[shagie]

An interesting problem is "how much energy would it take to launch from a super-earth?"

https://www.space.com/40375-super-earth-exoplanets-hard-alie...

> To launch the equivalent of an Apollo moon mission, a rocket on a super-Earth would need to have a mass of about 440,000 tons (400,000 metric tons), due to fuel requirements, the study said. That's on the order of the mass of the Great Pyramid of Giza in Egypt.

We may be in a Goldilocks zone not only for distance from the primary star (lots of systems are binary which gets problematic), but also in terms of size of the planet (not to light to loose atmosphere, not to heavy to trap everything), and we've got a nice big moon to keep things sloshing around and stabilize the tilt.

I am of the opinion that life is rare because we live in a surprisingly boring solar system with a terrestrial planet in the right range, not to large and not too small, and with a massive moon.

[pfdietz]

A super earth would require exponentially more energy with chemical rockets, due to that darn rocket equation. Fortunately, Earth isn't that bad.

[foobarian]

> A super earth would require exponentially more

Polynomially, right?

[pfdietz]

No, exponentially. I used that word precisely, because the mass ratio in the rocket equation is exponential in (delta V)/(exhaust velocity).

[foobarian]

Thanks for the reply. Context: there are so few exponential processes in nature, especially around classical physics that it really stuck out here, so I made the comment. I should have searched and read up on it instead, but thought there is still some value in others seeing the conversation.

And lastly: that is really depressing :-(

[nayuki]

This phenomenon is called "The Tyranny of the Rocket Equation": https://www.nasa.gov/mission_pages/station/expeditions/exped...

[ncmncm]

Saturn, Uranus, and Neptune all have surface (well, cloud level) gravity nearly equal to Earth's. But try launching anything to orbit from any of them!

(Strangely, with hydrogen and helium the majority of all three atmospheres, they have water and ammonia clouds. What holds them up?)

[pfdietz]

That exponential dependence comes from the use of chemical rockets, which have a pretty firm upper limit on their exhaust velocity. Once in space, lower thrust could be used, which admits the possibility of higher exhaust velocity. If one can vary the exhaust velocity then the problem can be evaded (although the specific power (power/mass) of the vehicle must be allowed to become very large, or else acceleration will decrease.)

Recent example: https://phys.org/news/2022-02-laser-mars.html

Note the very high specific power. High Isp rocketry is all about the heat dissipation.

[saiya-jin]

I agree with rare earth theory too, but even with it, the observable universe is so huge that even those tiny fractions of various probabilities should at the end materialize on a massive scale.

And its not like with higher G or some other singular aspect moving into more challenging territory, everything becomes suddenly impossible. Think about how many obstacles mankind could conquer in a relatively short amount of time, say a million years.

[GoblinSlayer]

We see only super earthes because they are easier to see. Small planets can be more numerous.

[shagie]

They can... but a challenge with a small planet in the habitable zone of a star is that it also has a weaker magnetic field and is more prone to having its atmosphere stripped away.

Consider that the moon had an atmosphere - https://en.wikipedia.org/wiki/Atmosphere_of_the_Moon#Ancient...

> In October 2017, NASA scientists at the Marshall Space Flight Center and the Lunar and Planetary Institute in Houston announced their finding, based on studies of Moon magma samples retrieved by the Apollo missions, that the Moon had once possessed a relatively thick atmosphere for a period of 70 million years between 3 and 4 billion years ago. This atmosphere, sourced from gases ejected from lunar volcanic eruptions, was twice the thickness of that of present-day Mars.

http://time.com/4974580/nasa-moon-had-atmosphere-volcanoes/

Which brings us to

https://worldbuilding.stackexchange.com/questions/13583/what...

While smaller planets are certainly more numerous - many of them likely lack the mass and active magnetic field to retain an atmosphere in the habitable zone.