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What’s the connection between the brown dwarfs being cold and being exoplanets? Are planets definitionally not hot, or does it tell us something about how the d
by hyperpape 6y ago
What’s the connection between the brown dwarfs being cold and being exoplanets? Are planets definitionally not hot, or does it tell us something about how the dwarfs formed?
- T-hawk 6y agoThe 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.
- mynegation 6y agoI think the brown dwarf still fuse deuterium but then the question is - how can it be so cold? And if it is so cold - is it a planet. But then why is it so (relatively) hot? Could be that the new discovered object is (thus far) a missing piece on a spectrum between brown dwarfs and gas giants?
- qayxc 6y agoOne of the difficulties in astronomy is determining the age of astronomical objects. Brown dwarfs and small red dwarfs have about the same temperature in the early stages of their formation. The main difference between the two is the drop in temperature over time. A super cool brown dwarf might just be an old brown dwarf. The (arbitrary) line between planemos and brown dwarfs can be drawn via mass (e.g. no matter the temperature, if it's too small for deuterium fusion, it's not a brown dwarf) and composition (e.g. via testing for lithium), though both criteria are not perfect. A cool brown dwarf can basically just be very old.
- mturmon 6y agoOne place where brown dwarf (BD) and exoplanet science overlap is atmospheric characterization by spectral methods. If you get reasonably well-resolved spectra from the BD, you can invert to obtain information about its atmosphere's composition and density/temperature profile. The BD is going to be more emissive than an exoplanet (and a BD has no light contamination from a nearby host star) so the BD is not an exact analog, but BDs provide a workshop for development of inversion approaches. [e.g.: https://arxiv.org/pdf/1410.6512.pdf https://arxiv.org/pdf/1410.6512.pdf]