Jupiter vs. Brown Dwarfs
(This seems to get a lot of views, more than any other post. So I’ve edited it a bit, to tidy up the figure and the text a little bit. The comparison is now a T9, not a T7/8 as previous.)
Sometimes a picture is worth a thousand words. This is definitely true in astronomy, where pictures called ‘spectra’ are pretty much the basis of every useful thing we do.
(The image links to a larger version.)
You are looking at a spectrum of Jupiter (the thicker, black line) and a spectrum of ULAS J1335 (red line), a spectral type T9 brown dwarf (source: Burningham et al. 2008).
The spectra are in the so-called ‘near infra-red’. The wavelength is the x-axis of this plot; the plot cuts off at ~0.98 microns, so you can’t actually see the human optical range on here. (If I remember correctly, it’s about 0.4 – 0.77 microns.)
I’ve also marked on the YJHK near-IR photometric bands. This gives you an idea what we mean when we talk about ‘J-band peaks’ or ‘K-band magnitudes’. These are the red lines and text.
The y-axis is something called ‘normalised flux’. This is a comparison technique; it’s the flux as a fraction of that at a certain point, in this case 1.27 microns. By ‘normalising’ spectra to the same point, it’s possible to plot them against each other for comparison purposes. It’s just easier to look at that way – you can see the relative differences, rather than having (say) a hardly-visible squashed line at the bottom of the plot and one sat right at the top. And for spectroscopy, we’re often more concerned about the morphology then we are about specific absolute fluxes. (If you look closely, you’ll see that the peak-to-peak normalisation isn’t quite spot on – but this will do as an example.)
Anyway, plotting-techniques aside, you can see that Jupiter is a lot ‘brighter’ at short wavelengths. A lot of this is reflected sunlight; Jupiter is close to a bright star, whereas J1335 isn’t. (If the optical region was shown, you would be able to see that reflected sunlight completely swamps the NIR emission.) However, at longer wavelengths, they start looking a bit similar. As you can see, Jupiter’s J- and H-band peaks line up somewhat with the brown dwarf’s.
This isn’t coincidence.
Brown dwarfs emit where they do because they’re glowing from internal heat. The big gaps between the band-peaks, believe it or not, are actually massive molecular absorption features. The gulf between the J and H peaks, for instance, is caused by water and methane. (In fact these two are the culprits of most of the gaps between peaks in the brown dwarf spectrum.) Jupiter’s atmosphere also contains water and methane,so we see some similar features. Also, like brown dwarfs, Jupiter radiates more heat then it receives from the Sun. (This dominates the spectrum at longer wavelengths.)
One big, notable difference is that Jupiter doesn’t have a distinct K-band peak. There is a similar feature between about 1.8 and 1.95 microns, although this may be down to ammonia rather than methane or water. Ammonia is not really seen in T-dwarfs, beyond a couple of tentative detections. This is because their atmospheres are too hot to support it – and here we come to the next really big difference between J1335 and Jupiter. Temperature. J1335 is at something like ~600 K, whereas Jupiter is at ~160K, which obviously makes quite a difference.
Nonetheless, this gives you an idea why people are so keen on comparing Jupiter and T-dwarfs.