Astronomers have discovered a batch of nine new brown dwarfs, also known as “failed stars,” including the two smallest examples of these curious celestial objects ever seen.
The new find could help better define the mass dividing line between large planets and small brown dwarfs, as well as that between large brown dwarfs and small stars.
“The new brown dwarfs are the least massive known brown dwarfs,” lead researcher Kevin Luhman of Pennsylvania State University told Space.com. “They place a new constraint on the lowest mass at which brown dwarfs exist.”
“In addition, one of the brown dwarfs near two Jupiter masses exhibits evidence of a disk of gas and dust, indicating that it may have the raw materials for making planets,” Luhman said. “So it is possible that the planetary systems exist in which the central ‘sun’ is only twice the mass of Jupiter.”
The team found the small brown dwarfs lurking among young stars in IC 348, a star-forming cluster in the Perseus Molecular Cloud around 1,000 light-years from Earth.
The stars that failed and the telescope that didn’t
Brown dwarfs get the slightly unfair nickname of “failed stars” from the fact that, though they form like stars from a collapsing cloud of gas and dust, they can’t gather enough mass from their prenatal envelopes to trigger the fusion of hydrogen to helium within their cores.
This is the nuclear process that defines what a main sequence star is; hence, brown dwarfs are cast as “failed stars.” That’s despite the fact that they conduct some forms of nuclear fusion within their bodies.
Currently, the mass limit of brown dwarfs is considered to be between 13 to 60 times the mass of Jupiter, or 0.013 to 0.08 times the mass of the sun.
The new discovery — of two brown dwarfs with masses around twice the mass of Jupiter, or about 0.002 times the mass of the sun — radically widens that mass scale.
“It is also surprising that the process that makes stars is able to produce objects down to only twice the mass of Jupiter, 500 times smaller in mass than the sun,” Luhman said.
If the discovery of such small brown dwarfs was a surprise, then something else the team discovered about these failed stars in IC 348 was a complete shock.
The new brown dwarfs also exhibit signals from an unidentified hydrocarbon, a chemical compound composed solely of hydrogen and carbon atoms. Its origin is a mystery, team members said.
“The presence of an unidentified, non-methane hydrocarbon is completely unexpected and unexplained,” Luhman said. “Because of the presence of that hydrocarbon, we have proposed a new spectral class (H) that is defined by the presence of that species.”
These hydrocarbons have only previously been observed in the atmospheres of Saturn and its largest moon, Titan, according to Luhman.
The cool atmosphere of the brown dwarfs was integral to the detection of these hydrocarbons by the JWST, which has been anything but a failure when it comes to studying these failed stars.
“Because they are cool, brown dwarfs are brightest at infrared wavelengths, and JWST is the most sensitive infrared telescope to date,” Luhman said. “The next steps for this research include performing new JWST spectroscopy at higher resolution to better constrain the species of hydrocarbon that have been detected.
“In addition, we need theorists to develop models of the atmospheres of brown dwarfs that can explain why our new brown dwarfs have the unidentified hydrocarbon, but don’t have methane, which is the hydrocarbon normally observed in older brown dwarfs.”
Additionally, the team will analyze the JWST spectra of the remaining brown dwarf candidates they identified to confirm that they are brown dwarfs.
Two of these candidates could have masses as low as the mass of Jupiter, hinting at a further shakeup of our concept of the brown dwarf mass range.
Luhman is confident that, if such tiny, paradigm-shifting brown dwarfs are out there, especially in IC 348, JWST will find them.
“Much deeper JWST images of the cluster that we studied could potentially detect brown dwarfs below the mass of Jupiter, if they exist at those masses,” the researcher concluded.
The team’s research was published on Tuesday (June 10) in The Astrophysical Journal Letters.
