It’s tantalizing to speculate that there might be a brown dwarf system nearer to us than the Alpha Centauri stars. The odds seem long, but the discovery of a pair of brown dwarfs that are each no more than a millionth as bright as the Sun makes for exciting reading. The objects were originally cataloged by the Two Micron All Sky Survey (2MASS) as a single brown dwarf identified as 2MASS J09393548-2448279, but Adam Burgasser (Massachusetts Institute of Technology) has been able to show that the ‘object’ is actually a pair of the faint dwarfs.
Here the Spitzer Space Telescope was the instrument of choice, showing that 2M 0939’s brightness was twice what would have been expected from its temperature, which was determined to be in the range of 565 to 635 Kelvin (560 to 680 degrees Fahrenheit). The implication was that this is a brown dwarf binary, two dwarfs each with a mass some thirty to forty times that of Jupiter. And while the objects are a million times fainter than the Sun in total light, they’re a billion times fainter in visible light alone.
2M 0939 becomes the fifth closest known brown dwarf, but what gets my attention is Burgasser’s comment that this discovery may be the tip of the iceberg. In an MIT news release, he puts it this way:
“These brown dwarfs are the lowest power stellar light bulbs in the sky that we know of. In this regime [of faintness] we expect to find the bulk of the brown dwarfs that have formed over the lifetime of the galaxy. So in that sense these objects are the first of these ‘most common’ brown dwarfs, which haven’t been found yet because they are simply really faint.”
Faint enough for there to be examples closer to Earth than this one, and indeed closer than the Centauri stars? It seems unlikely, but it’s a stimulating thought, and the work on 2M 0939 shows just how difficult it may be to be sure. After all, Burgasser’s team spent three years studying the object with data from the Anglo-Australian Observatory before they could come up with a definitive read on its distance. That work was crucial, when combined with the Spitzer infrared observations, in measuring the object’s brightness, and thus deducing its true nature as a binary from the contrast with observed temperatures.
The vastness of interstellar space naturally staggers the imagination, but it’s striking that as we learn more, we realize how many objects are out there that we know little about. A true stellar census of the Milky Way, for example, would need to include brown dwarfs, but we aren’t prepared at this point to say how common they are. We do know that the larger and hotter M-dwarfs comprise more than 70 percent of all stars in the galaxy, and we also believe there must be planets ejected from their solar systems wandering through the interstellar depths. Get us the technology to make it to the Oort Cloud and we may find that the next truly interesting destination isn’t necessarily four light years away.
The paper is Burgasser et al., “2MASS J09393548−2448279: The Coldest and Least Luminous Brown Dwarf Binary Known?” Astrophysical Journal Letters 689 (December 10 2008), pp. L53–L56 (abstract).