Introduction
Do we inhabit the best of all possible worlds (Leibnitz Reference Leibnitz1710)? From a variety of habitable worlds that may exist, Earth might well turn out as one that is marginally habitable. Other, more habitable (‘superhabitable’) worlds might exist (Heller and Armstrong Reference Heller and Armstrong2014). Planets more massive than Earth can have a higher surface gravity, which can hold a thicker atmosphere and thus better shielding for life on the surface against harmful cosmic rays. Increased surface erosion and flatter topography could result in an ‘archipelago planet’ of shallow oceans ideally suited for biodiversity. There is apparently no limit for habitability as a function of surface gravity as such (Dorn et al. Reference Dorn, Bower and Rozel2017). Size limits arise from the transition between Terran and Neptunian worlds about 
$2\pm 0.6\,R_{\oplus }$ (Chen and Kipping, Reference Chen and Kipping2017). The largest rocky planets known so far are 
$\sim 1.87\,R_{\oplus }$, 
$\sim 9.7\,M_{\oplus }$ (Kepler-20 b, Buchhave et al., Reference Buchhave, Dressing, Dumusque, Rice, Vanderburg, Mortier, Lopez-Morales, Lopez, Lundkvist, Kjeldsen, Affer, Bonomo, Charbonneau, Collier Cameron, Cosentino, Figueira, Fiorenzano, Harutyunyan, Haywood, Johnson, Latham, Lovis, Malavolta, Mayor, Micela, Molinari, Motalebi, Nascimbeni, Pepe, Phillips, Piotto, Pollacco, Queloz, Sasselov, Ségransan, Sozzetti, Udry and Watson2016). When such planets are in the habitable zone, they may be inhabited. Can‘Super-Earthlings’ still use chemical rockets to leave their planet? This question is relevant for the search for extraterrestrial intelligence (SETI) and space colonization (Lingam, Reference Lingam2016; Forgan, Reference Forgan2016, Reference Forgan2017 and Dutil and Dumas, Reference Dutil and Dumas2010).
Method
At our current technological level, spaceflight requires a rocket launch to provide the thrust needed to overcome Earth's force of gravity. Chemical rockets are powered by exothermic reactions of the propellant, such as hydrogen and oxygen. Other propulsion technologies with high specific impulses exist, such as nuclear thermal rockets (e.g., NERVA, Arnold and Rice, Reference Arnold and Rice1969), but have been abandoned due to political issues. Rockets suffer from the Tsiolkovsky equation (Tsiolkovsky, Reference Tsiolkovsky1903): if a rocket carries its own fuel, the ratio of total rocket mass versus final velocity is an exponential function, making high speeds (or heavy payloads) increasingly expensive (Plastino and Muzzio, Reference Plastino and Muzzio1992).
The achievable maximum velocity change of a chemical rocket is
$$ \Delta v = v_{\rm{ex}} \ln \displaystyle{{m_0} \over m_{\rm f}} $$where m 0 is the initial total mass (including fuel), m f is the final total mass without fuel (the dry mass) and v ex is the exhaust velocity. We can substitute v ex = g 0 I sp where 
$g_{0}={\rm GM}_{\oplus }/R^2_{\oplus }\sim 9.81\,$m s−1 is the standard gravity and I sp is the specific impulse (total impulse per unit of propellant), typically ~350…450 s for hydrogen/oxygen.
To leave Earth's gravitational influence, a rocket needs to achieve at minimum the escape velocity
$$ v_{\rm esc} = \sqrt{\displaystyle{2{\rm GM}_{\oplus}} \over {R_{\oplus}}} \sim 11.2\,{\rm km}\,{\rm s}^{-1} $$for Earth and v esc ~27.1 km s−1 for a 
$10\,M_{\oplus }$, 
$1.7\,R_{\oplus }$ Super-Earth is similar to Kepler-20 b.
Results
We consider a single-stage rocket with I sp = 350 s and wish to achieve Δv > v esc. The mass ratio of the vehicle becomes
$$ {\displaystyle{m_0} \over {m_{\rm f}}} \gt {\rm exp} \left( \displaystyle{v_{\rm{esc}}} \over {v_{\rm{ex}}} \right). $$which evaluates to a mass ratio of ~26 on Earth and ~2, 700 on Kepler-20 b. Consequently, a single-stage rocket on Kepler-20 b must burn 104× as much fuel for the same payload (~2, 700 t of fuel for each t of payload).
This example neglects the weight of the rocket structure itself and, is therefore, a never achievable lower limit. In reality, rockets are multistage and have typical mass ratios (to Earth escape velocity) of 50 … 150. For example, the Saturn V had a total weight of 3,050 t for a lunar payload of 45 t, so that the ratio is 68. The Falcon Heavy has a total weight of 1,400 t and a payload of 16.8 t, so that the ratio is 83 (i.e., the payload fraction is ~1%).
