Distant and dim
Sounds like my school report, circa 1980 something
Distant and dim, the brown dwarf WISE J085510.83-071442.5 (snappy name, there) could well have passed unnoticed since it's not large enough to ignite nuclear fuel to radiate visible light. However, NASA's WISE (Wide-field Infrared Survey Explorer) has spotted it, and further work by the Spitzer Space Telescope yielded a …
For a planet, gravity also releases gravitational potential energy and the gases are mildly radioactive. These are certainly reckoned to be the important factors for Earth (http://en.wikipedia.org/wiki/Earth%27s_internal_heat_budget) and are presumably even more important for a gas giant because gas is a rather better conductor of heat (via convection) than solid rock.
There's a third possible heat source: a matter phase change in the core. Inside the earth, a solid(ish) iron(ish) core is believed to be crystallizing as an on-going process, and releasing heat as it does so. Heaven only knows what corresponding process might be going on in the core of a brown dwarf.
Before things like radioactivity were known about, Lord Kelvin estimated the age of the Earth, based upon the cooling of the inside of the planet, after having been heated up by the compression of formation, at 100 million years. We now know this was a little bit out (by around 1.5 orders of magnitude) - and that heat from radioactive decay plays a significant role.
Other factors affecting celestial bodies can include gravitational effects from companion bodies (for example Jupiter's moon Io is very volcanic because it has an elliptical orbit around its parent planet which squishes and squashes it).
At 3-10 times the size of Jupiter, I imagine it is a gas giant.
Lord Kelvin estimated that Earth's "heat of formation" would have taken about 100Mya to wear off. A rocky planet 3-10 times the size of Jupiter would have a surface area to volume ratio many times less than Earth and so would presumably cool more slowly. I think the surface would still be molten.
From what I am given to understand, this body, with a mass some 3 - 10 times that of Jupiter, would, in fact, possess a volume or «size» less than that of the latter planet. Choosing to call it a brown dwarf rather than a Jovian body due to what is believed to be the greater frequency of the former in the galaxy seems odd to me ; unless there is something in its spectral distribution which indicates otherwise, as a body of less than 13 Jupiter masses, it would hardly seem to qualify as a brown dwarf....
Is Jovian body an actual thing? I have heard about Jovian planet, but this is clearly not a planet. We could classify it as a rouge planet, but should the past history of a body really determine it's class instead of it's actual current situation? And it wouldn't make it a Jovian planet anyway, since jovian planets are planets while rouge planets are not. Like dwarf planets are not planets.
As I understand it, in astronomy they make up terms as more and more information come to light, and they also need to revisit earlier definitions to see if they are sensible and useful in describing the universe as it actual turns out to be, not how it was perceived ten or fifty years ago. Brown dwarfs seem to be one of those they are debating. Personally I am in favour of too low mass to sustain hydrogen-1 fusion, but high enough that have or had some type of fusion going at some point.
But then we need a new word for all those minor systems that is probably scattered all over the place. May I coin black dwarf?
Mine, that is.
"At 7.2 light years distant, it's nearly twice as far away as our nearest neighbour Alpha Centauri,"
No it isn't. This mistake is commonly repeated (I've seen it on the Reg before). Alpha Centauri isn't even a star, it is a binary system. The closest star is Proxima Centauri which may or may not be gravitationally bound to the Alpha Centauri binary.
small enough that there's a chance it's a gas giant planet similar to Jupiter that's been ejected from its system. However, since brown dwarfs are far more common, that's the explanation preferred by Luhman
If they're so hard to detect that it's taken us this long to detect it, how do they know that brown drawfs are far more common, there might be twice as many gas giant planets as brown drawfs, we just can't see them.
About 25 years ago, in a lecture that had strayed onto the subject of Dark Matter, our prof suggested that the true explanation might not be the various forms of exotic, supersymmetric or wacko particles that were then in vogue. It might just be a lot of bog-standard matter, too cool to see. I think he was semi-serious. This is the sort of stuff he had in mind, but I don't know how common these objects would have to be in order to make the numbers work.
"This is the sort of stuff he had in mind, but I don't know how common these objects would have to be in order to make the numbers work."
"Only about 4% of the total energy density in the universe (as inferred from gravitational effects) can be seen directly. About 22% is thought to be composed of dark matter. The remaining 74% is thought to consist of Dark energy, an even stranger component, distributed diffusely in space."
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