Summary
A study of 37 nearby brown dwarfs used NASA’s SPHEREx observations to identify water, carbon dioxide, carbon monoxide and methane in their atmospheres. The findings also show that existing models struggle to match spectra during some cloudy atmospheric transitions.
NASA’s SPHEREx space telescope is helping astronomers examine the atmospheres of brown dwarfs, objects that form from collapsing gas clouds like stars but are not massive enough to sustain hydrogen fusion in their cores. In a study published in The Astrophysical Journal, researchers analysed observations of 37 nearby brown dwarfs and found spectral signatures of water, carbon dioxide, carbon monoxide and methane.
The objects span a broad temperature range, from about 4,000 to minus 10 degrees Fahrenheit (2,200 to minus 20 degrees Celsius). NASA’s Jet Propulsion Laboratory (JPL) says SPHEREx is also finding thousands of brown dwarfs as it surveys the sky, with further observations being analysed.
How SPHEREx reads brown dwarf atmospheres
SPHEREx measures the brightness of objects in 102 different colours, covering deep-red visible light and infrared wavelengths. The result is a spectrum: a record of how much light an object emits at different wavelengths. Molecules in an atmosphere absorb particular wavelengths, leaving patterns that can reveal which gases are present.
That infrared coverage is important because water in Earth’s atmosphere absorbs some of the wavelengths SPHEREx observes, making them difficult to study from the ground. From orbit, the telescope can detect those signals in the light coming from brown dwarfs.
Brown dwarfs are dim, warm objects that share some characteristics with giant planets such as Jupiter and Saturn. Unlike planets orbiting stars, free-floating brown dwarfs are not lit by a host star; they are heated from within. Their spectra offer a way to study how atmospheric chemistry changes across objects with different temperatures.
Clouds and chemistry vary between objects
The researchers found that the molecular signatures change from one brown dwarf to another across the temperature range. JPL reports that some of the observed objects are at a stage where their clouds thin, giving way to methane-rich atmospheres. Even brown dwarfs at similar temperatures can have distinct spectra.
The study’s authors say current models reproduce broad chemical trends but have difficulty matching observations during these cloudy transitions. The new measurements therefore provide more data for understanding the atmospheres and evolution of these objects, while highlighting a part of the transition that models need to describe better.
Detailed space-based studies have so far covered only a few dozen brown dwarfs, according to JPL. SPHEREx’s wide spectral coverage and sky survey are expanding the available observations: the study focused on 37 objects, while thousands more are in the process of being analysed.