Artwork by by NASA/JPL-Caltech/T. Pyle

Jul 28, 2015 Kepler-452b - what we really know

Sen—The astronomy world (or worlds?) was abuzz last week with news that an Earth-sized exoplanet had been found orbiting a star very much like the Sun. Called Kepler-452b, the planet was hailed by many as the closest analog yet found to Earth.

… except, in my opinion, not so much. We really have no idea just how much like our own Earth this planet is. In truth we hardly know anything at all about it besides its size and orbital period, and what we do know about planets like it is that Earth-like they ain’t.

OK, let me start from the beginning.

We have several methods for finding planets orbiting other stars. The most successful to date is the transit method: If the planet’s orbit is edge-on as seen from Earth, then once per orbit we see it passing directly in front of the star, blocking a small amount of starlight. The dip in light detected is small, less than 1% (and usually a lot less), so careful observations need to be made.

That’s why NASA built and launched the Kepler spacecraft; it stares at one patch of sky containing about 150,000 stars, continuously measuring their output. Stars that fade on a regular schedule, a repeating, periodic pattern, reveal the tell-tale signs of transiting planets.

Thousands of exoplanets (the technical term for an alien world orbiting an alien star) have been discovered in the Kepler data, and many thousands more await confirmation.

The transit method has some biases. For example, it favors big planets, since those block more starlight and are easier to see in the data. Or, the flipside of that, is that it favors smaller stars. A planet of a given size blocks more light from a small star than a big one.

As time goes on and analysis methods get better, we find smaller planets. Quite a few Earth-sized planets have been discovered, and of these, about a dozen have been found at the right distance from their star to have liquid water on their surfaces. But all of them orbit red dwarf stars, smaller and cooler than the Sun.

Enter Kepler-452b. It’s the first exoplanet seen that’s close to the size of Earth and orbiting a star very much like the Sun.

So of course the NASA announcement last week made a lot of people very excited. Headlines claimed astronomers found “Earth’s twin”, or the marginally better “Earth’s cousin”. Lots of people speculated about its habitability, and whether it has life on it.

But there’s a problem with all this. All we really know about Kepler-452b is how big it is—1.6 times the Earth’s diameter—and how long its orbit is (385 days, just a hair more than Earth’s year). That’s it. We don’t know the single most important thing about it: Its density.

The density tells you what the planet is made of. Ice is very low density, water somewhat higher, rock even more, and metal the most. The Earth is a mix of rock and metal, and has a density of 5.5 grams per cc. Jupiter is mostly gas and has a density of about 1.3.

If Kepler-452b is made of the same mix of rock and metal Earth is, then it will have 1.6 times the surface gravity Earth does. That’s substantial, and some models of how planets form indicate that’s enough for it to grow and hold onto a thick atmosphere, making it more like a mini-Neptune than a super-Earth.

Even if it avoided that fate, there’s no guarantee it even has an atmosphere! If it doesn’t have a magnetic field, then its atmosphere is exposed to the solar wind from its star, which, over billions of years, can strip the air away. That’s what happened to Mars, we think. I’ll note that the planet and star are over 6 billion years in age, by the way, 1.5 billion years older than the Sun and Earth.

Does it even have water? It receives more light and heat from its star than Earth does from the Sun; how hot is it? Did any water boil away?

We have far, far more questions than facts about this planet.

Let me be clear: This is a very cool discovery, and an important one. It shows the Earth isn’t a fluke! At least one other star like the Sun managed to make a planet like ours! That’s amazing, and brings hope for finding more.

But Earth-like? We literally have no idea. Exoplanet discoveries truly are one of the most revolutionary aspects of modern astronomy (the first was only discovered in 1992!), and as more of them roll in there will be plenty of chances for us to be awe-stricken. It’s perfectly fine to be excited about this, but let’s not let our excitement exceed our knowledge. 

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