Over the last couple of decades, astronomers have identified thousands of planets outside our solar system. However, they have been unable to find confirmation of a moon revolving around any of the distant worlds, mainly because the generally smaller satellites are harder to spot. Now, some researchers from Columbia University believe they have found evidence of an alien moon orbiting a gaseous exoplanet which, lies almost 8,000 light years away.
For their study, astronomers Alex Teachey and David Kipping, who have been searching for alien moons for nearly a decade, focused on the 284 exoplanets discovered by the planet-hunting Kepler spacecraft. The distant worlds were considered the best candidates for potential exomoons due to their relatively wide orbits that span more than 30 days.
Specifically, the scientists were searching for two dips in light – one caused by the exoplanet as it passed in front of its star and the second by another, most likely smaller, celestial body. After sifting through the data collected by Kepler, the researchers were able to identify one exoplanet, Kepler-1625b, that looked promising. “We saw little deviations and wobbles in the light curve that caught our attention,” Kipping said.
To confirm their suspicions, the team used the Hubble telescope to monitor the planet before and during its 19-hour-long transit across the face of its star. Sure enough, about 3.5 hours after the Kepler-1625b had passed, Hubble detected a second, much smaller decrease in the star’s brightness, which Kipping said was consistent with “a moon trailing the planet like a dog following its owner on a leash.”
Their exomoon hypothesis was further confirmed when the planet began its transit 1.25 hours earlier than predicted. This happens only when an orbiting moon pulls at the planet and causes it to wobble, as is the case with our own moon. “An extraterrestrial civilization watching the Earth and Moon transit the Sun would note similar anomalies in the timing of Earth’s transit,” Kipping explained.
The researchers, who published their findings in the journal Science Advances on October 3, 2018, estimate that the newly-discovered moon has a diameter of 30,600 miles (49,000 km). In comparison, our solar system's biggest moon - Jupiter’s Ganymede – boasts a diameter of about 3,270 miles (5,260 km). However, the exomoon is tiny compared to Kepler-1625b which is believed to be several times bigger than Jupiter. The researchers estimate the satellite's mass to be just 1.5 percent that of the planet it orbits, about the same ratio as the Earth and our moon. Though the host planet and its moon lie within the solar mass star’s (Kepler 1625) habitable zone, which has moderate temperatures and hence the possibility of water on planets that feature a solid rocky surface, there is no chance of finding aliens. As Kipping explains, “Both bodies are considered to be gaseous and therefore unsuitable for life as we know it.”
The team’s conclusion still has to be verified by follow-up Hubble observations of the moon’s transit. If confirmed, the discovery could provide vital clues about the development of planetary systems and may cause experts to revisit theories of how moons form around planets.