<p>Brown dwarfs occupy the gap between the highest mass planets and the lowest mass stars. They do not attain internal temperatures high enough to ignite the nuclear burning of hydrogen, but they are <a href="https://en.wikipedia.org/wiki/Lithium_burning" rel="nofollow noreferrer">typically</a> thought to be able to burn lithium.</p>
<p>Starspots are regions on the surface of a star that are visibly darker than the surrounded areas because it is cooler. The current paradigm for understanding the emergence of starspots is magnetic reconnection:</p>
<p>IT is possible that in the future, infrared observations could be used to measure the polarimetry of a brown dwarf's surface, <a href="https://academic.oup.com/pasj/article/53/5/923/1426754" rel="nofollow noreferrer">as it is done for the Sun's surface</a>, to try to look for sunspots, since seeing them visually will be very difficult since brown dwarfs radiate in infrared, predominantly. (And since the starspot is darker region of the surface, it'll be even harder to see visually).</p>
<p><a href="https://science.nasa.gov/science-news/science-at-nasa/2000/ast12jul_1m" rel="nofollow noreferrer">Solar flares from brown dwarfs</a> have been observed for a couple decades, which helped to reinvigorate studies into the internal dynamics of brown dwarfs, along with other discoveries. This field is very active today, for example nearly <a href="https://www.space.com/citizen-scientists-discover-95-brown-dwarfs.html" rel="nofollow noreferrer">a hundred</a> brown dwarfs have been discovered near our Sun (I accidentally said galactic center here earlier, my bad!), and with the prospect for things like <a href="https://arxiv.org/abs/1903.02049" rel="nofollow noreferrer">the LISA mission</a> to observe them near the galactic center makes the future very bright for this field!</p>
<p>Anyway, since solar flares are thought to be a result of starspot activity, observations of brown dwarf solar flares indicate that there might be starspots on the surfaces of brown dwarfs. This would not be totally surprising since brown dwarfs are thought to typically harbor strong magnetic fields: &quot;<a href="https://chandra.harvard.edu/xray_sources/pdf/brown_dwarfs.pdf" rel="nofollow noreferrer">Because</a> it has no strong central nuclear energy source, the interior of a brown dwarf is in a rapid boiling, or convective motion. When combined with the rapid rotation that most brown dwarfs exhibit, convection sets up conditions for the development of a strong, tangled magnetic field near the surface.&quot; This site also has some great content about observed flares from brown dwarfs.</p>
<p>There are high mass brown dwarfs that are <a href="https://iopscience.iop.org/article/10.3847/1538-4357/aa866b" rel="nofollow noreferrer">observed</a> to have <a href="https://www.sciencedaily.com/releases/2018/08/180803103336.htm" rel="nofollow noreferrer">very strong magnetic fields</a>, which, in the magnetic reconnection paradigm, is required for starspots to emerge. This implies that, together with obseravtions of solar flares, it would not be totally surprising to find starspots on brown dwarfs.</p>
<p>Theoretically, simulations of the emergence, evolution, and stability of magnetic fields in brown dwarfs are very uncertain currently - hence the need for more observations! - but these simulations suggest that the strength of the dwarf's magnetic field depends strongly on deuterium burning.</p>
<p>For example, in <a href="https://www.aanda.org/articles/aa/pdf/2010/14/aa14251-10.pdf" rel="nofollow noreferrer">this</a> study they assume the dwarfs are rotating fast, and find that &quot;massive brown dwarfs of about 70 Jupiter masses can have fields of a few kilo-Gauss during the first few hundred Million years.&quot; The fields weaken over time after deuterium burning ceases, but such field strengths are likely sufficient for producing starspots, since the Sun's starspots are on the order of kilo-Gauss as well.</p>
<p>If we observe these features on brown dwarf, then they might need a more general name than &quot;starspot,&quot; perhaps &quot;darkspot&quot; or something would be better? Idk ;)</p>

