As icy as it is in NASA's Image seen all over the web...
I would bet that it looks more like this illustration of how Earth might have been 850 million years ago.
So why do I say Kepler-186f is most likely a snowball Earth if it is habitable at all?
Without knowing the composition of it's atmosphere we can still make an educated guesses, hypotheses to be tested.
As we all know the temperature of a planet depends on it's atmosphere as well as the solar radiation it receives. Over geologic time Earth has been much hotter than it is now. Notably during the Paleocene-Eocene thermal maximum, and the Cryogenian "Snowball Earth" epoc. During the PETM 56 million years ago Earth had a stronger greenhouse effect, with continents about the same positions as they are now. What changed is that a large amount of methane was released into the atmosphere which heated the Earth to levels it had not seen since the impact that killed off the dinosaurs.
It was so warm on Earth during the PETM that the polar ocean and Antarctica were as warm as the tropics.
During the Cryogenian (850-635 million years ago) the continents were in a very different place...so it's not as good of a point of comparison to the Earth we know. However the atmosphere was very different with hardly any green house gasses. Oxygen levels were very high due to the recent appearance of blue-green and then green algae. With no animals larger than a single cell or perhaps a embryo like collection of cells to use oxygen and excrete carbon dioxide the Earth was much cooler for about 200 million years.
Twice Earth was frozen over almost totally. During that period perhaps Earth had a band of non-frozen ocean at the equator.
Now compare what we know about this new planet Kepler-186f. At high noon 186f receives as much light as near the artic circle in fall or spring at high noon, and is Earth sized and Earth massed.
With so much less solar radiation 186f has to be much colder than Earth if it's atmosphere is the same as Earth during the PETM. Rich in strong greenhouse gasses like methane but much like ours now. If it lacks greenhouse gasses it is much colder than Earth is now. So with as much greenhouse gas as Earth has ever seen, like during the PETM, 186-f is perhaps as warm at the equator at high noon as near the Arctic circle on Earth during twilight.
This means that if 186f has an Earth like atmosphere it is in a condition similar to Earth during Cryogenian Epoc. At best it is habitable in a way where, with a warm coat...we could walk around. Like near the Arctic circle during the Spring or fall twilight. This video talking about a Snow Ball Earth could stand in for a documentary about the condition of Kepler 186f.
To put it in sci-fi terms, think Hoth, or Andoria. Very cold, where only heavily insulated or otherwise cold tolerant macroscopic animals could live. Any life there could be massively different than ours.
This is a very safe hypothesis assuming the planet is habitable at all. It does not have to be. It could have an atmosphere of 100% nitrogen with some chlorine gas mixed in for all we know.
- Kepler-186f: An Earth-Sized Planet In A Habitable Zone
- Snowball Earth Hypothesis Contradicted By CO2 Research
- What Makes One Alternate Earth More Habitable Than Another?
- It's Misleading To Compare Exoplanet Kepler-452b To Earth
- A better than 50/50 chance Kepler-186f has technological life. (Updated)