Exoplanets are being found so frequently now that they have become commonplace. But what about exomoons? As might be expected, they are much harder to detect, being typically much smaller than most planets. There have been tantalizing hints but nothing conclusive so far. That may be about to change soon, however. Astronomers from Columbia University have reported the possible discovery of the first exomoon – but they stress that it is still unconfirmed and just a candidate at this point.
The tentative moon orbits a planet called Kepler-1625b, which orbits the star Kepler-1625. The star is about 4,000 light-years from Earth, and similar in size to the Sun, but with a mass 1.8 times that of the Sun. The planet is about the same size as Jupiter, but 10 times as massive. According to the analysis done so far, the moon would be the size of Neptune! There are no moons in our Solar System of that size, so this would be an unusual discovery if it pans out. The evidence for the possible moon is in the form of asymmetries in the transits observed as the planet passed in front of its star, as seen from Earth.
According to the new paper, “This candidate has passed a thorough preliminary inspection, but we emphasize again our position that the Kepler data are insufficient to make a conclusive statement about the existence of this moon. Only after the HST observation is made should any claim about this moon’s existence be given much credence.”
Little else is known about this putative moon, but there will be additional observations soon to help answer the question one way or the other – the Hubble Space Telescope is scheduled to study Kepler-1625b further in October. If the moon really is there, HST should be able to confirm it.
The new paper also discusses the overall search for exomoons around planets close in to their stars, mostly giant planets (i.e. “warm Jupiters”). The results so far suggest that moons orbiting those planets will be fewer in number than expected. The giant gas and ice planets in our Solar System have dozens of moons, although they are much farther out from the Sun. Only two of the four inner planets, Earth and Mars, have moons, and only one and two respectively. But larger planets closer to their stars were still expected to have more moons than this study suggests, so there is a bit of a puzzle to figure out.
With regard to the new exomoon candidate, the researchers want to emphasize that it still needs to be confirmed (or not). They wrote the Scientific American article to try to mitigate any premature sensationalistic media coverage, since the plans to observe again with Hubble had already been publicized. The paper has been submitted, but not yet accepted, in the Astrophysical Journal. From the article:
“Peer review is a critical part of the scientific process, and we are not terribly comfortable putting out our results before they have been examined by a qualified referee. Unfortunately, we feel the circumstances have forced us to make our results freely available to the public before such a review, so that everyone may see for themselves what we are claiming and what we are not. While David and I are both big proponents of engaging with the public and boosting interest in the incredible things happening every day in astronomy, we have serious concerns about the potential for sensational headlines misleading the public into thinking a discovery has been made when it is really too early to say that for sure. ”
So for now, the findings are tantalizing, but we will need to wait until October before hopefully popping any champagne bottles.