5 ms·
The definition of a brown dwarf is enough mass to cause nuclear fusion of deuterium (roughly 13x Jupiter's mass) but not enough for hydrogen-1 (roughly 75x Jupi
by T-hawk 6y ago
The definition of a brown dwarf is enough mass to cause nuclear fusion of deuterium (roughly 13x Jupiter's mass) but not enough for hydrogen-1 (roughly 75x Jupiter or 0.08x the Sun.)
Planets by definition are anything that doesn't meet the deuterium threshold. Stars meet the hydrogen threshold. "Cold" in this context means relative to hydrogen-fusing stars. It's referring to the object's own energy production, not like measuring the surface temperature of a planet illuminated by a star.
That we are finding "cold" brown dwarfs now is observational bias. We already found the hotter ones because they're more luminous and detectable. "Near the sun" (up to about 250 light-years here) is also observational bias, we just can't detect cold ones any farther.
- throwaway316943 6y agoI wonder what the ratio is of dark bodies to the star systems we can see? Are there vast numbers of invisible worlds hiding between the stars?
- zlynx 6y agoYes, as far as I know, astronomers think so. Those are exoplanets. Planets without any nearby stars. Even in our own solar system we don't know whats out in our Kuiper Belt. Probably nothing too big or we'd be able to measure the gravity effect. But in the Oort cloud, we could have multiple Pluto sized things out there and have no idea.
- kdmccormick 6y agoAn exoplanet is just any planet outisde our own star system. You're thinking of "rogue planets".
- mixmastamyk 6y agoSounds like we've found our dark matter.
- Igelau 6y agoOne of the proposed dark matter explanations is "massive astrophysical compact halo objects" (MACHOs). Brown dwarfs are among some of the possible MACHO candidates.