[syhprum (OP)]

Planets are considered to be capable of supporting life if they orbit their star within the goldilocks zone where liquid water can exist.
By my reckoning the Earth lies outside this zone with a black body temperature of 250°K where water can only exist in a solid form.
Would alien astronomers write it off ?

[Bored chemist]

Not if they had any sense.
It would be possible for a planet to be tidally locked to the star in which case the side nearest to the star would be much hotter than the average and the far side much cooler.
So the "goldilocks" range is rather larger than you would predict from a simple average black body temperature.
They would probably also allow quite a lot of leeway because it's very difficult to accurately measure the distance of an exoplanet from its star.

Who knows?  They might even consider the effect of an atmosphere.

[alancalverd]

By my reckoning the Earth lies outside this zone with a black body temperature of 250°K where water can only exist in a solid form.

Then your reckoning is wrong.

It's true that there isn't much life in those parts of the world where the surface temperature is 240K or less, but there's obviously a lot of surface covered with liquid water.

[evan_au]

As I understand it, Venus is considered at the inner edge of the Sun's habitable zone, while Mars is considered at the outer edge. So an Earth astronomer would not write it off.

• Venus has a runaway greenhouse effect, which makes it far too hot
• Mars has almost no atmosphere, and very little greenhouse effect, so it is too cold for more than midsummer thaws in the permafrost.
• If Mars had an atmosphere more similar to Venus, it might have year-round liquid water

There are many reasons that liquid water can exist, apart from distance from the star and temperature of the star. But when all you have is the orbit of a planet and its approximate size (eg from the Kepler mission), you would tend to consider the factors that you know, rather than what you don't know.

Some of the other factors have to do with:

• The mass of the planet. If it is too small, it won't hold enough atmosphere to produce liquid water. So the Moon is at the same average distance from the Sun, but has no atmosphere, and does not have liquid water at the surface.
• The greenhouse effect - the atmospheric composition.
• The presence of X-Ray flares (which would tend to break apart organic chemicals)
• The lifetime of a star (10 million years is considered too short).
• Moons like Enceledus have subsurface water due to tidal heating, despite being outside the habitable zone.

Of course, here we are only discussing "Life as we know it" - organic molecules floating in a water broth. Who knows what "Life, but not as we know it" would look like (and what it would find to be too hot or too cold)? I imagine that for quantum computers, even Pluto might be a tad on the warm side!