How Big Must a Planet Be to Block Space Travel?
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- James Arnold calculated that a planet with Earth's average density (~5,500 kg/m³) whose escape velocity equalled the speed of light would need a radius of about 170 million km — roughly Earth's orbital distance from the sun — and would in fact be a black hole
- Linda Phillips argued that just 1.2 g of surface gravity would suffice to prevent space travel using known rocket technology, compared with Mars at 0.38 g and the Moon at 0.17 g
- Phillips identified 0.8 g as the ideal gravity — strong enough to retain an atmosphere (which requires at least 0.6 g) while still allowing practical rocket launches
- Phillips noted that roughly 40% of exoplanets with estimated gravity fall within the 0.6–1.2 g range, suggesting high gravity is unlikely to be what blocks interplanetary travel if life is plentiful
- Both answers rested on a density assumption originally raised by earlier correspondent Alex McDowell (11 July), without which the size calculation has no fixed answer
Why it matters: Phillips's 0.6–1.2 g window reframes the search for habitable worlds: roughly 40% of exoplanets with measured gravity already fall in a band that keeps an atmosphere and permits rocket flight, so gravity alone is unlikely to be the bottleneck for any civilisations that do arise.
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