We think that most of the exoplanets out there in the universe are likely to be smaller than Earth. However, existing data doesn't support this. Since it's quite difficult to see small planets in the infinity of space, most exoplanets that we've so far observed are larger than Earth. 

But for the first time, astronomers have used a creative method to measure the mass and size of a planet smaller than Earth that is located nearly 200 light years from our solar system. In fact, it is closer to the size of Mars (which has roughly half the radius of Earth). The planet is named Kepler-138b, and it is one of three orbiting a central red giant star called Kepler-138. 

Daniel Jontof-Hutter, a research associate in astronomy at Penn State, who led the study published in Nature, describes how they measured the size of Kepler-138b: "Each time a planet transits the star, it blocks a small fraction of the star's light, allowing us to measure the size of the planet."

As planets orbit a central star, their gravitational attraction makes the star wobble. Massive planets like gas giants (similar to Jupiter) make the central star wobble a lot, whereas small, rocky planets with a low mass only have a miniscule gravitational effect on the host star. By measuring how much they affect the star's orbit, astronomers can trace back and figure out the mass of the planet.

It's especially difficult to measure the masses of rocky planets because they are usually smaller and less massive than Jupiter-like gas giants. However, astronomers have found a way around this issue. Instead of measuring a rocky planet's effect on a central star, they measured the planets' effects on each other.

"We also measured the gravity of all three planets, using data from NASA's Kepler mission, by precisely observing the times of each transit," Jontof-Hutter said. The astronomers were also able to measure the masses of these planets. "Each planet periodically slows down and accelerates ever so slightly from the gravity of its neighboring planets. This slight change in time between transits allowed us to measure the masses of the planets," he added.

Measuring the mass and the size of an exoplanet means that scientists can start to calculate more of its features, such as its density, bulk composition, and maybe even if it could harbor life.

[Via Penn State, Nature]