No, the calculation I made did not assume constant density. I just used the direct Newtonian formula for surface gravity and plugged in the known mass and radius of the planet. (You could also use that known mass and radius to calculate the average density. But you don't need to do that to calculate the surface gravity.)
> If the planet were 8x mass but with same radius
But we know it isn't. We know the planet's radius is 2.6 times the Earth's radius. That's stated in the article.
No, the calculation I made did not assume constant density. I just used the direct Newtonian formula for surface gravity and plugged in the known mass and radius of the planet. (You could also use that known mass and radius to calculate the average density. But you don't need to do that to calculate the surface gravity.)
> If the planet were 8x mass but with same radius
But we know it isn't. We know the planet's radius is 2.6 times the Earth's radius. That's stated in the article.