First evidence of moon outside our solar system: Astronomers

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What popped out was a planet called Kepler-1625b, a Jupiter-sized world that orbits a star around 8,000 light-years away.

While Kepler 1625b's wobble could be caused by the gravitational pull of another planet orbiting the parent star rather than by an orbiting moon, Teachey and Kipping believe a moon is the most likely explanation. It orbits about 1.8 million miles from its Jupiter-size planet.

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. The researchers say this may yield new insights into the development of planetary systems and may cause experts to revisit theories of how moons form around planets.

They were hoping to find a moon that was the size of Jupiter's large moons, and then requested for a more detailed observation with the Hubble telescope.

"An extraterrestrial civilization watching the Earth and Moon transit the Sun would note similar anomalies in the timing of Earth's transit", Kipping said. The observations measured the momentary dimming of starlight as a planet passed in front of its star, called a transit.

The researchers requested time on the Hubble Space Telescope, and their request, which was public, generated a bit of excitement among those who follow new planet discoveries.

David Kipping, the study's second author, said, 'We saw little deviations and wobbles in the light curve that caught our attention'. This small decrease is consistent with a gravitationally-bound moon trailing the planet, much like a dog following after its owner.

Unfortunately, the scientists' time on Hubble ended before they could completely observe the second transit.

"You could argue that because larger objects are easier to detect than smaller ones, this is really the lowest-hanging fruit, so it might not be wholly unexpected that the first exomoon detection would be among the largest possible", Teachey said.

Spotting an exomoon is done in largely the same way, and if the team's measurements are on point we're looking at an absolutely enormous moon and an even larger host planet. "Furthermore, the size we've calculated for this moon, about the size of Neptune, has hardly been anticipated and so that, too, is a reason to be careful here", said Alex Teachey. Kepler 1625b is roughly the same distance from its star as Earth is from the sun.

In July 2017, researchers started observing Kepler-1625b, an exoplanet orbiting the star Kepler-1625 in the constellation Cygnus.

However, in the case of the Earth-Moon system and the Pluto-Charon system - the largest of the five known natural satellites of the dwarf planet Pluto - an early collision with a larger body is hypothesised to have blasted off material that later coalesced into a moon.

Exomoons are hard to find because they are very small, so the dip they cause in light is obviously weaker than a planet that is, by comparison, much larger. "But we knew our job was to keep a level head testing every conceivable way in which the data could be tricking us until we were left with no other explanation". They made a decision to look at exoplanets with the widest orbits, or those that take about 30 days to circle their stars. These observations have to be done from space; the rotation of Earth means that ground-based telescopes spin away from their targets before they can capture a whole event. In 2021 NASA is going to launch into space telescope James Webb to take a closer explore these planets - for example, to look for biosignatures that would indicate the presence of life on them.