Is Kepler-1625b The First Exoplanet With A Moon To Be Discovered?

Is Kepler-1625b The First Exoplanet With A Moon To Be Discovered?
Image credit: Dan Durda

The image above shows an artists’ impression of the star Kepler- 1265, with the bigger of the black dots representing its associated gas planet, designated Kepler- 1265b, while the smaller dot represents the planet’s putative moon, respectively.  The system is located about 8,000 light years away in the constellation Cygnus.

The late Carl Sagan once remarked that extraordinary claims require extraordinary proof. While much had been said and written about the notion that a moon that is about the size of Neptune might be orbiting a giant gas planet that is in its turn, more than 11 times as big as Earth, conclusive, and extraordinary proof that the moon actually exists is still lacking.

In a paper that was published in the journal Science Advances, two astronomers, David Kipping and Alex Teachy, both from Columbia University, describe how they used the Hubble Space Telescope to observe the planet Kepler-1265b over a period of 40 hours, and how their observations may, or may not indicate the presence of a giant gaseous moon orbiting the planet.

Somewhat ironically, however, their observations were an attempt to validate previous observations made with the Kepler orbiting telescope that appeared to indicate the presence of a moon, and even though the Hubble Space Telescope is 3.8 times more sensitive than the Kepler instrument, the Hubble data created more questions than it answered.

Essentially, the Hubble data revealed a secondary dip in the plant’s brightness as the planet passed in front of its parent star, but towards the tail end of the transit. Moreover, the planet’s transit of the star started more than hour earlier than predicted, which may or may not be indicative of a massive body that is perturbing the planet’s orbit around its parent star.

Although technical aspects of how the investigators modeled the data to reveal the secondary dip in brightness falls outside the scope of this brief article, the fact that the secondary dip exists is beyond question. While the authors of the paper are the first to admit that the current observations were not extensive enough to prove the existence (or otherwise) of a moon around the planet, they also go to some trouble to explain that the presence of a moon offers the most plausible explanation for the secondary dip in brightness.

As a practical matter, the possibility that exomoons might exist is not disputed, and is therefore to be expected just like exoplanets were expected to exist, but from a scientific perspective, this particular moon seems to defy the current models of moon formation. At issue is the fact that while current models of moon formation are capable of explaining the formation of solid bodies, the moon orbiting Kepler-1265B is almost certainly gaseous- a condition for which no precedent exists.

So what is it- a moon or a planet?

Given the proposed moon’s size and mass, its presence represents a conundrum in the sense that it either should not be there, since the migration of giant planets tend to disrupt or destroy their satellites, or, that it is not a moon at all, but a member of a binary planetary system.

However, the planet/moon mass ratio of around 1.5% falls well within the range of bodies that had formed in situ from a gas-poor dust disc, but this mass ratio also falls in the extreme upper range of possibilities for this model. In addition, the moons’ orbital plane is inclined by about 45% relative to the planet’s rotational plane, which requires a separate explanation since gas discs are usually not that far inclined from a central bodies’ rotational plane.

Other theories about the nature of the object include the possibility that it might have been captured by the gas planet, notwithstanding the fact that its current separation from the planet cannot be reproduced with advanced computer modelling techniques. Captured objects usually orbit very close the objects that captured them, and in this case, the moon’s distance from the planet falls outside of the range of possible orbital separations.

One further theory holds that the moon was somehow exchanged between objects in a binary system, but since this mechanism presupposes the presence of at least one initial binary system, the absence of other nearby planets (or other objects) in the immediate vicinity of Kepler-1265b seems to rule out this scenario. However, given that the parent star is between 9 and 10 billion years old, the possibility that other members of a planetary system could have been ejected from the system by orbital instabilities and/or conflicts cannot (yet) be ruled out entirely.

In the interest of fair reporting though, it must be stated that the authors of the paper agree with their peers who entertain grave doubts about the existence of this massive gaseous moon. The authors have gone to some lengths to point out that their initial results do not represent definitive proof of the existence of the moon, and that they need additional observing time on the Hubble instrument to both develop and refine their data, which they will no doubt do when they next observe Kepler-1265b during the first half of 2019.

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