We typically think of stars and planets as very different types of objects. While it’s true that the Sun and Earth are clearly different, the line between the largest of planets and smallest of stars is a bit fuzzy. An interesting demonstration of this is a newly discovered storm (similar to Jupiter’s great red spot) on an object that some consider a star.

The object in question is known as W1906+40, and it is an L-dwarf. The usual definition of a star is that it must fuse elements in its core. Objects larger than about 78 Jupiter masses have enough mass to fuse hydrogen, and is a common dividing mass between red dwarf stars, and brown dwarfs. But objects between 65 and 78 Jupiter masses can fuse deuterium, and so are at least star-like in terms of fusion. An L-dwarf star is closest to a red dwarf. Though roughly the size of Jupiter, it they have surface temperatures around 2,000 Kelvin, and so would typically look rather star-like if we saw one close up.

This particular L-dwarf is massive enough and old enough that it’s just on the edge of being a star. However with a surface temperature of 2,200 Kelvin it is still cool enough that it has an atmosphere rather than an ionized photosphere. As a result, weather patterns and storms can form, similar to the way they form on Jupiter. Observations of this star found evidence of a large dark spot near one of its poles. For a star this would be a starspot, driven by strong magnetic fields. But we can observe the effects of magnetic fields near starspots, and observations by the Spitzer infrared telescope found no corresponding magnetic field. So this dark region isn’t a starspot, but rather an atmospheric storm.

It’s been supposed that brown dwarfs could have such storms on their surface, but this is the first observational indication that they actually occur.

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Brown dwarfs are like the Pluto of stars. While they are large enough to produce heat like a star, they are not large enough to fuse hydrogen in their cores like our Sun and other stars. They typically have a mass between 20 and 70 Jupiters, and one of the central questions has been whether they are more planet-like or star-like. New research published in Nature points to a more planetary nature by discovering bright aurora on a brown dwarf. 

Aurora, commonly known as northern lights, occur when high energy charged particles strike the Earth’s upper atmosphere, causing it to glow. They occur largely at the polar regions because of an interaction between the charged particles and the Earth’s magnetic field. While they are common on Earth, they are also found on other planets like Jupiter that have a strong magnetic field. Stars, on the other hand, don’t have aurora.

We’ve known for quite a while that the surface temperatures are rather cool. The most massive brown dwarfs can have temperatures about half that of the Sun, while the smallest brown dwarfs can have surface temperatures no warmer than an oven. But whether their atmospheres are more like that of stars or planets has been an unanswered question.  In this new work, the team noticed a brown dwarf that emitted bursts of strong radio energy about once every 2.8 hours. This pulsar-like behavior could be caused by charged particles interacting with the dwarf’s strong magnetic field, or it could be due to interactions with its atmosphere. To find out the team observed the object in the visible spectrum. What they found was that the spectrum matched that of hydrogen that has been struck by charged particles. In other words, these bursts are due to very bright aurora.

This is the first case of aurora being observed on an object outside our solar system. Combined with other research that shows brown dwarfs can have clouds, it’s clear that the atmospheres of brown dwarfs are more planet-like than star-like.

Paper: G. Hallinan et al. Magnetospherically driven optical and radio aurorae at the end of the stellar main sequence. Nature. Vol. 523, p. 568. doi: 10.1038/nature14619 (2015)

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A brown dwarf is larger than a planet, but not large enough to be considered a star.  Stars undergo fusion in their cores, but a brown dwarf lacks the mass necessary to initiate fusion.  At the same time, they are more like stars than planets, and can have at atmospheric temperature of 2000 K.  So these objects occupy a middle ground between star and planet.

Historically, brown dwarfs have been difficult to observe.  They are small and relatively cool, which makes them fairly dim in the range of visible light.  However with the rise infrared astronomy and all-sky surveys these brown dwarfs are now fairly easy to detect.  This is particularly true for brown dwarfs which are companions to other stars, since the methods used to discover planets would also easily detect brown dwarfs.

Credit: HubbleSite

This has given rise to a bit of a mystery, known as the brown dwarf desert.  You can see this in the figure, which shows the distribution of stars by spectral type for stars within 32 light years of us.  As you go from left to right, the mass of the stars go from large to small.  You can see that with smaller mass comes greater numbers.  There are much more red dwarf stars (M-class) than there are sun-like stars (G-class).  But once you get to brown dwarf mass, the number drops off significantly.  Given the way stars form, one would expect even more brown dwarfs than red dwarfs.

This particular chart is for stars within 32 light years.  At that range, the drop off is not simply due to brown dwarfs being dimmer and harder to detect.  We’ve done all-sky surveys in the infrared range, and if the brown dwarfs were there we would have observed them.  This also agrees with searches for extrasolar planets.  The methods we use to detect planets work best when the planets are more massive.  This is why, for example, the planets we’ve found so far tend to be more Jupiter sized than Earth sized.  If there were lots of brown dwarfs we would expect to have found them orbiting other stars.  We don’t see that.

So for some reason there seems to be much fewer brown dwarfs in the universe that we might expect.  Why this might be is still a bit of a mystery, but one possibility comes from the fact that larger planets form earlier than smaller planets in the accretion disk of a young star.  Since brown dwarfs are larger than Jupiter-size planets, they would start to form very early in the accretion disk of a star, and the drag of the accretion disk would cause their orbits to decay.  As a result they would fall into the young star.  Smaller planets, forming later, wouldn’t have the same problem.  So there would seem to be an upper constraint to the mass of planets.  This agrees with statistical studies of exoplanets.

Of course that doesn’t explain why there aren’t more single brown dwarfs in our neighborhood.  It might be that given the dynamics of a stellar nursery brown-dwarf mass objects don’t tend to form.  Instead they tend to coalesce into larger stars early.  We still aren’t sure.

Some things remain a mystery for now.

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