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<\/p>\nThoughts on Brown Dwarfs, Science Fiction and New Telescopes
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Here\u2019s an interesting notion, courtesy of science fiction writer Jack McDevitt. Jack\u2019s story \u201cLighthouse\u201d (available in Cryptic: The Best Short Fiction of Jack McDevitt<\/em> (Subterranean Press, 2009) posits an apparently normal brown dwarf that, upon closer study, turns out to be deeply anomalous, in ways that seem designed to call attention to it. There appears to be no intent to establish contact but rather to warn anyone traveling near the object that there is a solitary black hole, all but undetectable, in its immediate vicinity.<\/p>\n And there is not just one brown dwarf, but several thousand that display similar anomalies in their spectra, as determined by a survey of these stars throughout the galaxy. Could these markers be a warning system for interstellar travelers? Written with Michael Shara, the story is a nice, compact bit of hard science fiction, working with realistic spectroscopic analysis and ingenious speculation. It\u2019s also realistic about the kind of non-scientific pressures astronomers can face. Here the characters are wrangling over the unusual role deuterium plays in the spectra as they try to make sense of what they are seeing:<\/p>\n \u201cI don\u2019t buy it, Kristi,\u201d Greg said. She had him on the vid relay, from his office. \u201cThere\u2019s got to be a natural mechanism at work. Maybe high pressure chemistry. Maybe magnetic fields. Maybe radiative levitation. Geochemical processes can concentrate minerals by orders of magnitude on Earth, so why not deuterium on the surfaces of brown dwarfs under far more exotic conditions?\u201d<\/p>\n<\/blockquote>\n He sounded really concerned. It was why she liked him so much. He was worried about what would happen to her career if she tried to go public with her notion. She imagined it was hard for him not to say, You\u2019re skating at the edge of academic disaster. Blow your reputation now and you\u2019ll always be at the fringe.<\/p>\n<\/blockquote>\n I like that bit. It wasn\u2019t so long ago that writing about starships in a scientific journal was unusual, but these days we have many theorists pushing propulsion concepts and speculating on Fermi questions. Anyway, I notice that Jack\u2019s story \u201cCool Neighbor\u201d is freely available online<\/a>. It\u2019s another look at a brown dwarf, this one close indeed, and it reminds me of my own certainty that we would find a dim object like this within a light year or less. Alas, it was not to be, as the WISE data made clear, but we did find a lot of interesting objects, including Luhman 16 whose members are the closest brown dwarfs in the inventory.<\/p>\n Image:<\/strong> Untold numbers of brown dwarfs roam the cosmos, like the one in this artist\u2019s concept. But only a few dozen have been studied in detail with space-based telescopes. With the help of NASA\u2019s SPHEREx, scientists are in the process of analyzing thousands more. Credit: NASA\/JPL-Caltech.<\/p>\n Luhman 16 B, is to my knowledge the first brown dwarf to have its weather mapped, which fits nicely with the recent news about WISE 0855, a brown dwarf not all that much further away than Luhman 16 at 7.5 light years. In the just released study, WISE 0855 is subjected to atmospheric analysis that shows water clouds changing in thickness as the time series work proceeds. Brittany Miles (Steward Observatory, University of Arizona, Tucson) took spectra of the object every 15 minutes for 11 hours, to reveal not just the movement in the water clouds but processes of deep convection bringing up gases from below., <\/p>\n We have JWST to thank for the find, as Miles notes:<\/p>\n \u201cThis is the first time we\u2019ve been able to confirm that water clouds are becoming thinner and thicker on a nearby world. Before JWST, we only had photometry, which mixed up the effects of clouds, chemistry, and temperature all together. Now we can actually distinguish them.\u201d<\/p>\n<\/blockquote>\n WISE 0855 is hard to distinguish from a free-floating planet, given that its effective temperature (265 K) is not all that much warmer than Earth\u2019s. Churning carbon monoxide and phosphine are what is currently identified in the data as rising from below thanks to the heat within the object. The same process of convective churn is seen on Jupiter and is tracked on this brown dwarf with the greatest clarity yet, a molecule-by-molecule variation seen in real time. WISE 0855 doesn\u2019t look like Jupiter but it displays similar weather patterns. <\/p>\n Brown dwarfs are ubiquitous these days, but I still remember the announcement of Teide 1. This object, in the Pleiades cluster, turned up in data from the IAC-80 telescope in Tenerife (Canary Islands), with the paper published in 1995 in Nature<\/em>. Shortly followed by Gliese 229B, the object launched the study of remarkably low temperature stars, now classified in ways that sometimes muddle the distinction between planet and star. How far we\u2019ve come since then. Have a look at the just released image from SPHEREx, which is only a partial look at the thousands of brown dwarfs that are being identified.