So five times the mass of Earth. Assuming the same density as Earth, how large would this make it and what would be the surface gravity?
Same density as Earth and five times the mass yields a radius around 10,900 km.
Surface gravity about 1.7 times Earth’s.
Ugh! It’d be even tougher to get out of bed in the morning!
Well, it would be difficult to adjust to and we’d experience a lot more medical issues, but that isn’t insurmountable.
Thanks Xema.
Hmmm… A planet not too far away from here (as interstellar distances go). Higher gravity than Earth’s. A red sun. Even the fact that it’s far older than Earth (and its people must therefore have a very ancient and advanced civilization, natch.)
Kapteyn b is Krypton!
I call the lakefront properties.
Question:
Other than receiving a radio signal (e.g “Greetings from Kapteyn b! This place is here, wish you were beautiful.”), what would be required to establish (with, say, 95% probability) that this planet supports life? Could this be done without sending a probe there?
Only 1.7? Luxury!
This planet recently found has 17 times Earth’s mass, and 2.3 times its diameter (so about 1.4 times the Earth’s density). It would have about 3.2 times the Earth’s gravity at the surface.
The presence of free oxygen (O2) in the atmosphere would be pretty strong evidence. It’s an extremely unlikely molecule in a universe that is generally strongly reducing. In principle you could detect it by spectroscopy, but the light from the star would make it hard, you’d need a very sensitive and very discriminating detector. It’s not that far outside present tech, though. I could see it within the half century.
I am fascinated by how the history, climate and culture of a civilization would be affected by the planet’s sheer size. Were Earth of that size, it would probably have defeated most if not all the early explorers at sea, delayed the development of technology, or even given rise to multiple modern civilizations entirely unaware of one another’s existance.
Maybe this topic deserves a thread of its own. I think I will start one! 
Assume it has a radius of 2.31 Earths, (the lower bound) and a mass of 19 Earths, (the upper bound), then it has a surface gravity of 3,58 gees and an escape velocity of 32.1km/s.
The atmosphere on a planet like that would retain hydrogen, even at the planet’s likely average temperature of 485K. Looking at the density, at 8.5g/cm3 this is more dense than Earth, but actually less dense than a ball of Earth-like material at that mass (compression would raise the density of such a ball to 9.1g/cm3). So this planet is made of material less dense than Earth’s composition.
What does this mean? I think it means it has a moderately thick atmosphere and probably a layer of hot or supercritical water; a hot-water world, in fact.
Alternately it might have no metallic core and be predominately silicates, with a hot, relatively dry atmosphere - but this seems less likely. Any other options? A massive carbon world with no metal core perhaps? Carbon worlds are expected to have no surface water at all.
Note that it is quite difficult to pin down the exact characteristics of a planet, even when you know the radius and mass (not forgetting the error bars).
Edit - The planet I was talking about is Kepler 10c, by the way; the one mentioned by Zenbeam. Kapteyn b is smaller and has much lower gravity, but still would be uncomfortable for humans.
Why would presence of Oxygen be required to support life?
I think it’s the other way round: oxygen is a by-product of life.
It’s not. However, it’s presence would be a strong indicator of life since nothing else is likely to produce unbound oxygen in large amounts in an atmosphere.
Watch out for falling kryptonite! And falling Kryptonians!
This star has a very low metallicity, much lower than the sun. It’s planets are also likely to be low in elements heavier than helium. So I’d expect a planet with an extensive atmosphere, something more like Uranus or Neptune than a rocky planet.
could work as a gym planet, high gravity helps build muscles
On a water-world, with no easily accessible crust, the water itself could produce free oxygen by photolysis alone. On our world photolysis does not produce enough oxygen to be detectable, but on a waterworld with an ice mantle there would be no process to remove it except dissolving it in the water itself.
If we ever discover any oxygen-rich worlds we will have to discriminate between worlds with biotic oxygen and others with abiotic oxygen like waterworlds.
This article gives a reality check on Kapteyn b’s habitability.
News articles often run away with a single estimate for the characteristics of such worlds and ignore the error-bars.