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The famous picture taken by Event Horizon Telescope (EHT) showed a dark disk in surrounded by luminous bodies. Astrophysicists claimed it is a black hole.

But how sure are we that this is actually a black hole? Could it just be a planet?

Do we have evidence to believe it's a black hole?

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  • $\begingroup$ Have you worked out how big the "disk" is? How could that be a planet!? $\endgroup$
    – ProfRob
    Nov 19, 2020 at 22:42
  • $\begingroup$ Can't planets that big exist? $\endgroup$
    – user279760
    Nov 19, 2020 at 22:45
  • $\begingroup$ I have heard of some planet 40 per cent larger than the size of Jupiter, perhaps there exist larger ones. $\endgroup$
    – user279760
    Nov 19, 2020 at 22:53
  • $\begingroup$ Also, correct me if I am wrong, but I know that the event horizon of a black hole is generally very small. I think about 30 km. $\endgroup$
    – user279760
    Nov 19, 2020 at 22:57
  • $\begingroup$ @Kthamil That is the case for stellar mass black holes. But this one is the size of the solar system. $\endgroup$
    – Javier
    Nov 19, 2020 at 22:59

1 Answer 1

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The "disk" is actually a projection of the photon ring around the black hole (blurred by the finite instrumental resolution). At the 16.9 Mpc distance of M87, its angular diameter of about 42 microarcseconds implies a size of just over $10^{14}$ m. How could this be a planet?

The presence of a supermassive black hole had already been inferred from the rapid motion of gas in the surroundings of the black hole and its mass had been estimated as $(3-7)\times 10^9$ solar masses (e.g. Gebhardt et al. 2011; Walsh et al. 2013).

The Schwarzschild radius of such a black hole is about $2GM/c^2 \simeq 1-2\times 10^{13}$m, the "Schwarzschild diameter" would therefore be $2-4\times 10^{13}$ m and the remaining factor is roughly equal to the $\sqrt{27}/2$ expected from the gravitationally lensed image of the photon ring which is at 1.5 times the Schwarzschild radius.

i.e. The Event Horizon Telescope image is absolutely consistent with the expectations from General Relativity and the presence of a black hole with the mass indicated by the gas dynamics in its vicinity.

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  • $\begingroup$ Note that what we see is not quite the photon ring (see papers by Sam Gralla). $\endgroup$
    – mmeent
    Nov 20, 2020 at 6:59
  • $\begingroup$ @mmeent I'm not sure what your point is and it is confusing to those with little knowledge of the subject. Is Gralla disputing that the EHT images are consistent with a black hole producing a photon ring, or is in fact he just saying that "the ring" is in fact much more complex and the complexity has not yet been observed? $\endgroup$
    – ProfRob
    Nov 20, 2020 at 8:45
  • $\begingroup$ Gralla points out that what we are seeing is mostly just the primary image of the accretion disk, with maybe a small contribution from a secondary image (i.e. light that has been bent by more than 180 degrees to come in our direction) of the disk. But most importantly that the EHT does not have the resolution to distinguish the two. $\endgroup$
    – mmeent
    Nov 20, 2020 at 9:01
  • $\begingroup$ @mmeent The accretion disk is enormous - far, far bigger than the EHT picture. You mean we are seeing photons from the accretion disc that have been bent into the line of sight, but have not "orbited" the black hole? I had always assumed that this was the dominant component and hadn't realised that was even a debatable issue. But this still marks where the photon ring is right - or were everyone's pre-EHT simulations incorrect? $\endgroup$
    – ProfRob
    Nov 20, 2020 at 9:09

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