Article 1
Summary by Kevin A. Walker
Planet detection techniques were
very successful in finding large gas giants and small rocky planets very close
to their stars. Kepler 186f became the first Earth-sized rocky planet found to
lie within the habitable zone of its star, meaning that it can sustain liquid
water. Unsurprisingly, Kepler 186f orbits an M-Dwarf, by far the most common
type of star in the galaxy. The creatively named star Kepler 186 is only about
half the size of the Sun and much cooler. Kepler 186f has four other planets in
its solar system, all of which lie within its orbit.
Confirming Kepler 186f as a
planetary candidate proved to be very challenging for astronomers. The planet
was first discovered using the transit method in which the brightness of the
host star drops while the planet passes in front and obscures part of the light.
However, similar changes in brightness can be explained in many ways.
Astronomers first had to rule out the possibility that fluctuations in the
brightness of background stars could have mimicked a planetary transit. They
found that this had just a 0.5% chance of occurring relative to change being
due to a transit. Astronomers also had to rule out the possibility that Kepler
186 was in a binary system. The probability of this being true was determined
to be less than 0.2%. Astronomers could then confidently agree that there were
indeed five planets orbiting Kepler 186.
Although we know that Kepler 186f
has a radius about 1.11 times larger that the Earth’s, we cannot determine its
mass. Estimates range from 0.32 to 3.77 times the mass of the Earth, based on
various possible compositions of this planet. Due to what we know about
planetary formation and stellar evolution, it is believed that Kepler 186f does
not have an atmosphere, or at lease one that is not hydrogen-rich.
All of the planets that we know to
orbit Kepler 186 lie in the same plane and orbit in the same direction, leading
astronomers to believe that these planets formed in a proto-planetary disk
similar to the one Earth formed in. Unlike our solar system however, the four
interior planets of the Kepler 186 system have all become tidal locked. Nothing
is known about the rotation rate of Kepler 186f, but even if it were tidal
locked, this would not exclude it from being able to sustain liquid water.
Kepler 186 remains relatively
undisturbed by the orbit of its planets, meaning that the planets must have
regular orbits.
Article 2
The paper suggests that the planets either formed at their current position or formed at a more distant radius and migrated inwards. One inconsistency that was observed between formation models and the observed system is the spacing of the planets. All the simulations predicted a planet between Kepler 186-e and Kepler 186-f but there are no observations of such a planet. If the system formed further out and drifted in, the gap could be explained. On the other hand if the system formed close to its current location, there is most likely at least one planet in between e and f. Such a planet might be a few degrees off of the orbital plane of the system, accounting for the lack of observation of the planet.
The two theories on planet formation also affect the likelihood of water on the planets in the system. If the planets formed in their current location, they likely do not have much water. Even 186-f is just inside the predicted frost-line, the point after which water is solid, of the system. Water would have to reach the planets from some other means. If, however the planets formed further out, they could have gathered water and brought it closer in when they migrated.
The possibility of water on Kepler 186-f and the fact that its orbit that falls in the habitable zone makes it a candidate for sustaining life. It lies near the far end of the habitable zone so liquid water is not guaranteed. Whether or not the water would be liquid depends its atmosphere and whether or not the planet can maintain a strong greenhouse effect. Further analysis of the planet’s environment has suggested that photosynthesis would be possible given the right atmosphere. The paper points out that the habitable zone is not necessarily a great way to determine whether life is possible or likely. Many factors other than the presence of liquid water can add and subtract to the likelihood of life.
The paper also looks at the rotation and stability of the different planets. The simulations found that the inner four planets are most likely tidally locked, with 186-f having the strong possibility of being tidally locked. The models found that the system is very stable, even in cases where the planets were more massive than they most likely are.
This paper used models and a variety of input parameters to show the workings of the Kepler 186 system. It shed light on the possibility of life in the system and suggested different explanations for how the system was formed.