Can the sun emit a solar flare large enough to scorch Mercury and Venus? And if so, could one observe this with the naked eye from Earth?

Going into detail, we must first define the term “solar flare” accurately: A solar flare is a sudden flash of radiation, both visible light and Extra-UV, as X-ray, etc.

The largest solar flare known to us is the so-called Carrington solar flare, named after the scientist Carrington who observed this solar flare in 1851. Even this flare only slightly pushed up the Sun’s ‘normal’ radiant flux and thus had no appreciable direct impact on the planets. Note also that a solar flare originates and develops in the low solar atmosphere, it is max several hundred thousand km above the solar surface. And Mercury orbits the Sun with a radius of 58 million km (the Earth is at 150 million km).

However, a solar flare can indirectly affect planets by causing disturbances in the solar wind. These disturbances can take the form of high-energy particles or so-called ‘coronal mass ejections’. These can cause a lot of damage to technology in space and on Earth.

However, space weather as we know it cannot wipe out an entire planet. But who knows what the Sun is still hiding from us. There is no law of nature that says the Carrington flare is the largest the Sun can produce.