How hard will it be to measure Planet Nine?

First, find it, which is hard. Second, see if it occludes stars, which is really, really hard

Planet 9 will be easier to find if we know what we're looking for, so a French astronomer has set himself the task of trying to wrap the enigma in some parameters.

When the idea of a ninth planet (using the guidelines that excluded Pluto from planet status) was mooted last year, the CalTech group that ran the orbital maths reckoned it would mass about ten times more than Earth.

That estimate came with some uncertainty; it could be as big as 30 times Earth's mass. That's what Jean Schneider of Paris University looks at here.

Since we haven't seen the planet yet – a crowd effort is now underway – Schneider can't offer hard numbers, but stresses the importance of being able to run the calculations when we do get an observation.

Direct imaging, he says, will deliver the object's spectra, but “cannot provide the size and mass of the planet. Without a value for the radius, one cannot infer the planet albedo from its brightness, while this parameter is essential to constrain physical characteristics of the planet surface,” the paper says.

Schneider proposes different mechanisms for each of radius and mass: when we spot the planet, we watch it occlude stars to estimate its radius; and when it passes a star, estimate its mass from microlensing (the effect of gravity on passing light, first predicted by Einstein).

How many stars might Planet Nine occlude in any given period? That's problematic: a ten-year survey of the region the planet's probably traversing would only yield a 20 per cent likelihood of an occlusion.

That estimate obviously improves if we get an observation of Planet Nine.

A successful observation would also give 30-metre telescopes a chance to get an observation of any atmosphere the planet might have, along with things like rings and satellites. ®