on whether your Exoplanet is Earthlike

This week, NASA announced the discovery of Kepler 452-b, the newest candidate for Most Earthlike Exoplanet Ever. Most of my astrophysical colleagues are bit annoyed about that. I want to be clear: It’s not like the Kepler people are lying… far from it; in the ways that Kepler looks for planets, this one really does match the Earth the best. (And this is exactly what the Kepler team promised they’d find back when they proposed the mission in the first place) My problem is that THEIR definition of Earthlike probably does not mean what you think it means.

Also: We’re talking about Kepler 452b, the most Earth-like planet Kepler has found (July 2015), not Kepler 186f, the first Earth-size planet in the habitable zone (April 2014). And of course, “most similar to Earth” means if you’re reading this after 2015, there’s probably a new, even more similar planet found. They’re finding and confirming boatloads of new planets all the time, and sooner or later they’ll find one that edges out the previous one in some notable way. There’s a pretty good rundown at the Kepler 452b link.

For starters:

Kepler 452b is 1.6 Earth radii, and orbiting a star with spectral type G2V (the Sun is also spectral type G2V) that’s a tiny bit more massive and hotter than the Sun, and its year is 385 Earth days long. That means if you plopped the Earth there, with our 1 Earth Atmosphere of pressure and Nitrogen/Oxygen atmosphere, our oceans would stay in the liquid state and most things about living on the planet would be the same (after the initial chaos as the weather and water cycle and other climate patterns re-equilibrated).

What we notably DON’T know is anything beyond that. This slightly larger planet orbiting a slightly larger star means the planet was only blocking about 1/10,000 of the star’s light as it passed between us and the star, and finding it is an amazing validation of what Kepler was built to do. But Kepler can’t really tell us what the planet is made of, what its atmosphere is, whether it has water, or whether it has life.

Getting even close to 1 Earth radii isn’t a guarantee of anything. Venus is almost exactly the same radius (and mass!) as the Earth, but it’s a toxic hellhole, with 90 atmospheres of carbon dioxide gas and surface temperatures that could melt lead (citation). Is Kepler 452b like that? Conversely, Saturn’s moon Titan is a little larger than our Moon, but it has 1.6 atmospheres of pressure and it’s the only other place in the Solar System (that I know of) where you can find liquid ANYTHING (methane, in that case).

And then there’s the whole habitable zone concept. You need a certain amount of energy from a star, adjusted for atmospheric greenhouse effect, to allow surface temperatures between 273 and 373 Kelvin (0 and 100 Celsius). But graphs like this: The Habitable Zone, as defined in a frequently used figure. tend to show the habitable zone based on the Earth’s atmosphere, right now.. There are many different ideas about what that would mean; some calculations based on the current Earth’s atmosphere have Mars in the Sun’s habitable zone, except that Mars’s wimpy atmosphere doesn’t let it retain heat or have the surface pressure necessary for liquid water. Change that out for a thicker atmosphere and it would work.

Ultimately, what I’m driving at is that the Kepler people are looking for an exact twin of Earth – planet of similar radius orbiting a star of similar mass. That’s not quite all of what I think people hope “earthlike” means. It is necessary if we want to find an actual twin of Earth (on which to set up New London, New Paris, and New New York), but do we really need to go that far? I think the more interesting point is to find life, and for that you don’t need (as far as I can conceive) an Earth-mass, Earth-radius planet around an Earthlike star. Other instruments like the Gemini Planet Imager and the AMNH/Palomar Project 1640 instrument that are pursuing the goals of determining planetary composition (they’re a bit too crude to detect life-bearing chemical signatures now, but wait for their successors) are where the real fun will be.

Update: Barbara Rojas Ayala, via this post from Elizabeth Tasker, alerted me to this paper by Leslie Ann Rogers named, aptly, “Most 1.6 Earth-radius Planets are Not Rocky“. To whit, Kepler 452b may be a tiny Neptune (which is itself only 3.9 Earth radii) rather than a giant Earth. So there’s that too.


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