For a mass ratio of 83, the minimum rocket (1 t to v esc) would carry 9, 000 t of fuel on Kepler-20 b, which is 3× larger than a Saturn V (which lifted 45 t). To lift a more useful payload of 6.2 t as required for the James Webb Space Telescope on Kepler-20 b, the fuel mass would increase to 55,000 t, about the mass of the largest ocean battleships. For a classical Apollo moon mission (45 t), the rocket would need to be considerably larger, ~400, 000 t. This is of the order the mass of the Pyramid of Cheops and is probably a realistic limit for chemical rockets regarding cost constraints.
Discussion
Launching from a mountain top
Rockets work better in space than in an atmosphere. One might consider launching the rocket from high mountains on Super-Earths. The rocket thrust is given by
$$ F = \dot{m}\,v_{\rm{ex}} + A_{\rm e}(P_1 - P_2) $$where 
$\dot {m}$ is the mass flow rate, A e is the cross-sectional area of the exhaust jet, P 1 is the static pressure inside the engine and P 2 is the atmospheric pressure. The exhaust velocity is maximized for zero atmospheric pressure, i.e. in a vacuum. Unfortunately, the effect is not very large in practice. For the Space Shuttle's main engine, the difference between sea level and vacuum is ~25 % (Boeing, Rocketdyne Propulsion & Power, 1998). Atmospheric pressure below 0.4 bar (Earth altitude 6,000 m) is not survivable long term for humans, and presumably neither for ‘Super-Earthlings’. Such low pressures are reached in lower heights on Super-Earths because the gravity pulls the air down.
One disadvantage is that the bigger something is, the less it can deviate from being smooth. Tall mountains will crush under their own weight (the ‘potato radius’ is ~238 km, Caplan, Reference Caplan2015). The largest mountains in our Solar System are on less massive bodies, such as the Rheasilvia central peak on Vesta (22 km) or Olympus Mons on Mars (21.9 km). Therefore, we expect more massive planets to have smaller mountains. This will be detectable through transit observations in future telescopes (McTier and Kipping, Reference McTier and Kipping2018). One option would be to build artificial mountains as launch platforms.
Launching rockets from water-worlds
Many habitable (and presumably, inhabited) planets might be waterworlds (Simpson, Reference Simpson2017) and intelligent life in water and sub-surface is plausible (Lingam and Loeb, Reference Lingam and Loeb2018). Can rockets be launched from such planets? We here neglect how chemical fuels and whole rockets, are assembled on such worlds.
Rockets on waterworlds could either be launched from floating pontoon-based structures, or directly out of the water. Underwater submarine rocket launches use classical explosives to flash-vaporize water into steam. The pressure of the expanding gas drives the missile upwards in a tube. This works well for ICBMs launched from submerged submarines.
These aquatic launch complications make the theory of oceanic rocket launches appear at first quite alien; presumably land-based launches seem equally human to alien rocket scientists.
Relevance of spaceflight for SETI
It is not trivial to estimate the longevity of civilizations and the ‘doomsday’ argument limits the remaining lifetime of our species to between 12 and 8 × 106 yrs at 95 % confidence (Gott, Reference Gott1993). If this argument is correct (Haussler, Reference Haussler2016), it would also apply to other civilizations and it appears reasonable to assume that population growth is intertwined with technological and philosophical progress. Indeed, the risk of catastrophic extinction has been estimated of order 0.2 % per year (Matheny, Reference Matheny2007; Simpson, Reference Simpson2016; Turchin and Denkenberger, Reference Turchin and Denkenberger2018) at our current technological level and might be applicable to other civilizations within less than a few orders of magnitude (Gerig, Reference Gerig2012; Gerig et al., Reference Gerig, Olum and Vilenkin2013). The possession of ever more powerful technology in the hands of (many) individuals is increasingly dangerous (Cooper, Reference Cooper2013).
The risk of extinction could be reduced by colonization of other planets in a stellar system, such as Mars. If space-flight is much more difficult due to high surface gravity, colonization would be temporally delayed and the chances of survival reduced. Then, Drake's L would be (on average) smaller for civilizations on Super-Earths (Drake, Reference Drake2013). Inversely, planets with lower surface gravity would make colonization cheaper and thus increase L.
Conclusion
For a payload to escape velocity, the required amount of chemical fuel scales as exp(g 0). Chemical rocket launches are still plausible for Super-Earths 
$\lesssim 10\,{\rm M}_{\oplus }$, but become unrealistic for more massive planets. On worlds with a surface gravity of 
$\gtrsim 10\,{\rm g}_0$, a sizable fraction of the planet would need to be used up as chemical fuel per launch, limiting the total number of flights. On such worlds, alternative launch methods such as nuclear-powered rockets or space elevators are required.
