Even lighter than planets, brown dwarfs recently discovered by the James Webb telescope


Reflection Nebula IC 348.
The reflection nebula IC 348 has been discovered, in which the three brightest brown dwarfs ever observed have been discovered. Acknowledgments: NASA, ESA, CSA, STScI and K. Luhman (Penn State University) and C. Alves de Oliveira (ESA)
Sergio Messina Sergio Messina 8 minutes

The stellar population is characterized by a wide range of masseslets go from stars with a mass of ten times the mass of the Sun to stars with a mass of only a few tenths of the mass of the Sun. The sun is considered a star because of its mass yellow dwarfbut they also exist red dwarfswith masses of several tenths of the mass of the Sun and further in the list of so-called brown dwarfs.

Brown dwarfs

Brown dwarfs are also called “failed stars”that is, during the process of formation, they were unable to reach high enough temperatures in their core to trigger f reactionsthermonuclear fusion of hydrogen into helium. However, they managed to temporarily start the burning of deuterium (an isotope of hydrogen, the nucleus of which consists of a proton and a neutron). So they are very boring subjects (hence the adjective brune) destined to cool slowly, starting with surface temperatures generally near or below 1000 degrees Kelvin.

The boundary between stars and brown dwarfs is not well definedin addition to mass, metallicity may or may not play a role in ignition. These objects are still being studied today and as we shall see, their study recently led to an unexpected discovery.

What was discovered

A team of astronomers studying brown dwarfs thought of looking for new brown dwarfs in a cluster in which mostly low-mass stars are forming.. It is aboutonstar cluster IC 348precisely the reflection nebula.

IC 348
An image of the nebula IC 348 observed in infrared light by the Spitzer telescope. Acknowledgments: NASA, ESA, J. Muzerolle (STScI), E. Furlan (NOAO and Caltech), K. Flaherty (Univ. of Arizona/Steward Observatory), Z. Balog (Max Planck Institute for Astronomy), and R. Gutermuth ( University of Massachusetts, Amherst):

Its brightness is due to the presence of very bright stars in the same nebula, whose light is reflected in the abundant interstellar gas.

IC 348 is classified as a reflection nebula. It is object number 348 in the Index Catalog (IC), a catalog of galaxies, nebulae and star clusters. It is visible in the direction of the constellation Perseus about 1,000 light years from Earth.

To search for new brown dwarfs, a team led by Caterina Alves de Oliveira, a scientist from ESA (European Space Agency), used James Webb Space Telescope (JWST). This telescope thanks to its very high spatial resolution and thanks to its NIRCam camera, which, thanks to observations in the near-infrared region, can penetrate through the dust present in the nebula, it is an ideal tool for hunting brown dwarfs.

And the expectations did not disappoint at all!

From analysis of images collected by JWST, scientists managed to identify three brown dwarfs, one of them is absolutely unique: it is the smallest brown dwarf ever observed, with a mass even lower than that of the most massive gas planets known to date. The results of this research were published in the Astrophysical Journal by KL Luhman.

Complications, but also opportunities

Until now the known mechanism of star formation was not thought ofthe collapse of the cloud by its own gravity, could work for such small masses. Those discoveries are 1 brown dwarf with a mass approximately 3-4 times that of Jupiter, while the other 2 have masses between 4-8 Jupiter masses.

The gas giant exoplanets known so far have a mass up to 20 times that of Jupiter.

But then why consider them brown dwarfs and not gas giant planets?

This is exactly the question that researchers asked themselves after their discovery: brown dwarfs or giant planets expelled from their own planetary system?

IC 348
Image IC 348 taken by JWST showing the position of three ultralight brown dwarfs. Acknowledgments: NASA, ESA, CSA, STScI and K. Luhman (Penn State University) and C. Alves de Oliveira (ESA)

The answer is that it is considered highly unlikely that they are ejected exoplanets, for two reasons.

The first reason is related to their size. As mentioned, IC 348 produces mostly low-mass stars, so it is highly unlikely that they will be able to generate such massive planets.

The second reason relates to their isolated position. Planets form inside disks that rotate around protostars, so-called protoplanetary disks. The three objects discovered are instead isolated from any forming star. However, there is a possibility that a planet could be ejected (gravitationally) from its planetary system and become what is called “free floating planet” therefore a free floating planet. However, considering that the region of formation in which these brown dwarfs are located is about 5 million years old, this is too short a time for this ejection process to end.

The existence of such light brown dwarfs it’s a complication for star formation models. However, at the same time it’s an opportunity which paves the way for a better understanding of star formation mechanisms and for improved models.

A study of these light brown dwarfs it is also relevant to understanding giant exoplanetsand with which these brown dwarfs should have common characteristics. The advantage is that while the study of exoplanets is hampered by the glare of their parent star, which makes observation difficult, in the case of pale brown dwarfs, which are isolated, they are easier to study.

In addition to the “brightest brown dwarf” record, these objects are the first objects in which the presence of hydrocarbons has been observed in the atmosphere, so far only in our solar system, on Saturn and its moon Titan.

The team that made the discovery plans to continue a careful scan of the entire nebula in the near future in search of other similar objects.


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