just 1,6 astronomical unit. This is the smallest known transitional disk — an object with an inner void comparable in size to Earth's orbit.
“For a long time, smaller disks looked far simpler than large ones,” notes the study's lead author, Pietro Curone. “We have shown that part of this difference was merely an illusion caused by insufficient resolving power.” When details separated by just a few astronomical units become distinguishable, the picture changes dramatically.
A comparison with previous ALMA surveys confirmed the dependence: the better a disk is resolved, the more often structures are found in it. Among the objects observed with sufficient detail, substructures were detected in roughly 44 percent of extremely compact disks with radii from 3 to 10 astronomical units, and in roughly 55 percent of those from 10 to 30. In disks larger than 30 units, the fraction approaches 95 percent. So previously, small disks simply “did not have time” to reveal their complexity.
These rings and gaps are not just beautiful patterns. Dust grains in a protoplanetary disk constantly lose angular momentum and tend to fall onto the star. Without obstacles, millimetre-sized grains would vanish long before the disk could live out its life. Rings and gaps point to variations in gas pressure that create traps. Material lingers, concentrates — and creates favourable conditions for planets to grow.
If some of the detected gaps really are carved by forming planets, then the masses of these bodies fall in the range of super-Earths and mini-Neptunes. It is precisely such planets that are most often found near stars, and it is precisely in such orbits that they have until now been especially hard to “see” in the process of birth.
Two disks — Sz 73 and Sz 130 — delivered yet another surprise. Beyond their bright inner core, faint outer rings turned up. Perhaps some “tiny” systems are in fact more extended than they seemed.
All of this paints a single picture. Large and compact disks are not different worlds but parts of one continuum. The same physical processes operate at different scales. Where previously only empty space was seen, traces of hidden architecture now emerge. Planets, it seems, begin to build their “homes” far closer to stars than previous observations allowed us to see.
Behind this work lies the remarkable precision of an instrument capable of distinguishing details the size of Earth's orbit at a distance of hundreds of light-years. And behind it lies an awareness of how complex and coherent the Universe is. In tiny clouds of dust and gas swirling around barely born stars, future worlds are already laid down. Somewhere, in regions comparable to the inner part of our Solar System, processes are underway right now that once led to the emergence of the Earth. The cosmos is not merely vast — it is incredibly intricately ordered, and every new look opens up this grand, almost incredible interconnectedness ever more deeply.