CC BY-SA 4.0
2021-07-08T14:05:03.067Z
Sunspots on other stars are actually called starspots. https://en.wikipedia.org/wiki/Starspot
John
CC BY-SA 4.0
2021-07-08T14:09:34.750Z
Thanks! I've updated my language.
Daddy Kropotkin
CC BY-SA 4.0
2021-07-08T15:00:36.040Z
Where are you getting this "hundreds of brown dwarfs are being discovered near the galactic center" claim (your linked paper mentions nothing about this)? Given that brown dwarfs are *faint*, finding them 8 kpc away in an extremely crowded field seems *very* unlikely.
Peter Erwin
CC BY-SA 4.0
2021-07-08T15:01:35.707Z
"prospect for things like the LISA mission to observe even more" -- how is LISA supposed to discover brown dwarfs?
Peter Erwin
CC BY-SA 4.0
2021-07-08T15:18:23.887Z
https://www.space.com/citizen-scientists-discover-95-brown-dwarfs.html and https://arxiv.org/abs/1903.02049 I've added these citations.
Daddy Kropotkin
CC BY-SA 4.0
2021-07-08T16:37:05.530Z
@PeterErwin Also, sorry for that mistake: I meant to say that many brown dwarfs were discovered near the Sun, and LISA will discover more near the galactic center. Good catch.
Daddy Kropotkin
CC BY-SA 4.0
2021-07-08T13:36:00.283Z
2021-07-08T15:20:54.430Z
2021-07-08T15:20:54.430Z
Daddy Kropotkin
<p>Do lithium-burning brown dwarfs (65+ Jupiter masses) have starspots too or do they occur only on hydrogen-burning stars?</p>

CC BY-SA 4.0
2021-07-08T14:13:17.683Z
I created the new tag [tag:starspot], just in case somebody feels like retagging some other questions on the topic...
B--rian
CC BY-SA 4.0
2021-07-08T21:45:25.873Z
@B--rian it seems that that has since happened :-)
uhoh
CC BY-SA 4.0
2021-07-09T04:47:40.880Z
I ain't sure if it's a good idea to create two separate tags sun- and starspot. The Sun _is_ a star. I suggest sunspot be made synonymous with starspot, and if they're ones concerning the Sun you can add the tag Sun to your question.
John
CC BY-SA 4.0
2021-07-09T11:34:38.130Z
@John Not a bad idea, but probably to be discussed in astronomy meta where I created a [post on that topic](https://astronomy.meta.stackexchange.com/questions/779/)
B--rian
<brown-dwarf><starspot>
Do brown dwarfs have starspots?
John
# Do brown dwarfs have starspots?

(Asked by: username_0 on 2021-07-08)

Do lithium-burning brown dwarfs (65+ Jupiter masses) have starspots too or do they occur only on hydrogen-burning stars?

**Tags:** brown-dwarf, starspot
> **Comments:**
> **username_1**: I created the new tag [tag:starspot], just in case somebody feels like retagging some other questions on the topic...  
> **username_2**: @B--rian it seems that that has since happened :-)  
> **username_0**: I ain't sure if it's a good idea to create two separate tags sun- and starspot. The Sun _is_ a star. I suggest sunspot be made synonymous with starspot, and if they're ones concerning the Sun you can add the tag Sun to your question.  
> **username_1**: @John Not a bad idea, but probably to be discussed in astronomy meta where I created a [post on that topic](https://astronomy.meta.stackexchange.com/questions/779/)  

---

## 1 answers

### Answer by username_3 on 2021-07-08. Score: 4

Brown dwarfs occupy the gap between the highest mass planets and the lowest mass stars. They do not attain internal temperatures high enough to ignite the nuclear burning of hydrogen, but they are [typically](https://en.wikipedia.org/wiki/Lithium_burning) thought to be able to burn lithium.

