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Planet Earth has radius 2x of planet Mars, while having 10x times of Mars mass and 60% higher surface gravity. Could there exist rocky exoplanet with 2x radius of Earth but similar surface gravity (+-10%). Not a gas giant or pure ocean world planet, but rocky planet with at least 1/3 of total surface area being land.

In the past it was thought that upper limit for rocky exoplanet is 1.6 radius of Earth, but in 2016 http://thescienceexplorer.com/universe/largest-rocky-world-was-discovered-and-scientists-are-baffled astronomers discovered BD+20594b, which is believed to be pure rocky planet. This exoplanet has radius 2.23 of Earth and 16.3 its mass.

But BD+20594b should have surface gravity more than 3 times of Earth, so I wonder if there could exist also this radius size habitable exoplanet on which people can land and live there permanently.

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    $\begingroup$ It is an interesting thought experiment but I am not convinced it makes sense to colonise such planets. If you are capable of getting there you don't need planets to live well. If it has no life it won't have breathable air and probably won't even if there is life. If there is native life there that may make it less habitable not more. And it may be far more valuable for that alien biology, uncontaminated, to study, than indulging primitive dreams of land ownership. $\endgroup$
    – Ken Fabian
    Jul 5, 2021 at 7:00
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    $\begingroup$ Why you think that planet with radius twice of Earth and same surface gravity can not have complex life with breathable air. Maybe not breathable for humans with different composition than on Earth, but every exoplanet will probably need some form of terraforming to adapt it for human living. $\endgroup$
    – David Cage
    Jul 5, 2021 at 8:59
  • $\begingroup$ Lot of cheap available land with 4x surface area can be good motivation for investors to invest in terraforming. Rich people on Earth always want to have several properties with own mansions for comfort and privacy. On this planet it can be available for everybody, while now on Earth we have available hardly 2000 m2 for one person. Living on planets will be always more interesting than living on some O'Neill cylinder and even with lower average density there other ways how planet can have abundance of metals, for example late asteroids bombardment of planet surface. $\endgroup$
    – David Cage
    Jul 5, 2021 at 9:08
  • $\begingroup$ Off-topic, but presumably you could build one, with aerogel-like stuff making up a lot of the interior. I'm not sure what the crush-point pressure for anything we can currently fabricate is. $\endgroup$
    – Carl Witthoft
    Jul 6, 2021 at 15:11

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For such a planet to exist, twice Earth radius and similar gravity to Earth would mean the average density of the planet would be nearly half that of Earth. (see below)

Earth's average density is 5.514 g/cm3. This is in part due to the large density solid core of iron & nickel. The average density of your proposed planet would be 2.575 g/cm3. This is slightly more than the average density of Eris (2.43 g/cm3).

Such a planet would have less heavy metals and would have far fewer resources that people could use.

$$a = \frac{GM}{r^2}$$

$$M = \frac{4}{3} \pi \rho r^3$$

$$a = \frac{4}{3} \frac{\pi \rho G r^3}{r^2} = \frac{4}{3} \pi \rho G r$$

which means for a fixed gravitational acceleration at the surface density varies as inverse radius;

$$\rho \sim 1/r$$

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  • $\begingroup$ To get there we would need become very advance spacefaring species anyway, so heavy metals can be imported from elsewhere. What would be escape velocity for planet with 2x time of Earth radius and average density around 2.5 g/cm3. Around 50% more than for reaching the orbit on Earth ? $\endgroup$
    – David Cage
    Jul 4, 2021 at 17:37
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    $\begingroup$ If it's 50% more than Earth then it might be very difficult for chemically propelled rockets to escape from the surface at all. On Earth the mass ratio of propellant to non-propellent in a rocket is of the order of 90% and most of the remaining 10% is engines and tanks, so there's not a lot of wriggle room for more propellant. $\endgroup$
    – Slarty
    Jul 4, 2021 at 18:23
  • $\begingroup$ From this link space.stackexchange.com/questions/5320/… to reach orbit on planet with 50% greater orbital velocity you would need only add third stage with about 1700t for first stage below rocket of F9 size and payload. Total mass will be only about 2200t, so still less than Saturn V. And to get there we will need propulsion vastly superior to chemical rockets anyway. $\endgroup$
    – David Cage
    Jul 4, 2021 at 18:57
  • $\begingroup$ @uhoh: I've undeleted my Cydonia region question. $\endgroup$
    – Fred
    Jul 5, 2021 at 9:56
  • $\begingroup$ @uhoh: Thanks for editing the answer. I have a spreadsheet which does the calcs & I just do "what-if scenarios". $\endgroup$
    – Fred
    Jul 5, 2021 at 10:36

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