1,647 research outputs found
The habitable zone for Earthlike exomoons orbiting Kepler-1625b
The recent announcement of a Neptune-sized exomoon candidate orbiting the
Jupiter-sized object Kepler-1625b has forced us to rethink our assumptions
regarding both exomoons and their host exoplanets. In this paper I describe
calculations of the habitable zone for Earthlike exomoons in orbit of
Kepler-1625b under a variety of assumptions. I find that the candidate exomoon,
Kepler-1625b-i, does not currently reside within the exomoon habitable zone,
but may have done so when Kepler-1625 occupied the main sequence. If it were to
possess its own moon (a "moon-moon") that was Earthlike, this could potentially
have been a habitable world. If other exomoons orbit Kepler-1625b, then there
are a range of possible semimajor axes/eccentricities that would permit a
habitable surface during the main sequence phase, while remaining dynamically
stable under the perturbations of Kepler-1625b-i. This is however contingent on
effective atmospheric CO regulation.Comment: 17 pages, 5 figures, accepted to IJA. arXiv admin note: text overlap
with arXiv:1601.00789, arXiv:1404.449
Can Collimated Extraterrestrial Signals be Intercepted?
The Optical Search for Extraterrestrial Intelligence (OSETI) attempts to
detect collimated, narrowband pulses of electromagnetic radiation. These pulses
may either consist of signals intentionally directed at the Earth, or signals
between two star systems with a vector that unintentionally intersects the
Solar System, allowing Earth to intercept the communication. But should we
expect to be able to intercept these unintentional signals? And what
constraints can we place upon the frequency of intelligent civilisations if we
do?
We carry out Monte Carlo Realisation simulations of interstellar
communications between civilisations in the Galactic Habitable Zone (GHZ) using
collimated beams. We measure the frequency with which beams between two stars
are intercepted by a third. The interception rate increases linearly with the
fraction of communicating civilisations, and as the cube of the beam opening
angle, which is somewhat stronger than theoretical expectations, which we argue
is due to the geometry of the GHZ. We find that for an annular GHZ containing
10,000 civilisations, intersections are unlikely unless the beams are
relatively uncollimated.
These results indicate that optical SETI is more likely to find signals
deliberately directed at the Earth than accidentally intercepting collimated
communications. Equally, civilisations wishing to establish a network of
communicating species may use weakly collimated beams to build up the network
through interception, if they are willing to pay a cost penalty that is lower
than that meted by fully isotropic beacons. Future SETI searches should
consider the possibility that communicating civilisations will attempt to
strike a balance between optimising costs and encouraging contact between
civilisations, and look for weakly collimated pulses as well as narrow-beam
pulses directed deliberately at the Earth.Comment: 12 pages, 7 figures, accepted for publication in JBI
Slingshot Dynamics for Self Replicating Probes and the Effect on Exploration Timescales
Interstellar probes can carry out slingshot manoeuvres around the stars they
visit, gaining a boost in velocity by extracting energy from the star's motion
around the Galactic Centre. These maneouvres carry little to no extra energy
cost, and in previous work it has been shown that a single Voyager-like probe
exploring the galaxy does so 100 times faster when carrying out these
slingshots than when navigating purely by powered flight (Forgan et al. 2012).
We expand on these results by repeating the experiment with self-replicating
probes. The probes explore a box of stars representative of the local Solar
neighbourhood, to investigate how self-replication affects exploration
timescales when compared with a single non-replicating probe.
We explore three different scenarios of probe behaviour: i) standard powered
flight to the nearest unvisited star (no slingshot techniques used), ii) flight
to the nearest unvisited star using slingshot techniques, and iii) flight to
the next unvisited star that will give the maximum velocity boost under a
slingshot trajectory.
In all three scenarios we find that as expected, using self-replicating
probes greatly reduces the exploration time, by up to three orders of magnitude
for scenario i) and iii) and two orders of magnitude for ii). The second case
(i.e. nearest-star slingshots) remains the most time effective way to explore a
population of stars. As the decision-making algorithms for the fleet are
simple, unanticipated "race conditions" amongst probes are set up, causing the
exploration time of the final stars to become much longer than necessary. From
the scaling of the probes' performance with star number, we conclude that a
fleet of self-replicating probes can indeed explore the Galaxy in a
sufficiently short time to warrant the existence of the Fermi Paradox.Comment: Accepted for publication in the International Journal of
Astrobiology, 13 pages, 7 figure
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