# How certain we are that the body shown in the picture of EHT is a black hole?

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?

• Have you worked out how big the "disk" is? How could that be a planet!? Nov 19, 2020 at 22:42
• Can't planets that big exist?
– user279760
Nov 19, 2020 at 22:45
• I have heard of some planet 40 per cent larger than the size of Jupiter, perhaps there exist larger ones.
– user279760
Nov 19, 2020 at 22:53
• 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.
– user279760
Nov 19, 2020 at 22:57
• @Kthamil That is the case for stellar mass black holes. But this one is the size of the solar system. Nov 19, 2020 at 22:59

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.