Six wandering planets with their own protoplanetary disks have been discovered in a young star cluster.

Around several thousand stars in our Galaxy, protoplanetary disks can be observed—gas and dust clouds where matter gradually concentrates, clumps together, and forms new planets. Naturally, the best places to search for such emerging systems are in young star clusters. Interestingly, brown dwarfs—objects that are intermediate between giant planets and low-mass stars—are also found in these clusters.

In their "youth," these objects still emit a faint intrinsic light, which fades over time. Their mass is less than 75 times that of Jupiter. For comparison, our Sun's small neighbor, Proxima Centauri, has a mass equivalent to 130 Jupiters.

It turns out that brown dwarfs also have their own protoplanetary disks, but this has led to another intriguing discovery. As astronomy progresses, increasingly smaller and lighter objects are being found. This has resulted in the identification of a peculiar phenomenon in space—free-floating planets, which are worlds that do not belong to any star and wander independently through the cosmos.

Scientists are continuing to determine the extent to which these are "nuggets," meaning they were born alone, versus those that were ejected from their systems for various reasons. The first of these two scenarios has clear support: some of these orphaned planets also wander not entirely alone, as they "harbor" their own satellites.

One interesting example is the object OTS 44 in the constellation Chameleon, located 530 light-years away from us. Formally, it is classified as a brown dwarf, but in reality, the object's mass is below the minimum threshold at which any nuclear fusion can begin in a celestial body—specifically, the "burning" of deuterium.

This minimum threshold is 13 times the mass of Jupiter. OTS 44 has a mass of less than twelve. Thus, it is technically a planet, albeit a giant one, and it is surrounded by a gas and dust disk. This disk is modest, with a total mass of all the material being no more than about 60 percent of Earth's mass. Nevertheless, this is sufficient for a dozen satellite worlds similar to Mercury.

Recently, astronomers from the University of St Andrews (UK) discovered six new examples of this unusual phenomenon. They shared their observations in an article available on the Cornell University preprint server (USA).

The detected objects were observed in the star cluster IC348, which is only a couple of million years old. It is located about a thousand light-years away from us, in the constellation Perseus.

There, based on the light characteristics of certain "stars," scientists see that these are not stars or even brown dwarfs, but celestial bodies "lighter" than the aforementioned deuterium limit. Their masses are approximately 10 times that of Jupiter. In other words, these are free-floating planets.

Nine of them showed a clear excess of infrared radiation. This occurs when a central object is surrounded by a lot of warm dust, that is, a protoplanetary disk. In six cases, the researchers confidently concluded that each of these planets represents the center of a self-forming system.

Protoplanetary disks typically evolve into fully formed planets and moons over several million years, but it is possible that this largely depends on the mass of the central object. For example, in the nearest OB association to the Sun—Scorpius-Centaurus—there is also a region of star formation that is significantly older than IC348. There, protoplanetary disks around low-mass stars and brown dwarfs are more common. Scientists believe this may indicate that such modest hosts retain these material disks for longer.