<\/p>\n Image:<\/strong> As it maps the sky, NASA\u2019s SPHEREx telescope is finding thousands of brown dwarfs hiding in the dark. A selection is shown in this collage of observations, listed by spectral type, distance, and temperature in degrees Kelvin. Credit: Zafar Rustamkulov\/NASA\/JPL-Caltech.<\/p>\n SPHEREx (Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer) is making the case that brown dwarfs have a range of atmospheres rich in chemistry that show striking similarities to gas giants. The image above is drawn from SPHEREx infrared observations of 37 nearby brown dwarfs with a wide range of temperatures, from 2200 degrees Celsius (2477 K) to -20 degrees Celsius (253 K). Water, carbon dioxide, carbon monoxide and methane turn up in the analysis of their atmospheres. <\/p>\n Launched in March of 2025, SPHEREx is engaged in mapping the entire sky in near-infrared light, taking on the order of 3600 images per day. The instrument is multi-purpose, its major goal being to study the distribution of galaxies in the distant universe. But the brown dwarf component makes it possible to observe atmospheric chemistry across a wide range of temperatures in these objects. Although the paper deals with a relatively small number, thousands more brown dwarfs are now being analyzed. Lead author Zafar Rustamkulov (Caltech) adds:<\/p>\n \u201cFrom orbit, SPHEREx sees wavelengths of light that are basically impossible to see with telescopes on the ground because water in Earth\u2019s atmosphere absorbs them. We are picking up light from the deep, red clouds of brown dwarfs all over the sky.\u201d<\/p>\n<\/blockquote>\n Image:<\/strong> Thanks to its spectral coverage, NASA\u2019s SPHEREx is revealing molecules like methane (CH4), carbon dioxide (CO2), and carbon monoxide (CO) in brown dwarf atmospheres. The telescope\u2019s view of three different brown dwarfs appears on the left; the depictions on the right imagine them as Jupiter-like worlds. Credit: Zafar Rustamkulov\/International Gemini Observatory \/ NOIRLab \/ NSF\/ AURA, M.H. Wong (UC Berkeley) et al. Acknowledgements: M. Zamani.<\/p>\n Rustamkulov also notes how the absorption patterns in these spectra change as brown dwarfs age and grow cooler, with some of the objects in the SPHEREx data showing thinning clouds and increasingly methane-rich atmospheres. He adds:<\/p>\n \u201cThe state-of-the-art models are capturing the general chemical trend, but when it comes to these cloudy transitions, the models are struggling to match the data. No two brown dwarfs are alike. Even at the same temperature, their spectra look quite distinct.\u201d<\/p>\n<\/blockquote>\n Indeed. The paper comments that \u201cstate-of-the-art brown dwarf model grids generally fail to trace their complex features.\u201d So exactly where are we right now? From the paper:<\/p>\n \u2026the models generally approximate the smoothly coherent abundance trends of CO and CO2 as they are converted to CH4 with declining temperature and increasing age. We also show that the addition of clouds does not improve Diamondback model fits (see Figures 14 and 15), especially at the cloudy L\/T transition (see Figure 7), implying the need for more theoretical work connecting clouds with chemistry. Optimistically, the models seem to qualitatively represent the low-gravity and low-metallicity L dwarfs (see Figures A2, A3, A4, and A5, which probe a more extreme regime of parameter space.<\/p>\n<\/blockquote>\n So we\u2019re beginning to unlock some of the secrets of brown dwarf atmospheric chemistry, but as you would expect, our existing models need to be tuned up in this new era of rapidly multiplying observational data. I\u2019m reminded by the last several posts here that any graduate student looking for abundant challenges might want to consider dwarf possibilities, from white to brown, as both categories are yielding data we are only now able to explore. The exoplanet landscape around these objects is particularly frothy with potential.<\/p>\n The paper on WISE 0855 is Miles, et al., \u201cWater Cloud and Chemical Modulations in the Coldest Brown Dwarf,\u201d accepted at The Astronomical Journal<\/em> (preprint<\/a>). The SPHEREx paper is Rustamkulov et al., \u201cSPHEREx 0.75\u20135 \u03bcm Spectra for a Sequence of Nearby Brown Dwarfs,\u201d The Astrophysical Journal<\/em> Vol. 1009, No. 2 (28 September 2026). Full text<\/a>.<\/p>\n\n
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