Wikipedia:Reference desk/Archives/Science/2023 October 13

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October 13

Imaging Proxima Centauri b

How far should Proxima b be from its red dwarf host to be observed and resolved in image with the same accuracy as known imaging techniques used to study the young gas giants like 2M1207b, if (1) it has an atmospheric temperature comparable to surface temperature of Venus and a Neptune-like radius and (2) it has an average atmospheric temperature of 298 K (≈ 25 degree C) and a Jupiter-like radius (of course both in the IR range)? Although I knew that the values for orbital period and semi-major axis of Proxima b have been measured very well since its discovery in 2016 (so they can't be another number), I asked this double-case question as I was trying to figure out how difficult is it to detect and study the exoplanets using direct imaging technique. 2402:800:63AD:CB66:5951:F4CF:25D:8D5F (talk) 15:32, 13 October 2023 (UTC)

Somehow I suspect this depends mainly on the angular resolution of the telescope. The article 2M1207b gives a wide range for the semi-major axis, which corresponds to an angular distance in the range of 1/46,000th to 1/450,000th of a radian. For Proxima Centauri b I find 1/5,500,000th of a radian, so requiring a higher resolution by one to two orders of magnitude.  --Lambiam 18:11, 13 October 2023 (UTC)

A really useful answer, thanks (I didn't even think that the angular resolution of the telescope does matter in the direct imaging procedure). 2402:800:63AD:CB66:5951:F4CF:25D:8D5F (talk) 18:26, 13 October 2023 (UTC)

Our best instrument for direct imaging will be the Extremely Large Telescope in a few years, using coronagraphy to suppress the glare of the star. The angular distance of Prox b from the star is about 4 milliarcseconds, which corresponds to the pixel scale of the MICADO near-infrared instrument. The best angular resolution of the ELT at the bluest wavelengths (say 1 micron) is a bit more than 5 milliarcseconds, which means that star and planet cannot be separated. The coronagraphs have inner working angles (i.e. the minimum distance so the planet can be detected) that are a bit larger than that, so that will not work. --Wrongfilter (talk) 21:47, 13 October 2023 (UTC)

Is the James Webb space telescope incapable of using coronagraphy? Is it not a valid tool for investigating exoplanets in the way OP was looking for? 192.77.12.11 (talk) 02:59, 17 October 2023 (UTC)

JWST has worse angular resolution than the ELT (due to the smaller mirror size). It has a coronagraph but the inner working angle (the smallest angular separation from the star where a planet could be detected) is given here as 0.2 to 0.4 arcsec, which is a factor of 100 larger than what would be required for Prox Cen b. --Wrongfilter (talk) 08:27, 17 October 2023 (UTC)

Actually the main problem with direct imaging of planets is quite often not the angular resolution of the telescope but the glow of the star. Even a very weak scattering inside the telescope's optics or by the Earth's atmosphere creates a background illumination in which a dim object like a planet can be easily lost. Coronographs can reduce but not completely eliminate this problem. As to 2M1207b, this planet is a young, warm and therefore very bright object in the infra-red. Such planets are much easier to observe than old planets around red dwarfs. Ruslik_Zero 16:25, 14 October 2023 (UTC)