A very strange object called WISE 0855−0714 lies just 7.2 light-years from the earth. Discovered by the Wide-field Infrared Survey Explorer (WISE), it is nominally one of those not-quite-planets-not-quite-stars known as a brown dwarf.

Because they are so much smaller and cooler than stars, brown dwarfs appear red and faint. But astronomer Kevin Luhman noticed that WISE J085510.83-071442.5 was very red and very faint…partly because it is small—perhaps only 2 to 10 times the mass of Jupiter—and partly because it is so cold.

It’s temperature, Luhman found, is only about 9° F (-13° C). That’s well below the freezing point of water. In other words, the brown dwarf is literally ice cold. The fact that it is so cold is a clue to its age. If it started out at a few thousand degrees it would have taken somewhere between 1 and 10 billion years to have cooled to its present temperature.

It may well be that instead of being a brown dwarf, this object may in reality be one of the half dozen or so mysterious rogue planets, the first of which was first observed in 2010. These are worlds that, as the result of some catastrophe, were ejected from their home systems and now orbit the galaxy directly, as our sun does. Luhman found that WISE J085510.83-071442.5 is moving in relation to the background of stars. Most stars do, but this one appeared to be really traveling. This speed combined with a measurement of the body’s parallax (the amount by which it appeared to shift against the background stars as the earth moved from one side of its orbit to the other) enabled Luhman to figure its distance. WISE J085510.83-071442.5 turned out to be the 7th closest star to our own. Observations of WISE J085510.83-071442.5 by the WISE and Spitzer telescopes reveal the rapid motion of the brown dwarf over the past four years.

Brown Dwarfs Brown dwarfs are objects which are too large to be called planets and too small to be stars. They have masses that range between twice the mass of Jupiter and the lower mass limit for nuclear reactions (0.08 times the mass of our sun). Brown dwarfs are thought to form in the same way that stars do – from a collapsing cloud of gas and dust. However, as the cloud collapses, it does not form an object which is dense enough at its core to trigger nuclear fusion. The conversion of hydrogen into helium by nuclear fusion is what fuels a star and causes it to shine. Brown dwarfs were only a theoretical concept until they were first discovered in 1995. It is now thought that there might be as many brown dwarfs as there are stars.

Brown dwarfs are very dim and cool compared with stars. The best hope for finding brown dwarfs is in using infrared telescopes, which can detect the heat from these objects even though they are too cool to radiate visible light. Many brown dwarfs have also been discovered embedded in large clouds of gas and dust. Since infrared radiation can penetrate through the dusty regions of space, brown dwarfs can be discovered by infrared telescopes, even deep within thick clouds.

Brown Dwarfs were only a theoretical concept when the Spitzer Space Telescope was first proposed. Since the mid-1990s, various infrared telescopes and surveys have identified a few hundred of these objects. Spitzer will devote much of its time to the discovery and characterization of brown dwarfs. It is expected that Spitzer will study thousands of these objects, including those only slightly larger than Jupiter. This will provide astronomers with enough data on brown dwarfs for good quality statisical studies.