Vega and its dust disk
A white and bluish star, Vega, located in the constellation Lyra, is approximately 40 times brighter than our Sun. Evolving just 25 light years from Earth, however, is much more than a simple shiny object. Like several other A-type stars, it is surrounded by a circumstellar disk, a sort of halo composed of dust and debris. This disk is made up of fine dust particles, resulting from repeated collisions between small celestial bodies such as asteroids and comets. These collisions generate a large quantity of material, creating a sort of “coat” of dust around the star, which can extend over several tens of billions of kilometers.
Circumstellar disks are particularly interesting for astronomers, because they provide clues to the history and evolution of planetary systems. When a star forms, it is usually surrounded by a dense disk of gas and dust, which provides the basic material for the formation of planets, moons, and other celestial bodies. However, in the case of older stars, like Vega, this disk is no longer a “cradle” for new planets; it is powered by debris left by the celestial bodies that orbit the star, testifying to the dynamics of the system and the collisions that continue to occur there.
The study of these disks is therefore essential for understanding how planetary systems evolve after their initial formation.
What the Hubble and Webb telescopes observed
Thanks to Hubble and Webb, two of the most powerful telescopes in NASA astronomers were recently able to obtain an unprecedented view of this ring of matter, which extends over 100 billion kilometers in diameter.
To the surprise of researchers, Vega's disk is remarkably smooth, almost devoid of the disturbances one would expect to see in a star system of this type. Usually, large planets present in the disk act a bit like “snow plows”: they move and redistribute dust, thus creating distinct bands or rings. In our own solar system, for example, giants like Jupiter and Saturn strongly influence the distribution of dust. However, around Vega, no sign of such massive planets has been detected, leaving this disk in an unusually homogeneous state.
These observations contrast notably with those made around Fomalhaut, a star similar in size and in age. This object indeed presents a very different circumstellar architecture, with three well-defined debris belts. Researchers believe that these belts are the result of the influence of invisible planets, which makes Fomalhaut even more complex and intriguing than Vega.
This difference between Fomalhaut and Vega therefore raises interesting questions: why does Was the first able to form planets which structure its disk, while the second seems to be devoid of them? Is this due to differences in disk composition, environmental conditions, or other factors related to the star itself?
Dust “stratification”
< p>The two telescopes also provided complementary perspectives on this debris disk. Hubble captured the outer halo, where the smallest dust grains, sometimes the size of smoke particles, reflect the star's light. For his part, Webb, who observes in infrared, revealed the glow of a disc of slightly larger particles orbiting closer to Vega.
These new observations also highlighted a fine organization of the particles. The star's intense light puts pressure on the dust grains, sorting the particles by size and creating distinct layers, where each type of dust grain is at a specific distance from the star. This “stratification” phenomenon is rare and intrigues researchers because it could offer crucial clues about the dynamics of circumstellar disks and the diversity of environments where planetary systems form.
[à gauche] Hubble Space Telescope false-color view of a 160-billion-kilometer-diameter dust disk around Vega. Hubble detects light reflected from dust the size of smoke particles, largely in a halo at the edge of the disk. [à droite] The James Webb Telescope resolves the glow of hot dust in a disk halo, 37 billion kilometers away. The outer disk (analogous to the Kuiper belt of the solar system) extends from 11 to 24 billion kilometers. The inner disk extends from the inner edge of the outer disk to the immediate vicinity of the star. There is a notable drop in the surface brightness of the inner disk from about 6 to 11.6 billion kilometers. Credits: NASA, ESA, CSA, STScI, S. Wolff (University of Arizona), K. Su (University of Arizona), A. Gáspár (University of Arizona)
Better understand the diversity of planetary systems
The new observations of Vega carried out by Hubble and Webb thus highlight an astonishing example of diversity in the universe of stellar systems. While some disks are structured by massive planets, others like that of Vega remain “smooth” and homogeneous.
If planets are present around Vega, they would probably be of relatively modest size. Smaller, lighter planets would exert less gravitational influence, allowing dust and particles to remain dispersed more evenly. These planets, if they exist, should occupy specific orbits, distant from the main debris disk, where their influence remains undetectable by current means.
Researchers hope that future observations of Webb may be able to detect these possible hidden planets. By discovering more systems with varied architectures, astronomers also hope to better understand how the diversity of sizes, masses and positions of planets influences the formation and structure of circumstellar disks.
Source: arxiv
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