A large team of astronomers — including two members of Yale’s Astronomy Department — has discovered the Milky Way’s smallest known planet after analyzing two years of data from NASA’s Kepler probe.

Smaller than Mercury and barely larger than Earth’s moon, the planet closely orbits the star Kepler-37, located over 200 light-years away from Earth. The planet, currently referred to as Kepler-37b, is part of a solar system that also includes a planet 70 percent the size of Earth and another that is twice as large as Earth — both of which were also discovered by the research team. These findings were announced in a paper published in the journal Nature on Feb. 20.

Thomas Barclay, a scientist at the NASA Ames Research Center, found the first evidence of the planet’s existence in data from Kepler, an orbiting spacecraft that surveys stars in a small region of space to find planets. Kepler determines the existence of planets by observing host stars and looking for transits, which occur when a planet moves in front of its host star as it orbits. The Kepler spacecraft is equipped with a photometer that can measure the tiny, regular decreases in the star’s brightness that occur during a transit.

Once the NASA scientists were confident that there were planets orbiting Kepler-37, professors at universities around the world worked to characterize the size, density and composition of the star and its planets.

“The only way you can measure the size of the orbiting planet is relative to the star it is going around,” said Yale astronomy professor Sarbani Basu, an author of the paper.

An expert on star seismology, Basu used Kepler data to measure the vibrations of the star’s gases. Using this information, Basu and the seismic analysis team were able to determine the radius of Kepler-37. They found that the star is smaller and lighter than the sun — and knowing this, they could infer that the planet Kepler-37b was even smaller than Mercury. Debra Fischer, another astronomy professor at Yale, helped determine the chemical makeup of the star, Basu said.

The discovery of Kepler-37b contributes to the rapidly changing perception of the size and number of planets in the Milky Way, she added.

“We now know that planets can come in any size,” Basu said, adding that the sizes span from small rocky planets to gas giants bigger than Jupiter.

Cornell astronomy professor Jonathan Lunine, who was not involved in the study, said the discovery will help scientists estimate the prevalence of Earth-like planets in the Milky Way, an important goal of the Kepler mission.

“The way that you determine the statistical frequency of Earth-like planets requires looking at planets of many different sizes,” he said. “This is a really important discovery because it allows us to extend the frequency of planets below the size of the Earth. It helps us get a better handle on the numbers.”

Lunine added that the seismology work done by Basu and her collaborators was a “novel” way to measure the size of the star.

However, he said he remains slightly skeptical of the discovery, as some “noise” in the photometry data may have obscured results. He added that the strength of the claim is “still a little shaky, but that’s the nature of the fact that they are working at the bottom end [of the size spectrum].”

Though Kepler-37b is almost certainly too hot and too small to host life on its surface, its discovery is an encouraging step in the search for an Earth-like planet in the habitable zone of its star — the “holy grail” of exoplanet research, Basu said.

Steve Howell, the NASA project scientist for the Kepler mission and an author of the paper, said the Kepler probe has shown that small planets are quite common in the galaxy.

“If Kepler had never found a small planet, we would think that Earth is pretty special,” he said. “That would be sad, but interesting. But we have found hundreds and hundreds of small planets in one space in the sky. What Kepler has found is that it’s pretty unlikely that the Earth is alone.”

Since its 2009 launch, the Kepler spacecraft has detected 114 confirmed extrasolar planets, with over 2,000 exoplanet candidates awaiting confirmation, according to the NASA Kepler website.