2023-11-19 14:17:09
By observing the dust disk around the young star SZ Chamaeleontis (SZ Cha) using the Spitzer and James-Webb space telescopes, a team of astronomers noticed abrupt changes in the space of just fifteen years, at as planetoids form in this young planetary system. This is due to the dynamics of the nascent protoplanetary disk, which is still very erratic. The authors believe that the study of this young star and its dust disk can allow us to better understand the formation and evolution of our Solar System.
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According to the models most commonly considered by scientists, a star is generally born from a gigantic cloud of interstellar gas mainly composed of hydrogen and helium, the two most abundant elements in the Universe. Small variations in density within the cloud lead to what is called gravitational collapse: the cloud begins to contract on itself, with gases being drawn toward the center to form the young star. If the very large part of the gases in the cloud are found clustered in the center, in the star (our Sun alone represents 99% of the mass of the Solar System), the rest begins to swirl around, forming what astronomers call the protoplanetary disk. In a few million years, the gases and dust that make up the protoplanetary disk begin to agglomerate through successive collisions. The dust expands into larger and larger blocks, until it forms planetesimals — embryonic planets that evolve in a protoplanetary disk.
According to scientists, this is also the scenario that appears to have played out around 4.6 billion years ago, when our Sun was born from an interstellar cloud. The resulting protoplanetary disk formed in a few million years the planets that we know today in our Solar System, and all the ingredients that subsequently formed the Earth (and allowed the appearance of the life) were already present within this disc; but our understanding of the formation and evolution of our Solar System remains very imprecise. To try to better understand its origins, astronomers seek to observe emerging stellar systems in the Universe, in order to draw possible analogies with ours.
This animation shows the evolution of a protoplanetary disk around a star. Holes appear as planets form, collecting gas and dust. © ExploreAstro
The birth of a star system a few hundred light years from us
If the James-Webb space telescope is today the star of space observatories peering into the confines of the Universe, other telescopes preceded it, and the discoveries they made are not eclipsed by those made thanks to their big brother. This is particularly the case for the Spitzer space telescope (Space Infrared Telescope Facility) from NASA, launched in 2003, which among other things carried out the first live observation of the process of planet formation in a protoplanetary disk around stars similar to the Sun. Among these nascent star systems observed, astronomers looked at the young star SZ Chamaeleontis (SZ Cha), located about 500 light years from us in the constellation Chameleon and surrounded by a disk of dust – a protoplanetary disk .
If the formation of planetary systems is not rare in the Universe, unearthing a planetary system in formation remains a feat, and the observation of the phenomena which take place there is valuable for astronomers. By analyzing data collected by the Spitzer Space Telescope in 2008 when it observed SZ Chamaeleontis, scientists detected the surprising presence of neon III, one of the natural isotopes of neon. They presented their analyzes in 2013 in the journal The Astrophysical Journal. Scientists were surprised by their discovery because, of the sixty protoplanetary disks characterized at the time, SZ Chamaeleontis was the only one to have such a high concentration of neon III, when the other systems had very little. The team of scientists deduced that the presence of neon III in the SZ Chamaeleontis system was a sign that the protoplanetary disk was bombarded mainly by ultraviolet radiation, unlike other systems mainly affected by X-rays.
An indicator of the life expectancy of the protoplanetary disk
The type of radiation to which the protoplanetary disk is mainly subjected is crucial for its evolution: according to numerical simulations, a disk dominated by X-rays erodes much more quickly than when it is dominated by ultraviolet radiation. The planets are in a sort of race against time to be able to form before the disk disappears. In the SZ Chamaeleontis system, however, numerical models indicated that the dominance of ultraviolet radiation allowed at least a million more years for planet formation before the disk of gas and dust evaporated forever.
If SZ Chamaeleontis already stood out during its observation by the Spitzer telescope, scientists were not at the end of their surprises: by observing the young star again, this time using the James-Webb space telescope , astronomers were surprised to notice that the signature of the presence of neon III in the system had almost disappeared, only fifteen years after the observations made by the Spitzer telescope. They present their new results in the journal The Astrophysical Journal. The two telescopes being extremely efficient, the scientists had ruled out the possibility of a measurement error.
Formerly dominated by ultraviolet radiation, the protoplanetary disk is now dominated by X-rays, limiting the time available for planets to form there. The researchers believe that the neon signature differences in the SZ Cha system are the result of a variable wind which, when present, absorbs UV light and leaves behind X-rays. Winds are indeed common in a system with a newly formed and energetic star; but it remains possible to observe the system during a calm, windless period — something the Spitzer telescope did fifteen years ago. The researchers are already planning to carry out further observations of the system as well as other young planetary systems to better understand their formation processes. According to them, brief periods of calm in a protoplanetary disk dominated by ultraviolet radiation could be common, but have been rarely observed.
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