2018-04-25 15:46:09 UTC
By Charles Q. Choi, Space.com Contributor |
April 23, 2018 07:15am ET
No Way Out? Aliens on 'Super-Earth' Planets May Be Trapped by Gravity
Artist's illustration of the super-Earth alien planet Kepler-69c.
"Super-Earth" planets are giant-size versions of Earth, and some
research has suggested that they're more likely to be habitable than
Earth-size worlds. But a new study reveals how difficult it would be for
any aliens on these exoplanets to explore space.
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.
"On more-massive planets, spaceflight would be exponentially more
expensive," said study author Michael Hippke, an independent researcher
affiliated with the Sonneberg Observatory in Germany. "Such
civilizations would not have satellite TV, a moon mission or a Hubble
Space Telescope." [10 Exoplanets That Could Host Alien Life]
As researchers have discovered alien worlds around other stars, one
class of exoplanets that popped up was the super-Earths, planets that
can reach up to 10 times the mass of our own. A number of super-Earths
apparently lie in the habitable zones of their stars, where temperatures
can theoretically support liquid water on the planetary surface and
thus, potentially, life as it is known on Earth.
Prior work suggested not only that worlds other than Earth-like ones
could offer circumstances suitable for life, but also that some could be
even more suitable than Earth-like planets. Super-Earths, researchers
have suggested, might be "super-habitable" — their greater mass giving
them stronger gravitational pulls, so they could hold thicker
atmospheres to better shield life from harmful cosmic rays.
If life did evolve on a distant super-Earth, such aliens could have
developed an advanced civilization capable of spaceflight. However, the
strong gravitational pull of such planets could also make it more
difficult for extraterrestrials to blast off their planets, Hippke said
in the new study.
To see how difficult it might be for super-Earthlings to launch a
conventional rocket, Hippke calculated the rocket sizes needed to escape
a super-Earth 70 percent wider than our planet and 10 times more
massive. Those are roughly the specs of the alien planet Kepler-20b,
which lies about 950 light-years from Earth. On such a world, the escape
velocity is about 2.4 times greater than on Earth.
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A big challenge for aliens on such a world would be the weight of the
fuel that conventional rockets carry. Launching a rocket off a planet
requires a lot of fuel, which makes the rockets heavy, which requires
more fuel, making the craft heavier, and so on.
"I am surprised to see how close we as humans are to end up on a planet
which is still reasonably lightweight to conduct spaceflight," Hippke
told Space.com. "Other civilizations, if they exist, might not be as
Assuming a rocket on the simulated super-Earth worked about as well as
SpaceX's Falcon Heavy, to launch a payload like NASA's upcoming James
Webb Space Telescope would require 60,000 tons (55,000 metric tons) of
fuel, about the mass of the largest ocean battleships, Hippke said. [In
Photos: SpaceX's 1st Falcon Heavy Rocket Test Launch Success!]
"Civilizations from super-Earths are much less likely to explore the
stars," Hippke said. "Instead, they would be to some extent arrested on
their home planet and, for example, make more use of lasers or radio
telescopes for interstellar communication instead of sending probes or
Rockets work better in the vacuum of space than in an atmosphere,
though. So, Hippke suggested that super-Earthlings might want to launch
from a mountaintop. However, the strong gravitational pull of
super-Earths would squash down their surfaces, leading to smaller
mountains. And on Earth, the benefit of launching at high altitudes is
not very large compared to launching at sea level, Hippke said.
There could be ways of reaching orbit other than via conventional
rockets, such as by using space elevators traveling on giant cables
rising out of the atmosphere. However, a key limiting factor of space
elevators is the strength of the cable material. The most suitable
material known today, carbon nanotubes, is just barely strong enough for
Earth's gravity, and it is unclear if stronger materials are physically
possible, making it difficult to predict if space elevators on
super-Earths could work.
Another possibility is nuclear pulse propulsion, which would involve
detonating a series of atom bombs behind a vehicle to hurl it through
space. This explosive strategy offers more lifting power than
conventional rockets, and might be the only way for a civilization to
leave a planet more than 10 times Earth's mass, Hippke said.
However, such a nuclear-powered spacecraft would pose not only technical
challenges but political ones as well, he said.
"A launch failure, which typically happens with a 1 percent risk, could
cause dramatic effects on the environment" for a nuclear-powered
spacecraft, Hippke said. "I could only imagine that a society takes
these risks in a flagship project where no other options are available,
but the desire is strong — for example, one single mission to leave
their planet and visit a moon."
Hippke detailed his findings online April 12 in a study submitted to the
International Journal of Astrobiology.
Follow Charles Q. Choi on Twitter @cqchoi. Follow us @Spacedotcom,
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