Researchers weigh up a Mars-sized exoplanet

So far, the most successful method of searching for exoplanets has been watching for the dimming they create as they pass in front of a distant a star from Earth's perspective. That's the method used by the Kepler probe, and it's identified thousands of potential worlds.

But this method doesn't tell us much about the planets. The amount of dimming tells us the radius, and the time it takes for an orbit gives us the distance between the planet and the star. But on rare occasions, we get lucky, and there are several planets packed closely enough to have significant gravitational interactions. This can tell us the mass and, combined with the radius, the density—which in turn tells us something about the composition.

Now, researchers have used this method to measure three planets in the Kepler-138, including one that's Mars sized and has a surprisingly low density.

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