“Earth-like” planets discovered by Kepler. Credit: NASA Ames/W. Stenzel

There’s been buzz in the news about the latest planet discovered by NASA’s Kepler space telescope, Kepler-452b. It’s being hailed as a major discovery. It’s the first ever Earth-sized planet found in the habitable zone of a…boy, this sounds familiar, doesn’t it?

If you’re a regular follower of NASA and exoplanets, you probably remember hearing them announce the discovery of the first “Earth-like” or “potentially habitable” or “Earth-sized” planet multiple times. It seems like every year, they make a groundbreaking discovery, and it’s the same discovery. So what’s really going on? It actually has to do with how “Earth-like” you want it to be.

To be fair to NASA, this is only the third time they’ve made this kind of announcement, and they recapped the other two. And also to be fair, this really is the first time they found a planet that they’re 100% sure is potentially habitable…well, make that 99%. Kepler-452b’s radius is 1.6 times the radius of Earth. That means it is probably rocky, or maybe made of water. No thick hydrogen atmosphere here, or at least we’re pretty sure.

NASA made two similar announcements in the past of planets that could possibly be “Earth-like”. First, in December of 2011, they announced the discovery of Kepler-22b, which at the time was the smallest planet yet found in the habitable zone of its star. The habitable zone is basically the range of distances where liquid water can exist, although it’s more complicated than that. The trouble is that Kepler-22b is 2.4 times the size of Earth. At the time, we weren’t sure whether it would have a thick hydrogen atmosphere or be rocky, but with better internal models, we now know that a planet that large must have a lot of hydrogen.

Then, in April of 2014, Kepler-186f was announced. This planet is practically the same size as Earth and is in the habitable zone where liquid water can form. It sounds like this should be the big discovery, but it’s not. Why not? Two reasons. First, despite being in the habitable zone, Kepler-186f is colder than Earth. It gets about the same amount of sunlight as Mars, which is really cold and doesn’t have liquid water. On the other hand, it’s bigger, so it might be able to hold onto a thick carbon dioxide atmosphere that keeps it warm. So we don’t actually know whether it’s the right temperature for life or not.

The other problem is that Kepler-186f orbits a red dwarf star. Red dwarfs are very cool stars, and planets have to orbit very close to them to stay warm. But planets that orbit so close become tidally locked, always keeping one side towards their stars so that they have a permanent hot day side and a permanent cold night side. We don’t know what that does to the weather and climate, and we don’t know whether it’s very hostile to life. (However, I’m an optimist on this matter. I believe that “Life, uh, finds a way.”)

So to be absolutely sure that we’ve found a planet that has the right characteristics to support life, it must be close to Earth-sized, the right temperature for liquid water to form, and orbiting a bigger, brighter star so that it doesn’t become tidally locked.

Kepler-452b is questionable on two of these counts. It’s 60% larger than Earth, so we’re not completely sure it’s rocky, and it’s a little hotter than Earth, meaning it could be at risk for a runaway greenhouse effect, like on Venus, that would make it uninhabitable. (Personally, I think the idea that planets can so easily tip into a runaway greenhouse is pessimistic, based on questionable models that are perilously close to saying that Earth is uninhabitable.)

As for the tidal locking question, Kepler-452b is not tidally locked. The trouble is, I’m not convinced it’s the first one. Look here:

Plot of planets with stellar type against amount of sunlight received, including the tidal locking radius (Yang et al. 2014).

This graph shows where planets fall in terms of the temperature and the amount of sunlight of various planets. Planets below the gray line are tidally locked, and green planets near the green line most definitely “Earth-like”. It looks like planets start to become tidally locked midway through the K-dwarfs, that is, the intermediate orange dwarf stars. As you can see, Kepler-283c (and that’s just from the Kepler planets) is also not tidally locked and could be potentially habitable. However, Kepler-283c is 1.86 times the size of Earth, and based on our current understanding, it is much less like to have a solid surface.

You have similar problems with other promising discoveries from Kepler–too hot, too cold, too big, or orbiting a star that’s too small. Except Kepler-442b. Kepler-442b is in a similar position to Kepler-283c–orbiting a cool star and a little colder than Earth, but not tidally locked. However, Kepler-442b is smaller at only 1.34 times the size of Earth. At that size, it pretty much has to be rocky, and with that, it fulfilled all of the requirements to be “potentially habitable” before this new discovery.

So why did Kepler-452b get the hype and not Kepler-442b? It’s pretty clear that it’s because Kepler-442b is orbiting a sun-like star rather than a red dwarf. However, it’s my opinion that the mission scientists are getting a little too hung up looking for a star so similar to the Sun, and the real test should be whether the planet is spinning freely.