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Comment by UI_at_80x24 | Hacker News Reader

225K = -48C

So not exactly cozy. I'm not sure what the other measurements mean.

What are the numbers for the temperature Earth would have without any greenhouse gases? The right atmosphere might make it work.

-19°C

Huh that's surprisingly warm!

Keep in mind these calcs discard the the greenhouse effect, but not the albedo, which reality can’t happen since the same factor is causing both

I don't either, but if its radius is the size of Jupiter, I imagine the gravity's a real buzz kill.

For that range of mass values, the surface gravity would be relatively close to that of earth, even lower at 90x.

Is it Jupiter that's unusually dense or this planet that's unusually light? Related, any idea what the feasible range of densities is for a planet of a given size? I always assumed something as large as Jupiter would be impossible for a human to set foot on due to being crushed.

A ball of foamed rock the size of a planet is an amusing thought but I have to assume that's physically impossible.

Jupiter is a gas giant. It's near the threshold where adding more mass to it makes it smaller, not larger, as the added gravitational pull would make it denser.

But what would make it larger is if it was warmer. The radius of a planet like jupiter scales to the 1.6th power of it's temperature. Jupiter is actually slowly shrinking in size as the primordial heat of its formation is radiated away.

Jupiter has no surface to set foot on, unless you count the hypothetical earth size rock inside all the gas. What would happen is that you would sink and get crushed long before you got to the rock.

Pedantry. (Well I suppose it's at least a valid point that without a solid surface there probably isn't anything for a human to want to bother visiting in the first place.)

This one presumably doesn't have a surface either, yet an earlier commenter spoke of surface gravity. Doesn't it surprise you in the slightest that two planets of the same size could have gravity that differs by such a wide margin?

I worked in the area a long time ago, so as it happens, no, but I can see how it's unintuitive!

Surface gravity in this case would be the visible radius - the same way we talk about the surface gravity of the sun, and mean the photosphere.

If those units mean what I think they mean, then the planet seems to be far less dense than Jupiter, which is a little more then 300x Earth mass.

It would be warmer than Mars I think, which is c. 210K. Still frozen, barren and hostile, but slightly warmer than Mars.

Depends on its atmosphere, if it has one

Well, heating is still easier than cooling - if there are suitable resources at hand, heated habitats could be easy to setup.

RJup is the radius of Jupiter. 1 MEarth is equal to one million times the mass of the Earth. I’m not sure about RV limits.

> not sure about RV limits

Radial velocity, how quickly a planet moves “back and forth towards an observer” as it revolved about its star [1]. Its amplitude suggests planetary mass, its spectral shape orbital eccentricity.

[1] https://arxiv.org/pdf/2308.00701

Radial Velocity

https://www.planetary.org/articles/color-shifting-stars-the-...

1 MEarth is 1 Earth mass. Even our sun is only a third of a million Earth masses. Jupiter is about 318 Earth masses.

...so it's got a mass 3x that of our sun, but it's the size of Jupiter? And it's a planet? ...What? The star it orbits is about the same size as our sun, yet a planet orbits it with 3x the mass? I'm missing something massive here, or the summary is terrible.

The M in this context stands for "Mass" not "Mega."

If you take a look at the linked PDF you'll see that the "Earth" portion of that term is a subscript, so it reads "90-150 Earth masses."

I made a mistake here on MEarth and didn't notice the correction until well after the update window on my comment, oops!

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