This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: For decades, astronomers lumped brown dwarfs into a single category defined by mass alone—too big to be classified as planets but too small to become stars. However, this definition ignores the two very different mechanisms that can form them: the direct collapse of gas clouds and formation within the accretion disks of massive stars.
Through a new analysis of NASA's Transiting Exoplanet Survey Satellite (TESS), astronomers led by Nino Ephremidze at Harvard University have made the clearest observation to date of a brown dwarf in orbit around a massive, aging star, potentially offering important new clues about how these planet-like bodies form. Their results have been posted to the arXiv preprint server. Some brown dwarfs are thought to form in a similar way to stars: A cloud of gas and dust collapses under its own gravity but never becomes massive enough to initiate nuclear fusion in its core.
Others form more like planets, gradually building up mass within the accretion disk surrounding a massive young star. In principle, these two pathways should leave different fingerprints. While cloud collapse tends to produce more eccentric, elongated orbits, planet-like accretion favors calmer, more circular ones.
However, tidal forces from a close-in host star can smooth out an eccentric orbit over time, erasing the evidence astronomers need to distinguish the two formation pathways. Since its launch in 2018, TESS has monitored the brightness of nearby stars. By capturing the tiny dimming that occurs when an object passes in front of its star from our perspective, the instrument enables astronomers to calculate both the size and orbit of the transiting object.
Combining TESS observations with detailed spectroscopy, Ephremidze's team was able to measure the properties of a brown dwarf named HIP 61637 b with unusual precision. Remarkably, HIP 61637 b sits in the middle of the "brown dwarf desert," a mass range where brown dwarfs orbiting close to their host stars are notably scarce. In addition, the body's host star is unusual: an aging A-type star nearly three times the sun's mass and the brightest, most massive star ever found to host a transiting brown dwarf.
Because this star is nearing the end of its hydrogen-burning life, Ephremidze's team could pin down the system's age with unusual confidence at roughly 396 million years—making HIP 61637 b just the seventh brown dwarf with a reliably known age. When combined with the star's unusually large mass, HIP 61637 b's size hints that it may have built up gradually within a disk, planet-style, rather than collapsing suddenly like a star. However, at 48 Jupiter masses, the brown dwarf strains the limits of what disk accretion is thought capable of producing, leaving an open question for future research.
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