Often dubbed as “failed stars,” brown dwarfs form akin to stars, undergoing gravitational collapse but never accumulating enough mass to initiate nuclear fusion. Ranging in mass, the smallest brown dwarfs can share similarities with giant planets. In a pursuit to uncover the tiniest brown dwarf, astronomers utilizing the NASA/ESA/CSA James Webb Space Telescope have identified a new record-holder—a celestial object with a mass merely three to four times that of Jupiter.
Straddling the line between stars and planets, brown dwarfs evolve similarly to stars, reaching a density that prompts gravitational collapse. However, they never achieve the density and heat required for hydrogen fusion, preventing them from becoming stars. Some brown dwarfs, at the lower end of the mass spectrum, are comparable in size to giant planets, weighing just a few times the mass of Jupiter.
The quest to determine the smallest object capable of forming like a star led an international team, employing the capabilities of the James Webb Space Telescope. The newfound record-holder, a small, free-floating brown dwarf with a mass of only three to four times that of Jupiter, was identified in the star cluster IC 348, situated around 1000 light-years away in the Perseus star-forming region. The youthfulness of this cluster, approximately five million years old, ensures that any brown dwarfs within it remain relatively bright in infrared light, emitting warmth from their formation.
Lead author Kevin Luhman from Pennsylvania State University emphasized the fundamental question the research aims to address: “One basic question you’ll find in every astronomy textbook is, what are the smallest stars? That’s what we’re trying to answer.” Leveraging Webb’s Near-Infrared Camera (NIRCam) to identify brown dwarf candidates based on brightness and colors, the team further investigated the most promising targets using Webb’s Near-Infrared Spectrograph (NIRSpec) microshutter array.
The James Webb Space Telescope’s (Webb) invaluable infrared sensitivity played a pivotal role, enabling the identification of fainter objects compared to ground-based telescopes. Webb’s precise vision further allowed the differentiation between pinpoint brown dwarfs and amorphous background galaxies.
Through this meticulous selection process, the team pinpointed three compelling targets with masses ranging from three to eight times that of Jupiter and surface temperatures between 830 and 1500 degrees Celsius. Remarkably, the smallest of these objects weighs merely three to four times Jupiter, as suggested by computer models.
Understanding the formation of such diminutive brown dwarfs poses theoretical challenges. While a dense gas cloud with ample gravity can collapse to form a star, the weaker gravity of a small cloud should theoretically hinder the collapse necessary for brown dwarf formation. This challenge is particularly pronounced for brown dwarfs with masses comparable to giant planets.
Catarina Alves de Oliveira of ESA, principal investigator on the observing program, emphasized the perplexity: “So we have to ask, how does the star formation process operate at such very, very small masses?”
Beyond shedding light on star formation processes, these minuscule brown dwarfs offer insights into exoplanets. Given their mass overlap with the largest exoplanets, studying free-floating brown dwarfs becomes crucial, as they are more accessible for analysis than exoplanets concealed within the glare of their host stars.
Two of the identified brown dwarfs exhibit a spectral signature of an unidentified hydrocarbon—a molecule containing hydrogen and carbon atoms. This infrared signature, also detected in Saturn’s atmosphere and Titan by NASA’s Cassini mission, and in the interstellar medium, is a novel finding beyond our solar system.
“This is the first time we’ve detected this molecule in the atmosphere of an object outside our solar system,” explained Alves de Oliveira. The unexpected discovery challenges existing models of brown dwarf atmospheres.
While these objects fall within the mass range of giant planets, the question arises whether they are true brown dwarfs or rogue planets ejected from planetary systems. While the team cannot definitively rule out the latter, they contend that brown dwarfs are more probable, given the rarity of ejected giant planets and the cluster’s youth, limiting the time for such ejections.
Future endeavors may involve longer surveys to detect fainter, smaller objects, potentially reaching one Jupiter mass. The ongoing search for similar objects within IC 348 may provide additional clarity, especially considering theories that suggest rogue planets are more likely in the outskirts of star clusters.
Conducted as part of Guaranteed Time Observation program #1229, these observations contribute to advancing our understanding of celestial bodies and were published in The Astronomical Journal.
Source: NASA