Don’t get me wrong. The discovery of Kepler-186f is a big deal: a near Earth-sized planet in the habitable zone of its star, neither too close nor too distant from it for liquid water to exist on the planet’s surface. But is it another Earth, Earth 2.0, the long-sought twin of our beautiful blue planet? Not quite. Not yet. But that hasn’t stopped artists from speculating on what this planet might look like close up.
What exactly is it that we are looking for? More precisely, what planetary characteristics are necessary to be considered a twin of Earth? Let’s start with the characteristics of Kepler-186f to see how it almost-but-not-quite qualifies.
The name itself tells you that this is a discovery of the Kepler Space Telescope, dedicated to finding planets around other stars by the dimming of those stars’ light when the planet passes in front of them. Here’s how that works.
Astronomers can deduce a surprising amount of information about a star from a detailed study of its light. We know how big it is and how hot. From an analysis of the planetary transit light curve, we can determine the orbital period of the planet and its size. Put all of this together, and what do we know about Kepler-186f?
It is only slightly larger (10%) than Earth, and just close enough to its host star for surface temperatures to fall in the temperature range allowing water to exist as a liquid (0 to 100 degrees Celsius). The host star itself is much smaller and dimmer than our Sun, so the planet must orbit much closer to its star than does the Earth to find this warmth.
Therein lies a key difference between this planet and our own: the host star is not Sun-like. It is smaller, dimmer, and redder. The close-in orbit leaves this planet more vulnerable to stellar flares, and if the planet were just a little closer, tidal interactions with the star would lock it into an orbit where one side would always be facing the star. A planet with one side in perpetual frozen night and the other always in hot daytime is hardly another Earth.
The search for Earth 2.0 is of course also a search for Earth-like life, or at least for a planet where life as we know it could arise. A small dim star is much more long-lived than our Sun, so life would have a longer time period to emerge, take hold, and evolve. That much is a positive.
Is there liquid water on the surface of Kepler-186f? That would require the planet to have an atmospheric blanket for protection. We don’t know if the planet has such an atmosphere, and it is too far away (500 light years) to be able to determine that. The planet’s size is such that it almost certainly has a rocky surface instead of being a gas giant like Jupiter, so we can’t rule out the possibility of lakes and oceans of water.
The ideal Earth 2.0 would orbit a star like our Sun, hot enough so that its habitable zone is at some distance from the star, putting a planet in that zone at less risk from solar flares, and eliminating the possibility of tidal locking. A star that is hotter will have a larger and more distant habitable zone, but stars that are very much hotter are short-lived and allow little time for life to emerge or evolve to any level of complexity. The planet should be small enough not to retain a huge atmosphere of light gases like hydrogen and helium, but large enough to hold onto some atmosphere and allow liquid water to exist on its presumably rocky surface. In short, it would look pretty much like this.
Kepler returned massive amounts of data before the failure of two reaction wheels used to precisely point the telescope. It takes some time to sort through all this information, and when a star hosts several planets it can take weeks of supercomputer time for a full analysis. Earth 2.0 candidates will orbit far enough from their stars to make transits relatively rare events; if Kepler were to observe the solar system under the most ideal conditions, the Earth would block the Sun’s light for only about half a day every year. Clearly you have to observe for a year see even one transit, and Kepler needs to see at least three to confirm that the star’s dimming was indeed caused by a planetary transit. The first results from Kepler were, as expected, dominated by large planets orbiting quite close to their stars. As more data are accumulated, we are starting to see more and more planets that are closer to being Earth-like. Earth 2.0 is quite likely just waiting to be discovered in a light curve stored on NASA’s computers.
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