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Frigid Brown Dwarf Found Only 7.2 Light-Years Away
An anonymous reader writes "Astronomer Kevin Luhman just found the 7th closest star to the sun. It's a mere 7.2 light-years away, discovered using NASA's Spitzer and WISE telescopes. How could it exist so close for so long without us knowing? It's a brown dwarf — barely a star at all. 'Brown dwarfs are star-like objects that are more massive than planets, but not quite massive enough to ignite sustained fusion in their cores. Hydrogen fusion is what powers the Sun, and makes it hot; it's the mighty pressure of the Sun's core that makes that happen. Brown dwarfs don't have the oomph needed to keep that going.' This small almost-star is downright chilly at around 225-260 Kelvin. That's -48 to -13 C (or -54 to 9 F). As Phil Plait points out, that's not much different from the temperature in the freezer in your kitchen. He adds, 'It implies this object is very old, too, because it would've been a few thousands degrees when it formed, and would take at least a billion years to cool down to its current chilly temperature. It's hard to determine how old it actually is, but it's most likely 1-10 billion years old. It has a very low mass, too, probably between 3 and 10 times the mass of Jupiter. That's pretty lightweight even for a brown dwarf. And here's another amazing thing about it: It might be a planet. What I mean is, it may have formed around a star like a planet does, then got ejected by gravitational interactions with other planets.'"
That would make it a rogue planet.
My personal experiences, have been very much, the opposite.
That's part of the MACHO hypothesis regarding dark matter. We could explain away dark matter with trillions of brown dwarfs but that doesn't seem satisfactory for astronomers and cosmologists. For some reason (big bang and cosmic background calculations etc.) we know think that baryonic (regular matter) is only about 4% of the universe's amount of mass-energy and about 25% of non-baryonic dark matter is needed to make it all fit. Not enough baryonic matter to have enough brown dwarfs playing the role of dark matter in/around galaxies.
Kudos for writing 225-260 Kelvin and not 'degree Kelvin' or 'Kelvins' in the summary. Slate f'ed up though. They wrote 'Kelvins'. I have seen even reputable scientific writings using degrees prefix with Kelvin. It's very disheartening to see that even some scientists don't get it that you don't use degrees when talking about absolute temperature.
The Sun is ~1 AU away, the three Alpha Centauri stars are 4.24 to 4.37 l.y. away, Bernard's star is just under 6 l.y. away, and the two Luhman 16 brown dwarfs are just over 6.5 l.y. away. The position of Earth in its orbit is not enough to make any of those exceed 7.2 light years. this is either the 8th closest star that we know of, or it is not considered a star at all if you don't want to count brown dwarfs (or might be pushing the lower limit of what is a brown dwarf).
It is not just the comological level, but searches have been done for small objects roaming around the galaxy by looking for micro lensing events and occlusion events where rogue planets or objects move in front of other stars. If a large portion of the missing mass needed for the galaxy rotation curve were these planets, you can work out the chances of such objects passing in front of the stars being observed for such effects, and find that we should have seen way more than was actually observed.