2023-11-06 00:16:32
In recent decades, research into exoplanets (planets orbiting stars other than the Sun) has experienced rapid progress, and son miles the planetary systems confirmed to date. So much so that the community’s efforts are already focused on the search for exoplanets similar to Earth in the habitable zone of their stars, where liquid water and conditions for life could exist.
However, before reaching this ambitious goal, there are still many unknowns to be revealed about how planetary systems form and how they evolve.
From grains of dust to planets
The great variety of exoplanets discovered suggests that their training mechanisms can vary significantly. The most accepted theory is that they arise in protoplanetary disks that surround stars during their formation, composed mainly of gas and dust.
Most models suggest that dust grains in the disk come together and grow through collisions, forming small planetary cores. Once formed, these germs of new worlds begin to accumulate gas and dust from the disk, as they orbit the star, until they form a planet. In particular, the accretion of large amounts of gas is essential for the formation of gas giants like Jupiter.
Other models suggest that it is also possible that fragmentation of protoplanetary disks occurs and that these pieces can collapse directly into planets.
Furthermore, regardless of their formation mechanism, planets can undergo migration in their orbits due to gravitational interactions with other planets or with the protoplanetary disk. This displacement can influence the final architecture of the system, and even lead to the expulsion of one or several planets, which are freed from the gravitational pull of their star and become objects that roam freely through interstellar space. They are the so-called floating objects of planetary mass (or PMO, for its acronym in English).
Target: Trapezium Cluster
Substellar objects (that do not become stars) below the limit of hydrogen and deuterium fusion (about 13 times the mass of Jupiter) never generate nuclear reactions and cool rapidly, becoming dimmer as they age. However, when they are young, they remain relatively luminous and easy to detect as they release gravitational energy as they contract.
For this reason, the near-Earth regions where stars form offer the best opportunity to identify PMOs. Given its population density, young age and proximity to the Sun, the Trapezium Cluster (at the center of the Orion Nebula) provides an ideal laboratory for studying the birth of stars and planets.
Telescopes such as Hubble have allowed astronomers to study the origin and formation of PMOs, as well as the composition and properties of their atmospheres. By studying their location in space and their relationship to stars and other objects, astronomers can gain clues about how they formed. Until recently, the most modest PMOs had between 3 and 5 times the mass of Jupiter, close to the minimum theoretical mass limit to be able to explain their formation by the mechanism of fragmentation and collapse of clouds of gas and dust (i.e., the same mechanism by which stars are formed).
The discovery of JuMBOs
Now, the enormous detection capabilities of the James Webb Space Telescope have revolutionized the ability to investigate these objects. In a mapping project of the Orion Nebula and the Trapezium Cluster, ESA astronomers have discovered 540 new PMOs. Its smallest masses are equivalent to half that of Jupiter, which makes it very difficult to explain how such low-mass isolated objects can form.
Even more interesting is the fact that 9% of these celestial bodies, dubbed JuMBOs (binary objects with a mass similar to Jupiter), are part of large binary systems. That is, they are gravitationally linked to each other.
The existence of JuMBOs challenges current theories of both star and planet formation. In particular, the multiplicity fraction, defined as the portion of stars that have at least one companion, tends to decrease with mass. That is, more massive stars form much more often in binary or multiple systems than less massive ones. However, it appears that this trend is unexpectedly reversed by JuMBOs that are at the lower end of the mass range.
A new training mechanism?
These properties suggest that new formation mechanisms must come into play. If JuMBOs arose through a “star-like” process, through the gravitational collapse of a cloud of gas and dust, then there must be some as-yet-unidentified physical process that encourages the creation of such low-mass objects.
But perhaps they were born through a “planet-like” process, in a disk around a host star, and were violently ejected. These events may be caused by dynamical interactions between stars, relatively common in dense star-forming regions such as the Trapezium Cluster. However, how these young planets can be ejected in pairs and remain gravitationally bound, it remains difficult to explain by the theoretical models we have available.
Understanding the quantity and distribution of PMOs and JuMBOs that the James Webb telescope has observed in the Trapezium Cluster raises a new mystery, which seems to suggest the possibility of a mixture of several scenarios. Or perhaps it even requires the existence of a new planet formation mechanism.
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