Brown dwarfs are sometimes called failed stars because they form stars through gravitational collapse but never gain enough mass to ignite nuclear fusion. The smallest brown dwarfs can overlap in mass with the giant planets. In the search for the smallest brown dwarf, astronomers using the James Webb Space Telescope have discovered a new record holder: an object that weighs only three to four times the mass of Jupiter.
This image from the NIRCam (Near Infrared Camera) instrument on NASA's James Webb Space Telescope shows the central portion of the star cluster IC 348. The erratic curtain in the image is interstellar material that reflects light from the cluster's stars -- a so-called reflection nebula. These substances also include carbon-containing molecules called polycyclic aromatic hydrocarbons (PAHs). Winds from the cluster's most massive stars may help form the large rings seen on the right side of the field of view. Source: NASA, ESA, CSA, STScI, Kevin Luhman (PSU), Catarina Alves de Oliveira (ESA)
Brown dwarfs are objects between stars and planets. They form like stars, becoming increasingly dense enough to collapse under their own gravity, but they never get dense and hot enough to start fusing hydrogen and turning into stars. Among low-level brown dwarfs, some are comparable to the giant planets, with masses only several times that of Jupiter.
Astronomers are trying to identify the smallest objects that could form in a star-like manner. A team of researchers using NASA's James Webb Space Telescope has identified the new record holder: a tiny, free-floating brown dwarf with only three to four times the mass of Jupiter.
This image from the Near Infrared Camera (NIRCam) instrument on NASA's James Webb Space Telescope shows the central portion of star cluster IC 348. Astronomers look for tiny, free-floating brown dwarfs in this cluster: objects too small to be stars but larger than most planets. They discovered three brown dwarfs with masses less than eight times that of Jupiter, which are circled in the main image and shown in the detailed image to the right. The smallest ones are only three to four times more massive than Jupiter, challenging theories of star formation.
The erratic curtain in the image is interstellar material that reflects light from the cluster's stars - a so-called reflection nebula. These substances also include carbon-containing molecules called polycyclic aromatic hydrocarbons (PAHs). The bright stars closest to the center of the frame are actually a pair of B-type stars in a binary system, the most massive stars in the cluster. Winds generated by these stars may help form the large ring on the right side of the field of view. Source: NASA, ESA, CSA, STScI, Kevin Luhman (PSU), Catarina Alves de Oliveira (ESA)
To find the newly discovered brown dwarf, Luhman and his colleague Catarina Alves de Oliveira chose to study the IC 348 star cluster, located about 1,000 light-years away in the Perseus star-forming region. The cluster is young, only about 5 million years old. Therefore, any brown dwarf will be relatively bright in infrared light because of the heat they gave off during their formation.
The research team first used the Webb Telescope's Near Infrared Camera (NIRCam) to image the center of the cluster and identify brown dwarf candidates in terms of brightness and color. They tracked the most promising targets using Webb's NIRSpec (near-infrared spectrograph) microshutter array.
The Webb telescope's infrared sensitivity is crucial, allowing the team to detect dimmer objects than ground-based telescopes can. In addition, Webb's keen vision allowed them to determine which red objects were precise brown dwarfs and which were spherical background galaxies.
After screening, they found three intriguing targets that weighed between 3 and 8 Jupiter masses and had surface temperatures between 1,500 and 2,800 degrees Fahrenheit (830 and 1,500 degrees Celsius). The smallest of them weighs only three to four times that of Jupiter, according to computer models.
This image of star cluster IC348, taken with the Webb Near Infrared Camera (NIRCam), shows compass arrows, scale bar, and chromatic key for reference.
Compass arrows to the north and east show the image's orientation in the sky. Note that the relationship between north and east in the sky (looking from the bottom up) is reversed relative to the directional arrows on the ground map (looking from the top down).
The units marked on the scale bars are light years, which is the distance that light travels in one Earth year. (Light travels a distance equal to the length of the scale bar, which takes 0.1 years). One light year is approximately 5.88 trillion miles or 9.46 trillion kilometers. The field of view shown here is about 0.5 light-years wide and 0.8 light-years high.
This image shows invisible near-infrared light wavelengths that have been converted into visible colors. The chroma key shows which NIRCam filters were used to capture these rays. The color of each filter name is used to represent the visible color of infrared light passing through the filter. Source: NASA, ESA, CSA, STScI, Kevin Luhman (PSU), Catarina Alves de Oliveira (ESA)
Explaining how such small brown dwarfs form is theoretically challenging. Heavy, dense clouds of gas have enough gravity to collapse to form stars. However, due to the weaker gravity of brown dwarfs, it should be more difficult for small gas clouds to collapse into brown dwarfs, especially for brown dwarfs with the mass of giant planets.
Catarina Alves de Oliveira of the European Space Agency (ESA) is the principal investigator of this observation project. He said: In this star cluster, this object is unlikely to form a disk, but forms like a star, and the mass of the three Jupiters is 300 times that of the sun. So we ask, how does the star formation process work when the mass is so small? "
In addition to providing clues about star formation processes, tiny brown dwarfs can help astronomers better understand exoplanets. The least massive brown dwarfs overlap with the most massive exoplanets, so they should have some similar properties. However, free-floating brown dwarfs are easier to study than giant exoplanets, which are hidden in the glare of their host stars.
Two of the brown dwarfs discovered in the survey showed spectral signatures of an unidentified hydrocarbon, a molecule containing hydrogen and carbon atoms. NASA's Cassini mission detected the same infrared signature in the atmospheres of Saturn and its moon Titan. Such infrared signatures have also been seen in the interstellar medium, or the gas between stars.
"This is the first time we have detected this molecule in the atmosphere of an extrasolar object," explains Alves de Oliveira. "Models of brown dwarf atmospheres don't predict their existence. We're looking at objects that are younger and less massive than before, and we're seeing something new and unexpected."
Since the masses of these objects are well within the mass range of the giant planets, this raises the question: Are they brown dwarfs or rogue planets ejected from the planetary system? While the team cannot rule out the latter possibility, they think they are far more likely to be brown dwarfs than ejected planets.
Ejected giant planets are unlikely for two reasons. First, such planets are generally less common than less massive planets. Second, most stars are low-mass stars, and giant planets are especially rare among these stars. Therefore, it is unlikely that most of the stars in IC 348 (which are all low-mass stars) would have produced such massive planets. Additionally, since the cluster is only 5 million years old, there may not have been enough time for the giant planets to form and then be ejected from their systems.
Discovering more such objects will help clarify their status. The theory is that rogue planets are more likely to be found on the outskirts of star clusters, so if there are such planets in IC348, expanding the search range may find them.
Future work may also include longer surveys to detect fainter, smaller objects. The team's short-term sky survey is expected to detect objects as small as twice the mass of Jupiter. Longer surveys could easily reach a Jupiter mass.
These observations are part of Guaranteed Time Observation Program No. 1229. The observations were published in the Astronomical Journal.