An Earth-sized planet in the habitable zone of a cool star

In the spirit of last week’s poetic post, here are two more stanzas for your reading pleasure….

There is a new exoplanet in town.
This planet has only just now been found.
Why should you care?  It’s only one more.
Well this is one planet we’d love to explore.

This planet’s orbit is really just right.
It isn’t too cold.  You won’t freeze at night.
And it isn’t too hot.  You won’t get sun-baked.
In fact you could go take a dip in a lake.

OK, the rhyming stops there…. Let’s get to business. 

I want to introduce a newly-discovered planet called Kepler-186 f.  I was lucky enough to be part of the team that found this planet (it was led by scientists at NASA and the SETI Institute).  What is special is that the planet has the right temperature for water to be liquid on its surface.  All life on Earth depends on liquid water, so this is a pretty big deal!  Disclaimer: this planet needs to have the right kind of atmosphere and the right composition to actually have liquid water on its surface.  We don’t know anything about the planet’s surface or atmosphere.

Kepler-186 f is the most distant planet in the Kepler-186 system.  The system was discovered by the remarkable Kepler space telescope.  Each of the five planets in the system is slightly bigger than Earth, but none is more than 40% larger.  This means that the planets are probably rocky or at least have solid surfaces.  Only bigger planets — with sizes larger than about 1.5 to 2 times the size of Earth — are likely to be gaseous “mini-Neptunes”.

Below is an artist’s view of Kepler-186f.  You can see the four other planets closer to the star.  One is even about to transit in front of the star!

We don’t know anything about the planet apart from its size and orbit.  So this is just speculation.  Still, let’s let our imagination run wild.  To be consistent we need to take into account the fact that the planet’s host star (Kepler-186) is different than the Sun.  Kepler-186 f’s sky is not blue because there is not enough blue light reaching the planet.  There are also different kinds of clouds than on Earth.  As we’ll see below, this planet needs a decent amount of greenhouse heating to maintain liquid water on its surface.  This heating could very well come from a relatively dense atmosphere containing carbon dioxide (CO2).  If that is the case, then complex climate simulations tell us that Kepler-186 f should have multiple cloud layers.  A layer of water clouds should exist at a similar altitude as on Earth.  Another layer of CO2 ice clouds should be located much higher up and cover a significant fraction of the planet’s surface.  But watch out — the clouds can’t cover up the surface and make the image boring!  Everyone wants the planet to have vast oceans but some continents!

This image compares the orbital layout of the Kepler-186 system with the Solar System:

Kepler-186 f’s orbit is located in the outer parts of the habitable zone.  It receives one quarter to one third as much energy as the Earth does from the Sun (within the errors bars on the observations).  This is less than Mars.  The planet needs an atmosphere to heat it up.  This heating can come from greenhouse gases in its atmosphere.  The most common greenhouse gases in the Solar System are water vapor and CO2.  An atmosphere including CO2 (as on Mars and Venus) and a little Nitrogen (as on Earth) can heat the planet’s surface sufficiently for water to be liquid.  The exact amount of CO2 and Nitrogen that are needed depend on exactly how much energy the planet receives from the star.  It’s comparable to the density of Earth’s atmosphere.  Punchline: Kepler-186 f does indeed belong in the habitable zone.

Below is a comparison between the layout of Kepler-186 and three other planetary systems that contain a planet in the habitable zone.

Kepler-186 f is not the hottest planet out there.  It is indeed toward the outer parts of the habitable zone.  However, it does receive a little more energy from its star than the outermost planet in the GJ 581 system (GJ 581 d).  That planet has been studied like crazy.  It has been convincingly shown that it represents a potentially habitable planet, as a reasonable atmosphere (with CO2) can heat the planet enough for water to be liquid on its surface.

What makes this new planet special?  Well, it’s not the first planet discovered in the habitable zone.  And it’s not the first probably-rocky planet discovered in the habitable zone.  It is the smallest planet found in the habitable zone so far.  But there will certainly be even smaller ones found in the near future.  There is no reason to get caught up on labeling this planet the best or more interesting.  In my opinion, this is an important planet because it is a stepping stone on the path to finding true Earth analogs around other stars.

Also of interest: simulations of the formation of the Kepler-186 system systematically predict that an additional planet should exist between planets e and f.  Another planet could easily “hide” in there and not be detected by Kepler.  Such a planet could be in the inner parts of the star’s habitable zone.  Pretty exciting (but speculative).

More information: The paper presenting the discovery of Kepler-186 f (led by Elisa Quintana) comes out in Science April 18, 2o14 and can be accessed here.  The companion paper presenting a study of the system’s formation, tidal evolution and habitability (led by Emeline Bolmont) was submitted to the Astrophysical Journal and can be accessed here.

Note: I think there may very well be another planet in the Kepler-186 system.  See my arguments (in poem form) here.