Starspots are regions on the surface of a star that are visibly darker than the surrounded areas because it is cooler. The current paradigm for understanding the emergence of starspots is magnetic reconnection:

IT is possible that in the future, infrared observations could be used to measure the polarimetry of a brown dwarf's surface, [as it is done for the Sun's surface](https://academic.oup.com/pasj/article/53/5/923/1426754), to try to look for sunspots, since seeing them visually will be very difficult since brown dwarfs radiate in infrared, predominantly. (And since the starspot is darker region of the surface, it'll be even harder to see visually).

[Solar flares from brown dwarfs](https://science.nasa.gov/science-news/science-at-nasa/2000/ast12jul_1m) have been observed for a couple decades, which helped to reinvigorate studies into the internal dynamics of brown dwarfs, along with other discoveries. This field is very active today, for example nearly [a hundred](https://www.space.com/citizen-scientists-discover-95-brown-dwarfs.html) brown dwarfs have been discovered near our Sun (I accidentally said galactic center here earlier, my bad!), and with the prospect for things like [the LISA mission](https://arxiv.org/abs/1903.02049) to observe them near the galactic center makes the future very bright for this field!

Anyway, since solar flares are thought to be a result of starspot activity, observations of brown dwarf solar flares indicate that there might be starspots on the surfaces of brown dwarfs. This would not be totally surprising since brown dwarfs are thought to typically harbor strong magnetic fields: "[Because](https://chandra.harvard.edu/xray_sources/pdf/brown_dwarfs.pdf) it has no strong central nuclear energy source, the interior of a brown dwarf is in a rapid boiling, or convective motion. When combined with the rapid rotation that most brown dwarfs exhibit, convection sets up conditions for the development of a strong, tangled magnetic field near the surface." This site also has some great content about observed flares from brown dwarfs.

There are high mass brown dwarfs that are [observed](https://iopscience.iop.org/article/10.3847/1538-4357/aa866b) to have [very strong magnetic fields](https://www.sciencedaily.com/releases/2018/08/180803103336.htm), which, in the magnetic reconnection paradigm, is required for starspots to emerge. This implies that, together with obseravtions of solar flares, it would not be totally surprising to find starspots on brown dwarfs.

Theoretically, simulations of the emergence, evolution, and stability of magnetic fields in brown dwarfs are very uncertain currently - hence the need for more observations! - but these simulations suggest that the strength of the dwarf's magnetic field depends strongly on deuterium burning.

For example, in [this](https://www.aanda.org/articles/aa/pdf/2010/14/aa14251-10.pdf) study they assume the dwarfs are rotating fast, and find that "massive brown dwarfs of about 70 Jupiter masses can have fields of a few kilo-Gauss during the first few hundred Million years." The fields weaken over time after deuterium burning ceases, but such field strengths are likely sufficient for producing starspots, since the Sun's starspots are on the order of kilo-Gauss as well.

If we observe these features on brown dwarf, then they might need a more general name than "starspot," perhaps "darkspot" or something would be better? Idk ;)

> **Comments:**
> **username_0**: Sunspots on other stars are actually called starspots. https://en.wikipedia.org/wiki/Starspot  
> **username_3**: Thanks! I've updated my language.  
> **username_4**: Where are you getting this "hundreds of brown dwarfs are being discovered near the galactic center" claim (your linked paper mentions nothing about this)? Given that brown dwarfs are *faint*, finding them 8 kpc away in an extremely crowded field seems *very* unlikely.  
> **username_4**: "prospect for things like the LISA mission to observe even more" -- how is LISA supposed to discover brown dwarfs?  
> **username_3**: https://www.space.com/citizen-scientists-discover-95-brown-dwarfs.html and https://arxiv.org/abs/1903.02049 I've added these citations.  
> **username_3**: @PeterErwin Also, sorry for that mistake: I meant to say that many brown dwarfs were discovered near the Sun, and LISA will discover more near the galactic center. Good catch.  

---

