Exoplanets – planets orbiting other stars – have been found by the thousands now, with many more to come. But what about exomoons? So far there haven’t been any confirmed yet, and they are much more difficult to detect, but that may change soon. A new paper, just published in Science Advances, provides a very interesting update about a possible exomoon that we’ve heard of before – orbiting the planet Kepler-1625b, 8,000 light-years away. The paper, by researchers Alex Teachey and David Kipping at the University of Columbia, is an update to earlier work and gives new support to the possibility that the object really is a moon. If so, this moon is huge – about the size and mass of Neptune, while the planet is several masses larger than Jupiter!
“This intriguing finding shows how NASA’s missions work together to uncover incredible mysteries in our cosmos,” said Thomas Zurbuchen, associate administrator of NASA’s Science Mission Directorate in Washington. “If confirmed, this finding could completely shake up our understanding of how moons are formed and what they can be made of.”
From the new paper:
“Exomoons are the natural satellites of planets orbiting stars outside our solar system, of which there are currently no confirmed examples. We present new observations of a candidate exomoon associated with Kepler-1625b using the Hubble Space Telescope to validate or refute the moon’s presence. We find evidence in favor of the moon hypothesis, based on timing deviations and a flux decrement from the star consistent with a large transiting exomoon. Self-
AQ3 consistent photodynamical modeling suggests that the planet is likely several Jupiter masses, while the exomoon has a mass and radius similar to Neptune. Since our inference is dominated by a single but highly precise Hubble epoch, we advocate for future monitoring of the system to check model predictions and confirm repetition of the moon-like signal.”
“This would be the first case of detecting a moon outside our Solar System,” said Kipping, an assistant professor of astronomy at Columbia. “If confirmed by follow-up Hubble observations, the finding could provide vital clues about the development of planetary systems and may cause experts to revisit theories of how moons form around planets.”
Teachey and Kipping analyzed data from 284 Kepler-discovered planets which were in comparatively wide orbits, with periods of greater than 30 days, around their host star. As with many other exoplanets, they measured the amount of dimming that a star would experience as a planet transited in front of it. In the case of Kepler-1625b, they noticed some additional anomalies.
“We saw little deviations and wobbles in the light curve that caught our attention,” Kipping said.
Based on those observations, the researchers were able to get 40 additional hours on the Hubble Space Telescope to follow-up on the findings. The data obtained by Hubble was four times more precise than that from Kepler.
Hubble detected a second and much smaller decrease in the star’s brightness 3.5 hours after the initial transit of the planet, consistent with “a moon trailing the planet like a dog following its owner on a leash,” Kipping said. “Unfortunately, the scheduled Hubble observations ended before the complete transit of the moon could be measured.”
Hubble also found that the planet began its transit 1.25 hours earlier than had been predicted. This is consistent with the planet and moon orbiting a common center of gravity (barycenter) that would cause the planet to wobble from its predicted location.
“An extraterrestrial civilization watching the Earth and Moon transit the Sun would note similar anomalies in the timing of Earth’s transit,” Kipping noted.
It’s possible that the anomaly is caused by a second planet, but Kepler did not find any evidence of a second planet in that system, making that possibility unlikely.
“A companion moon is the simplest and most natural explanation for the second dip in the light curve and the orbit-timing deviation,” said lead author Teachey, an NSF Graduate Fellow in astronomy at Columbia. “It was a shocking moment to see that light curve, my heart started beating a little faster and I just kept looking at that signature. But we knew our job was to keep a level head testing every conceivable way in which the data could be tricking us until we were left with no other explanation.”
There are no moons anywhere near that size in our Solar System, but there are still some planet-moon systems that are otherwise similar, such as Pluto-Charon and our own Earth-Moon system, with similar mass-ratios. Both Charon and our Moon are very large in comparison to their host planet. But while Kepler-1625b’s tentative moon is huge in size compared to its planet, its mass is estimated to be only 1.5 that of the planet. Also, given the moon’s size, it is likely to be gaseous rather than rocky, something else that does not exist in our own Solar System – kind of like a gas giant planet orbiting an even larger gas giant planet.
“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.”
Now, the newest data from Hubble strengthens the case for a moon considerably. From the new paper:
“Together, a detailed investigation of a suite of models tested in this work suggests that the exomoon hypothesis is the best explanation for the available observations. The two main pieces of information driving this result are (i) a strong case for TTVs, in particular a 77.8-min early transit observed during our HST observations, and (ii) a moon-like transit signature occurring after the planetary transit. We also note that we find a modestly improved evidence when including additional dynamical effects induced by moons aside from TTVs.
The exomoon hypothesis is further strengthened by our analysis that demonstrates that (i) the moon-like transit is not due to an instrumental common mode, residual pixel sensitivity variations, or chromatic systematics; (ii) the moon-like transit occurs at the correct phase position to also explain the observed TTV; and (iii) simultaneous detrending and photodynamical modeling retrieves a solution that is not only favored by the data but is also physically self-consistent.
Together, these lines of evidence all support the hypothesis of an AQ20 exomoon orbiting Kepler-1625b. It also represents the simplest hypothesis to explain both the TTV and the post-transit flux decrease, since other solutions would require two separate and unconnected explanations for these two observations.”
As of now, it would seem that the first exomoon has probably been found, but more observations are still needed to fully confirm the finding. The discovery also suggests that there should be many other moons out there, including of course smaller, rocky ones like in our Solar System.