Cool that we're finding interesting planets though! An actually human-habitable planet would, I think, have a significant cultural impact here on Earth.
Volume scales cubically, yes. And therefore, also does the gravitational pull by the object at the same distance to the center. But due to the larger radius, you are also further away. Gravity respects the inverse square law. This means that the surface gravity of an object that maintains the same density increases linearly with the object's size.
In other words, you would have 1.36g on the planet's surface, not 2.5g. But that's assuming same density. Even within our own solar system this is not true: Mars's surface gravity is 0.38 of earth's gravity but 0.53 of earth's radius. The gravity is lower than it would be if Mars had the same density as earth.
g ~ density x radius [0]
I wonder what happens if a human child grows up in a 2.5g environment. I imagine they grow up to be short, stocky, and extremely muscular.
Adaptation probably plays a big role here. If you give a 150lb person a suit that weighs 225 lbs and tell them to wear it all the time, they probably give up after a few days. But if you tell a 150lb person to eat more until they're 375lbs, their body has time to adapt.
Since the new planet is 100 light years away, even at the speed of light we're talking about a 100-year journey minimum. Maybe the generation ship used to make the journey could use artificial gravity to smoothly transition from 1g to 2.5g over the course of 100 years. Loading the ship with people who are stocky & muscular is another option.
I don't think living in enhanced gravity needs to be terrible. Astronauts have to exercise more in order to maintain fitness in low-gravity environments. It appears to follow that if you're living in an enhanced-gravity environment, you can get away with exercising a lot less and still maintain fitness :-)
Red dwarfs are known for flares that would destroy the atmosphere of any nearby planet.
"Although this planet orbits very close to its star, at a distance about 10 times shorter than that of Mercury around our Sun, the amount of stellar irradiation it receives is still low... the star LP 890-9 is about 6.5 times smaller than the Sun and has a surface temperature half that of our star."
Live on the other side? or a narrow habitable band between the 2 hemispheres
You could pick a planet where the noonday sun is not too hot, but then the ice would be even further away.
Noplace on Earth is so dry that people are tempted to collect ice from Antarctica, even with shipping today cheaper than it has ever been.
There's a table in that link of how much reaction mass is needed to reach orbit. At 1.5g it requires about 500% as much rocket as Earth. At 2.5g the number is ... much larger.
Muscles and bone density will need to increase, but we do that as part of adaptation, evolution will favor the trait long term but regular use will already compensate in part or totally the need.
Possibly ankles and knees will need to get stronger as well
Apparently surface gravity is proportional to mass/radius^2 [0]
Assuming constant density as per the grandparent, the mass increases with the cube of the radius.
Which simplifies to surface gravity being proportional to the radius of the planet.