Thousands of planets have been spotted outside of our solar system over the past few decades, but for the first time, scientists may have discovered a moon.

Exomoons, or moons outside of our solar system, are difficult to spot because of their expected smaller size compared to planets. It took a while, but scientists may have confirmed the first known exomoon, located thousands of light-years from Earth.

First Known Exomoon Discovered

Last year, an international team of researchers analyzed data from 284 exoplanets that were discovered by Kepler Space Telescope. They were looking for exomoons, and they believe that they may have found a Neptune-sized exomoon orbiting the planet known as Kepler-1625b.

The scientists needed more time and data to confirm the discovery though, and they got it through the Hubble telescope. Through the Hubble, the team studied Kepler-1625b as it passes between the star that it is orbiting, which is Kepler-1625, and the Earth.

The team found two signs, which suggested that Kepler-1625b has an exomoon. First, after Kepler-1625b passes by Kepler-1625, the brightness of the star dims a second time by a smaller amount, which may mean that the exoplanet had an exomoon in tow. Second, the transit time of Kepler-1625b happened earlier than expected, which may have been caused by an exomoon that makes the exoplanet wobble, similar to what the moon does to the Earth.

"A companion moon is the simplest and most natural explanation for the second dip in the light curve and the orbit-timing deviation," said Alex Teachey, a graduate student at Columbia University and one of the researchers on the project.

Kepler-1625b Exomoon vs Solar System Moons

The exomoon is much larger than the biggest moon of the solar system, Jupiter's Ganymede. While Ganymede has a diameter of about 3,270 miles, the Kepler-1625b exomoon is believed to be as big as Neptune, with a diameter of about 30,600 miles, making it several times bigger.

The mass ratio between the exomoon and Kepler-1625b is similar to the ratio between the moon and Earth. However, unlike the rocky moons in the solar system, the exomoon is gaseous, which means that it may have been created through a different process.