Astrophysics

Portrait of a dynamic neighbour

Brown dwarfs are celestial objects that lack the mass to become fully fledged stars. High-resolution maps of one such object add to the evidence that these exotic worlds have highly dynamic weather and climate. See Letter p.654

Humanity's study of stars and planets stretches back centuries, but our understanding of intermediate objects — brown dwarfs — is relatively primitive. Brown dwarfs are fluid, hydrogen-dominated objects that are generally presumed to form like stars, but that contain insufficient mass to fuse hydrogen into helium. As links between planets and stars, these dwarfs provide clues about the processes of star and planet formation, the physics of interior structure and the behaviour of atmospheres under exotic conditions. But because they are so far away, brown dwarfs are seen as only unresolved points of light in telescope images. So far, observations of these objects have all been measurements of the combined light from their Earth-facing hemisphere that preclude any detailed view of what they look like. That has now changed: on page 654 of this issue, Crossfield et al.1 present the first spatially resolved maps of the visible surface of a nearby brown dwarf.

Crossfield and colleagues' maps of the brown dwarf, dubbed Luhman 16B, show large-scale bright and dark regions suggestive of patchy clouds. As such, the maps provide constraints on the dominant length scales of the meteorological motions and the overall nature of the atmospheric circulation on these exotic worlds. Luhman 16B was discovered in 2013 and lies a mere 2 parsecs away2, making it and its companion brown dwarf (Luhman 16A) the third-closest stellar or sub-stellar system to Earth, after α-Centauri and Barnard's star. Still glowing from the heat of its formation billions of years ago, the brown dwarf's atmospheric temperatures reach a baking 1,200 kelvin.

Given that brown dwarfs are unresolved points of light in the sky, how did Crossfield et al. construct these maps? The answer lies in the technique of Doppler imaging3 (Fig. 1). Brown dwarfs rotate rapidly — Luhman 16B rotates once every 4.9 hours. This fast rotation leads to movement of the atmospheric gas towards Earth on one side of the object and away from Earth on the other side. These rotational motions result in a change in frequency (Doppler shift) of emitted light, which, in turn, causes significant broadening of the emission lines observed in infrared spectra. If the visible surface of the brown dwarf were featureless, the spectral lines would be approximately symmetrical. But discrete, bright or dark atmospheric features that move across the dwarf's Earth-facing hemisphere during its rotation cause time-dependent asymmetries in the shape of the spectral lines that can be inverted to create a map, in longitude and latitude, of this surface patchiness. The technique has long been applied to stars3, but Crossfield and colleagues' study is the first to apply it to brown dwarfs.

Figure 1: The Doppler imaging technique.
figure1

The rotation of a rapidly spinning star or brown dwarf causes a significant broadening of spectral lines through the Doppler shift in the frequency of emitted light. For a featureless brown dwarf, the broadened line is mirror symmetrical in wavelength with respect to the line centre (orange curves). However, the presence of discrete spots causes perturbations in the line shape (white curves). Such perturbations will move from the left to the right wing of the line as the spots move across the Earth-facing hemisphere owing to the rotation of the brown dwarf (left to right panels). Because the rotational velocities, and hence Doppler shifts, are greatest in low-latitude regions of the brown dwarf, a spot near the equator produces a perturbation that migrates from the extreme left wing to the extreme right wing. Spots at higher latitude exhibit smaller Doppler shifts and thus produce perturbations that begin and end closer to the line centre. In this way, time-resolved spectra, such as those obtained by Crossfield et al.1, can be used to construct a map of the spot distributions on the surface of the brown dwarf. (Figure adapted from ref. 3.)

The observations add to a growing body of evidence demonstrating that brown dwarfs exhibit highly dynamic weather and climate. Atmospheric motions have long been hinted at from the presence of clouds and disequilibrium chemistry the result of vertical mixing of atmospheric gas — that are inferred from infrared spectra of brown dwarfs. The first spectacular evidence for weather on brown dwarfs emerged4 in 2009, when it became clear that the infrared emission of many brown dwarfs shows strong variability in integrated brightness on timescales of hours to days. Several lines of evidence indicate that this variability results from relatively cloudy and cloud-free patches coming into or out of view as the brown dwarf rotates. Luhman 16B is no exception, and recent observations5,6 indicate that it exhibits peak-to-peak brightness variations of about 5–20%, fluctuating in time as the weather evolves. Although tantalizing, such variability provides only loose constraints on the size, shape and configuration of atmospheric features, rendering any direct assessment of atmospheric circulation for these objects difficult. In this context, Crossfield and colleagues' maps are potentially game changing.

Brown dwarfs generally, and Luhman 16B specifically, occupy a key position in our grand effort to understand the mechanisms and behaviour of atmospheric circulation over a wide range of conditions. As on planets such as Earth and Jupiter, the rapid rotation of brown dwarfs ensures that their atmospheric dynamics are rotationally dominated at large scales7. Unlike most known planets, however, brown dwarfs receive negligible external irradiation. Earth's global-scale weather is driven primarily by the contrast in solar heating between the Equator and the poles, a type of climate forcing that is ruled out for brown dwarfs such as Luhman 16B. Theories suggest that the vigorous convection that takes place in a brown dwarf's interior, which is necessary to transport the enormous heat flux that they radiate into space, will trigger waves and turbulence in the atmosphere7,8 that could potentially organize into coherent, large-scale weather features such as those seen in Crossfield and co-workers' maps. Jupiter's Great Red Spot — a vast, centuries-old vortex — and Saturn's recent massive convective storm9 provide useful analogies.

That said, it is currently unclear how far the analogy with Jupiter extends. Although brown dwarfs are Jupiter-like in many ways, they radiate heat fluxes that are orders of magnitude greater. Recent work10 suggests that, under these radiative conditions, the atmospheric circulation may comprise turbulence and vortices with no preferred directionality, rather than a banded pattern with multiple east–west jet streams like that of Jupiter and Saturn. Unfortunately, Crossfield and colleagues' analysis does not resolve this crucial issue; a well-known bias makes it a particular challenge to confidently infer banded patterns with the Doppler-imaging technique. Still, future attempts will be welcome, and, if successful, they could have implications for the interpretation of brown-dwarf variability as well as theories of atmospheric dynamics generally, including the multi-decade effort to build a theory for Jupiter's and Saturn's jet streams.

There are other caveats. The signal-to-noise ratio in the authors' maps is modest, and only a few of the largest atmospheric structures in the maps are statistically robust. The observations — which are based on carbon monoxide spectral lines at a wavelength near 2 micrometres — do not establish whether the patchiness results from spatial variations of clouds, temperature or chemistry, although the first is most likely, and observations at other wavelengths can break this degeneracy. Moreover, because Luhman 16B and its companion are the brightest brown dwarfs in the sky, they are the only ones to which the Doppler-imaging technique can currently be applied. Despite the caveats, these are exciting times for brown-dwarf science. The next few years should see the workings of these fascinating worlds gradually come into focus.

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Correspondence to Adam P. Showman.

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Showman, A. Portrait of a dynamic neighbour. Nature 505, 625–626 (2014) doi:10.1038/505625a

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