SPHEREx study tests atmospheric models against nearby brown dwarfs

A study of nearby brown dwarfs is giving astronomers new evidence against which to test models of their atmospheres. NASA’s October 8 account describes observations from SPHEREx that reveal molecular signatures and differences among objects occupying the space between familiar categories of stars and planets. Brown dwarfs form from collapsing gas clouds but lack enough mass to sustain hydrogen fusion in their cores, NASA explains. Their atmospheres can share characteristics with giant planets. The agency identifies water, carbon dioxide, carbon monoxide and methane among the substances revealed in the study’s spectra. The researchers analyzed 37 nearby brown dwarfs. That number describes the paper’s sample, not the entire population that SPHEREx has detected or that the mission’s scientists expect to investigate. The research abstract gives a more specific account of the comparison. It describes spectra spanning 0.75 to 5 micrometers and a sequence from spectral class L0 to Y4. The authors compare the observations with theoretical model grids and report persistent difficulties in matching several spectral features simultaneously. The models broadly reproduce trends but show important disagreements, including around chemistry-sensitive carbon dioxide and carbon monoxide features. The abstract identifies the observations as a resource for improving models, rather than evidence that atmospheric theory has already become complete. A spectrum is important here because the analysis goes beyond how a brown dwarf looks in a conventional picture. The researchers examine patterns in light at different wavelengths. The relationship between those measured patterns and a model’s predictions allows them to assess how well an explanation fits the observations. For readers, two scales should remain distinct. A sky survey can identify many objects, while a particular study analyzes a selected group in detail. A finding from that group requires the sample and method to be understood before it is generalized. The work also illustrates a productive role for disagreement between observation and prediction. A mismatch does not automatically make every element of a model useless. It identifies where the account may need refinement and where further evidence can be valuable. NASA’s announcement and the research abstract therefore answer related questions. The announcement places the work within the mission; the abstract identifies the sample and specific model-comparison problem. Together, they support a more precise description than a claim that the telescope has simply discovered a new class of objects. NASA’s account adds detail about why the observations matter. Brown dwarfs were first discovered in the 1990s, and, unlike most planets, they drift without a host star, heated entirely from within. The agency notes that only a few dozen have been studied in detail with space-based telescopes, and that much of what scientists understand about their makeup, storminess and evolution comes from theoretical models. The findings were published in The Astrophysical Journal. The 37 objects in the study span the full brown dwarf temperature range, from about 4,000 degrees Fahrenheit down to minus 10 degrees Fahrenheit (about 2,200 to minus 20 degrees Celsius), according to NASA. SPHEREx measures their brightness in 102 different colors, from the deepest visible red into infrared light. Lead author Zafar Rustamkulov, a scientist at IPAC, Caltech’s science and data center in Pasadena, explained the advantage of observing from orbit: “SPHEREx sees wavelengths of light that are basically impossible to see with telescopes on the ground because water in Earth’s atmosphere absorbs them.” Coauthor J. Davy Kirkpatrick, also of IPAC, said the team is seeing molecular signatures change “from object to object across the entire temperature regime.” He added that the paper “concentrated on just three dozen, but we have thousands more that we are in the process of analyzing.” That statement describes work in progress; conclusions about the wider population will depend on results not yet published. The next scientific task is to use these and further observations to improve understanding of brown dwarf atmospheres. The present result is a comparison that makes remaining difficulties visible.