New confirmations that the exoplanet Kepler-1625b has a moon

Artist's concept of the planet Kepler-1625b with its moon and its star in the background (Image NASA, ESA, and L. Hustak (STScI))
Artist’s concept of the planet Kepler-1625b with its moon and its star in the background (Image NASA, ESA, and L. Hustak (STScI))

An article published in the journal “Science Advances” presents the evidence of the existence of a exomoon, a moon orbiting a planet of another solar system, named Kepler-1625b-I. David Kipping and Alex Teachey of Columbia University used observations of the Kepler and Hubble space telescopes to examine the traces left by the exoplanet Kepler-1625b in front of its star, similar to the Sun. The first indications of the discovery of the exomoon candidate were revealed in July 2017, follow-up observations carried out with the Hubble Space Telescope provided new confirmations.

The search for exomoons represents one of the next frontiers of astronomy but discovering one is much more difficult than discovering exoplanets. The Kepler space telescope allowed to discover a few thousand exoplanets, and others have yet to be confirmed, observing the tiny drop in their star’s brightness when they pass in front of it. In the event that an exomoon passes in front of its star as well, that drop would be much more limited and difficult to distinguish from an instrumental limitation or another cause.

A few years ago astronomer David Kipping founded the Hunt for Exomoons with Kepler (HEK) program, affiliated to the Harvard-Smithsonian Center for Astrophysics, with the aim of searching for exomoons using the observations carried out by the Kepler space telescope. The analysis of the data on the exoplanet Kepler-1625b revealed some anomalies in its transits that could be due to a smaller object near the planet.

The request for follow-up observations with the Hubble Space Telescope caused a news leak leading to the publication of the article in July 2017. Between October 28 and 29, 2017 Hubble allowed to observe the transit of the exoplanet Kepler-1625b, a gas giant with a mass several times Jupiter’s while having a similar size. Hubble can obtain data four times more accurate than Kepler’s for that kind of research and the results were positive.

First of all, the transit of the exoplanet Kepler-1625b started 1,25 hours earlier than predicted and this suggests an planet’s wobble caused by another object like a moon. During the 19 hours of transit, the light traces showed some drops that were extremely reduced but consistent with the presence of a large moon around the planet. The first data suggested the surprising possibility that the exomoon was really huge compared to those in the solar system, perhaps even Neptune-sized, and the new data confirmed that hypothesis.

Theoretically, the detected effects could be caused by another planet in the Kepler-1625 system. It’s a hypothesis that must be taken into consideration but it’s strange that in four years of observations with the Kepler space telescope it was never spotted. The presence of a moon orbiting the planet Kepler-1625b is the simplest explanation for the data collected.

The first exomoon discovered might be really strange, different from all the ones existing in the solar system. In fact its estimated mass is about 1.5% of the planet Kepler-1625b and from this point of view it’s not so strange. The fact is that the models indicate that in the solar system very large moons were created as a result of cosmic collisions that created the Earth-Moon and Pluto-Charon pairs. These are rocky objects while Kepler-1625b is a gas giant so a planet that hit it would’ve passed through it without stripping enough gas to form a Neptune-sized moon.

David Kipping and Alex Teachey advocate further monitoring of the Kepler-1625 system to obtain more data that can provide a final answer. It’s normal for the first exomoon candidate to require many confirmations for its existence to be accepted and this will help further research into other systems. Such massive moons are the ones most likely to be detected, in the coming years other anomalies found in the data collected by Kepler will be examined and this new frontier of astronomy could really open up.

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