The Line is Blurring

You’ve probably heard the term “brown dwarf”.  It’s an object in space that is considered too cool to be a star and too hot to be a planet. Some new observations of these odd little objects answers the question “How cold does one of these things have to be before it’s not considered a star anymore?”  NASA’s WISE telescope sent back data about some intriguing brown dwarfs found in the region of the Orion Nebula, and astronomers studied them using Spitzer Space Telescope as well, to determine their temperatures. It turns out they’re only 250-350 degrees F at their surfaces. That’s warm enough to bake a cake!

Now, we know that “normal” stars spend much of their lives being very hot, with temps soaring to tens of thousands or millions of degrees. That’s a far cry from the few-hundred-degree surface temperature of a brown dwarf, which is typically  only a few times the mass of Jupiter. These objects don’t have fusion going on in their cores, as “normal” stars do. They are warmed by gravitational contraction—that is, by the object shrinking in on itself, compressing its interior—which causes heating. Brown dwarfs form on their own in star birth regions, but if you happened to spot one orbiting another star and didn’t know much more about it, you might call it a planet.

So, how do you figure out if something this cool is a super Jupiter (for example) or a brown dwarf?  It’s challenging because such an object is not bright like a star. It’s small and radiates a great deal of its light in the infrared. After

Astronomers selected the dwarfs found by WISE in Orion and used the infrared-sensitive Spitzer Space Telescope to study their targets over time to determine their parallax. That helped them determine a distance of between 20 and 50 light-years from us. The results were then used to calculate temperatures of the objects. They[re still hot by Earth surface temperature standards, but are they hot enough to be brown dwarfs or in the right temperature range to be supergiant planets? It’s a good question, and the answer is still not clear.

Knowing the temperatures doesn’t explain everything that astronomers see when they study these objects. There are other properties that seem to make these cool objects more like planets. It’s possible that some atmospheric processes are driving interesting chemical reactions on the object’s “surface”. There’s also some evidence for disappearing alkali elements that are likely getting incorporated into noxious clouds hovering around the objects. Noxious clouds almost seem to imply a planet-like atmospheric characteristic.

For now, the answer is that these objects are very close to the dividing line between what we define as  a star or as a hot giant planet. There’s plenty of data yet to study from these observations with the WISE mission. It’s quite possible something else will turn up that further refines the definition between the two types of objects. If you’re interested in learning more, check out Science Express for a paper called “Distances, Luminosities, and Temperatures of the Coldest Known Substellar Objects” by Dupuy and Kraus.