A second exomoon discovered in another solar system

Despite the discovery of more than 10,000 candidates forexoplanets, the observation of exomoons is very rare: yet this is the challenge that a team ofastronomers from Cool World Labs from Columbia University in New York who, after deferring a first candidate fromexomoon in 2017, thinks to have detected an exomoon again in a planetary system at more than 5,000 light years from us, in the direction of constellation of the Swan and the Lyre. This new exomoon, orbiting around Kepler-1708b, a exoplanet of the size of Jupiter, would be comparable in size to two-thirds of our Neptune. The first exomoon detected by the team, the size of Neptune, orbit meanwhile around Kepler-1625b, an exoplanet comparable to Kepler-1708b.

Gaseous exomoons captured by more massive exoplanets?

According to the team of astronomers, the two newly discovered supermoons are likely to be mainly composed of gas, like Neptune. If their hypothesis is correct, these moons could have actually started life as planets which, after accreting a certain amount of matter, would have been captured by the gravitational pull of more massive planets. The great distance which separates Kepler-1708b and Kepler-1625b from their respective stars confirms their idea, the gravitational attraction exerted by the planets on their exomoons probably dominating on that exerted by their stars.

It is moreover around this type ofexoplanets – resembling Jupiter or Saturn, and with fairly wide orbits – that the team of astronomers concentrates in its hunt for the exomoon: if we rely on what we know in our Solar system (Jupiter and Saturn have more than a hundred moons around them), it is around these planets that exomoons would be most likely to hide.

Confirmation pending

It is by using the method of transits that the team of astronomers detected the exomoon, in the same way as for the detection of the exoplanet around which it orbits: when the brightness measured from the star decreases periodically, the phenomenon is associated with the passage of an exoplanet in front of the star. But these variations are generally weak and difficult to detect in the case of an exoplanet, making them even more minute in the case of an exomoon.

As a result, the existence of these exomoons is still highly debated: the first, detected by the team in 2017, is still unconfirmed after more than four years of debate. Some astronomers, skeptical of the discovery, argue that the signal interpreted as the presence of an exomoon may in fact be a simple fluctuation in the data due to the observed star, or simply noise originating in the observing instrument himself. In the case of the new exomoon candidate detected around Kepler-1708b, further observations, for example using the hubble telescope, will be needed to confirm its existence.

But the team of astronomers remains largely enthusiastic, as the search for exomoons offers an almost pristine new field of exploration, which could mark advances in various fields of space exploration, including the search for life outside of space. our Solar system.