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Some of the ultra-hot Jupiters (WASP-33b, KELT-9b) have dayside surface temperatures similar to low-mass stars. Would they be capable of producing flares? To be specific, I am asking about the possibility of flaring on the planet itself, rather than the planet inducing flares on the star.

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  • $\begingroup$ To get flares, you need nonhomogeneity somewhere. EIther a large atmospheric storm - analogous to tornado -- or strong magnetic fields, as happens in the sun. $\endgroup$ – Carl Witthoft Nov 19 '18 at 20:15
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Flares in space are known for coming from stars. Sunlike stars, red dwarfs, etc, almost any kind of star has flares.

Hot Jupiters can be the size of Jupiter,(hence the name). They can be bigger; however, if they get too big, they become brown dwarfs. This means that hot Jupiters are too small to have flares.

In addition, many hot Jupiters have unstable atmospheres. They are so close to their parent stars, that they lose their atmospheres. No atmosphere, no flares.

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    $\begingroup$ Just because hot Jupiters are smaller than brown dwarfs it does not necessarily follow that they are too small to have flares, unless you can show that the mechanism is mass-dependent. Incidentally flares have been observed from brown dwarfs (e.g. LP 944-20) so being smaller than a star is apparently not a blocker. Furthermore the estimated lifetimes of hot Jupiters in the face of evaporation tend to be in the 10^10–10^11 yr scale, which means that for G-type and earlier host stars they will likely last the entire main sequence lifetime. Hot Neptunes would be another matter obviously. $\endgroup$ – antispinwards Jan 27 '19 at 20:03
  • $\begingroup$ Yes, but hot Jupiters are planets, while brown dwarfs aren't. Can gas giants produce flares? $\endgroup$ – user35971 Jan 28 '19 at 0:52
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    $\begingroup$ that's a substantial part of the original question. You assert they cannot because they are not stars, but semantics probably doesn't have much bearing on the physical mechanisms involved. $\endgroup$ – antispinwards Jan 28 '19 at 8:26

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