Hot Rocks: Hints About the Smallest Exoplanets Yet Found

Comparison of the sizes of Kepler-20e, Kepler-20f, Earth, and Venus (in an artist's impression).

I might give the impression that I’m a Grinch about exciting science news. My first reaction to all these stories is to find the hype, and quickly debunk. And usually there’s something big to debunk, unfortunately: it’s not that I want to be the Grinch, it’s just that I’d rather people know the truth than get excited for the wrong reason.

So here we go: the Kepler mission has found two possible planets that are not terribly different from Earth in size. The star system is designated Kepler-20; the star is a yellow star (class G for those following at home), slightly less hot than the Sun, but of the same type. Five planets have been identified orbiting it, and given the letters “b”, “c”, “d”, “e”, and “f”. The smallest two, Kepler-20e and Kepler-20f, are estimated to be 87% and 103% of Earth’s radius, making them the smallest yet found orbiting a Sun-like star. That size means they’re probably rocky, though because their masses are not yet known, we don’t know anything about their density or composition.

However, they are not Earthlike in anything but size: they both orbit closer to their host star than Mercury does in our Solar System, so they are nowhere close to being in the habitable zone. As Phil Plait points out, the surface temperature on Kepler-20e is hot enough to melt tin, and even though Kepler-20f is farther out, any water on its surface would boil to vapor. To make the system weirder, the Earth-sized worlds are interleaved with larger planets, unlike our Solar System where the rocky planets are in the inner system and the gaseous worlds in the outer regions. How the planets ended up like this is an intriguing mystery: my friends who study such things tell me the planets likely formed farther from the star and migrated inward, but how this actually happened is puzzling.

So how did the Kepler mission even find these planets? It’s not easy: the basic method uses the transit of the planet across its star, creating a momentary dip in the brightness of the star. When the exoplanet is relatively large, the eclipse is strong enough that there isn’t much ambiguity, but when the planet is very small in size, things get complicated. Stars don’t always produce the same amount of light: there are small fluctuations from activity (like sunspots), and there might be wisps of dust that pass between us and the star. Maybe the eclipse isn’t caused by a planet but a white dwarf, which is roughly Earth-sized, but a lot more massive. Astronomers have to rule those options out before they can say definitively this is an exoplanet.

To that end, researchers at the Harvard-Smithsonian Center for Astrophysics developed a software tool called BLENDER, because it models the light we see from the star as a blend from these different possibilities. By adjusting the relative strength of contributions from different possible reasons for dimming the star’s light, they can determine if there really is an exoplanet present, or if something else might be the culprit. I can’t claim I know the details of how the software works, but we use a similar technique in cosmology: we match the “thumbprint” of the Cosmic Microwave Background with various possible combinations of contents in the universe until we get the correct proportions. Using BLENDER, the Kepler scientists have great confidence that Kepler-20e and Kepler-20f really are planets.

So, if you can forgive me for being Grinchy in similar posts I’ve written, I’m rather pleased with this find. We’ll know more when the official paper comes out (and hopefully Caleb Scharf, who deserves credit for the “Hot Rocks” headline, will write about it soon), and of course serious follow-up observations will need to be done, but things do look pretty good to me so far. Now if we can just keep headline writers from calling these planets “Earthlike”, I’ll be a happy Grinch.

4 responses to “Hot Rocks: Hints About the Smallest Exoplanets Yet Found”

  1. The article about this discovery says that once the transit method had indicated the possibility of an exo-planet, “The researchers then used ground-based observatories to confirm that the planets actually exist by measuring minute wobbles in the star’s position caused by gravitational tugs from its planets.”

    Would this be general practice that the author is referencing, or do you know if this was actually done in this specific case?

    1. That’s an effective method if the planet is relatively massive, but as far as I can tell it won’t be good for Kepler-20. The larger planets in the system would have a much larger effect than the Earth-sized planets. They can only put an upper limit on the mass of the smaller planets based on how much they would make the star wobble (which does help rule out stuff like white dwarfs). The BLENDER technique uses the light fluctuations (photometry, for the astro-heads in the crowd), and that’s how they found the smaller worlds in the data.

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  3. Eric That’s the radial velocity method I guess

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