Astronomers discover first alien exomoon in another star system

Astronomers believe they have made the first discovery of a moon orbiting a planet in another star system, 4,000 light-years away.

An artist’s impression of the exoplanet Kepler-1625b with its newly identified exomoon. Image credit: Dan Durda

NASA’s Kepler Space Telescope has discovered thousands of so-called exoplanets, or worlds around distant suns, since it began operations in 2009.

Now a team of researchers has zoomed in on one of these planets with the Hubble Space Telescope, operated jointly by ESA and NASA, and found evidence of an exomoon circling it.

The team of astronomers caution that they need more follow-up observations of the exoplanet, labelled Kepler-1625b, before the discovery can be confirmed. It lies about 4,000 light-years away in the constellation of Cygnus.

But if their initial findings are correct, it will be a profound moment in the search for worlds elsewhere in the Galaxy. Astrobiologists suggest that exomoons could be fertile locations in which to find alien life.

The possible exomoon’s parent planet, Kepler-1625b, is a Jupiter-like gaseous world, so probably unfriendly to life as we know it. However, our own Jupiter and the other gas giants in our Solar System have moons which are predominantly rocky, so other Jupiter-like planets should have too.

Researchers Alex Teachey and David Kipping, of Columbia University’s Department of Astronomy, found the exomoon candidate when they made follow-up observations of results from Kepler.

Kepler surveyed a sample of 284 planets that it had discovered when they transited, or passed in front of, their parent stars, dimming their light.

That survey had hinted at the possibility that Kepler-1625b might have an exomoon in its orbit. Teachey and Kipping were given time to use the Hubble Space Telescope to study the exoplanet to see if they could find more evidence for the exomoon.

They looked for two signals to help indicate that the exomoon was really there – a smaller dimming of the starlight as the exomoon made a transit, and any effects on the orbit of the exoplanet itself by the exomoon’s gravitational pull. The team say they found evidence for both.

About three and a half hours after a transit of Kepler-1625b across the star, a second smaller dimming was recorded, suggesting an exoplanet trailing the planet.

Hubble also showed that the exoplanet’s own transit in front of the star occurred nearly 80 minutes earlier than they expected, which could be due to the pull of the exomoon. It is estimated to be about 10 times as massive as our own Moon.

It is quite obvious that the Milky Way must be filled with exomoons as well as exoplanets, as our own Solar System is unlikely to be a special case. It has long been thought that they will be identified in time as technology and observing techniques improve.

In April, a new spacecraft, called TESS, launched to begin a fresh search for worlds around other stars.

The evidence is very good that the exomoon orbiting Kepler-1625b is real, and astronomers will be eager to make more follow-up observations.

Related: Conditions ideal for life could exist on moons of Saturn and Jupiter

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